Developmental changes in nuclear lamina components during germ cell differentiation

The nuclear lamina (NL) changes composition for regulation of nuclear events. We investigated changes that occur in Drosophila oogenesis, revealing switches in NL composition during germ cell differentiation. Germline stem cells (GSCs) express only LamB and predominantly emerin, whereas differentiating nurse cells predominantly express LamC and emerin2. A change in LamC-specific localization also occurs, wherein phosphorylated LamC redistributes to the nuclear interior only in the oocyte, prior to transcriptional reactivation of the meiotic genome. These changes support existing concepts that LamC promotes differentiation, a premise that was tested. Remarkably ectopic LamC production in GSCs did not promote premature differentiation. Increased LamC levels in differentiating germ cells altered internal nuclear structure, increased RNA production, and reduced female fertility due to defects in eggshell formation. These studies suggest differences between Drosophila lamins are regulatory, not functional, and reveal an unexpected robustness to level changes of a major scaffolding component of the NL.

Granting access: Development of a formal course to demystify and promote predoctoral fellowship applications for graduate students

Strong scientific writing skills are the foundation of a successful research career and require training and practice. Although these skills are critical for completing a PhD, most students receive little formal writing instruction prior to joining a graduate program. In 2015, the University of Iowa Medical Scientist Training Program (MSTP) addressed this issue by developing the scientific writing course Grant Writing Basics (GWB). Here we describe the structure of this course and its effectiveness. GWB is an interactive, workshop-based course that uses a National Institutes of Health (NIH) F30 predoctoral fellowship proposal as a platform for building writing expertise. GWB incorporates established pedagogical principles of adult learning, including flipped classrooms, peer teaching, and reiterative evaluation. Time spent in class centers on active student analysis of previously submitted fellowship applications, discussion of writing resources, active writing, facilitated small group discussion of critiques of student writing samples, revision, and a discussion with a panel of experienced study section members and a student who completed a fellowship submission. Outcomes of GWB include a substantial increase in the number of applications submitted and fellowships awarded. Rigorous evaluation provides evidence that learning objectives were met and that students gained confidence in both their scientific writing skills and their ability to give constructive feedback. Our findings show that investment in formal training in written scientific communication provides a foundation for good writing habits, and the knowledge and skills needed to succeed in this vital aspect of a scientific research career. Furthermore, they highlight that evaluation is valuable in guiding course evolution. Strategies embedded in GWB can be adapted for use in any graduate program to advance scientific writing skills among its trainees.

Guidelines for organizing accessible and inclusive research retreats

Research retreats are elements of scientific graduate training programs. Although expected to provide strong educational value, some students are reluctant to attend. Here, we identify participation barriers and provide guidelines for retreat design that minimize obstacles and establish an inclusive environment to improve attendance and enrichment for all attendees.

Immunohistochemical Analysis of Nuclear Lamina Structures in the Drosophila Ovary Using CRISPR-Tagged Genes

The Drosophila ovary represents an outstanding model for investigating tissue homeostasis. Females continuously produce oocytes throughout their lifetime. However, as females age, fecundity declines, in part, due to changes in ovarian niche function and germline stem cell (GSC) homeostasis. Understanding the dynamics of GSC maintenance will provide needed insights into how coordinated tissue homeostasis is lost during aging. Critical regulators of GSC maintenance are proteins that reside in the nuclear lamina (NL), including the NL proteins emerin and Barrier-to-Autointegration Factor (BAF). Continued investigation of how emerin, BAF, and other NL proteins contribute to GSC function depends upon the availability of antibodies for NL proteins, a limiting resource. In this chapter, we discuss strategies for using clustered regularly interspaced short palindromic repeats (CRISPR) genomic editing to produce endogenously tagged NL genes to circumvent this obstacle, using the generation of the gfp-baf allele as an example. We describe strategies for validation of tagged alleles. Finally, we outline methods for immunohistochemical analysis of resulting tagged-NL proteins.

Checkpoint activation drives global gene expression changes in Drosophila nuclear lamina mutants

The nuclear lamina (NL) lines the inner nuclear membrane. This extensive protein network organizes chromatin and contributes to the regulation of transcription, DNA replication, and repair. Lap2-emerin-MAN1 domain (LEM-D) proteins are key members of the NL, representing proteins that connect the NL to the genome through shared interactions with the chromatin-binding protein Barrier-to-Autointegration Factor (BAF). Functions of the LEM-D protein emerin and BAF are essential during Drosophila melanogaster oogenesis. Indeed, loss of either emerin or BAF blocks germ cell development and causes loss of germline stem cells, defects linked to the deformation of NL structure, and non-canonical activation of Checkpoint kinase 2 (Chk2). Here, we investigate the contributions of emerin and BAF to gene expression in the ovary. Profiling RNAs from emerin and baf mutant ovaries revealed that nearly all baf misregulated genes were shared with emerin mutants, defining a set of NL-regulated genes. Strikingly, loss of Chk2 restored the expression of most NL-regulated genes, identifying a large class of Chk2-dependent genes (CDGs). Nonetheless, some genes remained misexpressed upon Chk2 loss, identifying a smaller class of emerin-dependent genes (EDGs). Properties of EDGs suggest a shared role for emerin and BAF in the repression of developmental genes. Properties of CDGs demonstrate that Chk2 activation drives global misexpression of genes in the emerin and baf mutant backgrounds. Notably, CDGs were found upregulated in lamin-B mutant backgrounds. These observations predict that Chk2 activation might have a general role in gene expression changes found in NL-associated diseases, such as laminopathies.

Survival of Drosophila germline stem cells requires the chromatin-binding protein Barrier-to-autointegration factor

The nuclear lamina (NL) is an extensive protein network that underlies the inner nuclear envelope. This network includes LAP2-emerin-MAN1 domain (LEM-D) proteins that associate with the chromatin and DNA-binding protein Barrier-to-autointegration factor (BAF). Here, we investigate the partnership between three NL Drosophila LEM-D proteins and BAF. In most tissues, only Emerin/Otefin is required for NL enrichment of BAF, revealing an unexpected dependence on a single LEM-D protein. Prompted by these observations, we studied BAF contributions in the ovary, a tissue where Emerin/Otefin function is essential. We show that germ cell-specific BAF knockdown causes phenotypes that mirror emerin/otefin mutants. Loss of BAF disrupts NL structure, blocks differentiation and promotes germ cell loss, phenotypes that are partially rescued by inactivation of the ATR and Chk2 kinases. These data suggest that, similar to emerin/otefin mutants, BAF depletion activates the NL checkpoint that causes germ cell loss. Taken together, our findings provide evidence for a prominent NL partnership between the LEM-D protein Emerin/Otefin and BAF, revealing that BAF functions with this partner in the maintenance of an adult stem cell population.

Nuclear architecture as an intrinsic regulator of Drosophila female germline stem cell maintenance

Homeostasis of Drosophila germline stem cells (GSC) depends upon the integration of intrinsic and extrinsic signals. This review highlights emerging data that support nuclear architecture as an intrinsic regulator of GSC maintenance and germ cell differentiation. Here, we focus on the nuclear lamina (NL) and the nucleolus, two compartments that undergo alterations in composition upon germ cell differentiation. Loss of NL or nucleolar components leads to GSC loss, resulting from activation of GSC quality control checkpoint pathways. We suggest that the NL and nucleolus integrate signals needed for the switch between GSC maintenance and germ cell differentiation, and propose regulation of nuclear actin pools as one mechanism that connects these compartments.

Nuclear lamina dysfunction triggers a germline stem cell checkpoint

LEM domain (LEM-D) proteins are conserved components of the nuclear lamina (NL) that contribute to stem cell maintenance through poorly understood mechanisms. The Drosophila emerin homolog Otefin (Ote) is required for maintenance of germline stem cells (GSCs) and gametogenesis. Here, we show that ote mutants carry germ cell-specific changes in nuclear architecture that are linked to GSC loss. Strikingly, we found that both GSC death and gametogenesis are rescued by inactivation of the DNA damage response (DDR) kinases, ATR and Chk2. Whereas the germline checkpoint draws from components of the DDR pathway, genetic and cytological features of the GSC checkpoint differ from the canonical pathway. Instead, structural deformation of the NL correlates with checkpoint activation. Despite remarkably normal oogenesis, rescued oocytes do not support embryogenesis. Taken together, these data suggest that NL dysfunction caused by Otefin loss triggers a GSC-specific checkpoint that contributes to maintenance of gamete quality.

Otefin is a nuclear lamina protein required for survival of Drosophila germ stem cells. Here the authors show that nuclear lamina dysfunction resulting from loss of Otefin activates a DNA damage-independent germ stem cell-specific checkpoint, mediated by the ATR and Chk2 kinases, which ensures that healthy gametes are passed on to the next generation.

Deciphering the DNA code for the function of the Drosophila polydactyl zinc finger protein Suppressor of Hairy-wing

Polydactyl zinc finger (ZF) proteins have prominent roles in gene regulation and often execute multiple regulatory functions. To understand how these proteins perform varied regulation, we studiedDrosophila Suppressor of Hairy-wing [Su(Hw)], an exemplar multifunctional polydactyl ZF protein. We identified separation-of-function (SOF) alleles that encode proteins disrupted in a single ZF that retain one of the Su(Hw) regulatory activities. Through extended in vitro analyses of the Su(Hw) ZF domain, we show that clusters of ZFs bind individual modules within a compound DNA consensus sequence. Through in vivo analysis of SOF mutants, we find that Su(Hw) genomic sites separate into sequence subclasses comprised of combinations of modules, with subclasses enriched for different chromatin features. These data suggest a Su(Hw) code, wherein DNA binding dictates its cofactor recruitment and regulatory output. We propose that similar DNA codes might be used to confer multiple regulatory functions of other polydactyl ZF proteins.

Drosophila male and female germline stem cell niches require the nuclear lamina protein Otefin

The nuclear lamina is an extensive protein network that underlies the inner nuclear envelope. This network includes the LAP2-emerin-MAN1-domain (LEM-D) protein family, proteins that share an association with the chromatin binding protein Barrier-to-autointegration factor (BAF). Loss of individual LEM-D proteins causes progressive, tissue-restricted diseases, known as laminopathies. Mechanisms associated with laminopathies are not yet understood. Here we present our studies of one of the Drosophila nuclear lamina LEM-D proteins, Otefin (Ote), a homologue of emerin. Previous studies have shown that Ote is autonomously required for the survival of female germline stem cells (GSCs). We demonstrate that Ote is also required for survival of somatic cells in the ovarian niche, with loss of Ote causing a decrease in cap cell number and altered signal transduction. We show germ cell-restricted expression of Ote rescues these defects, revealing a non-autonomous function for Ote in niche maintenance and emphasizing that GSCs contribute to the maintenance of their own niches. Further, we investigate the requirement of Ote in the male fertility. We show that ote mutant males become prematurely sterile as they age. Parallel to observations in females, this sterility is associated with GSC loss and changes in somatic cells of the niche, phenotypes that are largely rescued by germ cell-restricted Ote expression. Taken together, our studies demonstrate that Ote is required autonomously for survival of two stem cell populations, as well as non-autonomously for maintenance of two somatic niches. Finally, our data add to growing evidence that LEM-D proteins have critical roles in stem cell survival and tissue homeostasis.

Networking in the nucleus: a spotlight on LEM-domain proteins

Proteins resident in the inner nuclear membrane and underlying nuclear lamina form a network that regulates nuclear functions. This review highlights a prominent family of nuclear lamina proteins that carries the LAP2-emerin-MAN1-domain (LEM-D). LEM-D proteins share an ability to bind lamins and tether repressive chromatin at the nuclear periphery. The importance of this family is underscored by findings that loss of individual LEM-D proteins causes progressive, tissue-restricted diseases, known as laminopathies. Diverse functions of LEM-D proteins are linked to interactions with unique and overlapping partners including signal transduction effectors, transcription factors and architectural proteins. Recent investigations suggest that LEM-D proteins form hubs within the nuclear lamina that integrate external signals important for tissue homeostasis and maintenance of progenitor cell populations.

The Drosophila nuclear lamina protein otefin is required for germline stem cell survival

LEM domain (LEM-D) proteins are components of an extensive protein network that assembles beneath the inner nuclear envelope. Defects in LEM-D proteins cause tissue-restricted human diseases associated with altered stem cell homeostasis. Otefin (Ote) is a Drosophila LEM-D protein that is intrinsically required for female germline stem cell (GSC) maintenance. Previous studies linked Ote loss with transcriptional activation of the key differentiation gene bag-of-marbles (bam), leading to the model in which Ote tethers the bam gene to the nuclear periphery for gene silencing. Using genetic and phenotypic analyses of multiple ote(-/-) backgrounds, we obtained evidence that is inconsistent with this model. We show that bam repression is maintained in ote(-/-) GSCs and that germ cell loss persists in ote(-/-), bam(-/-) mutants, together demonstrating that GSC loss is independent of bam transcription. We show that the primary defect in ote(-/-) GSCs is a block of differentiation, which ultimately leads to germ cell death.

The insulator protein Suppressor of Hairy-wing is an essential transcriptional repressor in the Drosophila ovary

Suppressor of Hairy-wing [Su(Hw)] is a DNA-binding factor required for gypsy insulator function and female germline development in Drosophila. The insulator function of the gypsy retrotransposon depends on Su(Hw) binding to clustered Su(Hw) binding sites (SBSs) and recruitment of the insulator proteins Centrosomal Protein 190 kD (CP190) and Modifier of mdg4 67.2 kD (Mod67.2). By contrast, the Su(Hw) germline function involves binding to non-clustered SBSs and does not require CP190 or Mod67.2. Here, we identify Su(Hw) target genes, using genome-wide analyses in the ovary to uncover genes with an ovary-bound SBS that are misregulated upon Su(Hw) loss. Most Su(Hw) target genes demonstrate enriched expression in the wild-type CNS. Loss of Su(Hw) leads to increased expression of these CNS-enriched target genes in the ovary and other tissues, suggesting that Su(Hw) is a repressor of neural genes in non-neural tissues. Among the Su(Hw) target genes is RNA-binding protein 9 (Rbp9), a member of the ELAV/Hu gene family. Su(Hw) regulation of Rbp9 appears to be insulator independent, as Rbp9 expression is unchanged in a genetic background that compromises the functions of the CP190 and Mod67.2 insulator proteins, even though both localize to Rbp9 SBSs. Rbp9 misregulation is central to su(Hw)(-/-) sterility, as Rbp9(+/-), su(Hw)(-/-) females are fertile. Eggs produced by Rbp9(+/-), su(Hw)(-/-) females show patterning defects, revealing a somatic requirement for Su(Hw) in the ovary. Our studies demonstrate that Su(Hw) is a versatile transcriptional regulatory protein with an essential developmental function involving transcriptional repression.

Genome-wide studies of the multi-zinc finger Drosophila Suppressor of Hairy-wing protein in the ovary

The Drosophila Suppressor of Hairy-wing [Su(Hw)] protein is a globally expressed, multi-zinc finger (ZnF) DNA-binding protein. Su(Hw) forms a classic insulator when bound to the gypsy retrotransposon and is essential for female germline development. These functions are genetically separable, as exemplified by Su(Hw)(f) that carries a defective ZnF10, causing a loss of insulator but not germline function. Here, we completed the first genome-wide analysis of Su(Hw)-binding sites (SBSs) in the ovary, showing that tissue-specific binding is not responsible for the restricted developmental requirements for Su(Hw). Mapping of ovary Su(Hw)(f) SBSs revealed that female fertility requires binding to only one third of the wild-type sites. We demonstrate that Su(Hw)(f) retention correlates with binding site affinity and partnership with Modifier of (mdg4) 67.2 protein. Finally, we identify clusters of co-regulated ovary genes flanked by Su(Hw)(f) bound sites and show that loss of Su(Hw) has limited effects on transcription of these genes. These data imply that the fertility function of Su(Hw) may not depend upon the demarcation of transcriptional domains. Our studies establish a framework for understanding the germline Su(Hw) function and provide insights into how chromatin occupancy is achieved by multi-ZnF proteins, the most common transcription factor class in metazoans.

The role of the Suppressor of Hairy-wing insulator protein in Drosophila oogenesis

The Drosophila Suppressor of Hairy wing [Su(Hw)] insulator protein has an essential role in the development of the female germline. Here we investigate the function of Su(Hw) in the ovary. We show that Su(Hw) is universally expressed in somatic cells, while germ cell expression is dynamic. Robust levels accumulate in post-mitotic germ cells, where Su(Hw) localization is limited to chromosomes within nurse cells, the specialized cells that support oocyte growth. Although loss of Su(Hw) causes global defects in nurse cell chromosome structure, we demonstrate that these architectural changes are not responsible for the block in oogenesis. Connections between the fertility and insulator functions of Su(Hw) were investigated through studies of the two gypsy insulator proteins, Modifier of (mdg4)67.2 (Mod67.2) and Centrosomal Protein of 190kDa (CP190). Accumulation of these proteins is distinct from Su(Hw), with Mod67.2 and CP190 showing uniform expression in all cells during early stages of oogenesis that diminishes in later stages. Although Mod67.2 and CP190 extensively co-localize with Su(Hw) on nurse cell chromosomes, neither protein is required for nurse cell chromosome development or oocyte production. These data indicate that while the gypsy insulator function requires both Mod67.2 and CP190, these proteins are not essential for oogenesis. These studies represent the first molecular investigations of Su(Hw) function in the germline, which uncover distinct requirements for Su(Hw) insulator and ovary functions.

Nuclear organization: taking a position on gene expression

Eukaryotic genomes are divided into chromosomes that occupy defined regions or territories within the nucleus. These chromosome territories (CTs) are arranged based on the transcriptional activity and chromatin landscape of domains. In general, transcriptionally silent domains reside at the nuclear periphery, whereas active domains locate within the interior. Changes in nuclear position are observed for stress-induced and developmentally regulated tissue-specific genes. Upon activation, these genes move away from a CT to inter-chromosomal space containing nuclear bodies enriched in gene expression machinery. Gene activation is not always accompanied by movement, as positioning is dictated by many determinants, including gene structure and the local genomic environment. Collectively, tissue-specific nuclear organization results from a culmination of inputs that result in proper transcriptional regulation.

The role of Drosophila Lamin C in muscle function and gene expression

The inner side of the nuclear envelope (NE) is lined with lamins, a meshwork of intermediate filaments that provides structural support for the nucleus and plays roles in many nuclear processes. Lamins, classified as A- or B-types on the basis of biochemical properties, have a conserved globular head, central rod and C-terminal domain that includes an Ig-fold structural motif. In humans, mutations in A-type lamins give rise to diseases that exhibit tissue-specific defects, such as Emery-Dreifuss muscular dystrophy. Drosophila is being used as a model to determine tissue-specific functions of A-type lamins in development, with implications for understanding human disease mechanisms. The GAL4-UAS system was used to express wild-type and mutant forms of Lamin C (the presumed Drosophila A-type lamin), in an otherwise wild-type background. Larval muscle-specific expression of wild type Drosophila Lamin C caused no overt phenotype. By contrast, larval muscle-specific expression of a truncated form of Lamin C lacking the N-terminal head (Lamin C DeltaN) caused muscle defects and semi-lethality, with adult 'escapers' possessing malformed legs. The leg defects were due to a lack of larval muscle function and alterations in hormone-regulated gene expression. The consequences of Lamin C association at a gene were tested directly by targeting a Lamin C DNA-binding domain fusion protein upstream of a reporter gene. Association of Lamin C correlated with localization of the reporter gene at the nuclear periphery and gene repression. These data demonstrate connections among the Drosophila A-type lamin, hormone-induced gene expression and muscle function.

A comparative study of Drosophila and human A-type lamins

Nuclear intermediate filament proteins, called lamins, form a meshwork that lines the inner surface of the nuclear envelope. Lamins contain three domains: an N-terminal head, a central rod and a C-terminal tail domain possessing an Ig-fold structural motif. Lamins are classified as either A- or B-type based on structure and expression pattern. The Drosophila genome possesses two genes encoding lamins, Lamin C and lamin Dm₀, which have been designated A- and B-type, respectively, based on their expression profile and structural features. In humans, mutations in the gene encoding A-type lamins are associated with a spectrum of predominantly tissue-specific diseases known as laminopathies. Linking the disease phenotypes to cellular functions of lamins has been a major challenge. Drosophila is being used as a model system to identify the roles of lamins in development. Towards this end, we performed a comparative study of Drosophila and human A-type lamins. Analysis of transgenic flies showed that human lamins localize predictably within the Drosophila nucleus. Consistent with this finding, yeast two-hybrid data demonstrated conservation of partner-protein interactions. Drosophila lacking A-type lamin show nuclear envelope defects similar to those observed with human laminopathies. Expression of mutant forms of the A-type Drosophila lamin modeled after human disease-causing amino acid substitutions revealed an essential role for the N-terminal head and the Ig-fold in larval muscle tissue. This tissue-restricted sensitivity suggests a conserved role for lamins in muscle biology. In conclusion, we show that (1) localization of A-type lamins and protein-partner interactions are conserved between Drosophila and humans, (2) loss of the Drosophila A-type lamin causes nuclear defects and (3) muscle tissue is sensitive to the expression of mutant forms of A-type lamin modeled after those causing disease in humans. These studies provide new insights on the role of lamins in nuclear biology and support Drosophila as a model for studies of human laminopathies involving muscle dysfunction.

Context differences reveal insulator and activator functions of a Su(Hw) binding region

Insulators are DNA elements that divide chromosomes into independent transcriptional domains. The Drosophila genome contains hundreds of binding sites for the Suppressor of Hairy-wing [Su(Hw)] insulator protein, corresponding to locations of the retroviral gypsy insulator and non-gypsy binding regions (BRs). The first non-gypsy BR identified, 1A-2, resides in cytological region 1A. Using a quantitative transgene system, we show that 1A-2 is a composite insulator containing enhancer blocking and facilitator elements. We discovered that 1A-2 separates the yellow (y) gene from a previously unannotated, non-coding RNA gene, named yar for y-achaete (ac) intergenic RNA. The role of 1A-2 was elucidated using homologous recombination to excise these sequences from the natural location, representing the first deletion of any Su(Hw) BR in the genome. Loss of 1A-2 reduced yar RNA accumulation, without affecting mRNA levels from the neighboring y and ac genes. These data indicate that within the 1A region, 1A-2 acts an activator of yar transcription. Taken together, these studies reveal that the properties of 1A-2 are context-dependent, as this element has both insulator and enhancer activities. These findings imply that the function of non-gypsy Su(Hw) BRs depends on the genomic environment, predicting that Su(Hw) BRs represent a diverse collection of genomic regulatory elements.

Identification of genomic sites that bind the Drosophila suppressor of Hairy-wing insulator protein

Eukaryotic genomes are divided into independent transcriptional domains by DNA elements known as insulators. The gypsy insulator, a 350-bp element isolated from the Drosophila gypsy retrovirus, contains twelve degenerate binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein. Su(Hw) associates with over 500 non-gypsy genomic sites, the functions of which are largely unknown. Using a bioinformatics approach, we identified 37 putative Su(Hw) insulators (pSIs) that represent regions containing clustered matches to the gypsy insulator Su(Hw) consensus binding sequence. The majority of these pSIs contain fewer than four Su(Hw) binding sites, with only seven showing in vivo Su(Hw) association, as demonstrated by chromatin immunoprecipitation. To understand the properties of the pSIs, these elements were tested for enhancer-blocking capabilities using a transgene assay system. In a complementary set of experiments, effects of the pSIs on transcriptional regulation of genes at the natural genomic location were determined. Our data suggest that pSIs have complex genomic functions and, in some cases, establish insulators. These studies provide the first direct evidence that the Su(Hw) protein contributes to the regulation of gene expression in the Drosophila genome through the establishment of endogenous insulators.

TFIIIC boxes in the genome

In this issue of Cell, Noma et al. (2006) show that B-boxes and TFIIIC limit the spread of heterochromatin at the silent mat region in the fission yeast genome. Global analysis of TFIIIC distribution revealed dispersed sites of association that coalesce at the nuclear periphery, suggesting that TFIIIC may act as a barrier throughout the genome.

A cis-regulatory sequence within the yellow locus of Drosophila melanogaster required for normal male mating success

Drosophila melanogaster males perform a courtship ritual consisting of a series of dependent fixed-action patterns. The yellow (y) gene is required for normal male courtship behavior and subsequent mating success. To better characterize the requirement for y in the manifestation of innate male sexual behavior, we measured the male mating success (MMS) of 12 hypomorphic y mutants and matched-outbred-background controls using a y+ rescue element on a freely segregating minichromosome. We found that 4 hypomorphs significantly reduced MMS to varying degrees. Reduced MMS was largely independent of adult pigmentation patterns. These mutations defined a 300-bp regulatory region upstream of the transcription start, the mating-success regulatory sequence (MRS), whose function is required for normal MMS. Visualization of gene action via GFP and a Yellow antibody suggests that the MRS directs y transcription in a small number of cells in the third instar CNS, the developmental stage previously implicated in the role of y with regard to male courtship behavior. The presence of Yellow protein in these cells positively correlates with MMS in a subset of mutants. The MRS contains a regulatory sequence controlling larval pigmentation and a 35-bp sequence that is highly conserved within the genus Drosophila and is predicted to bind known transcription factors.

Molecular genetic analysis of the nested Drosophila melanogaster lamin C gene

Lamins are intermediate filaments that line the inner surface of the nuclear envelope, providing structural support and making contacts with chromatin. There are two types of lamins, A- and B-types, which differ in structure and expression. Drosophila possesses both lamin types, encoded by the LamC (A-type) and lamin Dm0 (B-type) genes. LamC is nested within an intron of the essential gene ttv. We demonstrate that null mutations in LamC are lethal, and expression of a wild-type LamC transgene rescues lethality of LamC but not ttv mutants. Mutations in the human A-type lamin gene lead to diseases called laminopathies. To determine if Drosophila might serve as a useful model to study lamin biology and disease mechanisms, we generated transgenic flies expressing mutant LamC proteins modeled after human disease-causing lamins. These transgenic animals display a nuclear lamin aggregation phenotype remarkably similar to that observed when human mutant A-type lamins are expressed in mammalian cells. LamC aggregates also cause disorganization of lamin Dm0, indicating interdependence of both lamin types for proper lamina assembly. Taken together, these data provide the first detailed genetic analysis of the LamC gene and support using Drosophila as a model to study the role of lamins in disease.

Studies of the role of the Drosophila scs and scs' insulators in defining boundaries of a chromosome puff

Insulators are DNA elements that establish independent transcriptional domains within eukaryotic genomes. The Drosophila scs and scs' insulators localize near the borders of a structural domain in the polytene chromosomes, known as a puff, produced by transcription of the 87A heat shock protein (hsp) genes. It has been suggested that scs and scs' are boundary elements that delimit this decondensed chromatin domain, reflecting the mechanism by which these sequences act to constrain regulatory interactions. This model was tested using transposons that carried a yellow gene to assess enhancer blocking and an hsp70-lacZ gene to examine the structure of a heat shock puff in the presence and absence of insulators. We found that although scs and scs' blocked enhancer function, these sequences did not prevent the spread of decondensation resulting from hsp70-lacZ transcription. Further analysis of the endogenous 87A locus demonstrated that scs and scs' reside within, not at, the borders of the puff. Taken together, our studies suggest that scs and scs' are not boundary elements that block the propagation of an altered chromatin state associated with puff formation. We propose that these insulators may have a direct role in limiting regulatory interactions in the gene-dense 87A region.

Regulation of ribosomal protein mRNA content and translation in growth-stimulated mouse fibroblasts

When resting (G0) mouse 3T6 fibroblasts are serum stimulated to reenter the cell cycle, the rates of synthesis of rRNA and ribosomal proteins increase, resulting in an increase in ribosome content beginning about 6 h after stimulation. In this study, we monitored the content, metabolism, and translation of ribosomal protein mRNA (rp mRNA) in resting, exponentially growing, and serum-stimulated 3T6 cells. Cloned cDNAs for seven rp mRNAs were used in DNA-excess filter hybridization studies to assay rp mRNA. We found that about 85% of rp mRNA is polyadenylated under all growth conditions. The rate of labeling of rp mRNA relative to total polyadenylated mRNA changed very little after stimulation. The half-life of rp mRNA was about 11 h in resting cells and about 8 h in exponentially growing cells, values which are similar to the half-lives of total mRNA in resting and growing cells (about 9 h). The content of rp mRNA relative to total mRNA was about the same in resting and growing 3T6 cells. Furthermore, the total amount of rp mRNA did not begin to increase until about 6 h after stimulation. Since an increase in rp mRNA content did not appear to be responsible for the increase in ribosomal protein synthesis, we determined the efficiency of translation of rp mRNA under different conditions. We found that about 85% of pulse-labeled rp mRNA was associated with polysomes in exponentially growing cells. In resting cells, however, only about half was associated with polysomes, and about 30% was found in the monosomal fraction. The distribution shifted to that found in growing cells within 3 h after serum stimulation. Similar results were obtained when cells were labeled for 10.5 h. About 70% of total polyadenylated mRNA was in the polysome fraction in all growth states regardless of labeling time, indicating that the shift in mRNA distribution was species specific. These results indicate that the content and metabolism of rp mRNA do not change significantly after growth stimulation. The rate of ribosomal protein synthesis appears to be controlled during the resting-growing transition by an alteration of the efficiency of translation of rp mRNA, possibly at the level of protein synthesis initiation.

An endogenous suppressor of hairy-wing insulator separates regulatory domains in Drosophila

Insulators define independent domains of gene function throughout the genome. The Drosophila gypsy insulator was isolated from the gypsy retrotransposon as a region that contains a cluster of binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein. To study the effects of the gypsy insulator on gene expression within a single genomic domain, targeted gene replacement was used to exchange the endogenous yellow gene, located at cytological location 1A, with a set of gypsy-modified yellow genes. Replaced yellow genes carried a gypsy insulator positioned between the yellow promoter and either the upstream or the downstream tissue-specific enhancers. Whereas the gypsy insulator blocked the function of the upstream enhancers at the endogenous location, the downstream enhancers were not blocked. Investigation of the 1A region revealed two clustered Su(Hw)-binding sites downstream of the yellow gene, named 1A-2, that bind Su(Hw) in vivo and possess enhancer blocking function. We propose that interaction between 1A-2 and the gypsy insulator permits activation of yellow expression by enhancers in the neighboring achaete-scute complex, causing an apparent absence of the block of the downstream yellow enhancers. Based on these data, we suggest that 1A-2 is an endogenous Su(Hw) insulator that separates regulatory domains within the Drosophila genome.

Genomic insulators: connecting properties to mechanism

Insulators are regulatory elements that establish independent domains of transcriptional activity within eukaryotic genomes. Insulators possess two properties: an anti-enhancer activity that blocks enhancer-promoter communication, and an anti-silencer activity that prevents the spread of repressive chromatin. Some insulators are composite elements with separable activities, while others employ a single mechanism to confer both properties. Recent studies focus on elucidating the molecular mechanisms of insulator function. Emerging themes support connections between insulators, transcriptional activators and topological chromosomal domains. Understanding these processes will provide insights into prevention of inappropriate regulatory interactions, knowledge that can be applied to gene therapies.

A test of insulator interactions in Drosophila

Insulators are a class of elements that define independent domains of gene function. The Drosophila gypsy insulator is proposed to establish regulatory isolation by forming loop domains that constrain interactions between transcriptional control elements. This supposition is based upon the observation that insertion of a single gypsy insulator between an enhancer and promoter blocks enhancer function, while insertion of two gypsy insulators promotes enhancer bypass and activation of transcription. To investigate this model, we determined whether non-gypsy insulators interacted with each other and with the gypsy insulator. Pairs of scs or scs' insulators blocked enhancer function. Further, an intervening scs insulator did not block gypsy insulator interactions. Taken together, these data suggest that not all Drosophila insulators interact, with this property restricted to some insulators, such as gypsy. Three gypsy insulators inserted between an enhancer and promoter blocked enhancer function, indicating that gypsy insulator interactions may be restricted to pairs. Our studies imply that formation of loop domains may represent one of many mechanisms used by insulators to impart regulatory isolation.

Protecting against promiscuity: the regulatory role of insulators

Eukaryotic genomes contain transcriptional regulatory elements that alter promoter activity through long-range interactions. Many control elements show a broad range of promoter interactions, suggesting that these elements are capable of inappropriate transcription. The identification of a novel class of directing regulatory elements, called insulators, has provided clues into mechanisms used in eukaryotic genomes to maintain transcription fidelity. Insulators contribute to the organization of independent domains of gene function by restricting enhancer and silencer function. This review describes the properties of insulators and related elements that have been isolated from several eukaryotic genomes. Two classes of models of insulator function are considered. These models provide insights into possible mechanisms used by these diverse elements to provide regulatory autonomy.

Enhancer action in trans is permitted throughout the Drosophila genome

Interactions between paired homologous genes can lead to changes in gene expression. Such trans-regulatory effects exemplify transvection and are displayed by many genes in Drosophila, in which homologous chromosomes are paired somatically. Transvection involving the yellow cuticle pigmentation gene can occur by at least two mechanisms, one involving the trans-action of enhancers on a paired promoter and a second involving pairing-mediated bypass of a chromatin insulator. A system was developed to evaluate whether the action of the yellow enhancers in trans could be reconstituted outside of the natural near telomeric location of the yellow gene. To this end, transgenic flies were generated that carried a yellow gene modified by the inclusion of strategically placed recognition sites for the Cre and FLP recombinases. Independent action of the recombinases produced a pair of derivative alleles, one enhancerless and the other promoterless, at each transgene location. Transvection between the derivatives was assessed by the degree of interallelic complementation. Complementation was observed at all eight sites tested. These studies demonstrate that yellow transvection can occur at multiple genomic locations and indicate that the Drosophila genome generally is permissive to enhancer action in trans.

Long-range repression by multiple polycomb group (PcG) proteins targeted by fusion to a defined DNA-binding domain in Drosophila

A tethering assay was developed to study the effects of Polycomb group (PcG) proteins on gene expression in vivo. This system employed the Su(Hw) DNA-binding domain (ZnF) to direct PcG proteins to transposons that carried the white and yellow reporter genes. These reporters constituted naive sensors of PcG effects, as bona fide PcG response elements (PREs) were absent from the constructs. To assess the effects of different genomic environments, reporter transposons integrated at nearly 40 chromosomal sites were analyzed. Three PcG fusion proteins, ZnF-PC, ZnF-SCM, and ZnF-ESC, were studied, since biochemical analyses place these PcG proteins in distinct complexes. Tethered ZnF-PcG proteins repressed white and yellow expression at the majority of sites tested, with each fusion protein displaying a characteristic degree of silencing. Repression by ZnF-PC was stronger than ZnF-SCM, which was stronger than ZnF-ESC, as judged by the percentage of insertion lines affected and the magnitude of the conferred repression. ZnF-PcG repression was more effective at centric and telomeric reporter insertion sites, as compared to euchromatic sites. ZnF-PcG proteins tethered as far as 3.0 kb away from the target promoter produced silencing, indicating that these effects were long range. Repression by ZnF-SCM required a protein interaction domain, the SPM domain, which suggests that this domain is not primarily used to direct SCM to chromosomal loci. This targeting system is useful for studying protein domains and mechanisms involved in PcG repression in vivo.

Differences in insulator properties revealed by enhancer blocking assays on episomes

Insulators are genomic elements that define domains of transcriptional autonomy. Although a large number of insulators have been isolated, it is unclear whether these elements function by shared molecular mechanisms. Novel applications of FLP recombinase technology were used to dissect and compare the function of the Drosophila: gypsy and scs insulators. Inter actions between FLP monomers bound to chromosomally integrated FRT sites were unimpeded by either insulator, demonstrating that these insulators do not establish a chromosomal environment capable of disrupting all types of protein-protein interactions. The gypsy insulator blocked enhancer-activated transcription on FLP-generated extra-chromosomal episomes, whereas the scs insulator displayed silencing effects. These data indicate that these insulators differ in the mechanisms used to prevent enhancer function. That the gypsy insulator blocked enhancer-promoter communication within small episomes suggests that these effects may be accomplished without a global reorganization of chromatin structure. Instead, the gypsy insulator may disrupt enhancer-activated transcription by direct interference with transmission of the enhancer signal to the promoter.

Enhancer blocking by the Drosophila gypsy insulator depends upon insulator anatomy and enhancer strength

Insulators are specialized DNA sequences that prevent enhancer-activated transcription only when interposed between an enhancer and its target promoter. The Drosophila gypsy retrotransposon contains an insulator composed of 12 degenerate binding sites for the Suppressor of Hairy-wing [Su(Hw)] protein that are separated by AT-rich DNA possessing sequence motifs common to matrix/scaffold attachment regions (MARs/SARs). To further understand mechanisms of insulator function, the parameters required for the gypsy insulator to prevent enhancer-activated transcription were examined. Synthetic binding regions were created by reiteration of a single Su(Hw) binding site that lacked the MAR/SAR motifs. These synthetic binding regions reconstituted insulator activity, suggesting that the property of enhancer blocking may be distinct from matrix association. We found that the number and spacing of Su(Hw) binding sites within the gypsy insulator, as well as the strength of the enhancer to be blocked, were important determinants of insulator function. These results provide a link between transcription and insulation, suggesting that these processes may be mechanistically interconnected.

An analysis of transvection at the yellow locus of Drosophila melanogaster

Studies of a wide variety of organisms have shown that homologous sequences can exert a significant impact on each other, resulting in changes in gene sequence, gene expression, chromatin structure, and global chromosome architecture. Our work has focused on transvection, a process that can cause genes to be sensitive to the proximity of a homologue. Transvection is seen at the yellow gene of Drosophila, where it mediates numerous cases of intragenic complementation. In this article, we describe two approaches that have characterized the process of transvection at yellow. The first entailed a screen for mutations that support intragenic complementation at yellow. The second involved the analysis of 53 yellow alleles, obtained from a variety of sources, with respect to complementation, molecular structure, and transcriptional competence. Our data suggest two ways in which transvection may be regulated at yellow: (1) a transcriptional mechanism, whereby the ability of an allele to support transvection is influenced by its transcriptional competency, and (2) a structural mechanism, whereby the pairing of structurally dissimilar homologues results in conformational changes that affect gene expression.

Core promoter elements can regulate transcription on a separate chromosome in trans

Transvection can cause the expression of a gene to be sensitive to the proximity of a homolog. It can account for many cases of intragenic complementation at the Drosophila yellow gene, where one mode of transvection involves the action of enhancers in trans on a promoter present on a separate chromosome. Our goal was to identify cis-acting elements that regulate the trans action of enhancers. Using gene replacement, we altered two core promoter elements at yellow and tested the resulting alleles for their ability to support transvection. We found that the TATA box and initiator element can regulate transvection.

Two modes of transvection: enhancer action in trans and bypass of a chromatin insulator in cis

Ed Lewis introduced the term "transvection" in 1954 to describe mechanisms that can cause the expression of a gene to be sensitive to the proximity of its homologue. Transvection since has been reported at an increasing number of loci in Drosophila, where homologous chromosomes are paired in somatic tissues, as well as at loci in other organisms. At the Drosophila yellow gene, transvection can explain intragenic complementation involving the yellow2 allele (y2). Here, transvection was proposed to occur by enhancers of one allele acting in trans on the promoter of a paired homologue. In this report, we describe two yellow alleles that strengthen this model and reveal an unexpected, second mechanism for transvection. Data suggest that, in addition to enhancer action in trans, transvection can occur by enhancer bypass of a chromatin insulator in cis. We propose that bypass results from the topology of paired genes. Finally, transvection at yellow can occur in genotypes not involving y2, implying that it is a feature of yellow itself and not an attribute of one particular allele.

Polycomb group repression is blocked by the Drosophila suppressor of Hairy-wing [su(Hw)] insulator

The suppressor of Hairy-wing [SU(HW)] binding region disrupts communication between a large number of enhancers and promoters and protects transgenes from chromosomal position effects. These properties classify the SU(HW) binding region as an insulator. While enhancers are blocked in a general manner, protection from repressors appears to be more variable. In these studies, we address whether repression resulting from the Polycomb group genes can be blocked by the SU(HW) binding region. The effects of this binding region on repression established by an Ultrabithorax Polycomb group Response Element were examined. A transposon carrying two reporter genes, the yellow and white genes, was used so that repression and insulation could be assayed simultaneously. We demonstrate that the SU(HW) binding region is effective at preventing Polycomb group repression. These studies suggest that one role of the su(Hw) protein may be to restrict the range of action of repressors, such as the Polycomb group proteins, throughout the euchromatic regions of the genome.

The role of insulator elements in defining domains of gene expression

Insulators are naturally occurring DNA sequences that protect transgenes from genomic position effects, thereby establishing independent functional domains within the chromosome. Recent studies have focused on the identification of the cis and trans requirements for insulator activity. These experiments demonstrate that insulators contain multiple components that cooperate to confer their unique properties. Additionally, they suggest that the mechanism of insulation is related to that of enhancer function. Two models of insulator can be considered: a domain boundary and a transcriptional decoy model.

Effects of the su(Hw) insulator protein on the expression of the divergently transcribed Drosophila yolk protein genes

The suppressor of Hairy-wing [su(Hw)] protein mediates the mutagenic effects of the gypsy retrotransposon by blocking enhancer activity. These repressive effects are general, can occur over long distances and require that the su(Hw) protein is bound between the affected enhancer and promoter. The effects of the su(Hw) binding region on yolk protein (yp) gene expression were determined. These genes are regulated by shared enhancers in the intergenic region, which provided a method to examine whether an enhancer blocked by the su(Hw) protein remained functional. We demonstrate that a blocked enhancer is completely active, supporting the proposal that the su(Hw) protein is an insulator protein that acts by forming a new boundary in a pre-existing chromatin domain, thereby preventing the interaction of regulatory elements located upstream of the insertion site with the promoter. In addition, we found that yp promoter function is not diminished by sharing enhancers, suggesting that these enhancers are not rate limiting for transcriptional activation. Lastly, our data indicate that yp promoter activity is interdependent, such that transcription from one promoter influences the level of activity of the linked promoter.

A Drosophila insulator protein facilitates dosage compensation of the X chromosome min-white gene located at autosomal insertion sites

The suppressor of Hairy-wing [su(Hw)] gene encodes a zinc finger protein that binds to a repeated motif in the gypsy retrotransposon. These DNA sequences, called the su(Hw)-binding region, have properties of an insulator region because they (1) disrupt enhancer/silencer function in a position-dependent manner and (2) protect the mini-white gene from both euchromatic and heterochromatic position effects. To gain further insights into the types of position effects that can be insulated, we determined the effects of the su(Hw)-binding region on dosage compensation of the X-linked mini-white gene. Dosage compensation is the process that equalizes the unequal content of X-linked genes in males and females by increasing the X-linked transcription level twofold in males. Transposition of X-linked genes to the autosomes commonly results in incomplete dosage compensation, indicating that the distinct male X chromatin environment is important for this process. We found that dosage compensation of autosomally integrated mini-white genes flanked by su(Hw)-binding regions was greatly improved, such that complete or nearly complete compensation was observed at the majority of insertion sites. The su(Hw) protein was essential for this enhanced dosage compensation because in a su(Hw) mutant background compensation was incomplete. These experiments provide evidence that the su(Hw)-binding region facilitates dosage compensation of the mini-white gene on the autosomes. This may result from protection of the mini-white gene from a negative autosomal chromatin environment.

A P element containing suppressor of hairy-wing binding regions has novel properties for mutagenesis in Drosophila melanogaster

P elements are widely used as insertional mutagens to tag genes, facilitating molecular cloning and analyses. We modified a P element so that it carried two copies of the suppressor of Hairy-wing [su(Hw)] binding regions isolated from the gypsy transposable element. This transposon was mobilized, and the genetic consequences of its insertion were analyzed. Gene expression can be altered by the su(Hw) protein as a result of blocking the interaction between enhancer/silencer elements and their promoter. These effects can occur over long distances and are general. Therefore, a composite transposon (SUPor-P for suppressor-P element) combines the mutagenic efficacy of the gypsy element with the controllable transposition of P elements. We show that, compared to standard P elements, this composite transposon causes an expanded repertoire of mutations and produces alleles that are suppressed by su(Hw) mutations. The large number of heterochromatic insertions obtained is unusual compared to other insertional mutagenesis procedures, indicating that the SUPor-P transposon may be useful for studying the structural and functional properties of heterochromatin.

Developmental analysis of the ovarian tumor gene during Drosophila oogenesis

Severe alleles of the ovarian tumor (otu) and ovo genes result in female sterility in Drosophila melanogaster, producing adult ovaries that completely lack egg chambers. We examined the developmental stage in which the agametic phenotype first becomes apparent. Germ cell development in embryos was studied using a strategy that allowed simultaneous labeling of pole cells with the determination of embryonic genotype. We found that ovo- or otu- XX embryonic germ cells were indistinguishable in number and morphology from those present in wild-type siblings. The effects of the mutations were not consistently manifested in the female germline until pupariation, and there was no evidence that either gene was required for germ cell viability at earlier stages of development. The requirement for otu function in the pupal and adult ovary is supported by temperature-shift experiments using a heat-inducible otu gene construct. We demonstrate that otu activity limited to prepupal stages was not sufficient to support oogenesis, while induction during the pupal and adult periods caused suppression of the otu mutant phenotype.

The somatic sex determines the requirement for ovarian tumor gene activity in the proliferation of the Drosophila germline

Gametogenesis in Drosophila requires sex-specific interactions between the soma and germline to control germ cell viability, proliferation, and differentiation. To determine what genetic components are involved in this interaction, we examined whether changes in the sexual identity of the soma affected the function of the ovarian tumor (otu) and ovo genes. These genes are required cell autonomously in the female germline for germ cell proliferation and differentiation. Mutations in otu and ovo cause a range of ovarian defects, including agametic ovaries and tumorous egg cysts, but do not affect spermatogenesis. We demonstrate that XY germ cells do not require otu when developing in testes, but become dependent on otu function for proliferation when placed in an ovary. This soma-induced requirement can be satisfied by the induced expression of the 98 × 10(3) M(r) OTU product, one of two isoforms produced by differential RNA splicing. These results indicate that the female somatic gonad can induce XY germ cells to become ‘female-like’ because they require an oogenesis-specific gene. In contrast, the requirement for ovo is dependent on a cell autonomous signal derived from the X:A ratio. We propose that differential regulation of the otu and ovo genes provides a mechanism for the female germline to incorporate both somatic and cell autonomous inputs required for oogenesis.

Molecular characterization of ovarian tumors in Drosophila

Certain female-sterile mutations in Drosophila result in the uncontrolled proliferation of X/X germ cells. It has been proposed that this ovarian tumor phenotype results from the sexual transformation of X/X germ cells to a male identity. We present findings inconsistent with this model. We demonstrate that the tumorous cells produced by mutations in the ovarian tumor (otu), Sex-lethal (Sxl) and sans fille (snf) genes are capable of female-specific transcription and RNA processing. This indicates that these ovarian tumor cells still retain some female identity. Therefore, we propose that mutations in these genes do not cause a male transformation of the X/X germ line but instead either cause an ambiguous sexual identity or block specific stages of oogenesis. Our findings indicate that while Sxl is the master sex determination gene in somatic cells, it appears to play a more subsidiary role in the germ line. Finally, we demonstrate that the germ line function of Sxl depends on the activity of a specific OTU isoform.

Genetic and molecular characterization of P element-induced mutations reveals that the Drosophila ovarian tumor gene has maternal activity and a variable null phenotype

The mutations in the ovarian tumor (otu) gene arrest oogenesis at several stages in development. A series of deletion mutations in the otu region were characterized, each of which causes the absence or reduction of the otu transcript. These alleles range from the most severe class, which results in ovaries lacking egg cysts, to relatively mild mutations that allow the development of late stage oocytes. Heteroallelic combinations of these mutations demonstrate that the phenotypic complexity of otu mutant ovaries is due to a dosage dependent requirement for otu activity. Reciprocal cross and developmental Northern blot studies suggest a maternal requirement for otu in the development of the female germline. In addition we demonstrate that the otu zygotic null phenotype is variable, ranging from the absence of cysts in the most extreme cases, to the presence of tumorous egg chambers.

The su(Hw) protein insulates expression of the Drosophila melanogaster white gene from chromosomal position-effects

Mutations in the suppressor of Hairy-wing [su(Hw)] locus reverse the phenotype of a number of tissue-specific mutations caused by insertion of a gypsy retrotransposon. The su(Hw) gene encodes a zinc finger protein which binds to a 430 bp region of gypsy shown to be both necessary and sufficient for its mutagenic effects. su(Hw) protein causes mutations by inactivation of enhancer elements only when a su(Hw) binding region is located between these regulatory sequences and a promoter. To understand the molecular basis of enhancer inactivation, we tested the effects of su(Hw) protein on expression of the mini-white gene. We find that su(Hw) protein stabilizes mini-white gene expression from chromosomal position-effects in euchromatic locations by inactivating negative and positive regulatory elements present in flanking DNA. Furthermore, the su(Hw) protein partially protects transposon insertions from the negative effects of heterochromatin. To explain our current results, we propose that su(Hw) protein alters the organization of chromatin by creating a new boundary in a pre-existing domain of higher order chromatin structure. This separates enhancers and silencers distal to the su(Hw) binding region into an independent unit of gene activity, thereby causing their inactivation.

Position-independent germline transformation in Drosophila using a cuticle pigmentation gene as a selectable marker

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