hobo is responsible for the induction of hybrid dysgenesis by strains of Drosophila melanogaster bearing the male recombination factor 23.5MRF

Levels of P-element-induced hybrid dysgenesis in Drosophila simulans are uncorrelated with levels of P-element piRNAs

Transposable elements (TEs) are genomic parasites that proliferate within host genomes, and which can also invade new species. The P-element, a DNA-based TE, recently invaded two Drosophila species: Drosophila melanogaster in the 20th century, and D. simulans in the 21st. In both species, lines collected before the invasion are susceptible to "hybrid dysgenesis", a syndrome of abnormal phenotypes apparently due to P-element-inflicted DNA damage. In D. melanogaster, lines collected after the invasion have evolved a maternally acting mechanism that suppresses hybrid dysgenesis, with extensive work showing that PIWI-interacting small RNAs (piRNAs) are a key factor in this suppression. Most of these studies use lines collected many generations after the initial P-element invasion. Here, we study D. simulans collected early, as well as late in the P-element invasion of this species. Like D. melanogaster, D. simulans from late in the invasion show strong resistance to hybrid dysgenesis and abundant P-element-derived piRNAs. Lines collected early in the invasion, however, show substantial variation in how much they suffer from hybrid dysgenesis, with some lines highly resistant. Surprisingly, although, these resistant lines do not show high levels of cognate maternal P-element piRNAs; in these lines, it may be that other mechanisms suppress hybrid dysgenesis.

Transposable Element Expression and Regulation Profile in Gonads of Interspecific Hybrids of Drosophila arizonae and Drosophila mojavensis wrigleyi

Interspecific hybridization may lead to sterility and/or inviability through differential expression of genes and transposable elements (TEs). In Drosophila, studies have reported massive TE mobilization in hybrids from interspecific crosses of species presenting high divergence times. However, few studies have examined the consequences of TE mobilization upon hybridization in recently diverged species, such as Drosophila arizonae and D. mojavensis. We have sequenced transcriptomes of D. arizonae and the subspecies D. m. wrigleyi and their reciprocal hybrids, as well as piRNAs, to analyze the impact of genomic stress on TE regulation. Our results revealed that the differential expression in both gonadal tissues of parental species was similar. Globally, ovaries and testes showed few deregulated TEs compared with both parental lines. Analyses of small RNA data showed that in ovaries, the TE upregulation is likely due to divergence of copies inherited from parental genomes and lack of piRNAs mapping to them. Nevertheless, in testes, the divergent expression of genes associated with chromatin state and piRNA pathway potentially indicates that TE differential expression is related to the divergence of regulatory genes that play a role in modulating transcriptional and post-transcriptional mechanisms.

P-elements strengthen reproductive isolation within the Drosophila simulans species complex

Determining mechanisms that underlie reproductive isolation (RI) is key to understanding how species boundaries are maintained in nature. Transposable elements (TEs) are ubiquitous across eukaryotic genomes. However, the role of TEs in modulating the strength of RI between species is poorly understood. Several species of Drosophila have been found to harbor P-elements (PEs), yet only D. simulans is known to be currently polymorphic for their presence in wild populations. PEs can cause RI between PE-containing (P) and PE-lacking (M) lineages of the same species. However, it is unclear whether they also contribute to the magnitude of RI between species. Here, we use the simulans species complex to assess whether differences in PE status between D. simulans and its sister species, which do not harbor PEs, contribute to multiple barriers to gene flow between species. We show that crosses involving a P D. simulans father and an M mother from a sister species exhibit lower F1 female fecundity than crosses involving an M D. simulans father and an M sister-species mother. We also find that another TE, I-element, might play a minor role in determining the frequency of dysgenesis between species. Our results suggest that the presence of PEs in a species can strengthen isolation from its sister species, providing evidence that TEs can play a role in RI.

Tirant Stealthily Invaded Natural Drosophila melanogaster Populations during the Last Century

It was long thought that solely three different transposable elements (TEs)-the I-element, the P-element, and hobo-invaded natural Drosophila melanogaster populations within the last century. By sequencing the "living fossils" of Drosophila research, that is, D. melanogaster strains sampled from natural populations at different time points, we show that a fourth TE, Tirant, invaded D. melanogaster populations during the past century. Tirant likely spread in D. melanogaster populations around 1938, followed by the I-element, hobo, and, lastly, the P-element. In addition to the recent insertions of the canonical Tirant, D. melanogaster strains harbor degraded Tirant sequences in the heterochromatin which are likely due to an ancient invasion, likely predating the split of D. melanogaster and D. simulans. These degraded insertions produce distinct piRNAs that were unable to prevent the novel Tirant invasion. In contrast to the I-element, P-element, and hobo, we did not find that Tirant induces any hybrid dysgenesis symptoms. This absence of apparent phenotypic effects may explain the late discovery of the Tirant invasion. Recent Tirant insertions were found in all investigated natural populations. Populations from Tasmania carry distinct Tirant sequences, likely due to a founder effect. By investigating the TE composition of natural populations and strains sampled at different time points, insertion site polymorphisms, piRNAs, and phenotypic effects, we provide a comprehensive study of a natural TE invasion.

Transposable elements in Drosophila

Drosophila has been studied as a biological model for many years and many discoveries in biology rely on this species. Research on transposable elements (TEs) is not an exception. Drosophila has contributed significantly to our knowledge on the mechanisms of transposition and their regulation, but above all, it was one of the first organisms on which genetic and genomic studies of populations were done. In this review article, in a very broad way, we will approach the TEs of Drosophila with a historical hindsight as well as recent discoveries in the field.

Paternally Inherited P-Element Copy Number Affects the Magnitude of Hybrid Dysgenesis in Drosophila simulans and D. melanogaster

Transposable elements (TEs) are repetitive regions of DNA that are able to self-replicate and reinsert themselves throughout host genomes. Since the discovery of TEs, a prevalent question has been whether increasing TE copy number has an effect on the fitness of their hosts. P-elements (PEs) in Drosophila are a well-studied TE that has strong phenotypic effects. When a female without PEs (M) is crossed to a male with them (P), the resulting females are often sterile, a phenomenon called hybrid dysgenesis (HD). Here, we used short- and long-read sequencing to infer the number of PEs in the genomes of dozens of isofemale lines from two Drosophila species and measured whether the magnitude of HD was correlated with the number of PEs in the paternal genome. Consistent with previous reports, we find evidence for a positive correlation between the paternal PE copy number and the magnitude of HD in progeny from ♀M × ♂ P crosses for both species. Other crosses are not affected by the number of PE copies. We also find that the correlation between the strength of HD and PE copy number differs between species, which suggests that there are genetic differences that might make some genomes more resilient to the potentially deleterious effects of TEs. Our results suggest that PE copy number interacts with other factors in the genome and the environment to cause HD and that the importance of these interactions is species specific.

Hybrid dysgenesis in Drosophila virilis results in clusters of mitotic recombination and loss-of-heterozygosity but leaves meiotic recombination unaltered

BACKGROUND

Transposable elements (TEs) are endogenous mutagens and their harmful effects are especially evident in syndromes of hybrid dysgenesis. In Drosophila virilis, hybrid dysgenesis is a syndrome of incomplete gonadal atrophy that occurs when males with multiple active TE families fertilize females that lack active copies of the same families. This has been demonstrated to cause the transposition of paternally inherited TE families, with gonadal atrophy driven by the death of germline stem cells. Because there are abundant, active TEs in the male inducer genome, that are not present in the female reactive genome, the D. virilis syndrome serves as an excellent model for understanding the effects of hybridization between individuals with asymmetric TE profiles.

RESULTS

Using the D. virilis syndrome of hybrid dysgenesis as a model, we sought to determine how the landscape of germline recombination is affected by parental TE asymmetry. Using a genotyping-by-sequencing approach, we generated a high-resolution genetic map of D. virilis and show that recombination rate and TE density are negatively correlated in this species. We then contrast recombination events in the germline of dysgenic versus non-dysgenic F1 females to show that the landscape of meiotic recombination is hardly perturbed during hybrid dysgenesis. In contrast, hybrid dysgenesis in the female germline increases transmission of chromosomes with mitotic recombination. Using a de novo PacBio assembly of the D. virilis inducer genome we show that clusters of mitotic recombination events in dysgenic females are associated with genomic regions with transposons implicated in hybrid dysgenesis.

CONCLUSIONS

Overall, we conclude that increased mitotic recombination is likely the result of early TE activation in dysgenic progeny, but a stable landscape of meiotic recombination indicates that either transposition is ameliorated in the adult female germline or that regulation of meiotic recombination is robust to ongoing transposition. These results indicate that the effects of parental TE asymmetry on recombination are likely sensitive to the timing of transposition.

Birth, School, Work, Death, and Resurrection: The Life Stages and Dynamics of Transposable Element Proliferation

Transposable elements (TEs) can be maintained in sexually reproducing species even if they are harmful. However, the evolutionary strategies that TEs employ during proliferation can modulate their impact. In this review, I outline the different life stages of a TE lineage, from birth to proliferation to extinction. Through their interactions with the host, TEs can exploit diverse strategies that range from long-term coexistence to recurrent movement across species boundaries by horizontal transfer. TEs can also engage in a poorly understood phenomenon of TE resurrection, where TE lineages can apparently go extinct, only to proliferate again. By determining how this is possible, we may obtain new insights into the evolutionary dynamics of TEs and how they shape the genomes of their hosts.

The Evolution of Small-RNA-Mediated Silencing of an Invading Transposable Element

Transposable elements (TEs) are genomic parasites that impose fitness costs on their hosts by producing deleterious mutations and disrupting gametogenesis. Host genomes avoid these costs by regulating TE activity, particularly in germline cells where new insertions are heritable and TEs are exceptionally active. However, the capacity of different TE-associated fitness costs to select for repression in the host, and the role of selection in the evolution of TE regulation more generally remain controversial. In this study, we use forward, individual-based simulations to examine the evolution of small-RNA-mediated TE regulation, a conserved mechanism for TE repression that is employed by both prokaryotes and eukaryotes. To design and parameterize a biologically realistic model, we drew on an extensive survey of empirical studies of the transposition and regulation of P-element DNA transposons in Drosophila melanogaster. We observed that even under conservative assumptions, where small-RNA-mediated regulation reduces transposition only, repression evolves rapidly and adaptively after the genome is invaded by a new TE in simulated populations. We further show that the spread of repressor alleles through simulated populations is greatly enhanced by two additional TE-imposed fitness costs: dysgenic sterility and ectopic recombination. Finally, we demonstrate that the adaptive mutation rate to repression is a critical parameter that influences both the evolutionary trajectory of host repression and the associated proliferation of TEs after invasion in simulated populations. Our findings suggest that adaptive evolution of TE regulation may be stronger and more prevalent than previously appreciated, and provide a framework for interpreting empirical data.

Horizontal acquisition of transposable elements and viral sequences: patterns and consequences

It is becoming clear that most eukaryotic transposable elements (TEs) owe their evolutionary success in part to horizontal transfer events, which enable them to invade new species. Recent large-scale studies are beginning to unravel the mechanisms and ecological factors underlying this mode of transmission. Viruses are increasingly recognized as vectors in the process but also as a direct source of genetic material horizontally acquired by eukaryotic organisms. Because TEs and endogenous viruses are major catalysts of variation and innovation in genomes, we argue that horizontal inheritance has had a more profound impact in eukaryotic evolution than is commonly appreciated. To support this proposal, we compile a list of examples, including some previously unrecognized, whereby new host functions and phenotypes can be directly attributed to horizontally acquired TE or viral sequences. We predict that the number of examples will rapidly grow in the future as the prevalence of horizontal transfer in the life cycle of TEs becomes even more apparent, firmly establishing this form of non-Mendelian inheritance as a consequential facet of eukaryotic evolution.

p53 is required for female germline stem cell maintenance in P-element hybrid dysgenesis

Hybrid dysgenesis is a sterility syndrome resulting from the mobilization of certain transposable elements in the Drosophila germline. Particularly extreme is the hybrid dysgenesis syndrome caused by P-element DNA transposons, in which dysgenic female ovaries often contain few or no germline cells. Those offspring that are produced from dysgenic germlines exhibit high rates of de novo mutation and recombination, implicating transposition-associated DNA damage as the cause of germline loss. However, how this loss occurs, in terms of the particular cellular response that is triggered (cell cycle arrest, senescence, or cell death) remains poorly understood. We demonstrate that two components of the DNA damage response, Checkpoint kinase 2 and its downstream target p53, determine the frequency of ovarian atrophy that is associated with P-element hybrid dysgenesis. We further show that p53 is strongly induced in the germline stem cells (GSCs) of dysgenic females, and is required for their maintenance. Our observations support the critical role for p53 in conferring tolerance of transposable element activity in stem cells.

Paternal Induction of Hybrid Dysgenesis in Drosophila melanogaster Is Weakly Correlated with Both P-Element and hobo Element Dosage

Transposable elements (TEs) are virtually ubiquitous components of genomes, yet they often impose significant fitness consequences on their hosts. In addition to producing specific deleterious mutations by insertional inactivation, TEs also impose general fitness costs by inducing DNA damage and participating in ectopic recombination. These latter fitness costs are often assumed to be dosage-dependent, with stronger effects occurring in the presence of higher TE copy numbers. We test this assumption in Drosophila melanogaster by considering the relationship between the copy number of two active DNA transposons, P-element and hobo element, and the incidence of hybrid dysgenesis, a sterility syndrome associated with transposon activity in the germline. By harnessing a subset of the Drosophila Genetic Reference Panel (DGRP), a group of fully-sequenced D. melanogaster strains, we describe quantitative and structural variation in P-elements and hobo elements among wild-derived genomes and associate these factors with hybrid dysgenesis. We find that the incidence of hybrid dysgenesis is associated with both P-element and hobo element copy number in a dosage-dependent manner. However, the relationship is weak for both TEs, suggesting that dosage alone explains only a small part of TE-associated fitness costs.

A First Insight into the Genome of the Filter-Feeder Mussel Mytilus galloprovincialis

Mussels belong to the phylum Mollusca, one of the largest and most diverse taxa in the animal kingdom. Despite their importance in aquaculture and in biology in general, genomic resources from mussels are still scarce. To broaden and increase the genomic knowledge in this family, we carried out a whole-genome sequencing study of the cosmopolitan Mediterranean mussel (Mytilus galloprovincialis). We sequenced its genome (32X depth of coverage) on the Illumina platform using three pair-end libraries with different insert sizes. The large number of contigs obtained pointed out a highly complex genome of 1.6 Gb where repeated elements seem to be widespread (~30% of the genome), a feature that is also shared with other marine molluscs. Notwithstanding the limitations of our genome sequencing, we were able to reconstruct two mitochondrial genomes and predict 10,891 putative genes. A comparative analysis with other molluscs revealed a gene enrichment of gene ontology categories related to multixenobiotic resistance, glutamate biosynthetic process, and the maintenance of ciliary structures.

The cellular basis of hybrid dysgenesis and Stellate regulation in Drosophila

During normal tissue development, the accumulation of unrepaired cellular and genomic damage can impair growth and ultimately leads to death. To preserve cellular integrity, cells employ a number of defense mechanisms including molecular checkpoints, during which development is halted while dedicated pathways attempt repair. This process is most critical in germline tissues where cellular damage directly threatens an organism's reproductive capacity and offspring viability. In the fruit fly, Drosophila melanogaster, germline development has been extensively studied for over a century and the breadth of our knowledge has flourished in the genomics age. Intriguingly, several peculiar phenomena that trigger catastrophic germline damage described decades ago, still endure only a partial understanding of the underlying molecular causes. A deeper reexamination using new molecular and genetic tools may greatly benefit our understanding of host system biology. Among these, and the focus of this concise review, are hybrid dysgenesis and an intragenomic conflict that pits the X and Y sex chromosomes against each other.

piRNAs Are Associated with Diverse Transgenerational Effects on Gene and Transposon Expression in a Hybrid Dysgenic Syndrome of D. virilis

Sexual reproduction allows transposable elements (TEs) to proliferate, leading to rapid divergence between populations and species. A significant outcome of divergence in the TE landscape is evident in hybrid dysgenic syndromes, a strong form of genomic incompatibility that can arise when (TE) family abundance differs between two parents. When TEs inherited from the father are absent in the mother's genome, TEs can become activated in the progeny, causing germline damage and sterility. Studies in Drosophila indicate that dysgenesis can occur when TEs inherited paternally are not matched with a pool of corresponding TE silencing PIWI-interacting RNAs (piRNAs) provisioned by the female germline. Using the D. virilis syndrome of hybrid dysgenesis as a model, we characterize the effects that divergence in TE profile between parents has on offspring. Overall, we show that divergence in the TE landscape is associated with persisting differences in germline TE expression when comparing genetically identical females of reciprocal crosses and these differences are transmitted to the next generation. Moreover, chronic and persisting TE expression coincides with increased levels of genic piRNAs associated with reduced gene expression. Combined with these effects, we further demonstrate that gene expression is idiosyncratically influenced by differences in the genic piRNA profile of the parents that arise though polymorphic TE insertions. Overall, these results support a model in which early germline events in dysgenesis establish a chronic, stable state of both TE and gene expression in the germline that is maintained through adulthood and transmitted to the next generation. This work demonstrates that divergence in the TE profile is associated with diverse piRNA-mediated transgenerational effects on gene expression within populations.

Transposable elements (TEs) are selfish elements that copy themselves. More than half of the human genome is comprised of such elements. Studies in the fruit flies Drosophila melanogaster and D. virilis have been important in demonstrating a role for RNA silencing by PIWI-interacting RNAs (piRNAs) in protecting the genome against these harmful elements. These small RNAs are capable of recognizing TE mRNAs and mediating their destruction. They are also transmitted by the female germline to offspring in order to maintain a stable genome across generations. When males carrying a particular TE family are crossed with females lacking the element, the mother is unable to provide genome defense via complementary piRNAs that target the element. This leads to excess TE activation in the germline and sterility, a phenomenon known as hybrid dysgenesis. In this article we characterize the genomic landscape of TE destabilization that occurs in dysgenic crosses of D. virilis. We demonstrate that this mobilization is associated with an increased level of germline TE expression that persists through adulthood. In addition, we find that TE activation is associated with diverse effects on normal gene expression that are also mediated by piRNAs.

Whole genome sequencing in Drosophila virilis identifies Polyphemus, a recently activated Tc1-like transposon with a possible role in hybrid dysgenesis

BACKGROUND

Hybrid dysgenic syndromes in Drosophila have been critical for characterizing host mechanisms of transposable element (TE) regulation. This is because a common feature of hybrid dysgenesis is germline TE mobilization that occurs when paternally inherited TEs are not matched with a maternal pool of silencing RNAs that maintain transgenerational TE control. In the face of this imbalance TEs become activated in the germline and can cause F1 sterility. The syndrome of hybrid dysgenesis in Drosophila virilis was the first to show that the mobilization of one dominant TE, the Penelope retrotransposon, may lead to the mobilization of other unrelated elements. However, it is not known how many different elements contribute and no exhaustive search has been performed to identify additional ones. To identify additional TEs that may contribute to hybrid dysgenesis in Drosophila virilis, I analyzed repeat content in genome sequences of inducer and non-inducer lines.

RESULTS

Here I describe Polyphemus, a novel Tc1-like DNA transposon, which is abundant in the inducer strain of D. virilis but highly degraded in the non-inducer strain. Polyphemus expression is also increased in the germline of progeny of the dysgenic cross relative to reciprocal progeny. Interestingly, like the Penelope element, it has experienced recent re-activation within the D. virilis lineage.

CONCLUSIONS

Here I present the results of a comprehensive search to identify additional factors that may cause hybrid dysgenesis in D. virilis. Polyphemus, a novel Tc1-like DNA transposon, has recently become re-activated in Drosophila virilis and likely contributes to the hybrid dysgenesis syndrome. It has been previously shown that the Penelope element has also been re-activated in the inducer strain. This suggests that TE co-reactivation within species may synergistically contribute to syndromes of hybrid dysgenesis.

A genome-wide survey of genetic instability by transposition in Drosophila hybrids

Hybridization between species is a genomic instability factor involved in increasing mutation rate and new chromosomal rearrangements. Evidence of a relationship between interspecific hybridization and transposable element mobilization has been reported in different organisms, but most studies are usually performed with particular TEs and do not discuss the real effect of hybridization on the whole genome. We have therefore studied whole genome instability of Drosophila interspecific hybrids, looking for the presence of new AFLP markers in hybrids. A high percentage (27–90%) of the instability markers detected corresponds to TEs belonging to classes I and II. Moreover, three transposable elements (Osvaldo, Helena and Galileo) representative of different families, showed an overall increase of transposition rate in hybrids compared to parental species. This research confirms the hypothesis that hybridization induces genomic instability by transposition bursts and suggests that genomic stress by transposition could contribute to a relaxation of mechanisms controlling TEs in the Drosophila genome.

What makes transposable elements move in the Drosophila genome?

Transposable elements (TEs), by their capacity of moving and inducing mutations in the genome, are considered important drivers of species evolution. The successful invasions of TEs in genomes, despite their mutational properties, are an apparent paradox. TEs' transposition is usually strongly regulated to low value, but in some cases these elements can also show high transposition rates, which has been associated sometimes to changes in environmental conditions. It is evident that factors susceptible to induce transpositions in natural populations contribute to TE perpetuation. Different factors were proposed as causative agents of TE mobilization in a wide range of organisms: biotic and abiotic stresses, inter- and intraspecific crosses and populational factors. However, there is no clear evidence of the factors capable of inducing TE mobilization in Drosophila, and data on laboratory stocks show contradictory results. The aim of this review is to have an update critical revision about mechanisms promoting transposition of TEs in Drosophila, and to provide to the readers a global vision of the dynamics of these genomic elements in the Drosophila genome.

Evolutionary Implications of Mechanistic Models of TE-Mediated Hybrid Incompatibility

New models of TE repression in plants (specifically Arabidopsis) have suggested specific mechanisms by which TE misregulation in hybrids might result in the expression of hybrid inviability. If true, these models suggest as yet undescribed consequences for (1) mechanistic connections between hybrid problems expressed at different postzygotic stages (e.g., inviability versus sterility), (2) the predicted strength, stage, and direction of isolation between diverging lineages that differ in TE activity, and (3) the association between species attributes that influence TE dynamics (e.g., mode of reproduction, geographical structure) and the rate at which they could accumulate incompatibilities. In this paper, we explore these implications and outline future empirical directions for generating data necessary to evaluate them.

Evolutionary dynamics of transposable elements in a small RNA world

Transposable elements (TEs) are selfish elements that cause harmful mutations, contribute to the structure of regulatory networks and shape the architecture of genomes. Natural selection against their harmful effects has long been considered the dominant force limiting their spread. It is now clear that a genome defense system of RNA-mediated silencing also plays a crucial role in limiting TE proliferation. A full understanding of TE evolutionary dynamics must consider how these forces jointly determine their proliferation within genomes. Here I consider these forces from two perspectives - dynamics within populations and evolutionary games within the germline. The analysis of TE dynamics from these two perspectives promises to provide new insight into their role in evolution.

Drosophila I-R hybrid dysgenesis is associated with catastrophic meiosis and abnormal zygote formation

The Drosophila I-R type of hybrid dysgenesis is a sterility syndrome (SF sterility) associated with the mobilization of the I retrotransposon in female germ cells. SF sterility results from a maternal-effect embryonic lethality whose origin has remained unclear since its discovery about 40 years ago. Here, we show that meiotic divisions in SF oocytes are catastrophic and systematically fail to produce a functional female pronucleus at fertilization. As a consequence, most embryos from SF females rapidly arrest their development with aneuploid or damaged nuclei, whereas others develop as non-viable, androgenetic haploid embryos. Finally, we show that, in contrast to mutants affecting the biogenesis of piRNAs, SF egg chambers do not accumulate persistent DNA double-strand breaks, suggesting that I-element activity might perturb the functional organization of meiotic chromosomes without triggering an early DNA damage response.

Detection of P element transcripts in embryos of Drosophila melanogaster and D. willistoni

The P element is one of the most thoroughly studied transposable elements (TE). Its mobilization causes the hybrid dysgenesis that was first described in Drosophila melanogaster. While studies of the P element have mainly been done in D. melanogaster, it is believed that Drosophila willistoni was the original host species of this TE and that P was transposed to the D. melanogaster genome by horizontal transfer. Our study sought to compare the transcriptional behavior of the P element in embryos of D. melanogaster, which is a recent host, with embryos of two strains of D. willistoni, a species that has contained the P element for a longer time. In both species, potential transcripts of transposase, the enzyme responsible for the TE mobilization, were detected, as were transcripts of the 66-kDa repressor, truncated and antisense sequences, which can have the ability to prevent TEs mobilization. The truncated transcripts reveal the truncated P elements present in the genome strains and whose number seems to be related to the invasion time of the genome by the TE. No qualitative differences in antisense transcripts were observed among the strains, even in the D. willistoni strain with the highest frequency of heterochromatic P elements.

The fitness consequences of P element insertion in Drosophila melanogaster

In this study we estimate the frequency at which P-element insertion events, as identified by in situ hybridization, generate lethal and mild viability mutations. The frequency of lethal mutations generated per insertion event was 0·004. Viability dropped an average of 1% per insertion event. Our results indicate that it is deletions and rearrangements resulting from the mobilization of P elements already in place and not the insertions per se that cause the drastic effects on viability and fitness observed in most studies of P–M dysgenesis-derived mutations. Elements of five other families (I, copia, 412, B104, and gypsy) were not mobilized in these crosses. Finally, we contrast the density of P elements on the X chromosome with the density on the four autosomal arms in a collection of thirty genomes from an African population. The relative number of P elements on the X chromosome is too high to be explained by either a hemizygous selection or a neutrality model. The possible reasons for the failure to detect selection are discussed.

The hobo-related elements in the melanogaster species group

The hobo-related sequences (hRSs) were considered as degenerate and inactive elements until recently, when one mobilizable copy was described. Using this sequence as the initial seed to search for homologous sequences in 12 available Drosophila genomes, in addition to searching for these sequences by PCR and Southern blot in nine other species, we found homologous sequences in every species of the Drosophila melanogaster species subgroup. Some evidence suggests that these non-autonomous sequences were kept mobilizable for at least 0.4 million years. Also, some very short sequences with miniature inverted-repeat transposable element (MITE) characteristics were found among these hRSs. These hRSs and their 'MITE-like' counterparts could provide a good example of the steps proposed in models that describe the MITEs origin.

Wiser (tsl): a recessive X-linked temperature-sensitive lethal mutation that affects the wings and the eyes in Drosophila melanogaster

In this study, we characterize a recessive X-linked temperature-sensitive mutation of the gene CG32711. The mutation, named wiser ( tsl ) (wings scalloped-eyes rough), was isolated from a dysgenic cross and is due to a natural P element insertion within the 5' regulatory region of the gene. Mutant (wiser ( tsl )) individuals exhibit wing notching, rough eyes, tarsal malformations and reduced life-span. At 29 degrees C they die at larval and late pharate stages or during eclosion. The CG32711 (wiser) gene is mainly expressed in the ventral midline cells, the peripheral neural system, the hemocytes and the tracheal system of embryos. It is also expressed in nurse cells of adult female ovaries. Our results show that the wiser gene is alternatively spliced generating two mRNAs, which share the same open reading frame, while western analysis identified two protein isoforms. Their expression pattern depends on the stage of development and the culture temperature. wiser ( tsl ) and wild-type individuals display different expression patterns of the two isoforms and this difference most probably accounts for the mutant phenotype. Our results indicate that wiser is a vital gene for the development of Drosophila melanogaster which has no orthologs outside the Drosophilidae.

Seasonal analysis of 23.5 MRF (hobo) and P-M hybrid dysgenesis determinants in a Greek natural population of Drosophila melanogaster

Genetic analysis of 23.5 MRF (hobo) and P-M hybrid dysgenesis determinants in a Greek natural population in six collections over 24 months, showed the existence of hobo activity in the population at rates higher than P activity. Moreover, seasonal differentiation in hobo GD-sterility potential and hobo repressor abilities were observed. The P activity was low in the population but some tendency for seasonal differentiation of the cytotype was detected. The two systems operate independently in nature. Analysis of isofemale lines, established from inseminated wild-caught females, showed rapid differentiation of their hybrid dysgenesis determinants in the laboratory. This shows that results obtained from isofemale lines do not necessarily reflect the original population structure. The seasonal differentiation may be correlated with seasonal environmental factors, and may be attributed to differences in structure and function of the elements that consequently affect their regulation and transposition.

Sperm competition can drive a male-biased mutation rate

A pattern of male-biased mutation has been found in a wide range of species. The standard explanation for this bias is that there are greater numbers of mitotic cell divisions in the history of the average sperm, compared to the average egg, and that mutations typically result from errors made during replication. However, this fails to provide an ultimate evolutionary explanation for why the male germline would tolerate more mutations that are typically deleterious. One possibility is that if there is a tradeoff between producing large numbers of sperm and expending energetic resources in maintaining a lower mutation rate, sperm competition would select for males that produce larger numbers of sperm despite a higher resulting mutation rate. Here I describe a model that jointly considers the fitness consequences of deleterious mutation and mating success in the face of sperm competition. I show that a moderate level of sperm competition can account for the observation that the male germline tolerates a higher mutation rate than the female germline.

Patterns of hobo elements and their effects in natural populations of Drosophila melanogaster in Japan

We studied the dynamics of hobo elements of Drosophila melanogaster in Japan with the goal of better understanding the invasion and evolution of transposons in natural populations. One hundred and twenty-six isofemale lines and 11 older stocks were tested for the presence and genetic phenotype of hobo elements. The oldest H strain, containing complete and deleted hobo elements, is Hikone-H (1957), but Hikone-R (1952) has no hobo-homologous sequences. The findings suggest that the hobo element invaded Japanese populations in the mid-1950s, at about the same time as the P element invasion in Japan. This chronology is consistent with the hypothesis of a recent worldwide hobo element invasion into D. melanogaster in the mid-1950s. In recently collected populations, H degrees strains (low hobo activity and high repression potency) are predominant, whereas H+ strains (high hobo activity and high repression potency) are predominant in the Sakishima Islands, the most southwestern islands of the Japanese archipelago. H' strains (high hobo activity and low repression potency) were first found in limited island populations. Japanese populations have not only full-size hobo elements and 1.5 kb Th elements but also characteristic deletion derivatives (1.6 and 1.8 kb XhoI fragments) that we have named Jh elements. These results are consistent with transgenic experiments with complete hobo elements, in which populations evolved to H+ or H degrees via H', and in which 1.8 kb fragments appeared. We conclude that hobo elements invaded the central region of Japan, spread to the far islands, and that the invasion is currently at an intermediate, nonequilibrium stage.

Evidence for maternally transmitted small interfering RNA in the repression of transposition in Drosophila virilis

Hybrid dysgenesis in Drosophila is a syndrome of gonadal atrophy, sterility, and male recombination, and it occurs in the progeny of crosses between males that harbor certain transposable elements (TEs) and females that lack them. Known examples of hybrid dysgenesis in Drosophila melanogaster result from mobilization of individual families of TEs, such as the P element, the I element, or hobo. An example of hybrid dysgenesis in Drosophila virilis is unique in that multiple, unrelated families of TEs become mobilized, but a TE designated Penelope appears to play a major role. In all known examples of hybrid dysgenesis, the paternal germ line transmits the TEs in an active state, whereas the female germ line maintains repression of the TEs. The mechanism of maternal maintenance of repression is not known. Recent evidence suggests that the molecular machinery of RNA interference may function as an important host defense against TEs. This protection is mediated by the action of endogenous small interfering RNAs (siRNAs) composed of dsRNA molecules of 21-25 nt that can target complementary transcripts for destruction. In this paper, we demonstrate that endogenous siRNA derived from the Penelope element is maternally loaded in embryos through the female germ line in D. virilis. We also present evidence that the maternal inheritance of these endogenous siRNAs may contribute to maternal repression of Penelope.

Cchobo, a hobo-related sequence in Ceratitis capitata

A hobo-related sequence, Cchobo, with high similarity to the Drosophila melanogaster HFL1 and hobo108 elements was isolated from the medfly. Thirteen PCR-derived clones, which share 97.9-100% DNA identity, were sequenced, seven of which do not show frame-shift or stop codon mutations in their conceptual translations. The consensus sequence has 99.7% DNA identity with the D. melanogaster hobo element HFLI. In a phylogenetic analysis with other hobo-related elements, Cchobo clusters with the HFL1 and hobo108 elements from D. melanogaster and hobo-related elements from D. simulans, D. mauritiana and Mamestra brassicae. These elements may have undergone horizontal transfer in the recent past. The genomic distribution of Cchobo was studied by FISH to mitotic and polytene chromosomes, which revealed that Cchobo is distributed within both the heterochromatin and euchromatin. Intra- and interstrain polymorphisms were detected both at euchromatic and heterochromatic sites. These findings suggest that active copies of the element may be present in the medfly genome.

Maintenance of a large pericentric inversion generated by the hobo transposable element in a transgenic line of Drosophila melanogaster

The impact of the hobo transposable element in the global reorganization of the Drosophila melanogaster genome has been investigated in transgenic lines generated by the injection of hobo elements into the Hikone strain, which lacked them previously. Extensive surveys of transgenic lines followed for 250 generations have identified 13 inversions with hobo inserts at most breakpoints. One of these inversions is pericentric on chromosome 2. It has been maintained in the line where it was discovered and in several sublines at frequencies from 0.19 to 0.45, generating stable chromosomal polymorphisms, similar to cosmopolitan paracentric inversions in natural populations. Individuals homozygous for this inversion were viable and fertile, allowing the creation of a new homozygous strain.

Distribution of P and hobo mobile elements in environmentally manipulated long-term Drosophila melanogaster cage populations

The copy number and the chromosome positions of the P and hobo insertions were determined by means of in situ hybridization to polytene chromosomes, in five long-term Drosophila melanogaster cage populations kept for 18 years under different culture conditions (temperature and relative humidity). The analysis revealed that the copy number of both P and hobo elements were similar between the populations kept under the same culture conditions and significantly different among the populations maintained under different culture conditions. A tendency for similar distribution of these elements along the major chromosome arms was also observed in the populations of the same environmental manipulation. The distribution of the insertions along the chromosomes was not random for both the P and hobo elements; sites with high insertion frequencies were found (hot spots of occupation). Some of them were common in all cage populations while others were characteristic of the populations kept under the same conditions. Finally, fixed sites of occupation were also observed in all populations and refer mostly to hobo distribution. The data are discussed on the basis of the possible involvement of the P and hobo elements, in some way, to the adaptation process and speciation.

Comparative studies of P- and hobo-element activity, fitness components and recombination parameters in two natural populations of Drosophila melanogaster in Moldova

The genomic frequency of crossovers, fitness components, and activity of P- and hobo-transposable elements were studied in two natural populations of Drosophila melanogaster from regions with different pesticide pressure (forest and orchard populations). A significant increase in rf (recombination frequency) in the orchard population (as compared with the forest population) has been found within segments al-b, b-cn, cn-vg (chromosome 2) and for some chromosome 3 segments, th-st, st-cu, cu-sr, sr-e. None of the populations exhibited P-properties. However, the forest population showed weak hobo activity. Both populations contained probably defective hobo copies. With respect to average fitness (as estimated using a compound-autosome strain) and fecundity, the forest population was superior to the orchard one, whereas the latter had a higher locomotor activity and resistance to etherization. The difference in rf between the populations may be due to differences in their environmental conditions. Selection for pesticide resistance may favor the propagation of genotypes with higher rf, thus, possibly, ensuring release of adaptively significant genetic variation.

A three-season comparative analysis of the chromosomal distribution of P and hobo mobile elements in a natural population of Drosophila melanogaster

An analysis on the chromosomal distribution of P and hobo elements in a Greek natural population extending over three seasons showed that the P elements were more abundant in the population than hobos. The copy number distribution per chromosome arm was in general random. The X chromosome had more P copies and the 3R arm more hobos in all three collections. Significant seasonal differences were not observed for these two elements in relation to the total number of insertions per haploid genome. There were, however, certain seasonal differences. They involved the copy number variability, the intra-arm distribution, the distribution along the chromosomes, and the spread and occupancy frequencies. There were no significant differences between the copy numbers of the two elements carried by the standard and the corresponding inverted regions for a number of inversions found in the population. Finally, three out of the five cosmopolitan inversions were found to have hobo insertions at or very near the one of the two breakpoints. Three out of the total had P insertions at or very near the one of the two breakpoints in some squashes and two of the three endemic inversions had a hobo insertion at or very near the one breakpoint, while the third had a P insertion.

Characterization of hobo element copy number and integrity in Brasilian populations of Drosophila melanogaster

We have determined the copy number and the presence of full-size hobo transposable elements in eight Brasilian strains of Drosophila melanogaster. Genomic DNA was digested with AvaII and XhoI restriction enzymes, respectively, and probed with a 963 bp sequence of the hobo element. Variable numbers of full-sized and defective elements were detected in all strains. The range of the copy number was 22.13 +/- 4.52. Blots showed the presence of a 2.6 kb fragment, corresponding to the complete element, in all strains exception of one and the 1.0 kb sequence, correponding to the Th1 and Th2 repressor elements. There was neither association among copy numbers of hobo elements and latitude nor the mean annual temperatures in the original geographical region of each strain.

Sequences homologous to the hobo transposable element in E strains of Drosophila melanogaster

Hobo is one of the three Drosophila melanogaster transposable elements, together with the P and I elements, that seem to have recently invaded the genome of this species. Surveys of the presence of hobo in strains from different geographical and temporal origins have shown that recently collected strains contain complete and deleted elements with high sequence similarity (H strains), but old strains lack hobo elements (E strains). Besides the canonical hobo sequences, both H and E strains show other poorly known hobo-related sequences. In the present work, we analyze the presence, cytogenetic location, and structure of some of these sequences in E strains of D. melanogaster. By in situ hybridization, we found that euchromatic hobo-related sequences were in fixed positions in all six E strains analyzed: 38C in the 2L arm; 42B and 55A in the 2R arm; 79E and 80B in the 3L arm; and 82C, 84C, and 84D in the 3R arm. Sequence comparison shows that some of the hobo-related sequences from Oregon-R and iso-1 strains are similar to the canonical hobo element, but their analysis reveals that they are substantially diverged and rearranged and cannot code for a functional transposase. Our results suggest that these ubiquitous hobo-homologous sequences are immobile and are distantly related to the modern hobo elements from D. melanogaster.

Temperature-dependent gonadal hybrid dysgenesis in Drosophila willistoni

Temperature-dependent gonadal dysgenesis was shown to occur in the progeny of both inter- and intrastrain crosses involving two populations of Drosophila willistoni, one of which was an old laboratory stock, and the other, freshly collected from a natural population. We propose that the phenomenon observed was caused by the mobilization of transposable elements, as occurs in several other Drosophila species.

hobo Induced rearrangements in the yellow locus influence the insulation effect of the gypsy su(Hw)-binding region in Drosophila melanogaster

The su(Hw) protein is responsible for the insulation mediated by the su(Hw)-binding region present in the gypsy retrotransposon. In the y2 mutant, su(Hw) protein partially inhibits yellow transcription by repressing the function of transcriptional enhancers located distally from the yellow promoter with respect to gypsy. y2 mutation derivatives have been induced by the insertion of two hobo copies on the both sides of gypsy: into the yellow intron and into the 5' regulatory region upstream of the wing and body enhancers. The hobo elements have the same structure and orientation, opposite to the direction of yellow transcription. In the sequence context, where two copies of hobo are separated by the su(Hw)-binding region, hobo-dependent rearrangements are frequently associated with duplications of the region between the hobo elements. Duplication of the su(Hw)-binding region strongly inhibits the insulation of the yellow promoter separated from the body and wing enhancers by gypsy. These results provide a better insight into mechanisms by which the su(Hw)-binding region affects the enhancer function.

Lack of correlation between dysgenic traits in the hobo system of hybrid dysgenesis in Drosophila melanogaster

Currently in the hobo system of hybrid dysgenesis, strain classification is based on the presence/absence of the 2.6 kb Xho I restriction fragment. Using this criterion, strains are classified as: (1) H strains when full-size elements are detected by presence of a 2.6 kb Xho I restriction fragment; they can also contain internally deleted elements; (2) DH strains when only deleted elements are detected (Xho I restriction fragment less than 2.6 kb); (3) E strains, devoid of any restriction fragment equal to or less than 2.6 kb in length. In addition, the strains can be classified on their ability to generate gonadal atrophy (GD sterility) when males of a studied strain are crossed with females from an E strain (dysgenic cross). Here we try to define the nature of the dysgenic cross, which leads us to analyse the different components of the dysgenic syndrome and to look for eventual correlations between them. Molecular analysis, GD sterility tests, hobo mobilization with the haw strain and the vg(al) strain, and hereditary transmission of the instability at the vg locus have been assayed in different strains. We show that the occurrence of GD sterility depends on the tested H strains as expected, but also on the E strains used. On the other hand we do not find any correlation between the different dysgenic parameters. Our data reveal that molecular and GD sterility tests are not sufficient to classify strains in the hobo system, and that all the components of the dysgenic syndrome must be taken into account. Our results are discussed with regard to active and full-size elements in relation to the structure of the S region where an amino acid sequence (TPE) presents a repetition polymorphism.