Sperm nuclear activation during fertilization

Developmental Hurdles That Can Compromise Pregnancy during the First Month of Gestation in Cattle

Several key developmental events are associated with early embryonic pregnancy losses in beef and dairy cows. These developmental problems are observed at a greater frequency in pregnancies generated from in-vitro-produced bovine embryos. This review describes critical problems that arise during oocyte maturation, fertilization, early embryonic development, compaction and blastulation, embryonic cell lineage specification, elongation, gastrulation, and placentation. Additionally, discussed are potential remediation strategies, but unfortunately, corrective actions are not available for several of the problems being discussed. Further research is needed to produce bovine embryos that have a greater likelihood of surviving to term.

Chromosomal positioning in spermatogenic cells is influenced by chromosomal factors associated with gene activity, bouquet formation and meiotic sex chromosome inactivation

Chromosome territoriality is not random along the cell cycle and it is mainly governed by intrinsic chromosome factors and gene expression patterns. Conversely, very few studies have explored the factors that determine chromosome territoriality and its influencing factors during meiosis. In this study, we analysed chromosome positioning in murine spermatogenic cells using three-dimensionally fluorescence in situ hybridization-based methodology, which allows the analysis of the entire karyotype. The main objective of the study was to decipher chromosome positioning in a radial axis (all analysed germ-cell nuclei) and longitudinal axis (only spermatozoa) and to identify the chromosomal factors that regulate such an arrangement. Results demonstrated that the radial positioning of chromosomes during spermatogenesis was cell-type specific and influenced by chromosomal factors associated to gene activity. Chromosomes with specific features that enhance transcription (high GC content, high gene density and high numbers of predicted expressed genes) were preferentially observed in the inner part of the nucleus in virtually all cell types. Moreover, the position of the sex chromosomes was influenced by their transcriptional status, from the periphery of the nucleus when its activity was repressed (pachytene) to a more internal position when it is partially activated (spermatid). At pachytene, chromosome positioning was also influenced by chromosome size due to the bouquet formation. Longitudinal chromosome positioning in the sperm nucleus was not random either, suggesting the importance of ordered longitudinal positioning for the release and activation of the paternal genome after fertilisation.

LINCking the Nuclear Envelope to Sperm Architecture

Nuclear architecture undergoes an extensive remodeling during spermatogenesis, especially at levels of spermatocytes (SPC) and spermatids (SPT). Interestingly, typical events of spermiogenesis, such as nuclear elongation, acrosome biogenesis, and flagellum formation, need a functional cooperation between proteins of the nuclear envelope and acroplaxome/manchette structures. In addition, nuclear envelope plays a key role in chromosome distribution. In this scenario, special attention has been focused on the LINC (linker of nucleoskeleton and cytoskeleton) complex, a nuclear envelope-bridge structure involved in the connection of the nucleoskeleton to the cytoskeleton, governing mechanotransduction. It includes two integral proteins: KASH- and SUN-domain proteins, on the outer (ONM) and inner (INM) nuclear membrane, respectively. The LINC complex is involved in several functions fundamental to the correct development of sperm cells such as head formation and head to tail connection, and, therefore, it seems to be important in determining male fertility. This review provides a global overview of the main LINC complex components, with a special attention to their subcellular localization in sperm cells, their roles in the regulation of sperm morphological maturation, and, lastly, LINC complex alterations associated to male infertility.

Epigenetic regulation in development: is the mouse a good model for the human?

BACKGROUND

Over the past few years, advances in molecular technologies have allowed unprecedented mapping of epigenetic modifications in gametes and during early embryonic development. This work is allowing a detailed genomic analysis, which for the first time can answer long-standing questions about epigenetic regulation and reprogramming, and highlights differences between mouse and human, the implications of which are only beginning to be explored.

OBJECTIVE AND RATIONALE

In this review, we summarise new low-cell molecular methods enabling the interrogation of epigenetic information in gametes and early embryos, the mechanistic insights these have provided, and contrast the findings in mouse and human.

SEARCH METHODS

Relevant studies were identified by PubMed search.

OUTCOMES

We discuss the levels of epigenetic regulation, from DNA modifications to chromatin organisation, during mouse gametogenesis, fertilisation and pre- and post-implantation development. The recently characterised features of the oocyte epigenome highlight its exceptionally unique regulatory landscape. The chromatin organisation and epigenetic landscape of both gametic genomes are rapidly reprogrammed after fertilisation. This extensive epigenetic remodelling is necessary for zygotic genome activation, but the mechanistic link remains unclear. While the vast majority of epigenetic information from the gametes is erased in pre-implantation development, new insights suggest that repressive histone modifications from the oocyte may mediate a novel mechanism of imprinting. To date, the characterisation of epigenetics in human development has been almost exclusively limited to DNA methylation profiling; these data reinforce that the global dynamics are conserved between mouse and human. However, as we look closer, it is becoming apparent that the mechanisms regulating these dynamics are distinct. These early findings emphasise the importance of investigations of fundamental epigenetic mechanisms in both mouse and humans.

WIDER IMPLICATIONS

Failures in epigenetic regulation have been implicated in human disease and infertility. With increasing maternal age and use of reproductive technologies in countries all over the world, it is becoming ever more important to understand the necessary processes required to establish a developmentally competent embryo. Furthermore, it is essential to evaluate the extent to which these epigenetic patterns are sensitive to such technologies and other adverse environmental exposures.

Nuclear Reformation at the End of Mitosis

Cells have developed highly sophisticated ways to accurately pass on their genetic information to the daughter cells. In animal cells, which undergo open mitosis, the nuclear envelope breaks down at the beginning of mitosis and the chromatin massively condenses to be captured and segregated by the mitotic spindle. These events have to be reverted in order to allow the reformation of a nucleus competent for DNA transcription and replication, as well as all other nuclear processes occurring in interphase. Here, we summarize our current knowledge of how, in animal cells, the highly compacted mitotic chromosomes are decondensed at the end of mitosis and how a nuclear envelope, including functional nuclear pore complexes, reassembles around these decondensing chromosomes.

The lysine demethylase LSD1 is required for nuclear envelope formation at the end of mitosis

The metazoan nucleus breaks down and reassembles during each cell division. Upon mitotic exit, the successful reestablishment of an interphase nucleus requires the coordinated reorganization of chromatin and formation of a functional nuclear envelope. Here, we report that the histone demethylase LSD1 (also known as KDM1A) plays a crucial role in nuclear assembly at the end of mitosis. Downregulation of LSD1 in cells extends telophase and impairs nuclear pore complex assembly. In vitro, LSD1 demethylase activity is required for the recruitment of MEL28 (also known as ELYS and AHCTF1) and nuclear envelope precursor vesicles to chromatin, crucial steps in nuclear reassembly. Accordingly, the formation of a closed nuclear envelope and nuclear pore complex assembly are impaired upon depletion of LSD1 or inhibition of its activity. Our results identify histone demethylation by LSD1 as a new regulatory mechanism linking the chromatin state and nuclear envelope formation at the end of mitosis.

Nuclear assembly shaped by microtubule dynamics

Maintenance of nuclear architecture is crucial for gene regulation, cell proliferation and tissue development. However, during every open mitosis and meiosis, chromosomes are exposed to cytoskeletal forces until they are fully reassembled into mature nuclei. Here we discuss our recent study of nuclear assembly in Xenopus egg extracts, where we showed that the DNA binding protein Developmental pluripotency associated 2 (Dppa2) directly inhibits microtubule polymerization during nuclear formation, and that this is essential for normal nuclear shape and replication. We explore mechanisms by which microtubule dynamics could regulate nuclear formation and morphology, and discuss the importance of both spatial and temporal regulation of microtubules in this process. Moreover, expression of Dppa2 is limited to the early embryo and pluripotent tissues, and we highlight the specific demands of mitosis in these often rapidly dividing cells, in which telophase nuclear assembly must be expedited and may facilitate developmental changes in nuclear architecture.

Chromatin-bound Xenopus Dppa2 shapes the nucleus by locally inhibiting microtubule assembly

Nuclear shape and size vary between species, during development, and in many tissue pathologies, but the causes and effects of these differences remain poorly understood. During fertilization, sperm nuclei undergo a dramatic conversion from a heavily compacted form into decondensed, spherical pronuclei, accompanied by rapid nucleation of microtubules from centrosomes. Here we report that the assembly of the spherical nucleus depends on a critical balance of microtubule dynamics, which is regulated by the chromatin-binding protein Developmental pluripotency-associated 2 (Dppa2). Whereas microtubules normally promote sperm pronuclear expansion, in Dppa2-depleted Xenopus egg extracts excess microtubules cause pronuclear assembly defects, leading to abnormal morphology and disorganized DNA replication. Dppa2 inhibits microtubule polymerization in vitro, and Dppa2 activity is needed at a precise time and location during nascent pronuclear formation. This demonstrates a strict spatiotemporal requirement for local suppression of microtubules during nuclear formation, fulfilled by chromatin-bound microtubule regulators.

Heterochromatin formation in the mouse embryo requires critical residues of the histone variant H3.3

In mammals, oocyte fertilization by sperm initiates development. This is followed by epigenetic reprogramming of both parental genomes, which involves the de novo establishment of chromatin domains. In the mouse embryo, methylation of histone H3 establishes an epigenetic asymmetry and is predominant in the maternal pronucleus. However, the roles of differential incorporation of histone H3 variants in the parental chromatin, and of modified residues within specific histone variants, have not been addressed. Here we show that the histone variant H3.3, and in particular lysine 27, is required for the establishment of heterochromatin in the mouse embryo. H3.3 localizes to paternal pericentromeric chromatin during S phase at the time of transcription of pericentromeric repeats. Mutation of H3.3 K27, but not of H3.1 K27, results in aberrant accumulation of pericentromeric transcripts, HP1 mislocalization, dysfunctional chromosome segregation and developmental arrest. This phenotype is rescued by injection of double-stranded RNA (dsRNA) derived from pericentromeric transcripts, indicating a functional link between H3.3K27 and the silencing of such regions by means of an RNA-interference (RNAi) pathway. Our work demonstrates a role for a modifiable residue within a histone-variant-specific context during reprogramming and identifies a novel function for mammalian H3.3 in the initial formation of dsRNA-dependent heterochromatin.

Parental origin of chromatin in human monopronuclear zygotes revealed by asymmetric histone methylation patterns, differs between IVF and ICSI

In mouse zygotes, many post-translational histone modifications are asymmetrically present in male and female pronuclei. We investigated whether this principle could be used to determine the genetic composition of monopronuclear human zygotes in conventional IVF and ICSI. First we determined whether male female asymmetry is conserved from mouse to human by staining polypronuclear zygotes with antibodies against a subset of histone N-tail post-translational modifications. To analyze human monopronuclear zygotes, a modification, H3K9me3, was selected that is present in the maternal chromatin. After IVF a total of 45 monopronuclear zygotes were obtained. In 39 (87%) of zygotes a nonuniform staining pattern was observed, proof of a bi-parental origin and assumed to result into a diploid conception. Two zygotes showed no staining for the modification, indicating that the single pronucleus was of paternal origin. Four zygotes contained only maternally derived chromatin. ICSI-derived monopronuclear zygotes (n = 33) could also be divided into three groups based on the staining pattern of their chromatin: (1) of maternal origin (n = 15), (2) of paternal origin (n = 8) or (3) consisting of two chromatin domains as dominating in IVF (n = 10). Our data show that monopronuclear zygotes originating from IVF generally arise through fusion of parental chromatin after sperm penetration. Monopronuclear zygotes derived from ICSI in most cases contain uni-parental chromatin. The fact that chromatin was of paternal origin in 24% of ICSI and in 4% of the IVF zygotes confirms earlier results obtained by FISH on cleavage stages. Our findings are of clinical importance in IVF and ICSI practice.

Cytoplasmic and nuclear determinants of the maternal-to-embryonic transition

Although improvements in culture systems have greatly enhanced in vitro embryo production, success rates under the best conditions are still far from ideal. The reasons for developmental arrest of the majority of in vitro produced embryos are unclear, but likely attributable, in part, to intrinsic and extrinsic influences on the cytoplasmic and/or nuclear environment of an oocyte and/or early embryo that impede normal progression through the maternal-to-embryonic transition. The maternal-to-embryonic transition is the time period during embryonic development spanning from fertilisation until when control of early embryogenesis changes from regulation by oocyte-derived factors to regulation by products of the embryonic genome. The products of numerous maternal effect genes transcribed and stored during oogenesis mediate this transition. Marked epigenetic changes to chromatin during this window of development significantly modulate embryonic gene expression. Depletion of maternal mRNA pools is also an obligatory event during the maternal-to-embryonic transition critical to subsequent development. An increased knowledge of the fundamental mechanisms and mediators of the maternal-to-embryonic transition is foundational to understanding the regulation of oocyte quality and future breakthroughs relevant to embryo production.

Equivalent Parental Contribution to Early Plant Zygotic Development

Hybrid vigor or heterosis results from the combination of genetically distant genomes at fertilization, and as well as being of major commercial importance, it is held to contribute significantly to fitness [1]. Activation of the paternal genome marks the transition from maternal to zygotic control of development, but a reported delay of paternal-genome activation in flowering plants [2-4] and animals [5, 6] excludes heterosis from impacting on very early development. We have analyzed the allele-specific expression of 25 genes after fertilization of the egg in maize and show immediate equivalent parental genomic contribution to the zygote. Every gene expressed before the first cell division of the zygotes showed paternal transcripts. Sequence comparisons indicate that these genes are involved in a range of processes and are distributed throughout the genome. Our findings confirm that some plant species have evolved a strategy to activate the paternal genome immediately after fertilization, in contrast to the situation in other plants and in animals. Such an extensive activation of the paternal genome very early in development is consonant with observations of high levels of heterosis in early hybrid maize embryos [7, 8], indicating a significant impact of this sexual strategy on fitness.

The essential role of Drosophila HIRA for de novo assembly of paternal chromatin at fertilization

In many animal species, the sperm DNA is packaged with male germ line–specific chromosomal proteins, including protamines. At fertilization, these non-histone proteins are removed from the decondensing sperm nucleus and replaced with maternally provided histones to form the DNA replication competent male pronucleus. By studying a point mutant allele of the Drosophila Hira gene, we previously showed that HIRA, a conserved replication-independent chromatin assembly factor, was essential for the assembly of paternal chromatin at fertilization. HIRA permits the specific assembly of nucleosomes containing the histone H3.3 variant on the decondensing male pronucleus. We report here the analysis of a new mutant allele of Drosophila Hira that was generated by homologous recombination. Surprisingly, phenotypic analysis of this loss of function allele revealed that the only essential function of HIRA is the assembly of paternal chromatin during male pronucleus formation. This HIRA-dependent assembly of H3.3 nucleosomes on paternal DNA does not require the histone chaperone ASF1. Moreover, analysis of this mutant established that protamines are correctly removed at fertilization in the absence of HIRA, thus demonstrating that protamine removal and histone deposition are two functionally distinct processes. Finally, we showed that H3.3 deposition is apparently not affected in Hira mutant embryos and adults, suggesting that different chromatin assembly machineries could deposit this histone variant.

Chromatin is composed of basic units called nucleosomes, in which DNA wraps around a core of histone proteins. HIRA is a histone chaperone that is specifically involved in the assembly of nucleosomes containing H3.3, a universally conserved type of histone 3. To understand the function of HIRA in vivo, the authors generated mutant fruit flies with a non-functional Hira gene. Surprisingly, mutant flies were viable, but females were completely sterile. By analysing the female fruit flies' eggs, the authors found that in the absence of HIRA protein, the sperm nucleus was unable to participate in the formation of the zygote. In Drosophila, as in many animals, the condensed sperm chromatin contains protamines instead of histones. The authors found that the only crucial role of HIRA in flies was to assemble nucleosomes containing H3.3 in the male pronucleus, after the removal of protamines. This fundamental process, which is presumably also controlled by HIRA in vertebrates, allows the paternal DNA to reconstitute its chromatin and participate in the development of the embryo.

p21 provides stage specific DNA damage control to preimplantation embryos

The early stage embryogenesis of higher eukaryotes lacks some of the damage response pathways such as G1/S checkpoint, G2/M checkpoint and apoptosis. We examined here the damage response of preimplantation stage embryos after fertilization with 6 Gy irradiated sperm. Sperm-irradiated embryos developed normally for the first 2.5 days, but started to exhibit a developmental delay at day 3.5. p21 was activated in the delayed embryos, which carried numerous micronuclei owing to delayed chromosome instability. Apoptosis was observed predominantly in the inner cell mass of the day 4.0 embryos. Sperm-irradiated p21-/- embryos lacked the delay, but chromosome instability and apoptosis were more pronounced than the corresponding p21 wild-type embryos. We conclude from the result that damage responses come in a stage-specific manner during preimplantation stage development; p53-dependent S checkpoint at the zygote stage, p21-mediated cell cycle arrest at the morula/blastocyst stages and apoptosis after the blastocyst stage in the inner cell mass.

γH2AX signalling during sperm chromatin remodelling in the mouse zygote

In the mouse, the paternal post-meiotic chromatin is assumed to be devoid of DNA repair after nuclear elongation and protamine-induced compaction. Hence, DNA lesions induced thereafter will have to be restored upon gamete fusion in the zygote. Misrepair of such lesions often results in chromosome type aberrations at the first cleavage division, suggesting that the repair event takes place prior to S-phase. During this stage of the zygotic cell cycle, the paternal chromatin transits from a protamine- to a nucleosome-based state. We addressed the question whether the canonical signalling pathway to DNA double strand breaks (DSBs), the phosphorylated form of histone H2AX (gammaH2AX) is active during chromatin restructuring of the male genetic complement in the zygote. Here, we describe the detailed characterization of gammaH2AX signalling in the early stages of zygotic development up to the appearance of the pronuclei. We have found the gammaH2AX signalling pathway to be already active during sperm chromatin remodelling after gamete fusion in a dose dependent manner, reflecting the amount of DSBs present in the sperm nucleus after in vivo male irradiation. Using DNA damaging compounds to induce lesions in the early zygote, differences in DSB sensitivity and gammaH2AX processing between paternal and maternal chromatin were found, suggesting differences in DNA repair capacity between the parental chromatin sets.

Transmission of modified nucleosomes from the mouse male germline to the zygote and subsequent remodeling of paternal chromatin

Rapidly after gamete fusion, the sperm nucleus loses its specific chromatin conformation and the DNA is repopulated with maternally derived nucleosomes. We evaluated the nature of paternally derived nucleosomes and the dynamics of sperm chromatin remodeling in the zygote directly after gamete fusion. We observed histone H4 acetylated at K8 or K12 already prior to full decondensation of the sperm nucleus, suggesting that these marks are transmitted by the spermatozoon. Tracking down the origin of H4K8ac and H4K12ac during spermiogenesis revealed the retention of nucleosomes with these modifications in the chromocenter of elongating spermatids. We show that sperm constitutive heterochromatin is enriched for nucleosomes carrying specific histone modifications which are transmitted to the zygote. Our results suggest an epigenetic mechanism for inheritance of chromosomal architecture. Furthermore, up to pronucleus formation, histone acetylation and phosphorylation build up in a cascade-like fashion in the paternal chromatin. After formation of the pronucleus, a subset of these marks is removed from the heterochromatin, which suggests a reestablishment of the euchromatin-heterochromatin partition.

The effect of epigenetic sperm abnormalities on early embryogenesis

Sperm are a highly specialized cell type derived to deliver the paternal haploid genome to the oocyte. The epigenetic, or gene regulatory, properties and mechanisms of the sperm assist in preparation of the paternal genome to contribute to embryogenesis and the genome of the zygote. Many recent studies have addressed the issue of altered epigenetic processes in the sperm. This review evaluates the current understanding of DNA damage, chromosome aneuploidy, reduced telomere length, malformations of the centrosome, genomic imprinting errors, altered mRNA profiles, and abnormal nuclear packaging in the sperm prior to fertilization and the observed effects on embryogenesis. Attention has also been given to understanding the underlying etiology of sperm with altered epigenetic mechanisms in humans.

2. Chromatin assembly with H3 histones: full throttle down multiple pathways

The typical eukaryotic genome packages roughly 6 feet of DNA into a nucleus about 5 mum in diameter, yet this compaction blocks access to the DNA. At the first level of compaction, DNA is wrapped around octamers of core histone proteins to form arrays of nucleosomes. Nucleosomes are sufficient to block access to DNA, and cells must therefore manipulate nucleosomes in the course of activating the genome. Dramatic progress has been made in understanding the mechanisms by which nucleosomes are manipulated. In addition to the major core histones, most eukaryotic genomes also encode additional variant histones, which have some structural similarity. These are targeted to specific loci by coupling specialized nucleosome assembly pathways to DNA replication, transcription, or to developmental processes. We review evidence that nucleosome assembly pathways are interlinked with histone-modification systems, and may thereby perpetuate epigenetic chromatin states.

Asymmetry in histone H3 variants and lysine methylation between paternal and maternal chromatin of the early mouse zygote

In mammalian fertilization, the paternal genome is delivered to the secondary oocyte by sperm with protamine compacted DNA, while the maternal genome is arrested in meiotic metaphase II. Thus, at the beginning of fertilization, the two gametic chromatin sets are strikingly different. We elaborate on this contrast by reporting asymmetry for histone H3 type in the pre-S-phase zygote when male chromatin is virtually devoid of histone H3.1/3.2. Localization of the histone H3.3/H4 assembly factor Hira with the paternal chromatin indicates the presence of histone H3.3. In conjunction with this, we performed a systematic immunofluorescence analysis of histone N-tail methylations at position H3K4, H3K9, H3K27 and H4K20 up to the young pronucleus stage and show that asymmetries reported earlier are systematic for virtually all di- and tri-methylations but not for mono-methylation of H3K4 and H4K20, the only marks studied present in the early male pronucleus. For H4K20 the expanding male chromatin is rapidly mono-methylated. This coincides with the formation of maternally derived nucleosomes, a process which is observed as early as sperm chromatin decondensation occurs. Absence of tri-methylated H3K9, tri-methylated H4K20 and presence of loosely anchored HP1-beta combined with the homogenous presence of mono-methylated H4K20 suggests the absence of a division of the paternal chromatin in eu- and heterochromatin. In summary the male, in contrast to female G1 chromatin, is uniform and contains predominantly histone H3.3 as histone H3 variant.

The histone H3.3 chaperone HIRA is essential for chromatin assembly in the male pronucleus

In sexually reproducing animals, a crucial step in zygote formation is the decondensation of the fertilizing sperm nucleus into a DNA replication-competent male pronucleus. Genome-wide nucleosome assembly on paternal DNA implies the replacement of sperm chromosomal proteins, such as protamines, by maternally provided histones. This fundamental process is specifically impaired in sésame (ssm), a unique Drosophila maternal effect mutant that prevents male pronucleus formation. Here we show that ssm is a point mutation in the Hira gene, thus demonstrating that the histone chaperone protein HIRA is required for nucleosome assembly during sperm nucleus decondensation. In vertebrates, HIRA has recently been shown to be critical for a nucleosome assembly pathway independent of DNA synthesis that specifically involves the H3.3 histone variant. We also show that nucleosomes containing H3.3, and not H3, are specifically assembled in paternal Drosophila chromatin before the first round of DNA replication. The exclusive marking of paternal chromosomes with H3.3 represents a primary epigenetic distinction between parental genomes in the zygote, and underlines an important consequence of the critical and highly specialized function of HIRA at fertilization.

Non-random chromosome positioning in mammalian sperm nuclei, with migration of the sex chromosomes during late spermatogenesis

Chromosomes are highly organized and compartmentalized in cell nuclei. The analysis of their position is a powerful way to monitor genome organization in different cell types and states. Evidence suggests that the organization of the genome could be functionally important for influencing different cellular and developmental processes, particularly at early stages of development (i.e. fertilization and the consequent entry of the sperm nucleus into the egg). The position of chromosomes in the sperm nucleus might be crucial, because their location could determine the time at which particular chromatin domains are decondensed and remodelled, allowing some epigenetic level of control or influence over subsequent paternal gene expression in the embryo. Here, we analyse genome organization by chromosome position in mammalian sperm nuclei from three breeds of pig, as a model species. We have mapped the preferential position of all chromosomes (bar one) in sperm nuclei in two dimensions and have established that the sex chromosomes are the most internally localized chromosomes in mature sperm. The distribution of two autosomes and chromosomes X and Y in sperm heads was compared in primary and secondary spermatocytes and spermatids in porcine testes. The sex chromosomes were found at the nuclear edge in primary spermatocytes, which correlates with the known position of the XY body and their position in somatic cells, whereas, in spermatids, the sex chromosomes were much more centrally located, mirroring the position of these chromosomes in ejaculated spermatozoa. This study reveals the temporal repositioning of chromosome territories in spermatogenesis.

Chromatin dynamics and Arabidopsis development

The plant life cycle involves a series of developmental phase transitions. These transitions require the regulation and highly co-ordinated expression of many genes. Epigenetic controls have now been shown to be a key element of this mechanism of regulation. In the model plant Arabidopsis, recent genetic and molecular studies on chromatin have begun to dissect the molecular basis of these epigenetic controls. Chromatin dynamics represent the emerging and exciting field of gene regulation notably involved in plant developmental transitions. By comparing plant and animal systems, new insights into the molecular complexes and mechanisms governing development can be delineated. We are now beginning to identify the components of chromatin complexes and their functions.

Implications of cloning technique for reproductive medicine

The birth of Dolly following the transfer of mammary gland nuclei into enucleated eggs established cloning as a feasible technique in mammals, but the moral implications and high incidence of developmental abnormalities associated with cloning have induced the majority of countries to legislate against its use with human gametes. Because of such negative connotations, restrictive political reactions could jeopardize the therapeutic and scientific promise that certain types of cloning may present. For example, in addition to its proposed use as a way of generating stem cells, the basic technique of nuclear transplantation has proven useful in other ways, including its application to immature eggs as a new approach to the prevention of the aneuploidy common in older women, and for some recent advances in preimplantation genetic diagnosis. Thus, while attempts at reproductive cloning in man would seem premature and even dangerous at present, this field will require rational rather than emotional reactions as a basis for legislation if the therapeutic promise of stem cell research and the experimental potential of nuclear transplantation techniques are to be fully realized.

Natural Selection and the Evolution of Genome Imprinting

Sexual reproduction results from the fusion of gametes in which the chromatin configuration of maternal and paternal chromosomes is distinct at fertilization. Although many of the differences are erased during successive cellular divisions and chromatin modifications, some are retained in both somatic and germline cells. These epigenetic modifications can confer different characteristics on maternal and paternal chromosomes and such differences can be selected during any process that has the ability to distinguish between homologues. The end result of these selective forces are parental origin effects, writ large. The range of effects observed, including transcriptional imprinting and effects on chromosome segregation and heterochromatization, reflects the diversity of selective forces in operation. However, a closer look at these effects suggests that parental origin-dependent differences in chromatin structure might be subject to some common forces and that these forces may explain many of the "nontranscriptional" parental origin effects observed in mammals.

Sperm chromatin remodelling and Wolbachia-induced cytoplasmic incompatibility in Drosophila

Wolbachia pipientis is an obligate bacterial endosymbiont, which has successfully invaded approximately 20% of all insect species by manipulating their normal developmental patterns. Wolbachia-induced phenotypes include parthenogenesis, male killing, and, most notably, cytoplasmic incompatibility. In the future these phenotypes might be useful in controlling or modifying insect populations but this will depend on our understanding of the basic molecular processes underlying insect fertilization and development. Wolbachia-infected Drosophila simulans express high levels of cytoplasmic incompatibility in which the sperm nucleus is modified and does not form a normal male pronucleus when fertilizing eggs from uninfected females. The sperm modification is somehow rescued in eggs infected with the same strain of Wolbachia. Thus, D. simulans has become an excellent model organism for investigating the manner in which endosymbionts can alter reproductive programs in insect hosts. This paper reviews the current knowledge of Drosophila early development and particularly sperm function. Developmental mutations in Drosophila that are known to affect sperm function will also be discussed.incompatibility.

The Drosophila misfire gene has an essential role in sperm activation during fertilization

The male sterile mutation, misfire (mfr), of Drosophila melanogaster is a novel paternal effect, fertilization defective mutant that effects sperm head decondensation. mfr sperm were motile, appeared normal morphologically and were transferred to the female during copulation. However, less than 0.1% of eggs laid by females mated to mfr males hatched. Although mfr sperm entered eggs at a high frequency (93%), 99% of the inseminated eggs did not initiate the first nuclear division. Unlike wild type fertilizing sperm, the position and shape of mfr sperm tails within the egg were not constant, but varied in a seemingly random manner. The heads of inseminating mutant sperm were always located near the surface of eggs just underlying the egg plasma membrane, and maintained their needle-like shape indicating the failure of nuclear decondensation. Further observations revealed that plasma membrane of inseminating sperm appeared intact, including the head region. These phenotypes were equivalent to those of sneaky (snky), another fertilization defective male sterile mutation. Our observations strongly suggest that mfr mutant males are sterile because their inseminating sperm fail to form a male pronucleus due to the inability of the sperm to properly respond to egg factors responsible for the breakdown of the plasma membrane. Although mfr and snky mutations were phenotypically identical, they mapped to cytologically distinct genetic loci and no genetic interactions were observed, suggesting that at least two distinct paternal gene products are involved in the early stages of pronuclear formation.

Epigenetic reprogramming in early mammalian development and following somatic nuclear transfer

Epigenetic modifications of the genome play a significant role in the elaboration of the genetic code as established at fertilisation. These modifications affect early growth and development through their influence on gene expression especially on imprinted genes. Genome-wide epigenetic reprogramming in germ cells is essential in order to reset the parent-of-origin specific marking of imprinted genes, but may have a more general role in the restoration of totipotency in the early embryo. In a similar way, on somatic nuclear cloning, a differentiated cell must become 'reprogrammed' restoring totipotency in order to undergo development. Here we discuss the dynamic epigenetic reprogramming that takes place during normal development and highlight those areas with relevance to somatic nuclear cloning and the possibility of improving the efficiency of this process. We propose the concept of 'epigenetic checkpoints' for normal progression of development and the loss of totipotency.

Unresolved Issues in Mammalian Fertilization

This review considers the role of the sperm in fertilization, addressing areas of misunderstanding and unfounded assumptions and taking particular advantage of the large body of data resulting from work with rodent species in vitro. Considerable attention is given to the appropriate use and interpretation of assays for capacitation, acrosomal exocytosis, hyperactivation, and sperm protein phosphorylation, as well as tests for sperm-zona and sperm-oocyte membrane interactions. The lack of general agreement on the means of sperm adhesion to and penetration of the zona pellucida is addressed, and the need for new approaches to this problem is pointed out. Some molecular advances in our understanding of specific steps in the process of fertilization are discussed in the context of intact cell-matrix and cell-cell interaction. This review should provide practical information for researchers just beginning the study of fertilization and interesting but not widely known observations to stimulate new ideas in experienced scientists.

Paternal mRNA and protein synthesis coincides with male chromatin decondensation in maize zygotes

Decondensation of the male genome after fertilization is a prerequisite for replication and transcription. Cytological analysis has revealed decondensation of the male chromatin to commence immediately after karyogamy and progress rapidly, pointing to an early start of transcription. To investigate early transcription from the paternal genome in maize zygotes, we generated transgenic plants containing green fluorescent protein (GFP) under control of the 35S promoter. Single transgenic sperm cells from these plants were used to fertilize isolated wild-type egg cells in vitro. These sperm cells did not contain gfp transcripts. Appearance of gfp mRNA, 4 h after fertilization, was coincident with decondensation of the male chromatin, and clearly demonstrates early accessibility to the transcriptional machinery of at least a part of the male genome. Translational activity in early zygotes was evident 6 h after fertilization, as demonstrated by measurable levels of GFP fluorescence signal. Using a similar strategy, we also demonstrated activity of the paternal genome early in endosperm development. These findings may exclude any global mechanism of silencing the entire paternal genome over this period, and make an almost immediate paternal contribution to zygote and early endosperm development conceivable. These data are also considered in the perspective of current views of genome activation in the zygotes and young embryos of animals.

Persistent induction of somatic reversions of the pink-eyed unstable mutation in F1 mice born to fathers irradiated at the spermatozoa stage

Untargeted mutation and delayed mutation are features of radiation-induced genomic instability and have been studied extensively in tissue culture cells. The mouse pink-eyed unstable (p(un)) mutation is due to an intragenic duplication of the pink-eyed dilution locus and frequently reverts back to the wild type in germ cells as well as in somatic cells. The reversion event can be detected in the retinal pigment epithelium as a cluster of pigmented cells (eye spot). We have investigated the reversion p(um) in F1 mice born to irradiated males. Spermatogonia-stage irradiation did not affect the frequency of the reversion in F1 mice. However, 6 Gy irradiation at the spermatozoa stage resulted in an approximately twofold increase in the number of eye spots in the retinal pigment epithelium of F1 mice. Somatic reversion occurred for the paternally derived p(un) alleles. In addition, the reversion also occurred for the maternally derived, unirradiated p(un) alleles at a frequency equal to that for the paternally derived allele. Detailed analyses of the number of pigmented cells per eye spot indicated that the frequency of reversion was persistently elevated during the proliferation cycle of the cells in the retinal pigment epithelium when the male parents were irradiated at the spermatozoa stage. The present study demonstrates the presence of a long-lasting memory of DNA damage and the persistent up-regulation of recombinogenic activity in the retinal pigment epithelium of the developing fetus.

DNA topoisomerase II is essential for preimplantation mouse development

Topoisomerase II (topo II) is an essential enzyme that alters DNA topology. This activity is important for a variety of chromosome functions including DNA replication, transcription, recombination, and chromosome condensation and segregation. Previously we localized topo II in mouse gametes and preimplantation embryos using isoform-specific antibodies demonstrating the presence of the enzyme in oocytes and embryos, but not sperm. To probe functions of topo II during preimplantation development, we treated mouse zygotes with 100 nM teniposide, and assessed embryo morphology and DNA replication. Teniposide blocked cleavage in 69% embryos; the remainder cleaved once but had abnormal nuclei. Teniposide-treated embryos were devoid of topo II immunofluorescence. Teniposide also prevented DNA replication, implicating topo II in this process. Embryos treated with a 2 hr pulse of teniposide recovered and developed to the blastocyst stage, indicating 100 nM teniposide did not induce apoptosis. To more specifically analyze topo IIalpha function, we treated zygotes with topo IIalpha-targeted antisense oligodeoxynucleotides. Most zygotes arrested at the 2-cell stage while controls developed into blastocysts indicating topo IIalpha is essential for preimplantation development. The absence of topo IIalpha, but not beta immunofluorescence in antisense-treated embryos confirms the specificity and impact of the treatment. In addition, topo IIalpha is newly synthesized at the 2-cell stage. These results establish an essential function for topo II in mouse preimplantation embryonic development.

DNA topoisomerase II distribution in mouse preimplantation embryos

DNA topoisomerase II (topo II) is an essential enzyme that mediates a variety of chromosome activities including DNA replication, transcription, recombination, and chromosome condensation and segregation. Isoform-specific anti-topo II antibodies were used to determine the distribution of topo II alpha and beta in mouse gametes and embryos. Immunoblot analysis with two anti-topo IIalpha antibodies revealed that a 170 kDa topo IIalpha band was present in ovary and testis. Mature sperm exhibited an 89 kDa band only, which may be a degradation product of topo IIalpha. Immunoblots probed with a monoclonal antibody that recognizes both isoforms, showed bands at 170 and 180 kDa, which correspond to topo IIalpha and beta, respectively. An additional 100 kDa band was also present in ovary and testis. Mature sperm did not exhibit staining with this antibody. We also localized topo II in mouse gametes and embryos up to the blastocyst stage using immunofluorescence microscopy. While both isoforms were found in nuclei and nucleoli of germinal vesicle oocytes, topo IIalpha localized to metaphase chromosomes during meiosis, and only to nucleoli during embryonic interphase. Topo IIbeta was absent from chromosomes of metaphase II oocytes, but localized to embryonic interphase nuclei. Both full-length isoforms were absent from sperm, indicating topo II is stored maternally. These results identify topo II as an important component of mouse oocyte and embryonic chromatin, and suggest its involvement in oocyte maturation and preimplantation embryonic development. The different immunofluorescent staining patterns indicate topo IIalpha and beta may serve different roles during the embryonic cell cycle.

The Drosophila nuclear lamina protein YA binds to DNA and histone H2B with four domains

Dramatic changes occur in nuclear organization and function during the critical developmental transition from meiosis to mitosis. The Drosophila nuclear lamina protein YA binds to chromatin and is uniquely required for this transition. In this study, we dissected YA's binding to chromatin. We found that YA can bind to chromatin directly and specifically. It binds to DNA but not RNA, with a preference for double-stranded DNA (linear or supercoiled) over single-stranded DNA. It also binds to histone H2B. YA's binding to DNA and histone H2B is mediated by four domains distributed along the length of the YA molecule. A model for YA function at the end of Drosophila female meiosis is proposed.

Epigenetic reprogramming in the mammalian embryo: struggle of the clones

During preimplantation development in mammals, distinct epigenetic marks on oocyte and sperm DNA are remodeled to an embryonic pattern. A recent study examining global methylation of repetitive elements in various mammals showed that the reprogramming that occurs during normal preimplantation development is aberrant in cloned embryos.

Paternal chromosome incorporation into the zygote nucleus is controlled by maternal haploid in Drosophila

maternal haploid (mh) is a strict maternal effect mutation that causes the production of haploid gynogenetic embryos (eggs are fertilized but only maternal chromosomes participate in development). We conducted a cytological analysis of fertilization and early development in mh eggs to elucidate the mechanism of paternal chromosome elimination. In mh eggs, as in wild-type eggs, male and female pronuclei migrate and appose, the first mitotic spindle forms, and both parental sets of chromosomes congress on the metaphase plate. In contrast to control eggs, mh paternal sister chromatids fail to separate in anaphase of the first division. As a consequence the paternal chromatin stretches and forms a bridge in telophase. During the first three embryonic divisions, damaged paternal chromosomes are progressively eliminated from the spindles that organize around maternal chromosomes. A majority of mh embryos do not survive the deleterious presence of aneuploid nuclei and rapidly arrest their development. The rest of mh embryos develop as haploid gynogenetic embryos and die before hatching. The mh phenotype is highly reminiscent of the early developmental defects observed in eggs fertilized by ms(3)K81 mutant males and in eggs produced in incompatible crosses of Drosophila harboring the endosymbiont bacteria Wolbachia.

The maternal effect mutation sésame affects the formation of the male pronucleus in Drosophila melanogaster

After entering the oocyte and before the formation of the diploid zygote, the sperm nucleus is transformed into a male pronucleus, a process that involves a series of conserved steps in sexually reproducing animals. Notably, a major modification of the male gamete lies in the decondensation of the highly compact sperm chromatin. We present here the phenotype of sésame (ssm), a maternal effect mutation which affects the formation of the male pronucleus in Drosophila melanogaster. Homozygous ssm(185b) females produce haploid embryos which develop with only the maternally derived chromosomes. These haploid embryos die at the end of embryogenesis. Cytological analyses of the fertilization in eggs laid by ssm(185b) mutant females showed that both pronuclear migration and pronuclear apposition occurred normally. However, a dramatic alteration of the male pronucleus by which its chromatin failed to fully decondense was systematically observed. Consequently, the affected male pronucleus does not enter the first mitotic spindle, which is organized around only the maternally derived chromosomes. Immunodetection of lamina antigens indicates that a male pronuclear envelope is able to form around the partially decondensed paternal chromatin. This suggests that the maternally provided sésame(+) function is required for a late stage of sperm chromatin remodeling.