Phylogenetic analysis and identification of pseudogenes reveal a progressive loss of zona pellucida genes during evolution of vertebrates

Oocytic expression of zona pellucida protein ZP4 in Japanese quail (Coturnix japonica)

The avian perivitelline layer, an extracellular matrix homologous to the zona pellucida (ZP) of mammalian oocytes, is composed mainly by zona pellucida gene family glycoproteins. Our previous studies in Japanese quail have demonstrated that the matrix's components, ZP3 and ZPD, are synthesized in ovarian granulosa cells. Another component, ZP1, is synthesized in the liver. Recently, we demonstrated that another minor constituent, ZP2 is produced in the oocytes of the immature follicles. In the present study, we report the isolation of complementary DNA encoding quail ZP4 and its expression and origin in the female birds. By ribonuclease protection assay and in situ hybridization, we demonstrated that ZP4 transcripts were transcribed in the oocytes of small white follicles. The expression level of ZP4 decreased dramatically during follicular development, and the highest expression was observed in the small white follicles. Western blot analysis using the specific antibody against ZP4 indicated that the immunoreactive 58.2 kDa protein was present in the lysates of the small white follicles. These results demonstrate for the first time that the avian ZP4 is expressed in the oocyte, and that the expression pattern of the gene is similar to that of ZP2.

Evolutionary patterns of two major reproduction candidate genes (Zp2 and Zp3) reveal no contribution to reproductive isolation between bovine species

BACKGROUND

It has been established that mammalian egg zona pellucida (ZP) glycoproteins are responsible for species-restricted binding of sperm to unfertilized eggs, inducing the sperm acrosome reaction, and preventing polyspermy. In mammals, ZP apparently represents a barrier to heterospecific fertilization and thus probably contributes to reproductive isolation between species. The evolutionary relationships between some members of the tribe Bovini are complex and highly debatable, particularly, those involving Bos and Bison species for which interspecific hybridization is extensively documented. Because reproductive isolation is known to be a major precursor of species divergence, testing evolutionary patterns of ZP glycoproteins may shed some light into the speciation process of these species. To this end, we have examined intraspecific and interspecific genetic variation of two ZP genes (Zp2 and Zp3) for seven representative species (111 individuals) from the Bovini tribe, including five species from Bos and Bison, and two species each from genera Bubalus and Syncerus.

RESULTS

A pattern of low levels of intraspecific polymorphism and interspecific divergence was detected for the two sequenced fragments each for Zp2 and Zp3. At intraspecific level, none of neutrality tests detected deviations from neutral equilibrium expectations for the two genes. Several haplotypes in both genes were shared by multiple species from Bos and Bison.

CONCLUSIONS

Here we argue that neither ancestral polymorphism nor introgressive hybridization alone can fully account for haplotype sharing among species from Bos and Bison, and that both scenarios have contributed to such a pattern of haplotype sharing observed here. Additionally, codon-based tests revealed strong evidence for purifying selection in the Zp3 coding haplotype sequences and weak evidence for purifying selection in the Zp2 coding haplotype sequences. Contrary to a general genetic pattern that genes or genomic regions contributing to reproductive isolation between species often evolve rapidly and show little or no gene flow between species, these results demonstrate that, particularly, those sequenced exons of the Zp2 and the Zp3 did not show any contribution to reproductive isolation between the bovine species studied here.

Cellular mechanisms regulating sperm-zona pellucida interaction

For mammalian spermatozoa to exhibit the ability to bind the zona pellucida (ZP) they must undergo three distinct phases of maturation, namely, spermatogenesis (testis), epididymal maturation (epididymis) and capacitation (female reproductive tract). An impressive array of spermatozoa surface remodeling events accompany these phases of maturation and appear critical for recognition and adhesion of the outer vestments of the oocyte, a structure known as the ZP. It is becoming increasingly apparent that species-specific zona adhesion is not mediated by a single receptor. Instead, compelling evidence now points toward models implicating a multiplicity of receptor-ligand interactions. This notion is in keeping with emerging research that has shown that there is a dynamic aggregation of proteins believed to be important in sperm-ZP recognition to the regions of sperm that mediate this binding event. Such remodeling may in turn facilitate the assembly of a multimeric zona recognition complex (MZRC). Though formation of MZRCs raises questions regarding the nature of the block to polyspermy, formation and assembly of such a structure would no doubt explain the strenuous maturation process that sperm endure on their sojourn to functional maturity.

Acrosome reaction: relevance of zona pellucida glycoproteins

During mammalian fertilisation, the zona pellucida (ZP) matrix surrounding the oocyte is responsible for the binding of the spermatozoa to the oocyte and induction of the acrosome reaction (AR) in the ZP-bound spermatozoon. The AR is crucial for the penetration of the ZP matrix by spermatozoa. The ZP matrix in mice is composed of three glycoproteins designated ZP1, ZP2 and ZP3, whereas in humans, it is composed of four (ZP1, ZP2, ZP3 and ZP4). ZP3 acts as the putative primary sperm receptor and is responsible for AR induction in mice, whereas in humans (in addition to ZP3), ZP1 and ZP4 also induce the AR. The ability of ZP3 to induce the AR resides in its C-terminal fragment. O-linked glycans are critical for the murine ZP3-mediated AR. However, N-linked glycans of human ZP1, ZP3 and ZP4 have important roles in the induction of the AR. Studies with pharmacological inhibitors showed that the ZP3-induced AR involves the activation of the G(i)-coupled receptor pathway, whereas ZP1- and ZP4-mediated ARs are independent of this pathway. The ZP3-induced AR involves the activation of T-type voltage-operated calcium channels (VOCCs), whereas ZP1- and ZP4-induced ARs involve both T- and L-type VOCCs. To conclude, in mice, ZP3 is primarily responsible for the binding of capacitated spermatozoa to the ZP matrix and induction of the AR, whereas in humans (in addition to ZP3), ZP1 and ZP4 also participate in these stages of fertilisation.

'ZP domain' of human zona pellucida glycoprotein-1 binds to human spermatozoa and induces acrosomal exocytosis

BACKGROUND

The human egg coat, zona pellucida (ZP), is composed of four glycoproteins designated as zona pellucida glycoprotein-1 (ZP1), -2 (ZP2), -3 (ZP3) and -4 (ZP4) respectively. The zona proteins possess the archetypal 'ZP domain', a signature domain comprised of approximately 260 amino acid (aa) residues. In the present manuscript, attempts have been made to delineate the functional significance of the 'ZP domain' module of human ZP1, corresponding to 273-551 aa fragment of human ZP1.

METHODS

Baculovirus-expressed, nickel-nitrilotriacetic acid affinity chromatography purified 'ZP domain' of human ZP1 was employed to assess its capability to bind and subsequently induce acrosomal exocytosis in capacitated human spermatozoa using tetramethyl rhodamine isothiocyanate conjugated Pisum sativum Agglutinin in absence or presence of various pharmacological inhibitors. Binding characteristics of ZP1 'ZP domain' were assessed employing fluorescein isothiocyanate (FITC) labelled recombinant protein.

RESULTS

SDS-PAGE and immunoblot characterization of the purified recombinant protein (both from cell lysate as well as culture supernatant) revealed a doublet ranging from ~35-40 kDa. FITC- labelled 'ZP domain' of ZP1 binds primarily to the acrosomal cap of the capacitated human spermatozoa. A dose dependent increase in acrosomal exocytosis was observed when capacitated sperm were incubated with recombinant 'ZP domain' of human ZP1. The acrosome reaction mediated by recombinant protein was independent of Gi protein-coupled receptor pathway, required extra cellular calcium and involved both T- and L-type voltage operated calcium channels.

CONCLUSIONS

Results described in the present study suggest that the 'ZP domain' module of human ZP1 has functional activity and may have a role during fertilization in humans.

Oviductal secretions: will they be key factors for the future ARTs?

A variety of evolutionary processes has led to the development of different organs to ensure that internal fertilization occur successfully. Fallopian tubes are a particularly interesting example of such organs. Some of the key events during fertilization and early embryo development occur in the oviduct. Knowledge of the different components described in the oviduct is extensive. Oviductal components include hormones, growth factors and their receptors that have important roles in the physiology of the oviduct and embryo development. Other oviductal factors protect the gamete and the embryos against oxidative stress and pathogens. Different proteins and enzymes are present in the oviductal fluid and have the ability to interact with the oocyte and the sperm before the fertilization occurs. Of special interest is the oviduct-specific glycoprotein (OVGP1), a glycoprotein that is conserved in different mammals, and its association with the zona pellucida (ZP). Interaction of the oocyte with oviductal secretions leads us to emphasize the concept of 'ZP maturation' within the oviduct. The ZP changes produced in the oviduct result in an increased efficiency of the in vitro fertilization technique in some animal models, contributing in particular to the control of polyspermy and suggesting that a similar role could be played by oviductal factors in human beings. Finally, attention should be given to the presence in the oviductal fluid of several embryotrophic factors and their importance in relation to the in vivo versus in vitro developmental ability of the embryos.

In humans, zona pellucida glycoprotein-1 binds to spermatozoa and induces acrosomal exocytosis

BACKGROUND

It has been suggested that the zona pellucida (ZP) may mediate species-specific fertilization. In human the ZP is composed of four glycoproteins: ZP1, ZP2, ZP3 and ZP4. In the present study, the expression profile of ZP1 in human oocytes and ovaries, and its role during fertilization, is presented.

METHODS

Human ZP1 (amino acid residues 26-551) was cloned and expressed in both non-glycosylated and glycosylated forms and its ability to bind to the capacitated human spermatozoa and to induce acrosomal exocytosis was studied. Monoclonal antibodies (MAbs), specific for human ZP1 and devoid of reactivity with ZP2, ZP3 and ZP4 were generated and used to localize native ZP1 in oocytes and ovarian tissues.

RESULTS

The MAbs generated against ZP1 recognized specifically the zona matrix of secondary and antral follicles, ovulated oocytes, atretic follicles and degenerating intravascular oocytes, but failed to react with the Fallopian tube, endometrium, ectocervix and kidney. Escherichia coli and baculovirus-expressed recombinant human ZP1 revealed bands of approximately 75 and approximately 85 kDa, respectively, in western blot. Lectin binding studies revealed the presence of both N- and O-linked glycosylation in baculovirus-expressed ZP1. Fluorescein isothiocyanate-labelled E. coli- and baculovirus-expressed recombinant ZP1 bound to the anterior head of capacitated spermatozoa, however, only baculovirus-expressed ZP1 induced acrosomal exocytosis in capacitated sperm suggesting the importance of glycosylation in mediating the acrosome reaction. The human ZP1-mediated acrosome reaction involved the activation of both T- and L-type voltage-operated calcium channels, but does not activate the G(i)-coupled receptor pathway. Inhibition of protein kinase A and C significantly also reduced the ZP1-mediated induction of the acrosome reaction.

CONCLUSION

These studies revealed for the first time that in humans ZP1, in addition to ZP3 and ZP4, binds to capacitated spermatozoa and induces acrosomal exocytosis.

Comparative genomic analysis of the zebra finch degradome provides new insights into evolution of proteases in birds and mammals

Background

The degradome -the complete repertoire of proteases in an organism- is involved in multiple key biological and pathological processes. Previous studies in several organisms have yielded sets of curated protease sequences which may be used to characterize the degradome in a novel genome by similarity. Differences between degradomes can then be related to physiological traits of the species under study. Therefore, the sequencing of the zebra finch genome allows the comparison between the degradomes of mammals and birds and may help to understand the biological peculiarities of the zebra finch.

Results

A set of curated protease sequences from humans and chicken was used to predict the sequences of 460 protease and protease-like genes in the zebra finch genome. This analysis revealed important differences in the evolution of mammalian and bird degradomes, including genomic expansions and deletions of caspases, cytotoxic proteases, kallikreins, matrix metalloproteases, and trypsin-like proteases. Furthermore, we found several zebra finch-specific features, such as duplications in CASP3 and BACE, and a large genomic expansion of acrosin.

Conclusions

We have compared the degradomes of zebra finch, chicken and several mammalian species, with the finding of multiple differences which illustrate the evolution of the protease complement of these organisms. Detailed analysis of these changes in zebra finch proteases has shown that they are mainly related to immunological, developmental, reproductive and neural functions.

Zona pellucida protein ZP2 is expressed in the oocyte of Japanese quail (Coturnix japonica)

The avian perivitelline layer (PL), a vestment homologous to the zona pellucida (ZP) of mammalian oocytes, is composed of at least three glycoproteins. Our previous studies have demonstrated that the matrix's components, ZP3 and ZPD, are synthesized in ovarian granulosa cells. Another component, ZP1, is synthesized in the liver and is transported to the ovary by blood circulation. In this study, we report the isolation of cDNA encoding quailZP2and its expression in the female bird. By RNase protection assay andin situhybridization, we demonstrate that ZP2 transcripts are restricted to the oocytes of small white follicles (SWF). The expression level ofZP2decreased dramatically during follicular development, and the highest expression was observed in the SWF. Western blot and immunohistochemical analyses using the specific antibody against ZP2 indicate that the 80 kDa protein is the authentic ZP2, and the immunoreactive ZP2 protein is also present in the oocytes. Moreover, ultrastructural analysis demonstrated that the immunoreactive ZP2 localizes to the zona radiata, the perivitelline space, and the oocyte cytoplasm in the SWF. By means of western blot analysis and immunofluorescence microscopy, we detected a possible interaction of the recombinant ZP2 with ZP3 and that this interaction might lead to the formation of amorphous structure on the cell surface. These results demonstrate for the first time that the avian ZP gene is expressed in the oocyte, and that the ZP2 protein in the oocyte might play a role for the PL formation in the immature follicles of the ovary.

What controls polyspermy in mammals, the oviduct or the oocyte?

A block to polyspermy is required for successful fertilisation and embryo survival in mammals. A higher incidence of polyspermy is observed during in vitro fertilisation (IVF) compared with the in vivo situation in several species. Two groups of mechanisms have traditionally been proposed as contributing to the block to polyspermy in mammals: oviduct-based mechanisms, avoiding a massive arrival of spermatozoa in the proximity of the oocyte, and egg-based mechanisms, including changes in the membrane and zona pellucida (ZP) in reaction to the fertilising sperm. Additionally, a mechanism has been described recently which involves modifications of the ZP in the oviduct before the oocyte interacts with spermatozoa, termed "pre-fertilisation zona pellucida hardening". This mechanism is mediated by the oviductal-specific glycoprotein (OVGP1) secreted by the oviductal epithelial cells around the time of ovulation, and is reinforced by heparin-like glycosaminoglycans (S-GAGs) present in oviductal fluid. Identification of the molecules contributing to the ZP modifications in the oviduct will improve our knowledge of the mechanisms of sperm-egg interaction and could help to increase the success of IVF systems in domestic animals and humans.

Gene expression during the oocyte-to-embryo transition in mammals

The seminal question in modern developmental biology is the origins of new life arising from the unification of sperm and egg. The roots of this question begin from 19th to 20th century embryologists studying fertilization and embryogenesis. Although the revolution of molecular biology has yielded significant insight into the complexity of this process, the overall orchestration of genes, molecules, and cells is still not fully formed. Early mammalian development, specifically the oocyte-to-embryo transition, is essentially under "maternal command" from factors deposited in the cytoplasm during oocyte growth, independent of de novo transcription from the nascent embryo. Many of the advances in understanding this developmental period occurred in tandem with application of new methods and techniques from molecular biology, from protein electrophoresis to sequencing and assemblies of whole genomes. From this bed of knowledge, it appears that precise control of mRNA translation is a key regulator coordinating the molecular and cellular events occurring during oocyte-to-embryo transition. Notably, oocyte transcriptomes share, yet retain some uniqueness, common genetic motifs among all chordates. The common genetic motifs typically define fundamental processes critical for cellular maintenance, whereas the unique genetic features may be a source of variation and a substrate for sexual selection, genetic drift, or gene flow. One purpose for this complex interplay among genes, proteins, and cells may allow for evolution to transform and act upon the underlying processes, at molecular, structural and organismal levels, to increase diversity, which is the ultimate goal of sexual reproduction.

New insights into the mechanisms of fertilization: comparison of the fertilization steps, composition, and structure of the zona pellucida between horses and pigs

The mechanism of fertilization remains largely enigmatic in mammals. Most studies exploring the molecular mechanism underlying fertilization have been restricted to a single species, generally the mouse, without a comparative approach. However, the identification of divergences between species could allow us to highlight key components in the mechanism of fertilization. In the pig, in vitro fertilization (IVF) and polyspermy rates are high, and spermatozoa penetrate easily through the zona pellucida (ZP). In contrast, IVF rates are low in the horse, and polyspermy is scarce. Our objective was to develop a comparative strategy between these two divergent models. First, we compared the role of equine and porcine gametes in the following five functions using intraspecific and interspecific IVF: ZP binding, acrosome reaction, penetration through the ZP, gamete fusion, and pronucleus formation. Under in vitro conditions, we showed that the ZP is a determining element in sperm-ZP attachment and penetration, whereas the capacity of the spermatozoa is of less importance. In contrast, the capacity of the spermatozoa is a key component of the acrosome reaction step. Second, we compared the composition and structure of the equine and porcine ZP. We observed differences in the number and localization of the ZP glycoproteins and in the mesh-like structure of the ZP between equine and porcine species. These differences might correlate with the differences in spermatozoal attachment and penetration rates. In conclusion, our comparative approach allows us to identify determining elements in the mechanism of fertilization.

Hamster zona pellucida is formed by four glycoproteins: ZP1, ZP2, ZP3, and ZP4

The zona pellucida (ZP) is an extracellular glycoprotein matrix that surrounds all mammalian oocytes. Recent data have shown the presence of four glycoproteins (ZP1, ZP2, ZP3, and ZP4) in the ZP of human and rat rather than the three glycoproteins proposed in the mouse model. In the hamster (Mesocricetus auratus), it was previously described that ZP was composed of three different glycoproteins, called ZP1, ZP2, and ZP3, even though only ZP2 and ZP3 have been cloned thus far. The aim of the study was to determine whether hamster might also express four, rather than three, ZP proteins. The full-length cDNAs encoding hamster ZP glycoproteins 1 and 4 were isolated using rapid amplification cDNA ends (RACE). The cDNA of ZP1 contains an open reading frame of 1851 nucleotides encoding a polypeptide of 616 amino acid residues. The amino acid sequence of ZP1 revealed a high homology with other mammalian species like human (66%), rat (80%), and mouse (80%). The cDNA of ZP4 contains an open reading frame of 1632 nucleotides encoding a polypeptide of 543 amino acid residues. The deduced amino acid sequence of ZP4 revealed high overall homology with rat (82%) and human (78%). Subsequent mass spectrometric analysis of the hamster ZP allowed identification of peptides from all four glycoproteins. The data presented in this study provide evidence, for the first time, that the hamster ZP matrix is composed of four glycoproteins.

Comprehensive egg coat proteome of the ascidian Ciona intestinalis reveals gamete recognition molecules involved in self-sterility

Despite central roles of egg coat proteins in gamete recognition, their functions and composition are poorly understood. Here, we report that the proteome of the egg coat in the solitary ascidian Ciona intestinalis, called vitelline coat (VC) fraction, contains more than 800 proteins identified by mass spectrometry-based analyses. Over 100 proteins were enriched in the VC fraction compared with the VC-free egg proteome. The most abundant component in the VC was an apolipoprotein-like protein. The VC contained multiple homologs of mammalian zona pellucida (ZP) proteins, the number of which was unexpectedly large and most of which possessed epidermal growth factor-like repeats. Furthermore, the present study revealed that two fibrinogen-like proteins, v-Themis-A and -B, both of which are expressed in the VC, are the molecules responsible for the two self-sterility loci that were identified by our previous genetic study in this species.

Imaging the zona pellucida of canine and feline oocytes using scanning electron microscopy

Ultrastructure of the zona pellucida (ZP) of canine and feline oocytes has not been fully investigated. The objective of the study was to evaluate the potential use of the low vacuum scanning electron microscope (LVSEM) with oocytes. This required development of a method to prepare canine and feline cumulus-oocyte complexes (COCs) for LVSEM to provide ultrastructural information on the ZP. COCs were collected from ovaries, and cumulus cells were either partially or completely removed to reveal the ZP. COCs and zona-intact oocytes were fixed at 4 degrees C for 1 h in 2.5% glutaraldehyde in 0.1 M sodium cacodylate buffer, pH 7.4 and subsequently viewed wet or further processed by critical point drying, and viewed uncoated or sputter coated with gold. Although the spongy surface of the ZP was visible at low vacuum in uncoated oocytes, coated oocytes had more details at high vacuum. The ZP surface of canine and feline oocytes contained numerous various-sized, spherical or elliptical pores that narrowed centripetally splitting into several smaller, deep pores. The round to oblong cumulus cells tightly surrounded the ZP. Each corona radiata cumulus cell tapered into a thin projection that entered the ZP. Our detailed techniques will enable future studies connecting ultrastructural and molecular aspects of oocyte maturation and development in mammals.

Molecular characterization of egg envelope glycoprotein ZPD in the ovary of Japanese quail (Coturnix japonica)

The egg envelope surrounding avian oocytes exhibits a three-dimensional network of coarse fibers between the granulosa cells and the oocyte. Our previous studies have demonstrated that one of the matrix's components, ZP3, is synthesized in the ovarian granulosa cells. Another component, ZP1, which is critically involved in triggering the sperm acrosome reaction, is synthesized in the liver. We have previously isolated cDNAs encoding quail ZP3 and ZP1, and we now report the isolation of cDNA encoding quail ZPD. By RNase protection assay and in situ hybridization, we have demonstrated that ZPD transcripts are restricted to the granulosa cells of preovulatory follicles. The expression level of ZPD increased progressively during follicular development, and the highest expression was observed in the largest follicles. Western blot analyses using the specific antibody against ZPD indicate that the 40 kDa protein is the authentic ZPD, and the contents of ZPD protein also increased during follicular development. Moreover, we found that the addition of FSH to the culture media enhances the ZPD secretion in the cultured granulosa cells. Two-dimensional gel electrophoresis revealed the presence of several ZPD isoforms with different pI values ranging from 5.5 to 7. Immunohistochemical analyses indicate that the materials recognized with anti-quail ZPD antibody were accumulated in the egg envelope of large yellow follicles. These results demonstrate the presence of ZPD protein in the egg envelope, and that the amount of ZPD in the egg envelope as well as the mRNA in the cells increases at the latter stages of folliculogenesis.

GPR50 is the mammalian ortholog of Mel1c: evidence of rapid evolution in mammals

BACKGROUND

The melatonin receptor subfamily contains three members Mel1a, Mel1b and Mel1c, found in all vertebrates except for Mel1c which is found only in fish, Xenopus species and the chicken. Another receptor, the melatonin related receptor known as GPR50, found exclusively in mammals and later identified as a member of the melatonin receptor subfamily because of its identity to the three melatonin receptors despite its absence of affinity for melatonin. The aim of this study was to describe the evolutionary relationships between GPR50 and the three other members of the melatonin receptor subfamily.

RESULTS

Using an in silico approach, we demonstrated that GPR50 is the ortholog of the high affinity Mel1c receptor. It was necessary to also study the synteny of this gene to reach this conclusion because classical mathematical models that estimate orthology and build phylogenetic trees were not sufficient. The receptor has been deeply remodelled through evolution by the mutation of numerous amino acids and by the addition of a long C-terminal tail. These alterations have modified its affinity for melatonin and probably affected its interactions with the other two known melatonin receptors MT1 and MT2 that are encoded by Mel1a and Mel1b genes respectively. Evolutionary studies provided evidence that the GPR50 group evolved under different selective pressure as compared to the orthologous groups Me11 a, b, and c.

CONCLUSION

This study demonstrated that there are only three members in the melatonin receptor subfamily with one of them (Me11c) undergoing rapid evolution from fishes and birds to mammals. Further studies are necessary to investigate the physiological roles of this receptor.

Use of combined in silico expression data and phylogenetic analysis to identify new oocyte genes encoding RNA binding proteins in the mouse

During folliculogenesis, oocytes accumulate maternal mRNAs in preparation for the first steps of early embryogenesis. The processing of oocyte mRNAs is ensured by heterogeneous nuclear ribonucleoproteins (hnRNPs) genes that encode RNA binding proteins implied in mRNA biogenesis, translation, alternative splicing, nuclear exportation, and degradation. In the present work, by combining phylogenetic analyses and, when available, in silico expression data, we have identified three new oocyte-expressed genes encoding RNA binding proteins by using two strategies. Firstly, we have identified mouse orthologs of the Car1 gene, known to be involved in regulation of germ cell apoptosis in C. elegans, and of the Squid gene, required for the establishment of anteroposterior polarity in the Drosophila oocyte. Secondly, we have identified, among genes whose ESTs are highly represented in oocyte libraries, a paralog of Poly(A) binding protein--Interacting Protein 2 (Paip2) gene, known to inhibit the interaction of the Poly(A)-Binding Protein with Poly(A) tails of mRNAs. For all of these genes, the expression in oocyte was verified by in situ hybridization. Overall, this work underlines the efficiency of in silico methodologies to identify new genes involved in biological processes such as oogenesis.

Search for the genes involved in oocyte maturation and early embryo development in the hen

Background

The initial stages of development depend on mRNA and proteins accumulated in the oocyte, and during these stages, certain genes are essential for fertilization, first cleavage and embryonic genome activation. The aim of this study was first to search for avian oocyte-specific genes using an in silico and a microarray approaches, then to investigate the temporal and spatial dynamics of the expression of some of these genes during follicular maturation and early embryogenesis.

Results

The in silico approach allowed us to identify 18 chicken homologs of mouse potential oocyte genes found by digital differential display. Using the chicken Affymetrix microarray, we identified 461 genes overexpressed in granulosa cells (GCs) and 250 genes overexpressed in the germinal disc (GD) of the hen oocyte. Six genes were identified using both in silico and microarray approaches. Based on GO annotations, GC and GD genes were differentially involved in biological processes, reflecting different physiological destinations of these two cell layers. Finally we studied the spatial and temporal dynamics of the expression of 21 chicken genes. According to their expression patterns all these genes are involved in different stages of final follicular maturation and/or early embryogenesis in the chicken. Among them, 8 genes (btg4, chkmos, wee, zpA, dazL, cvh, zar1 and ktfn) were preferentially expressed in the maturing occyte and cvh, zar1 and ktfn were also highly expressed in the early embryo.

Conclusion

We showed that in silico and Affymetrix microarray approaches were relevant and complementary in order to find new avian genes potentially involved in oocyte maturation and/or early embryo development, and allowed the discovery of new potential chicken mature oocyte and chicken granulosa cell markers for future studies. Moreover, detailed study of the expression of some of these genes revealed promising candidates for maternal effect genes in the chicken. Finally, the finding concerning the different state of rRNA compared to that of mRNA during the postovulatory period shed light on some mechanisms through which oocyte to embryo transition occurs in the hen.