Gametogenesis in the Pacific Oyster Crassostrea gigas: A Microarrays-Based Analysis Identifies Sex and Stage Specific Genes

Abstract

BackgroundThe Pacific oyster Crassostrea gigas (Mollusca, Lophotrochozoa) is an alternative and irregular protandrous hermaphrodite: most individuals mature first as males and then change sex several times. Little is known about genetic and phenotypic basis of sex differentiation in oysters, and little more about the molecular pathways regulating reproduction. We have recently developed and validated a microarray containing 31,918 oligomers (Dheilly et al., 2011) representing the oyster transcriptome. The application of this microarray to the study of mollusk gametogenesis should provide a better understanding of the key factors involved in sex differentiation and the regulation of oyster reproduction.Methodology/Principal FindingsGene expression was studied in gonads of oysters cultured over a yearly reproductive cycle. Principal component analysis and hierarchical clustering showed a significant divergence in gene expression patterns of males and females coinciding with the start of gonial mitosis. ANOVA analysis of the data revealed 2,482 genes differentially expressed during the course of males and/or females gametogenesis. The expression of 434 genes could be localized in either germ cells or somatic cells of the gonad by comparing the transcriptome of female gonads to the transcriptome of stripped oocytes and somatic tissues. Analysis of the annotated genes revealed conserved molecular mechanisms between mollusks and mammals: genes involved in chromatin condensation, DNA replication and repair, mitosis and meiosis regulation, transcription, translation and apoptosis were expressed in both male and female gonads. Most interestingly, early expressed male-specific genes included bindin and a dpy-30 homolog and female-specific genes included foxL2, nanos homolog 3, a pancreatic lipase related protein, cd63 and vitellogenin. Further functional analyses are now required in order to investigate their role in sex differentiation in oysters.Conclusions/SignificanceThis study allowed us to identify potential markers of early sex differentiation in the oyster C. gigas, an alternative hermaphrodite mollusk. We also provided new highly valuable information on genes specifically expressed by mature spermatozoids and mature oocytes.

Additive transcriptomic variation associated with reproductive traits suggest local adaptation in a recently settled population of the Pacific oyster, Crassostrea gigas

“…Despite strong difference in expression levels between sexes, as previously reported [53], we nonetheless detected a clear difference in expression levels between progenies of oysters originated from France and Denmark, and their hybrids. From the RDA approach on microarrays we found that 4.3 % of the explained variance is ascribed by the progeny in a mainly orthogonal way to sex differences, which means that despite the huge transcriptomic differences between sexes, the more modest differences between progenies proved to be mainly independent of the sex.…”

Sex-Specific Regulation of AMP-Activated Protein Kinase (AMPK) in the Pacific Oyster Crassostrea gigas1

“…When these signaling pathways are deregulated, animals are less fertile [28,29]. In the past, gonad differentiation and development in oysters were studied by measuring the expression of genes potentially involved in reproduction (for example, CG-FOXL2 [30,31], CG-DML [32], vasa (termed oyvlg) [5], or OG-TGF beta [33,34]) or elements belonging to the insulin pathway [16][17][18], and recently by a genome-wide expression profiling that identified gonad-and sex-specific genes and potential markers of early sex differentiation in C. gigas [35]. Despite this work, however, our understanding of the mechanisms of the regulation of gonad development versus storage tissue is still limited [36].…”

Involvement of FOXL2 and RSPO1 in Ovarian Determination, Development, and Maintenance in Mammals

“…Since these first reports, the role of FOXL2 as a master regulator of ovarian differentiation has been abundantly described in various vertebrate species, such as fish [Baron et al, 2004;Nakamoto et al, 2006;Liu et al, 2007;Yamaguchi et al, 2007;Amberg et al, 2010;Hu et al, 2014;Liu H et al, 2015;Wang et al, 2015], amphibians [Oshima et al, 2008;Okada et al, 2009], reptiles [Rhen et al, 2007;Alam et al, 2008;Moore et al, 2010;Shoemaker-Daly et al, 2010], and birds [Govoroun et al, 2004]. More recently, FOXL2 ovarian expression was also reported in invertebrate species, such as crustaceans [Ma et al, 2012;Meng et al, 2015] and molluscs [Dheilly et al, 2012;Liu et al, 2012;Zhang et al, 2014;Shi et al, 2015]. In total, ovary-specific FOXL2 expression has been described in more than 40 species.…”