Abstract

Four pairs of XX and XY gonads from Nile tilapia were sequenced at four developmental stages, 5, 30, 90, and 180 days after hatching (dah) using Illumina HiseqTM technology. This produced 28 Gb sequences, which were mapped to 21,334 genes. Of these, 259 genes were found to be specifically expressed in XY gonads, and 69 were found to be specific to XX gonads. Totally, 187 XX- and 1,358 XY-enhanced genes were identified, and 2,978 genes were found to be co-expressed in XX and XY gonads. Almost all steroidogenic enzymes, including cyp19a1a, were up-regulated in XX gonads at 5 dah; but in XY gonads these enzymes, including cyp11b2, were significantly up-regulated at 90 dah, indicating that, at a time critical to sex determination, the XX fish produced estrogen and the XY fish did not produce androgens. The most pronounced expression of steroidogenic enzyme genes was observed at 30 and 90 dah for XX and XY gonads, corresponding to the initiation of germ cell meiosis in the female and male gonads, respectively. Both estrogen and androgen receptors were found to be expressed in XX gonads, but only estrogen receptors were expressed in XY gonads at 5 dah. This could explain why exogenous steroid treatment induced XX and XY sex reversal. The XX-enhanced expression of cyp19a1a and cyp19a1b at all stages suggests an important role for estrogen in female sex determination and maintenance of phenotypic sex. This work is the largest collection of gonadal transcriptome data in tilapia and lays the foundation for future studies into the molecular mechanisms of sex determination and maintenance of phenotypic sex in non-model teleosts.

Highlights

  • Sex determination and maintenance involve complex processes with many interacting events

  • XX and XY fish were dissected to produce testicular and ovarian samples for RNA isolation at four critical stages of tilapia gonadal development: the critical time for sex determination and differentiation (5 days after hatching [dah]; 300 gonads pooled for each sex), initiation of germ cell meiosis and oogenesis in XX gonads (30 dah, 150 gonads pooled for each sex), initiation of germ cell meiosis or spermatogenesis in XY gonads (90 dah, 3 gonads pooled for each sex), and sperm maturation and vitellogenesis in XY and XX gonads (180 dah, 3 gonads pooled for each sex)

  • Each stage was represented by approximately 45 million reads, a tag density sufficient for the quantitative analysis of gene expression profiles

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Summary

Introduction

Sex determination and maintenance involve complex processes with many interacting events. A few genes have been identified as candidate master sex determining genes These include SRY/Sry in eutherian mammals [2], Dmrt in chickens [3], dmy in medaka [4], Dmw in the African clawed frog [5], amhy in the Patagonian pejerrey [6], and irf9y in the rainbow trout [7] (see Table S1 for the symbols and full names of all genes mentioned in the text). In addition to those candidate sex-determining genes, a number of relatively conserved genes have been shown to be involved in sex determination. These include sox9 [8], wt1 [9], foxl2 [10], dax

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