Zebrafish (Danio rerio) and the egg size versus egg number trade off: effects of ration size on fecundity are not mediated by orthologues of the Fec gene

Few studies have demonstrated plasticity of egg size within the confines of an egg size-number trade-off in response to trophic conditions in fishes. Moreover, the physiological mechanisms that govern this plasticity are not known. Growth differentiation factor 9 (Gdf9) and bone morphogenetic factor 15 (Bmp15) are oocyte-specific factors implicated in follicular growth and ovulation in mammals. In order to investigate whether expression levels of these genes were correlated with ration-dependent changes in fecundity in fish, zebrafish (Danio rerio) were subjected to four different feeding regimens. Counts of spawned eggs or vitellogenic follicles were used to estimate fecundity, whereas quantitative polymerase chain reaction analyses were performed to determine Gdf9 and Bmp15 mRNA levels in response to changes in ration size. Both relative fecundity and gonadosomatic index increased significantly with increased ration size, whereas egg size and hatching rate decreased significantly. No significant differences in Gdf9 or Bmp15 transcript abundance were evident between feeding regimens, suggesting that these growth factors do not govern fecundity in fish. However, favourable trophic conditions markedly affected follicle or egg size and number, with important implications for downstream egg quality and survival.