Rivulus is a hermaphroditic, self-fertilizing fish species that possesses an ovotestis and, in

the wild, exist as androdioecious populations comprised of hermaphrodites and males.

Following embryonic incubation at an ambient temperature of 25 °C, rivulus develop

normally as hermaphrodites. Embryos exposed to a lower temperature of 20 °C during a

critical phase of embryogenesis develop as males. In this study, rivulus embryos were

maintained at ambient (25 °C), low (20 °C), or high (31 °C) temperatures during various

stages of embryogenesis. The expression of seven evolutionary conserved genes with

known relevance to gonadal differentiation including figá, foxl2, cyp19a1b, cyp19a1a,

dmrt1, sox9a, and sox9b was measured using real-time PCR. The expression of

cyp19a1a was downregulated at 20 °C and the expression of ovarian-specific genes

increased throughout embryogenesis. These results provide the first data documenting

how temperature affects the expression of genes relevant to sex determination during

embryogenesis in rivulus.