Effects of Constant and Fluctuating Temperatures on Gene Expression During Gonadal Development.

There is ample research demonstrating that temperature can have complex effects on biological processes, including the timing of when organisms respond to temperature; some responses occur rapidly while others require an extended exposure time. However, most of what we know about temperature effects comes from studies using constant temperature conditions, which are not reflective of natural, fluctuating temperatures. Species with temperature-dependent sex determination (TSD) present an ideal system to study the temporal aspects of the temperature response because prior research has established a number of temperature-responsive genes involved in TSD, albeit under constant temperatures. To investigate potential differences in timing of sexual development between constant and fluctuating incubation temperatures, we exposed Trachemys scripta embryos to two conditions that produce males (constant 26°C and 26 ± 3°C) and two that produce females (constant 31°C and 31 ± 3°C), and sampled embryonic gonads for gene expression analysisvia qPCR. We analyzed three genes involved in testis differentiation (Kdm6b,Dmrt1, andSox9) and two genes involved in ovary differentiation (Foxl2andCyp19A1). Results show that Kdm6b expression was significantly lower under fluctuating temperatures compared to constant temperatures. Foxl2 and Cyp19A1 expression were also lower under fluctuating temperatures, but not at all stages of development. These results suggest that constant temperatures caused increases in both Foxl2 and Cyp19A1 expression earlier (developmental stage 20) than fluctuating temperatures (stages 22 and 23). Dmrt1 and Sox9 expression did not differ between constant and fluctuating temperatures. These results highlight that not all genes in a temperature-dependent process respond to temperature in the same manner. Whether there are functional consequences of this variation remains to be determined.