Gene expression dynamics during temperature-dependent sex determination in a sea turtle.

Fifty years ago, researchers discovered a link between ambient temperature and the sex of turtle embryos. More recently, significant progress has been made in understanding the influence of temperature on freshwater turtles. However, our understanding of the key genetic factors in other turtle groups, such as sea turtles, remains limited. To address this gap, we conducted RNA-seq analyses on embryonic tissues from the sea olive ridley turtle during the thermosensitive period (stages 21-26) at temperatures known to produce males (26 °C) and females (33 °C). Our findings revealed that incubation temperatures primarily influence genes with broad expression across tissues due to differential cell division rates and later have an effect regulating gonad-specific transcripts. This effect is mostly related to gene activation rather than transcription repression. We performed transcriptome analyses following shifts in incubation temperatures of bi-potential gonads. This approach allowed us to identify genes that respond rapidly and may be closer to the beginning of the temperature-sensing pathway. Notably, we observed swift adaptations in the expression levels of chromatin modifiers JARID2 and KDM6B, as well as the splicing factor SRSF5, and transcription regulators THOC2, DDX3X and CBX3, but little impact in the overall gonad-specific pathways, indicating that temperature-sensing genes may change rapidly but the rewiring of the gonad's developmental fate is complex and resilient. AUTHOR SUMMARY: Sea turtles, one of the most iconic creatures of our oceans, confront a troubling reality of endangerment, a peril magnified by the looming specter of climate change. This climatic shift is gradually increasing the temperature of the nesting beaches thus causing dramatic male/female population biases. Conservation efforts will need genetic and molecular information to reverse the negative effects of climate change on the populations. In this study, we conducted the first transcriptomic analysis of embryonic tissues, including gonads, brain, liver, and mesonephros, in the olive ridley sea turtle during the critical thermosensitive period spanning stages 21-26. We examined both male-producing (26 °C) and female-producing (33 °C) temperatures and found that incubation temperatures influence temperature-sensitive genes that are either expressed globally or specifically associated with the gonads. These findings indicate that incubation temperatures predominantly sway genes with broad expression patterns due to differential cell division rates. This natural process was opted in the gonads to drive sex determination. We also identified genes that are rapidly capable of sensing temperature changes and that could play a role in the activation of the sex determination pathway. Overall, our study sheds light on the intricate interplay between temperature and gene expression during sea turtle development, revealing dynamic changes in the transcriptome and highlighting the involvement of key genetic players in sex determination.

A sliver of the past: The decimation of the genetic diversity of the Mexican wolf.

The endangered Mexican wolf (Canis lupus baileyi) is known to carry exceedingly low levels of genetic diversity. This could be (i) the result of long-term evolutionary patterns as they exist at the southernmost limit of the species distribution at a relatively reduced effective size, or (ii) due to rapid population decline caused by human persecution over the last century. If the former, purifying selection is expected to have minimized the impact of inbreeding. If the latter, rapid and recent declines in genetic diversity may have resulted in severe fitness consequences. To differentiate these hypotheses, we conducted comparative whole-genome analyses of five historical Mexican wolves (1907-1917) and 18 contemporary Mexican and grey wolves from North America and Eurasia. Based on whole-genome data, historical and modern Mexican wolves together form a discrete unit. Moreover, we found that modern Mexican wolves have reduced genetic diversity and increased inbreeding relative to the historical population, which was widespread across the southwestern United States and not restricted to Mexico as previously assumed. Finally, although Mexican wolves have evolved in sympatry with coyotes (C. latrans), we observed lower introgression between historical Mexican wolves and coyotes than with modern Mexican wolves, despite similarities in body size. Taken together, our data show that recent population declines probably caused the reduced level of genetic diversity, but not the observed differentiation of the Mexican wolves from other North American wolves.