New research conducted at Duke University has uncovered an intriguing link between warmer temperatures, sex determination in turtles, and egg production. The study, published in the journal Current Biology on June 23, reveals that higher incubation temperatures not only result in more female hatchlings but also enhance their capacity for egg production even before their sex is determined.
The researchers discovered that higher temperatures lead to an increase in the number of “germ cells” or pre-eggs present in the embryos. These germ cells are believed to play a role in determining the embryo’s sex. Boris Tezak, the lead researcher from the Capel lab, explained that the abundance of germ cells drives the feminization process, as the temperatures conducive to female development also lead to a higher number of germ cells.
Interestingly, similar findings have been observed in fish, where increased germ cell numbers also contribute to female development. To validate the connection between germ cells and female turtles, the researchers removed some germ cells from red-eared slider turtle embryos incubated at an intermediate temperature that should have produced an equal ratio of males and females. However, they found a higher number of males than expected, further supporting the relationship between germ cells and sex determination.
Temperature-dependent sex determination has been known to scientists for many years and has been observed across various species. It appears to have evolved independently multiple times through different mechanisms. Considering the potential risks associated with relying on temperature for sex determination, particularly in the face of climate change and weather variations, the persistence of this system has remained a puzzle.
The researchers propose that temperature-dependent sex determination offers a reproductive advantage. Females hatched with a higher number of germ cells are likely to be more reproductively fit, as it increases their potential for carrying more eggs. By establishing a link between the female pathway and the increased number of germ cells, the researchers believe they have made significant progress in explaining the persistence of temperature-dependent sex determination.
However, as global temperatures continue to rise, the implications for temperature-sensitive species, including turtles, become concerning. The researchers plan to investigate how further increases in temperature will affect the pool of germ cells and whether it will result in less-fit females.
To explore these questions, the team carefully nurtures clutches of red-eared slider turtle eggs obtained from a breeder in Louisiana. They incubate the eggs at different temperatures in the lab, with one incubator set at 26 degrees Celsius (producing more males) and another at 31 degrees Celsius (the optimal temperature for producing more females). By observing the development of the embryos under a bright light, they notice that warmer incubation leads to larger and more active embryos.
The researchers propose the existence of a temperature “sweet spot” within a specific range where a higher number of germ cells is produced. Beyond this range, declines in germ cell numbers are expected. Interestingly, incubating eggs at a slightly higher temperature of 33.5 degrees Celsius (only two and a half degrees above the optimal temperature for females) resulted in abnormal embryos with deformities like cyclops and two-headed structures. The researchers plan to investigate the germ cell count in these embryos as well.
The lab’s upcoming experiments involve studying alligator eggs. Unlike red-eared slider turtles, alligators produce females at low temperatures and males at high temperatures. However, the temperature at which alligators produce females aligns with the high-temperature range for turtle females (31 degrees Celsius). By comparing germ cell numbers in both species at this temperature, the researchers aim to gain further insights into the relationship between temperature, germ cells, and sex determination.
Overall, this research sheds light on the intricate mechanisms underlying temperature-dependent sex determination in turtles and provides a glimpse into the potential consequences in a warming world.
Source: Duke University