Animals can be said to be strange in terms of gender, and there are always some wonderful existences that are eye-opening. For example, some snails are hermaphrodites that can lay eggs and sperm at the same time; fish in the family Ronaldidae are initially females when they hatch, but as they age, they become males (this phenomenon is called 'female-first-mature hermaphrodites'); and others, such as some lizards, simply abandon males altogether, and females reproduce their offspring through parthenogenesis and lay fertilized eggs.
But many creatures are human-like, either male or female, and their gender does not change after birth. It's so trivial that you might find it boring and seem like there's nothing to watch. But this is not the case.
Crescent moonfish, a species that females ripen first | Wikipedia
Until now, sex selection has been thought to be determined by genes passed down from parents (such as mammals with X and Y chromosomes) or influenced by environmental factors (such as temperature-dependent sex-determined turtles). But new research suggests that the truth is more complex.
Studies have found that unhatched baby turtles can control their own development, choosing their sex by moving positions in the eggs.
Temperature affects gender
The idea that oviparous reptiles are able to influence their gender in some way has long been thought to be impossible. But a study published in the journal Contemporary Biology in August provides clear and strong evidence for this idea.
Led by Du Weiguo of the Institute of Zoology of the Chinese Academy of Sciences and led by a team of scientists including Rick Shine, a biologist at Macquarie University, experimented by hatching a grass turtle in a semi-natural turtle nest (Mauremys reevesii).
A grass turtle (Mauremys reevesii) | Wikipedia
The sex of many reptiles depends heavily on the temperature of the nest, and grass turtles are no exception. This temperature-dependent sex is called temperature-dependent gender determination (TSD).
TSD has raised concerns among scientists about the survival of many reptile species. We all know that global warming is getting worse, which means that the earth's temperature is getting higher and higher every year. Scientists believe that when the temperature rises, the usual 1:1 sex ratio will deviate significantly, which will lead to a decrease in the reproductive rate of these species, making it difficult to maintain the continuation of the species.
However, not all evidence can lead to this conclusion. "Ancestral lineage reconstructions of many TSD species show that they have survived for millions of years," Schine said, "and some of these periods are much hotter than current climate change models predict." ”
Another grass turtle | harum.koh/Flickr
Such a tenacious vitality may be explained by some of the behaviors of the mother grass turtle, such as changing the spawning time or changing the structure of the nest to avoid the hottest time of the year. But once the eggs have been laid, adult reptiles are no longer able to protect their children from temperature changes.
That said, it's likely that there are some other factors that help species like grass turtles survive harsh temperature conditions. Scientists speculate that it may be that the grass turtle embryos themselves influenced their own sex. In order to verify this, Du Weiguo and others conducted experiments to control the conditions affecting grass turtle eggs.
Boys like cold, girls like warmth
They found that when heated by a heat source set at an angle, each egg produced a noticeable thermal gradient, with one end becoming warmer, at which point the embryos moved in the egg accordingly. Embryos that move toward the warmer end eventually develop into females, while those that move towards the colder end develop into males.
The researchers speculated that embryos detect temperature through built-in biosensors called ion channels, and in many animals, transient receptor potential ion channels (TRPs) act as temperature sensors. To test this, they used capsaicin receptor antagonists to block ionic activity and inhibit the body temperature regulation behavior of grass turtles without affecting the health of grass turtles.
Fig. A shows an embryo using capsaicin receptor antagonist resistance, which can be seen in the middle of the egg; Fig. B shows an embryo without capsaicin receptor antagonist resistance, and the embryo moves | to the warmer end Reference 2
It turned out that the treated embryos did not really move like the untreated embryos, and the sex ratio quickly became skewed. In colder conditions (May 2018), almost all grass turtles are males, while at higher temperatures (June 2017) females dominate.
For Du Weiguo, the study also supported his early work. A previous study by his team suggested that the movement of grass turtle embryos is triggered by temperature differences within the egg, but this conclusion has been questioned by other scientists in the field. Now, the new findings reinforce the conclusions he once reached.
"The discovery that embryos can affect their own development is really remarkable," says Hine, who is a rare "good news" in animal protection.
Tackling climate change
"Turtles around the world are in desperate trouble, they are being eaten in Asia, and Australia's habitat is being destroyed by river system management," Schine explains, "but it's encouraging that these turtles may be better than we expected in addressing the challenges posed by climate change." ”
"We've previously shown that reptile embryos can move within their eggs for thermoregulation, so it's curious if this affects their sex decisions," Du said, "and we wanted to know if this behavior could help mitigate the effects of global warming on the sex ratio of the offspring of these species, and exactly how it is done." ”
The effect of embryonic body temperature regulation on the sex ratio of offspring depends on weather, seasons and vegetation cover, and these factors determine the intensity of sunlight reaching the ground, thus the degree of heat gradient within the egg. At low latitudes, vegetation cover has a large influence on the sex ratio of offspring early in the breeding season, while at higher latitudes the effect is greatest later in the breeding season. In the absence of embryonic body temperature regulation, the team's model predicts that global warming will lead to an increasing proportion of females.
A grass turtle photographed in East Timor | Wikipedia
But overall, the trait characteristics of offspring are influenced by many factors, including environmental abiotic conditions and the duration of exposure to specific conditions. In addition, the characteristics of the species itself are also important. For example, the thermal changes caused by the thermoregulatory behavior of grass turtle embryos are very slight, and the range of thermal changes required to transition from 100% male to 100% female offspring is narrow (about 2 °C), while for species with a wide threshold, the thermal changes obtained through embryonic thermoregulation may be too insignificant to affect the sex of the offspring.
In addition, there is still a lot to be seen, especially to figure out why the embryo chose the temperature it prefers – to what extent it is to choose gender and to what extent it is for other reasons.
But what is certain is that the data and analysis produced by Du Weiguo's team show that for oviparous reptiles, the temperature regulation capacity within eggs plays an important role in their adaptation to the environment, helping to buffer the effects of climate change.
How, when, and why animals develop into males or females is much more complex than we think.
图 | Greg Peterson/Flickr
bibliography:
1.Gender bending: Baby turtles influence their own sex, MACQUARIE UNIVERSITY, https://www.eurekalert.org/pub_releases/2019-08/mu-gbb080419.php
2.The Embryos of Turtles Can Influence Their Own Sexual Destinies, Yin-Zi Ye, Liang Ma, Bao-Jun Sun, Yang Wang, Richard Shine, Wei-Guo Du, DOI:https://doi.org/10.1016/j.cub.2019.06.038
3.Seebacher, F., and Murray, S.A. (2007). Transient receptor potential ion channels control thermoregulatory behaviour in reptiles. PLoS ONE 2, e281.
4.Du, W.-G., Hu, L.-J., Lu, J.-L., and Zhu, L.-J. (2007). Effects of incubation temperature on embryonic development rate, sex ratio and post-hatching growth in the Chinese three-keeled pond turtle, Chinemys reevesii. Aquaculture 272, 747–753.
5.Ewert, M.A., Jackson, D.R., and Nelson, C.E. (1994). Patterns of temperature-dependent sex determination in turtles. J. Exp. Zool. 270, 3–15.
6.Turtle embryos can choose their own sex, shows new research – but why?, Peter Ellis, https://theconversation.com/turtle-embryos-can-choose-their-own-sex-shows-new-research-but-why-121074
Cover image from testudines.org
Author: Yakumo
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