Temperature-dependent Sex Determination In Gekko Japinicus

The sex-determining mechanisms (SDMs) of vertebrate occupy one of central positions in studies of evolutionary biology, because of its importance to both ontogeny and sex ratio of natural population. Sex-determining mechanisms can be categorized into genotypic sex determination (GSD) and environmental sex determination (ESD) according to the main causal agent of sex determination, and temperature-dependent sex determination (TSD) is the primary sex-determining mechanism in ESD. Most amniote vertebrates have a stable GSD mechanism, while there are various kinds of SDMs in reptiles, and even closely related species might have different mechanisms of SDM. Researchers have done many studies on the SDMs of vertebrate using reptiles as mode systems. Here, we summarize the latest research on TSD in reptiles, paying particular attention to the combined effects of temperature and hormone on TSD reptiles, the adaptive significance of TSD evolution, and the potential impacts of climate change on TSD reptiles. Efforts are made to point out the priority in further TSD studies in reptiles.There are7families,200species of Gekkota in the world, which is one of the oldest categories in squamate reptiles. And there are about27species are TSD lizards that is nearly20%of all. By studying SDMs of7species of Gekkota lizards in china, results showed that Goniurosaurus lichtenfelderi and the three Gekko lizards (G hokouensis, G. japonicus and G. swinhonis) are TSD lizards, while Gehyra mutilate, Hemidactylus bowringii and H. frenatus are GSD lizards. SDMs species were quite abundant in Gekkota, so they are excellent candidates as a model clade for the study of vertebrate sex determination and evolution. Based on phylogenetic tree using partial mitochondrial DNA, we reconstructed ancestral state of SDM in Gekkota, and found that (1) the ancestral state of SDMs are all TSD in Gekkota and its branches;(2) the number of transitions between GSD and TSD can be at least1-3times in evolutionary history of Gekkota.Japanese gecko (G. japonicus) is a small, nocturnal, oviparous gecko, which is widely distributed in Eastern China. G. japonicus does not have heteromorphic sex chromosomes in this distribution area, and it is also a TSD lizard with FMF pattern, which is ideal model system for studying TSD of Squamata reptiles. We will study TSD in G. japonicas through the following four aspects:geographic variation, temperature-sensitive period, adaptive significance of TSD as well as relationship between TSD and yolk steroid hormones.We collected G. japonicus from two geographically separated populations in Longnan (LN) and Chuzhou (CZ) of china as a model animal to study geographic variation of life-history and hatchling phenotypic traits.Adult females have larger SVL and abdomen length (AL) but smaller head length (HL) than adult males, moreover morphological traits of adult geckos varied between the two populations. Combined with reported data on adult SVL of G. japonicus, we found that the latitude was negative related with male SVL, while no correlation to female SVL. The average reproductive time of females from LN is earlier than the ones from CZ; SVL of reproductive female from LN is larger than the ones from CZ, on the other hand, postpartum body mass, clutch mass, egg mass and relative clutch mass are all smaller. Hatchling phenotypic traits are also significantly affected by population and incubation temperature.Incubation length was not different between the two populations, but there were significant differences among the three incubation temperatures. Based on sex ratios of G. japonicus from LN, CZ and Fukuoka (Japan) populations, we used TSD soft to construct three fitting curves, and found that(1) the curve model of sex ratio drift geographically; and (2) the FM and MF pivotal temperatures of G. japonicas were closed with the average temperatures and maximal temperatures of the habitats during their breeding season, respectively, in each population. Body mass, SVL, AL and HL of LN hatchlings were lager than those of CZ, however the growth rate of LN hatchlings was smaller. Tail length of hatchlings from28℃was larger than the ones from24℃; the growth rate of hatchlings from28℃was larger than the ones from24and32℃. No significantly difference showed in locomotor performance of hatchlings between LN and CZ among the three incubation temperatures.Thermal environment has significant impact to sex differentiation of developing embryonic gonad in TSD reptiles. We did the temperature-shift experiments, using the new laid eggs of G. japinicus, to compare difference of hatchling phenotypic traits in different thermal treatments. Results of our study showed that,(1) the longer time eggs are exposed at high temperature, the shorter incubation lengths are;(2) it takes5.9days to reach offspring sex ratio in balance of1:1at32℃, and TSP of G. japinicus is between about7-44%in the whole incubation length;(3) fore-limb length and growth of hatchlings were differed significantly among six temperature-shift treatments, but not other morphological traits and locomotor performance. Combined with reported data on sex-sensitive period (SSP) of TSD reptiles, we found that,(1) starting time of SSP in TSD reptiles are all early than the1/3period of incubation length;(2) especially in Squamata, the SSP range is larger and starting time is earlier than those in other reptiles.Why is the sex of many reptiles determined by the temperatures, rather than by their genes? The Charnov-Bull model (differential fitness hypothesis) suggests that the fitness between sexes is different under the same environment condition. TSD can enhance maternal fitness relative to offspring phenotype, which is affected by difference of nest temperatures for sons versus daughters. We experimentally test five separate models within the Charnov-Bull framework, using G. japinicus with TSD as an animal model. We applied an aromatase inhibitor to apart of the eggs to override thermal effects on sex determination, thus decoupling sex and incubation temperature, and identified the sex-specific effects of incubation temperature on offspring phenotypic traits. Based on our results about egg size,offspring morphology, survival, sex mature, reproductive ability, we were able to accepted five separate differential fitness models, which indicate especially after sex maturing, G. japinicus has more evident adaptive significance of TSD.We used Gekko japonicus, a TSD gecko, as the model system to test the hypothesis that maternal control over offspring sex by means of yolk steroid allocation is common, especially in reptiles with TSD. Eggs were incubated at24℃,28℃and32℃; yolk testosterone (T) and17β-estradiol (E2) levels were measured at three points during development, at oviposition,1/3of incubation and2/3of incubation. Eggs incubated at24℃produced mostly females, so did eggs incubated at32℃; eggs incubated at28℃produced mixed-sex hatchlings. Female-producing eggs on average were larger than male-producing eggs. Clutches in which eggs were incubated at the same temperature mostly produced same-sex siblings. Yolk E2arid T levels did not differ between eggs within a clutch. Yolk T level was negatively related to eggs mass, and yolk E2/T ratio was positively related to egg mass. Incubation temperature affected yolk E2level, T level and E2/T ratio; eggs dissected at1/3of incubation differed from those dissected at2/3of incubation in yolk E2and T levels. The mean yolk E2level was greater at32℃than at248, the mean yolk T level was greater at28℃than at24℃, the mean yolk E2/T ratio was greater at24℃than at28℃, and the mean yolk E2and T levels both were greater at2/3than at1/3of incubation. Taken together, our data show that:(1)incubation temperature affects the dynamics of developmental changes in yolk steroid hormones and thus the hormonal environment of developing embryos;(2) influences of yolk steroid hormones on offspring sex determination are secondary relative to incubation temperature effects; and (3) offspring sex depends on an interaction between incubation temperature and yolk steroid hormones.