The ends of a continuum: genetic and temperature-dependent sex determination in reptiles

Two prevailing paradigms explain the diversity of sex-determining modes in reptiles. Many researchers, particularly those who study reptiles, consider genetic and environmental sex-determining mechanisms to be fundamentally different, and that one can be demonstrated experimentally to the exclusion of the other. Other researchers, principally those who take a broader taxonomic perspective, argue that no clear boundaries exist between them. Indeed, we argue that genetic and environmental sex determination in reptiles should be seen as a continuum of states represented by species whose sex is determined primarily by genotype, species where genetic and environmental mechanisms coexist and interact in lesser or greater measure to bring about sex phenotypes, and species where sex is determined primarily by environment. To do otherwise limits the scope of investigations into the transition between the two and reduces opportunities to use studies of reptiles to advance understanding of vertebrate sex determination generally.     

First evidence of higher female recombination in a species with temperature-dependent sex determination: the saltwater crocodile

The first evidence of genetic linkage and sex-specific recombination in the order Crocodylia is reported. This study was conducted using a resource pedigree of saltwater crocodiles consisting of 16 known-breeding pairs (32 adults) and 101 juveniles. A total of 21 microsatellite loci were available for analysis. Ten of the 21 loci showed linkage with 4 linkage groups: 3 pairwise (Cj131/Cj127, CUD68/Cj101, and Cj107/Cp10) and 1 four-locus (Cj122, CUD78, Cj16, and Cj104) being found. Linkage analysis on the 21 loci revealed evidence of sex-specific differences in recombination rates. All 5 nonzero interlocus intervals were longer in females than in males, with the 4-loci linkage group 3-fold longer in females than in males (41.63 cM and 14.1 cM, respectively). This is the first report of sex-specific recombination rates in a species that exhibits temperature-dependent sex determination.     

Nest-site selection in two eublepharid gecko species with temperature-dependent sex determination and one with genotypic sex determination

At present, most turtles, all crocodilians, and several lizards are known to have temperature-dependent sex determination (TSD). Due to the dependence of sex determination on incubation temperature, the long-term survival of TSD species may be jeopardized by global climate changes. The current study was designed to assess the degree to which this concern is justified by examining nest-site selection in two species of Pattern II TSD geckos (Eublepharis macularius and Hemitheconyx caudicinctus) and comparing these preferences with those of a species with genotypic sex determination (GSD) (Coleonyx mitratus). Temperature preferences for nest sites were found to be both species-specific and female-specific. While H. caudicinctus females selected a mean nest-site temperature (32.4°) very close to the upper pivotal temperature (32°C) for the species, E. macularius females selected a mean nest-site temperature (28.7°C) well below this species' lower pivotal temperature (30.5°C). Thus, the resultant sex ratios are expected to differ between these two TSD species. Additionally, nest-site temperatures for the GSD species were significantly more variable (SE=+0.37) than were temperatures for either of the TSD species (E. macularius SE=±0.10; H. caudicinctus SE =+ 0.17), diereby further demonstrating temperature preferences within the TSD species.     

Microsatellite DNA loci identify individuals and provide no evidence for multiple paternity in wild tuatara (Sphenodon: Reptilia)

Six new microsatellite DNA loci are isolated from a genomic library of the sphenodontid reptile tuatara (Sphenodon) and presented here as a tool for identifying individuals for future paternity and kinship studies. These loci, in combination with four previously published loci, are sufficient to discriminate between clutch-mate siblings from Stephens Island and Brothers Island populations. These populations represent high and low levels of genetic diversity in tuatara populations respectively. An estimate of minimum number of fathers of each clutch found no evidence for multiple paternity in any clutch. These newly isolated loci complete the development of an array of genetic tools for use in tuatara to enhance ongoing conservation and management of wild, translocated and captive populations.     

Gonadal sex differentiation in Alligator mississippiensis,a species with temperature-dependent sex determination

Temperature of egg incubation determines sex in Alligator mississippiensis hatchlings. To define the timing and morphology of sexual differentiation, alligator gonads were examined histologically and ultrastructurally throughout embryogenesis. At the male-producing temperature (33° C), the onset of testis differentiation occurred in most embryos during developmental stages 21–22, when a number of somatic cells in the medulla of the gonad became enlarged, forming presumptive Sertoli cells. Some enlarged somatic cells were also observed at the female-producing temperature (30° C) during gonadogenesis, but they were less widespread than at 33° C. Ovarian differentiation at 30° C began slighlty later, during stage 22–23, and was characterised by proliferation of germs cells in the cortex of the gonad. Testis formation in alligators may depend upon presumptive Sertoli cells differentiating prior to a critical event in embryogenesis, such as germ cell proliferation and meiosis. If follows that ovary formation occurs if this requirement is not met, as at lower incubation temperatures.     

Segregating variation for temperature-dependent sex determination in a lizard

Temperature-dependent sex determination (TSD) was first reported in 1966 in an African lizard. It has since been shown that TSD occurs in some fish, several lizards, tuataras, numerous turtles and all crocodilians. Extreme temperatures can also cause sex reversal in several amphibians and lizards with genotypic sex determination. Research in TSD species indicates that estrogen signaling is important for ovary development and that orthologs of mammalian genes have a function in gonad differentiation. Nevertheless, the mechanism that actually transduces temperature into a biological signal for ovary versus testis development is not known in any species. Classical genetics could be used to identify the loci underlying TSD, but only if there is segregating variation for TSD. Here, we use the ‘animal model'' to analyze inheritance of sexual phenotype in a 13-generation pedigree of captive leopard geckos, Eublepharis macularius, a TSD reptile. We directly show genetic variance and genotype-by-temperature interactions for sex determination. Additive genetic variation was significant at a temperature that produces a female-biased sex ratio (30 °C), but not at a temperature that produces a male-biased sex ratio (32.5 °C). Conversely, dominance variance was significant at the male-biased temperature (32.5 °C), but not at the female-biased temperature (30 °C). Non-genetic maternal effects on sex determination were negligible in comparison with additive genetic variance, dominance variance and the primary effect of temperature. These data show for the first time that there is segregating variation for TSD in a reptile and consequently that a quantitative trait locus analysis would be practicable for identifying the genes underlying TSD.     

Physiological effects of a fish oil supplement on captive juvenile tuatara (Sphenodon punctatus)

Tuatara (Sphenodon, Order Sphenodontia) are rare New Zealand reptiles whose conservation involves captive breeding. Wild tuatara eat seabirds, which contain high levels of the long-chain n-3 polyunsaturated fatty acids (PUFAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These fatty acids are absent from the captive diet, and consequently, plasma fatty acid composition of wild and captive tuatara differs. This study investigated the effects of incorporating EPA and DHA into the diet of captive juvenile tuatara (Sphenodon punctatus) in an attempt to replicate the plasma fatty acid composition of wild tuatara. Tuatara receiving a fish oil supplement containing EPA and DHA showed overall changes in their plasma fatty acid composition. Phospholipid EPA and DHA increased markedly, reaching 10.0% and 5.9 mol%, respectively, by 18 mo (cf. 相似文献   

Differences in dietary and plasma fatty acids between wild and captive populations of a rare reptile, the tuatara (Sphenodon punctatus)

Tuatara (Sphenodon) are rare reptiles endemic to New Zealand. Wild tuatara on Stephens Island (study population) prey on insects as well as the eggs and chicks of a small nesting seabird, the fairy prion (Pachyptila turtur). Tuatara in captivity (zoos) are fed diets containing different insects and lacking seabirds. We compared the fatty acid composition of major dietary items and plasma of wild and captive tuatara. Fairy prions (eaten by tuatara in the wild) were rich in C20 and C22 polyunsaturated fatty acids (PUFA), especially the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). In contrast, items from the diet of captive tuatara contained no C20 and C22 PUFA and were higher in medium-chain and less unsaturated fatty acids. Plasma from wild tuatara was higher in n-3 PUFA [including alpha-linoleic acid (C18:3n-3), EPA and DHA], and generally lower in oleic acid (C18:1) and palmitic acid (C16:0), than plasma from captive tuatara in the various fractions (phospholipid, triacylglycerol, cholesterol ester and free fatty acids). Plasma from wild adult tuatara showed strong seasonal variation in fatty acid composition, reflecting seasonal consumption of fairy prions. Differences in the composition of diets and plasma between wild and captive tuatara may have consequences for growth and reproduction in captivity. Accepted: 3 August 1998     

Morphological changes in the corpus luteum of tuatara (Sphenodon punctatus) during gravidity

Tuatara (Sphenodon spp.) are rare reptiles, members of the reptilian order Sphenodontida, inhabiting small offshore islands of New Zealand. Females usually require about three years to yolk a clutch of eggs followed by an 8-month period of in utero egg shelling. As in other vertebrates, the post-ovulatory follicle forms a transitory endocrine structure, the corpus luteum. The tuatara Sphenodon punctatus exhibits a corpus luteum having several unusual morphological features as compared to turtles and squamate reptiles. Like the crocodilians, the tuatara has a corpus luteum in which the luteal cell mass never fills the central cavity and in which the thecal fibroblasts do not close the ovulation aperture. As in all oviparous reptiles examined, however, the corpus luteum appears to persist throughout gravidity based on its histological appearance. During gravidity, plasma progesterone concentrations are detectable, even though gravidity lasts an exceptionally long time (8 months) for an oviparous species. Luteolysis is initiated within two months following oviposition. The initial stages of luteolysis appear rapid, but luteal scar tissue is apparent in the ovaries of all adult females we examined and probably persists for many years post-oviposition. J Morphol 232:79–91, 1997. © 1997 Wiley-Liss, Inc.     

Karyotypes and sex determination in two species of laelapid mites (Acari: Gamasida)

The chromosome number and morphology of two species of the family Laelapidae have been determined: Hypoaspis aculeifer Canestrini, 1887 has 9() and 18() chromosomes and Cosmolaelaps miles Berlese, 1914 7() and 14(). Both karyotypes have monokinetic chromosomes and show obvious similarity. The longest chromosome of both species consists of a normal and a heterochromatic arm. The two laelapid mites prove to be arrhenotokous, as unfertilized females lay eggs from which only males arise. A theory has been postulated that within the Gamasida an evolutionary line towards arrhenotoky is present.Arguments supporting an independent evolution of sex determining system in the two acarine taxa Actinotrichida and Anactinotrichida are discussed.     

Identification of a 130-kDa albumin in tuatara (Sphenodon) and detection of a novel albumin polymorphism

Electrophoretic, immunochemical, and protein sequence analyses were performed on plasma albumin of the tuatara (Sphenodon), a rare reptile endemic to New Zealand. The analyses revealed that, unlike other terrestrial vertebrates, tuatara do not seem to possess a 60- to 75-kDa plasma albumin. The common form of plasma albumin in this genus has an apparent molecular mass of 130 kDa, making it by far the largest albumin reported for any terrestrial vertebrate. Starch gel electrophoresis of samples from tuatara on 24 of the 30 islands inhabited by this genus resolved two forms of the 130-kDa albumin (albumins A and C). A third albumin of approximately 170 kDa (albumin B), reflecting a novel alloalbuminemia, was found in tuatara in three geographically isolated populations. Albumin A appears to be restricted to populations at the southern extremity of the tuatara's distribution, while albumin C was found in all but four (southern) populations. Possible explanations for the origin and distribution of these albumins are discussed.     

Conservation implications of a long-term decline in body condition of the Brothers Island tuatara (Sphenodon guntheri)

Understanding the factors that drive the dynamics of remnant populations of long-lived species presents a unique challenge for conservation management. The long-lived Brothers Island tuatara Sphenodon guntheri is represented by one natural, self-sustaining population on 4-ha North Brother Island, New Zealand, and two small, translocated populations. The North Brother Island population was almost driven to extinction by extreme habitat modification and collecting in the late 19th century. Analysis of a long-term (1957–2001) dataset, following the population's recovery, reveals a significant decline in tuatara body condition over time, which is more pronounced in females. Declining body condition, coupled with very low reproductive output, may be symptomatic of a density-dependent response to elevated population size exacerbated by resource limitation. Sex-specific effects that disadvantage females could compromise this small population, particularly as it exhibits a male-biased sex ratio. We recommend removal of infrequently used structures and habitat restoration to alleviate intense resource competition. Population-level manipulation should be considered if future monitoring indicates an increasingly male-biased sex ratio and continued decline of female body condition.     

Artificially induced sex-reversal leads to transition from genetic to temperature-dependent sex determination in fish species

正Dear Editor,Sex determination is highly plastic across vertebrates. The mechanisms of sex determination can be broadly divided into genetic sex determination (GSD) and environmental sex determination (ESD). GSD is the most popular sex-determining type in vertebrates, while ESD mainly occurred in     

Sex reversal of pejerrey (Odontesthes bonariensis), a species with temperature-dependent sex determination,in a seminatural environment

This study examined the changes in sex ratios and sex reversal rates in pejerrey Odontesthes bonariensis that occur with the progression of the spawning season in a seminatural setting. Four groups of hatchery-produced pejerrey larvae were stocked in floating cages in La Salada de Monasterio lake (Pampas region), a natural habitat of this species, and reared from hatching beyond gonadal sex determination with minimum human interference. Cage 1 was stocked at the beginning of the spring spawning season and the other cages were stocked with monthly delays until cage 4 in early summer. The genotypic (amhy+, XY/YY; amhy−, XX) and phenotypic (testis, male; ovary, female) sex ratios and proportions of genotype/phenotype mismatched individuals were estimated and their relation to water temperature and daylength during the experiment was analysed by generalized linear modelling. Water temperature varied between 11 and 30.5°C, and daylength duration between 11 h 22 min and 14 h 35 min. Sex genotyping revealed nearly balanced sex ratios of XY/YY (46%–49.1%) and XX (50.9%–54%) fish in cages 2–4 whereas the genotypic sex ratio in cage 1 was clearly biased towards XY/YY fish (60.6%). Phenotypic males ranged from 42% to 54.4% in cages 1–3. Cage 4, in turn, had significantly more phenotypic males (66%). The percentage of XX males (phenotypic male/genotypic female) was 23.1% in cage 1, decreased to a minimum of 5.4% in cage 2 and gradually increased in cages 3 and 4 to a maximum of 40.7% in the latter. The percentages of XY/YY females (phenotypic female/genotypic male) were highest in cage 1 (30%) and decreased progressively in the other cages to a significantly lower value (4.3%) in cage 4. These results generally support the findings of laboratory studies on the effect of temperature on the sex determination of this species and also provide novel evidence of a XX genotype-specific masculinizing effect of short daylength.