Sex Determining Temperatures in Turtles: A Geographic Comparison

Bull, James J.; Vogt, Richard C.; McCoy, Clarence J.

doi:10.1111/j.1558-5646.1982.tb05048.x

Cited by 117 publications

(56 citation statements)

References 12 publications

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“…A 3 °C shift of this reaction norm is well within the observed variation for laboratory‐estimated threshold temperature across turtle taxa; for example, Pelomedusa subrufa has a male to female mean population threshold temperature of > 32 °C (Ewert et al. , 2004), whereas C. picta and G. ouachitensis have corresponding temperatures of 27–29 °C (Bull et al. , 1982b; Ewert et al.…”

Section: Discussionsupporting

confidence: 64%

“…This weighting is supported by the observation that about one‐third of field‐incubated nests produce mixed‐sex clutches for C. picta (Janzen, 1994b). In other words, the sex determination threshold may play a substantially larger role in a microevolutionary response of TSD to a sex‐ratio bias in nature than previously thought ( contra Bull et al. , 1982a,b; Janzen, 1992; Doody et al.…”

Section: Discussionmentioning

confidence: 96%

“…Variance in nesting behaviour across latitudes of widely distributed species with TSD appears to be greater than that of variance in threshold temperatures of sex determination and, hence, to indicate a larger evolutionary role of nesting behaviour in response to local conditions (Bull et al. , 1982b; Ewert et al.…”

Section: Discussionmentioning

confidence: 99%

“…To understand how TSD will evolve in response to sex‐ratio bias in the field, where the sex determination threshold may be visible to selection in only a subset of nests, effective heritability must be considered (Bull et al. , 1982a,b).…”

Section: Discussionmentioning

confidence: 99%

“…single‐sex clutches) do not express the sex determination threshold. Much like sex‐limited expression of a trait can slow its response to selection relative to a trait expressed in the total population (Bull et al. , 1982a,b; Falconer & Mackay, 1996), environment‐limited expression of the sex determination threshold will retard its response to selection.…”

Section: Methodsmentioning

confidence: 99%

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Effective heritability of targets of sex-ratio selection under environmental sex determination

McGaugh

Janzen

2011

Journal of Evolutionary Biology

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Selection is expected to maintain primary sex ratios at an evolutionary equilibrium. In organisms with temperature‐dependent sex determination (TSD), targets of sex‐ratio selection include the thermal sensitivity of the sex‐determining pathway (hereafter, sex determination threshold) and nest‐site choice. However, offspring sex may be canalized for nests located in thermally extreme environments; thus, genetic variance for the sex determination threshold is not expressed and is invisible to direct selection. The concept of ‘effective heritability’ accounts for this dependence and provides a more realistic prediction of the expected evolutionary response to selection in the wild. Past estimates of effective heritability of the sex determination threshold, which were derived from laboratory data, suggested that the potential for the sex determination threshold to evolve in the wild was extremely low. We re‐evaluated original estimates of this parameter by analysing field‐collected measures of nest temperatures, vegetation cover and clutch sex ratios from nests in a population of painted turtles (Chrysemys picta). We coupled these data with measurements of broad‐sense heritability of the sex determination threshold in C. picta, using an experiment that splits clutches of eggs between a constant temperature (i.e. typical laboratory incubation) and a daily fluctuating temperature (i.e. similar to natural nests) with the same mean. We found that (i) the effective heritability of the sex determination threshold appears to have been historically underestimated and the effective heritability of nest‐site choice has been overestimated and (ii) significant family‐by‐incubation treatment interaction exists for sex for C. picta between constant‐ and fluctuating‐temperature regimes. Our results suggest that the thermal sensitivity of the sex‐determining pathway may play a larger, more complex role in the microevolution of TSD than traditionally thought.

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Section: Discussionsupporting

confidence: 64%

Section: Discussionmentioning

confidence: 96%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

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Effective heritability of targets of sex-ratio selection under environmental sex determination

McGaugh

Janzen

2011

Journal of Evolutionary Biology

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Thermal thresholds and critical period of thermolabile sex determination in two atherinid fishes,Odontesthes bonariensis andPatagonina hatcheri

et al. 1997

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Atherinidae is a family of fishes known to include at least three species with documented thermolabile sex determination. In Odontesthes bonariensis, fish exposed to about 17°C from hatching to juvenile stage become all-female, whereas groups exposed to about 25°C become male-biased. In contrast, sex in another atherinid, Patagonina hatcheri, appears to be under stronger genotypic control because the sex ratios are always balanced (1:1) within the same range of temperatures. The main purpose of this study was to determine the effect of temperature on the sex ratios of both species over their full range of viable temperatures. In addition, we determined the critical time of thermolabile sex determination in O. bonariensis at female-and male-forming temperatures. O. bonariensis larvae survived at temperatures between 13 and 29°C, but growth was negligible at 13 and 15°C. The proportions of females were 100% at 13-19°C, 95% at 21°C, 81.2% at 23°C, 29.4% at 25°C, 10% at 27°C, and 0% at 29°C. These results indicate a gradual transition from female-to male-forming temperatures evenly spread over the range of viable temperatures and hence the absence of any marked thermal threshold for thermolabile/ genotypic control of sex; they also suggest weak or no genotypic control of sex in this species. In addition, prolonged exposure to 29°C caused disappearance of germ cells. The critical time of thermolabile sex determination in O. bonariensis was estimated to be 3-5 wk, 2-4 wk, and 1-4 wk after hatching at 17, 19, and 27°C, respectively. Survival of P. hatcheri was limited to temperatures between 13 and 27°C (only 8.7% at the latter temperature) with slow but steady growth at 13 and 15°C. The proportions of females were 88.9 and 89.5% at 13 and 15°C, respectively, about 50% at temperatures between 17 and 23°C, and 30.8% at 25°C, hence, showing one clear threshold for thermolabile/genotypic control of sex at 15-17°C and possibly another at or around 25°C. These results confirmed that gonadal sex in this species is under genotypic control at intermediate temperatures but also showed that this control can be overridden at low (and possibly also at high) temperatures.

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Sex determination and primary sex differentiation in amphibians: Genetic and developmental mechanisms

1998

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Most amphibians lack morphologically distinguishable sex chromosomes, but a number of experimental techniques have shown that amphibian sex determination is controlled genetically. The few studies suggesting that environment influences sex determination in amphibians have all been conducted at temperatures outside of the range normally experienced by the species under study, and these effects probably do not occur under natural conditions. No sexdetermining genes have been described in amphibians, and sex differentiation can be altered by treatment with exogenous steroid hormones. The effects of sex steroids vary extensively between species, and a variety of steroids can alter the sex ratios of treated larvae. The role of endogenous sex steroids in gonadal differentiation has not been fully explored; thus the natural role of steroids in amphibian gonadal differentiation is unknown. Sex steroid receptors have not been examined in amphibian gonads, and the mechanism of steroid action on the gonad is unclear. In addition to steroids, the thyroid hormones may play a role in gonadal differentiation. Pituitary gonadotrop(h)ins affect gonadal growth, but not differentiation or maturation of gonads.In addition to the issue of resolving the mechanisms underlying hormone action in gonadal differentiation, other debates concerning interactions between the developing gonads and the invading germ cells, and even the origin of the medullary and cortical portions of the developing gonads, remain unresolved. Studies examining links between sex determination and gonadal differentiation are needed. In addition, examinations of variation in steroidal effects on gonadal development in a phylogenetic context are lacking.

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Sex Determining Temperatures in Turtles: A Geographic Comparison

Cited by 117 publications

References 12 publications

Effective heritability of targets of sex-ratio selection under environmental sex determination

Effective heritability of targets of sex-ratio selection under environmental sex determination

Thermal thresholds and critical period of thermolabile sex determination in two atherinid fishes,Odontesthes bonariensis andPatagonina hatcheri

Sex determination and primary sex differentiation in amphibians: Genetic and developmental mechanisms

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