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

Abstract

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.

Molecular Cytogenetic Search for Cryptic Sex Chromosomes in Painted Turtles <b><i>Chrysemys picta</i></b>

“…scripta , that exhibits a virtually identical pattern of TSD and gene expression Mork et al, 2014] and where heritability of the threshold for TSD has also been reported [Rhen and Lang, 1998;McGaugh and Janzen, 2011] consistent with the findings in T. scripta [Mork et al, 2014]. Thus, it would be expected that a GSD mechanism with cryptic sex chromosomes may be present in C. picta as well.…”

Characteristics and heritability analysis of head scales of the Hungarian meadow viper (Vipera ursinii rakosiensis, Méhely 1893)

“…Apart from snakes, reptiles with temperaturedependent sex determination (TSD) such as Chrysemys picta (Rhen and Lang, 1998;Mc-Gaugh and Janzen, 2011), Chelydra serpentina (Janzen, 1992;Rhen and Lang, 1998), anà Alligator mississipiensis (Rhen and Lang, 1998) provide an interesting opportunity for the study of "genetic variation to support an evolutionary response of TSD to sex-ratio selection" which acts at nest site choice and the sexdetermination threshold, i.e. the temperature at which a male producing genetic programme changes to produce females (McGaugh and Janzen, 2011). Our results show that although different head scales of V. ursinii rakosiensis are not equally heritable, indirect evidence suggests that most of them are quite well-buffered against the effects of the environment, mostly in agreement with the findings of the aforementioned studies.…”

Are sex ratios in wild European sea bass (Dicentrarchus labrax) populations biased?

“…In addition to temperature effects, between-families variation of sex ratio shows that genotypic effects also exist (Saillant et al 2002;Vandeputte et al 2007), and the distribution of family sex ratios was shown to be compatible with a polygenic system, but not with a "classical" genotypic sex determination (GSD) system with sex chromosomes (Vandeputte et al 2007). This type of sex determination system has seldom been evidenced in Vertebrates (McGaugh and Janzen 2011), and is believed to be evolutionarily unstable (Bulmer and Bull 1982;Rice 1986), as it should evolve in most cases towards a chromosomal system where sex is determined at conception. In some cases however, when the environment has differential effect on the fitness of both sexes (e.g., environment influences growth rate and females benefit more of a large size than males, as in Menidia menidia -Conover 1984), the polygenic system may in some rare cases be maintained or alternatively evolve towards a system where sex is determined by environmental factors only (Bulmer and Bull 1982).…”