PMC

Illustrating the continuum of genetic and environmental sex determination. Examples of possible effects of environmental factors (e.g. temperature or concentration of endocrine-disrupting micropollutants) on sex determination in (a) a hypothetical population with genetic sex determination and the female genotype being susceptible to environmental factors that masculinize (i.e. turning some XX or ZW individuals into males) and (b) a population with genetic sex-determining factors and the male genotype being susceptible to environmental factors that feminize (i.e. turning some XY or ZZ individuals into females). The shaded area indicates the within-population variance that could be due to additive genetic variance in the reaction norms or due to random effects at the start of the sex determination cascade. The hatched line gives the population sex ratio (proportion of males) if all clutches experience the same environmental conditions. This population sex ratio will equal adult sex ratio (ASR) if there is no sex-specific mortality.

Sex ratios predicted for each population from the DDC Model across the nesting season. Sex ratios are averaged across all years by each nesting day, and a sliding 3-day average is depicted. Louisiana is shown with red circles, South Carolina with grey triangles, and Illinois with blue squares. Estimates for Louisiana and South Carolina use the population average sex ratio reaction norm (Fig. ). Sex ratios in Illinois are portrayed using the population average sex ratio reaction norm (Fig. , dashed line), and the two season-specific reaction norms (Fig. , solid line with squares) to demonstrate the relative effect of the seasonal reaction norms on predicted sex ratios.

PCR amplicons of the Amelogenin and Sex determining region Y (SRY) gene from sperm genomic DNA. A. Schematic illustration of annealing of primers for specific amplification of X and Y chromosome in PCR reaction and B. A 167-bp and 107-bp fragment was amplified in PCR reaction from SRY and Amelogenin gene as indicators for sex determination. PCR; Polymerase chain reaction.

Hypothetical norms of reaction for XX and XY genotypes, with sex modelled as a threshold trait. The amount of a sex factor SF (e.g. a male hormone) produced by a given genotype increases with the environmental gradient E (e.g. temperature). For the environmental range considered, the XY genotype always produces enough of the sex factor to lie above the threshold (horizontal straight line), so that all XY individuals develop as males. At low environmental values (E₁), the amount of sex factor produced by the XX genotype always lies below the threshold, so that all XX individuals develop as females; sex determination is thus purely genetic (GSD). As the environmental gradient increases, an increasing proportion of XX individuals exceed the threshold, thus developing into ‘sex-reversed’ males. As a result of sex-ratio selection, the frequency of XY individuals progressively diminishes. At the extreme (E₃), all individuals are XX and sex determination becomes purely environmental (ESD). Adapted from [].