Quantitative genetics of sexual plasticity: The environmental threshold model and genotype-by-environment interaction for phallus development in the snail Bulinus truncatus
Mf. Ostrowski et al., Quantitative genetics of sexual plasticity: The environmental threshold model and genotype-by-environment interaction for phallus development in the snail Bulinus truncatus, EVOLUTION, 54(5), 2000, pp. 1614-1625
Sexual polymorphisms are model systems for analyzing the evolution of repro
ductive strategies. However, their plasticity and other binary traits have
rarely been studied, with respect to environmental variables. A possible re
ason is that, although threshold models offer an adequate quantitative gene
tics framework for binary traits in a single environment, analyzing their p
lasticity requires more refined empirical and theoretical approaches. The s
tatistical framework proposed here, based on the environmental threshold mo
del (ETM), should partially fill this gap. This methodology is applied to a
n empirical dataset on a plastic sexual polymorphism, aphally, in the snail
Bulinus truncatus. Aphally is characterized by the co-occurrence of regula
r hermaphrodites (euphallics) together with hermaphrodites deprived of the
male copulatory organ (aphallics). Reaction norms were determined for 40 in
bred lines, distributed at three temperatures, in a first experiment. A sec
ond experiment allowed us to rule out maternal effects. We confirmed the ex
istence of high broad-sense heritabilities as well as a positive effect of
high temperatures on aphally. However a significant genotype-by-environment
interaction was detected for the first time, suggesting that sexual plasti
city itself can respond to selection. A nested series of four ETM-like mode
ls was developed for estimating genetical effects on both mean aphally rate
and plasticity. These models were tested using a maximum-likelihood proced
ure and fitted to aphally data. Although no perfect fit of models to data w
as observed, the refined versions of ETM models conveniently reduce the ana
lysis of complex reaction norms of binary traits into standard quantitative
genetics parameters, such as genetic values and environmental variances.