The flexible stem hypothesis: evidence from genetic data.

Phenotypic plasticity, the ability of a given genotype to produce different phenotypes in response to distinct environmental conditions, is widely observed in the wild. It is believed to facilitate evolution and, under the "flexible stem hypothesis", it is thought that an ancestral plastic species can be at the origin of sister lineages with divergent phenotypes fixed by genetic assimilation of alternative morphs. We review here the genetic mechanisms underlying such phenomenon. We show several examples in which the same gene shows transcriptional plasticity in response to environmental factors and divergence of expression within or between species. Thus, the same gene is involved both in the plasticity of a trait and in the evolution of that trait. In a few cases, it can be traced down to cis-regulatory variation in this gene and, in one case, in the very same regulatory sequence whose activity is modulated by the environment. These data are compatible with the "flexible stem hypothesis" and also suggest that the evolution of the plasticity of a trait and the evolution of the trait are not completely uncoupled as they often involve the same locus. Furthermore, the "flexible stem hypothesis" implies that it is possible to canalize initially plastic phenotypes. Several studies have shown that it was possible through modification of chromatin regulation or hormonal signalling/response. Further studies of phenotypic plasticity in an evolutionary framework are needed to see how much the findings described in this review can be generalized.