No evidence for local adaptation between cytoplasmic male sterility and nuclear restorer genes in the gynodioecious species Thymus vulgaris L.

Abstract

In Thymus vulgaris L., sex determination involves both the nuclear and the cytoplasmic genomes: the cytoplasm is responsible for male sterility (the female phenotype) whereas specific nuclear genes may restore male fertility (the hermaphrodite phenotype). The evolutionary dynamics of cytoplasmic male-sterility genes and nuclear restorer genes represents a coevolutionary conflict. Here we draw a parallel between this conflict and the coevolutionary interaction between hosts and parasites. Local adaptation of parasites to their hosts may be predicted under some situations of relative evolutionary rates of the two partners in this conflict and is often observed in natural populations. We tested for local adaptation between nuclear restorer genes and cytoplasmic male-sterility factors using 12 female plants that were hand-pollinated with pollen from hermaphrodites from more or less distant populations, and measuring progeny sex ratios. No significant effects of the geographical distance between maternal and paternal parents could be detected, although highly significant differences among individual females for the restorability of their cytoplasms were found. We suggest that the same evolutionary processes giving rise to individual differences among cytoplasmic male-sterility types might account for the absence of local adaptation between nuclear and cytoplasmic factors in this system, and also discuss the absence of local adaptation in the light of a recent model of local adaptation in host-parasite metapopulations.