Résumé
Gynodioecy is a sexual dimorphism where females coexist with hermaphrodite
individuals. In most cases, this dimorphism involves the interaction of
cytoplasmic male sterility (CMS) genes and nuclear restorer genes. Two
scenarios can account for how these interactions maintain gynodioecy.
Either CMS genes recurrently enter populations at low frequency via
mutation or migration and go to fixation unimpeded (successive sweeps), or
CMS genes maintain polymorphism over evolutionary time through
interactions with a nuclear restorer allele (balanced polymorphism). To
distinguish between these scenarios, we used transcriptome sequencing in
gynodioecious Thymus vulgaris and surveyed genome-wide diversity in 18
naturally occurring individuals sampled from populations at a local
geographic scale. We contrast the amount and patterns of nucleotide
diversity in the nuclear and cytoplasmic genome, and find ample diversity
at the nuclear level (π = 0.019 at synonymous sites) but reduced genetic
diversity and an excess of rare polymorphisms in the cytoplasmic genome
relative to the nuclear genome. Our finding is incompatible with the
maintenance of gynodioecy via scenarios invoking long-term balancing
selection, and instead suggests the recent fixation of CMS lineages in the
populations studied.