Thymus vulgaris Transcriptomes (from study on cytoplasmic male sterility)

Gynodioecy is a sexual dimorphism where females co-exist with hermaphrodite individuals and represents a key transition stage from hermaphroditism to dioecy. In most cases, this dimorphism involves the interaction of cytoplasmic male sterility (CMS) genes and nuclear restorer genes. Much theory focuses on understanding the maintenance of gynodioecy. Two scenarios can account for this: either CMS genes regularly enter populations at low frequency via mutation/migration and go to fixation unchecked (successive sweeps), or CMS genes are maintained polymorphic over evolutionary times 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 amount and patterns of nucleotide diversity in the nuclear and cytoplasmic genome. Our finding is not compatible with maintenance of gynodioecy via scenarios invoking long-term balancing selection and instead suggest recent fixation of CMS lineages in the populations studied.