Reproductive biology and female frequencies of two co-occurring gynodioecious Thymus species

Gynodioecy, the co-occurrence of female and hermaphrodite plants in the same population is relatively common in the genus Thymus, where a high and variable female frequency has been reported in several species, populations and years. In most gynodioecious species, female plants produce more and/or better seeds than hermaphrodite plants, facilitating female maintenance in natural populations. Thymus × arundanus and T. granatensis are two sympatric gynodioecious species that inhabit the region of Andalucia, Spain. Here we studied reproductive components in two different years (1987 and 2016) as flower number, fruit-set, seed number and seed mass, and their possible relation to female frequencies in those years in natural populations of both species. In T. × arundanus, mean female frequency (59%) was ten times higher than that of T. granatensis (5%). Female frequency was relatively constant in T. × arundanus after almost 15 years (1987–2016), and in T. granatensis low female frequencies were observed (0–5%) with the exception of one population (18%). However, reproductive components were variable among species, years and genders, showing no consistent female fertility advantage. High female frequencies in T. × arundanus are not likely to be maintained by female fertility advantage but stochastic and pleiotropic effects on sex determination may play a relevant role in sex ratio variation in the studied populations of both species.