Evidence for effects of restorer genes on male and female reproductive functions of hermaphrodites in the gynodioecious species Thymus vulgaris L.

Ln Thymus, vulgaris L., sex determination involves both the nuclear and the cytoplasmic genomes: the:cytoplasm is responsible for male-sterility (the female phenotype) while specific nuclear genes may restore male fertility (the hermaphrodite phenotype). Previous observations have shown high variation among hermaphrodites for pollen and seed production. In order to investigate the origin of this variation, 12 female plants, four from each of three populations, were hand-pollinated with pollen from hermaphrodites from three different paternal populations. The sex-ratio (i.e. the frequency of hermaphrodites) produced and the reproductive functions of these offspring were measured. A strong positive correlation was observed between the sex-ratio within a family and both female and male reproductive functions of its hermaphrodites. No such-correlation was found for females. This result suggests that restorer genes may be directly or indirectly involved both in sex determination and in the efficiency of resource allocation to reproductive: functions. As a consequence, female advantage, i.e. the relative fecundity of females to hermaphrodites, is larger in families with low sex-ratio, and this might affect the evolution of this gynodioecious breeding system.