Spatial structure of nuclear factors involved in sex determination in the gynodioecious Thymus vulgaris L.

Theoretical models have shown that the maintenance of high frequencies of females in gynodioecious species can be explained by the spatial structure of cytoplasmic and nuclear genes involved in sex determination. Whereas spatial structure of cytoplasmic factors which cause male-sterility has been studied in several taxa, that of nuclear factors that restore male-fertility (restoration factors) has received little attention. In this paper, we estimate spatial variation in the frequency of restoration factors associated with different cytoplasmic male-sterilities in the gynodioecious Thymus vulgaris. Clonal replicates of five female plants bearing at least four different male-sterilities and originating from five different populations (i.e., five cytogenotypes), were reciprocally transplanted into the original populations. Following open pollination at each site, seeds were harvested, germinated and grown to flowering. The frequency of hermaphrodites in each progeny was used to estimate the frequency of restoration factors in each population. For all cytogenotypes, there was marked variation in the rate of restoration among populations, indicating that spatial structure in nuclear restoration factors may influence the variation in female frequency in T. vulgaris. For three out of the five cytogenotypes, higher restoration rates were obtained for females introduced into their original population, which agrees with the theoretical prediction, under some hypotheses, that restoration factors are selected for in populations that contain the associated male-sterility (Gouyon and Couvet, 1985). The very low restoration rate in the progeny of one female clone in three populations indicates that stochastic events, such as founder effects, could lead to a local absence of restoration factors and thus the high female frequency (up to 90%) observed in some populations. Differences among cytogenotypes suggest that some male-sterilities are associated with rarer restoration factors and are thus more likely to cause patches of females to develop in young populations.