SPATIAL STRUCTURE OF NUCLEAR FACTORS INVOLVED IN SEX DETERMINATION INTHE 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 s tructure oi cytoplasmic and nuclear genes involved in sex determinatio n. Whereas spatial structure of cytoplasmic factors which cause male-s terility has been studied in several taxa, that of nuclear factors tha t restore male-fertility (restoration factors) has received little att ention. In this paper, we estimate spatial variation in the frequency of restoration factors associated with different cytoplasmic male-ster ilities in the gynodioecious Thymus vulgaris. Clonal replicates of fiv e female plants bearing at least four different male-sterilities and o riginating from five different populations (i.e., five cytogenotypes), were reciprocally transplanted into the original populations. Followi ng open pollination at each site, seeds were harvested, germinated and grown to flowering. The frequency of hermaphrodites in each progeny w as used to estimate the frequency of restoration factors in each popul ation. For all cytogenotypes, there was marked variation in the rate o f restoration among populations, indicating that spatial structure In nuclear restoration factors may influence the variation in female freq uency in T. vulgaris. For three out of the five cytogenotypes, higher restoration rates were obtained for females introduced into their orig inal population, which agrees with the theoretical prediction, under s ome hypotheses, that restoration factors are selected for in populatio ns 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 e ffects, could lead to a local absence of restoration factors and thus the high female frequency (up to 90%) observed in some populations. Di fferences among cytogenotypes suggest that some male-sterilities are a ssociated with rarer restoration factors and are thus more likely to c ause patches of females to develop in young populations.