FOUNDER EFFECTS AND SEX-RATIO IN THE GYNODIOECIOUS THYMUS-VULGARIS L

Thymus vulgaris is a gynodioecious species (in which females and herma phrodites coexist) with a highly variable frequency of females among n atural populations (5-95%) and a high average female frequency (60%). Sex determination involves both cytoplasmic genes responsible for male sterility, i.e. the female phenotype, and specific nuclear factors re sponsible for the restoration of male fertility, and thus a hermaphrod ite phenotype. In this study, molecular markers of the mitochondrial g enome have been used to quantify the cytoplasmic diversity in 11 clump s of individuals observed in four recently founded populations. The ve ry low diversity within patches in conjunction with the strong diversi ty among patches strongly suggests that clumps of individuals are the result of single matrilinear families. In clumps that contain mainly f emales, all the analysed females showed the same cytoplasmic pattern. This pattern differed from that shown by neighbouring hermaphrodites, indicating that the determination of sex is locally cytoplasmic. A com parison of genetic diversity before and after fire in one population s howed that disturbances may cause a reduction in genetic diversity and a concurrent induction of local cytoplasmic determination of sex. Suc h cytoplasmic determination of sex in colonizing populations, together with the greater seed set of females, may largely improve the coloniz ing ability of the species.