Spatio-temporal variation of male sterile frequencies in two natural populations of Beta maritima

Abstract

The spatio-temporal variation of sex phenotype frequencies is investigated within two gynodioecious populations of Beta maritima located along the English Channel, in which the proportions of females differ significantly: 0·19 (Population A) and 0·62 (Population B). A genetical analysis of maternal progenies obtained from in situ open-pollinated plants (G1, generation) allows us to define two types of parents: segregating plants (females, intermediate-females and some hermaphrodites) which yield different types of progenies, and non-segregating plants, hermaphrodites which yield only hermaphrodites. Molecular analysis (Saumitou-Laprade, 1989) confirms the results of a preliminary study (Boutin et al., 1987), in which it is shown that cytoplasmic type is related to segregation in the maternal progeny. A differentiation in space has been pointed out by the comparison between the sex phenotype frequencies in the two populations. This differentiation is not due to a variation of cytoplasmic frequencies but to a variation in the nuclear genetic components of these populations: the level of restoration is higher in population A than in population B. A comparison of the G0 and G1 generations did not allow to predict any detectable temporal change in population A, whereas the frequency of females in population B has been predicted to decrease quickly, probably due to a rapid invasion of restorer genes. We suggest that pollen flow is an important factor which determines the rate of the dynamics of male sterility during the life-span of a population.