SEX-RATIO IN POPULATIONS OF SILENE OTITES IN RELATION TO VEGETATION COVER, POPULATION-SIZE AND FUNGAL INFECTION

In contrast to populations of most dioecious Silene species (which usu ally are female-biased), populations of Silene otites have been freque ntly reported to be male-biased. We describe sex ratio variation in 34 natural S. otites populations in Central Germany in relation to veget ation cover, population size and fungal infection. The overall sex rat io was unbiased in 1994 and only slightly male-biased in 1995. Sex rat io Varied among the populations from 26.6 % to 72.6 % females. The sex ratio of small populations varied strongly due to stochastic processe s. Furthermore, we found that populations in habitats with high vegeta tion cover contained a higher percentage of females. Hermaphroditic pl ants, theoretically, could increase male bias as they only produce mal e or hermaphroditic offspring. Their frequency in the populations, how ever, was far too low to affect sex ratio. In 1994 12.1 % and in 1995 17.0 % of the plants were infected by the smut fungus Ustilago major. Disease incidence in the population was not related to sex ratio, sugg esting equal susceptibility of males and females. The sex ratio of par tially infected plants did not deviate from the population sex ratio, both under field conditions and in a greenhouse laboratory experiment. The results suggest that the frequently reported male bias in Silene otites populations is not a general pattern, but is mainly caused by e nvironmental conditions.