Variable herkogamy and autofertility in marginal populations ofGentianella germanica in the Netherlands

Abstract

Gentianella germanica (Willd.) Borner, a rare plant species, occurs at the margin of its distribution area in the Netherlands. It is found only in isolated patches of chalk grassland surrounded by intensively-used agricultural land. In 1997 and 1998, we investigated the spatial and temporal separation of anthers and stigma in relation to autofertility. We performed our experiments in one large and two small populations to investigate the possible selection against herkogamy and for increased autofertility in marginal populations with pollination limitation. Dichogamy appeared to be very weak and varied within individuals. Homogamous and protandrous flowers were most frequent. Dichogamy is apparently no effective barrier against self-fertilization in this species. Herkogamy varied significantly between individuals and between populations, and plant and flower size were not correlated with anther-stigma separation. Both observations suggest a genetic basis for herkogamy. Autofertility was generally high in all three populations. Only in one small population was a correlation between autofertility and herkogamy found. In this population, most plants had the stigma positioned above the anthers (mean herkogamy +0.8 mm), while in the other two populations the stigma was positioned mainly in between or below the anthers (mean herkogamy −0.55 mm). In comparison with earlier data from 1991 and 1992, plants in the large population were less herkogamous and more self-fertile in 1998. Since 1993 this population was mown each year one month earlier than usual, leading to dramatic reductions (bottlenecks) in population size. In addition, the early mowing time may have caused a further reduction of the already scarce visitation by pollinating insects. We hypothesize that both adverse conditions may have caused a selection of plants with smaller anther-stigma separation and higher autofertility and thus showing that human influence can have profound effects on the reproductive success and evolution of rare plant species, even if it occurs in the context of nature conservation.