Consequences of prolonged clonal growth on local and regional genetic structure and fruiting success of the forest perennial Maianthemum bifolium

The balance between clonal propagation and sexual reproduction varies among species. Although theory predicts an impact of clonal growth on both- within- and between population genetic structure, most empirical evidence available to date does not reveal sharp differences between sexually reproducing and clonal species. This has been attributed mainly to the fact that even low levels of sexual recruitment can maintain high levels of genetic diversity. Here we study the effects of prolonged clonal growth and very low rates of sexual recruitment on the genetic structure of the perennial Maianthemum bifolium, an outcrossing understorey species of temperate forests. Average genotypic diversity (0.37) of the populations, as revealed by AFLP, was above the average values reported for species of similar characteristics, but some populations were extremely poor in genotypes. Fruiting success was positively correlated with genotypic diversity, probably as a result of shortage in mating types and compatible pollen in populations poor in genotypes. This was confirmed by a pollination experiment. Fruiting success increased by a factor three when individuals were hand-pollinated with pollen from a nearby population compared to hand-pollinations with pollen from the own population. Furthermore, the fruiting success after natural pollination (control individuals) was positively related to number of nearby populations which could act as pollen sources. Given the limited colonization capacity of the species (no seed flow), and the long time since fragmentation of the forest fragments studied, between-population genetic differentiation was relatively low (Phi(st)=0.14). Lack of genetic drift due to long generation times and very limited sexual recruitment is probably responsible for this. Our results show that prolonged clonal growth and lack of sexual recruitment may affect within- and between- population genetic structure and the capability for sexual reproduction.