Negative effect of inbreeding on fitness of an arctic–alpine Hieracium alpinum (Asteraceae), a species with a geographical parthenogenesis distribution pattern

Decreased fitness due to inbreeding in small populations can be a considerable disadvantage in sexually reproducing organisms compared to their asexual relatives. Therefore, inbreeding depression of sexuals could contribute to geographical parthenogenesis (GP) distribution patterns, when sexuals have more restricted distribution than their asexual relatives. In this study, we tested whether biparental inbreeding will lead to decreased fitness in sexual diploids of Hieracium alpinum , a species showing a GP distribution pattern. We performed three types of crosses (inbred, outbred, and interpopulation outbred) and measured eleven fitness traits. A subset of plants was also grown with a natural competitor Nardus stricta to increase any negative effects of inbreeding. We found significant effects of inbreeding in two out of eleven measured traits in the F1 generation. Initial biomass was statistically significantly lower by 9.6% in inbred lineages compared to outbred lineages. Similar trends, i.e., lower values in inbred progeny when compared to outbred one, were found also in final biomass and survival rate (values decreased in inbred progeny by 17.5% and 11.5%, respectively), but these differences were statistically nonsignificant. However, the negative effect of inbreeding on these traits was not increased by the presence of Nardus stricta as a competitor. Our results suggest that biparental inbreeding could to some extent explain a lowered colonizing ability of sexual diploid of H. alpinum , but the GP pattern observed is also likely to be influenced by other processes and their interactions.