Effects of crossing distance on components of fitness in two rare rock plants.

To alleviate negative effects of genetic drift on rare plants, it has been suggested to artificially increase gene flow between isolated populations and create new populations by mixing different provenances to maximize genetic variation. However, there is also a risk of outbreeding depression. We studied the effects of cross type on components of fitness in two closely related taxa, the rare rock plants Saxifraga rosacea ssp. rosacea (octoploid) and S. rosacea ssp. sponhemica (hexaploid), a Central European endemic. Using plants from 30 populations, we carried out within-population crosses (WPC), between-population crosses (BPC) and between-taxon crosses (BTC), and measured performance over two generations in a common garden. We also studied backcrosses (BTC × WPC). Seed yield of BPC and survival of offspring were higher than that of both WPC and BTC, indicating genetic load in the isolated rock populations of both taxa. However, BPC offspring showed outbreeding depression for reproduction, while BTC offspring was nearly completely sterile, and backcrosses hardly produced any seeds. The mass and germination of the seeds produced by the F1 and the F2, and survival of the F2 were not negatively affected by BPC and BTC. Our results suggest caution in increasing gene flow between long-term isolated populations or mixing seeds for restoration projects. The nearly complete infertility of the F1-plants resulting from BTC strongly supports the notion that management measures resulting in heteroploid crosses should be avoided. Moreover, the sterility of the hybrids suggests that the two taxa should be treated as separate species.