Genetic structure of Polytrichum formosum in relation to the breeding system as revealed by microsatellites

Microsatellite variation was determined for three Danish and three Dutch po pulations of the haploid moss species Polytrichum formosum to gain insight into the relative importance of sexual vs. asexual reproduction for the amo unt and structure of genetic variation. In general, low levels of microsate llite variation were observed within this species. Even when estimated for polymorphic loci only, the levels of microsatellite variability (P=90.6, A = 4.3 and H-s = 0.468) within populations were on average lower than those reported for most other plant species. In contrast, genotypic diversity was high within each of the examined populations, indicating that sexual repro duction is a very important determinant of the genetic structure of P. form osum within populations. In agreement with previous findings; for allozyme data, no significant genetic differentiation (F-ST = 0.028 R-ST = 0.015) wa s observed neither between populations nor between regions approximately 45 0 km apart (Denmark vs. the Netherlands). These low levels of population di fferentiation observed for bath types of genetic markers are probably best explained by a high level of effective spore dispersal (gene flow) between populations. Therefore, also on a large geographical scale sexual reproduct ion is the most important determinant of the gene-tie structure of P. formo sum, despite the high potential to reproduce clonally.