Population size affects genetic diversity and fine-scale spatial genetic structure in the clonal distylous herb Menyanthes trifoliata

• The phalanx type of clonal growth dominates in the studied populations.
• Genetic diversity decreases with decreasing genet population size.
• Ramet fine-scale spatial genetic structure is high and affected by clonality.
• Fine-scale spatial genetic structure is stronger in small populations.

Small populations often show a low degree of genetic diversity. In distylous clonal plants, genetic diversity may also be influenced by other factors, such as floral morph aggregation, clonality and spatial genetic structure. Nevertheless, the relationships between population size, genetic diversity and spatial genetic structure are poorly understood. We tested for the effects of population size on genetic diversity and fine-scale spatial genetic structure (FSGS) in the clonal, distylous aquatic herb Menyanthes trifoliata. To investigate genetic diversity and structure, we used microsatellite markers and genotyped a total of 510 ramets from nine populations in southern Belgium. The populations showed a relatively high degree of genetic diversity (HEc = 0.80; A[12] = 4.45), even if both expected heterozygosity and allelic richness significantly decreased with decreasing population size (genet numbers). Populations were significantly genetically differentiated (FST = 0.14), indicating reduced gene flow among populations. Morph ratio for genets did not deviate from 1:1. The phalanx type of clonal growth dominated in the studied populations. Ramet FSGS (which was strongly affected by clonality) was significant in all populations. Sp statistic significantly decreased with increasing population size, indicating higher FSGS in smaller populations. Consequently, small populations may suffer from a higher rate of geitonogamous pollination (between ramets of the same genet). Since M. trifoliata is a strongly self-incompatible herb, no inbred seeds can be produced from self- (and geitonogamous) pollination. In this case, reproductive success of small and more spatially structured populations of M. trifoliata may be reduced due to clogging of stigmas with incompatible pollen. We conclude that understanding genetic diversity and fine-scale clonal structure is crucial for conservation of self-incompatible plant species with extensive clonal propagation.