Genetic diversity in peripheral and subcentral populations of Corrigiola litoralis L. (Illecebraceae)

Genetic diversity and differentiation were studied in Corrigiola litoralis L., an annual plant species growing on seasonally flooded river banks. Plant species that are restricted to river systems may consist of highly isolated populations. For this species, pronounced genetic differentiation among rivers was expected. Plants were sampled from the river Loire (France) representing subcentral populations and the rivers Rhine, Weser and Elbe (Germany) representing peripheral ones. Allozyme electrophoresis revealed 17 putative loci in II enzyme systems. at the species level, percentage polymorphic loci, mean number of alleles, observed heterozygosity and expected heterozygosity were P=29%, A=1.5 +/- 0.2, H-o=0.007 +/- 0.005 and H-e=0.065 +/- 0.035, respectively. Peripheral populations were smaller in number and showed decreased levels of genetic diversity relative to central populations. Corrigiola litoralis was highly inbreeding as indicated by a mean F-IS of 0.755. Genetic differentiation among populations was high with a mean F-ST-value of 0.585. Hierarchical F-statistics revealed that genetic variability was partitioned at 57% among sites, 52% among countries and 11% among sites within countries. Genetic distances between French and German populations were 0.08, indicative of considerable differentiation at the intraspecific level. The overall low level of allozyme diversity is attributed to the breeding system and to habitat conditions homogenized by regular flooding. The decrease in diversity from subcentral to peripheral populations is considered to be a result of drift and founder effects during postglacial recolonization. Peripheral populations were characterized by a single fixed allele at locus IDH, thus representing an evolutionarily significant unit.