Population genetics of some species of Potamogeton L.

Selected Potamogeton taxa were studied to establish the levels of gene flow within species, and to test the identity of putative hybrids. 390 individuals from twelve British populations of the anemophilous aquatic Potamogeton coloratus were analysed using starch gel electrophoresis of isozymes. Low levels of variability were found, with evidence of considerable inbreeding and / or clonal spread. Only two of the populations sampled are polymorphic; both inhabit sites with a long post-glacial history as wetlands. Populations of recent origin, as well as some of older vintage, contain only a single multi-locus isozyme genotype. Evidence for a duplicated IDH locus is presented. A further 647 individuals of Potamogeton coloratus from 60 ditches in the Gordano Valley, Somerset were analysed for variation at two polymorphic PGM loci. High levels of partitioning of genetic variation between ditches was observed with FST=0.575. A population of pondweeds from the River Stour in Dorset intermediate in morphology between Potamogeton natans L. and P. nodosus Poir. is shown by means of isozyme evidence to be the hybrid P. x schreberi G. Fisch. It is represented by a single multi-enzyme phenotype which, together with its sterility, suggests it reproduces vegetatively and may well constitute a single clone. It is genetically distinct from the morphologically similar hybrid between P. lucens L. and P. natans (P. x fluitans Roth). Genetic variation in Potamogeton pectinatus and P. filiformis was studied using isozymes. The overall levels of variability are similar to some other well-studied hydrophilous species, as well as to the average for terrestrial clonal species. Variation is shown to be distributed more between than within populations. Isozyme evidence supports the hypothesis that plants morphologically intermediate between Potamogeton pectinatus and P. filiformis are hybrids of these two species. Variation in enzyme banding patterns suggests that this hybrid has arisen on at least eleven occasions. In a review of molecular population genetic studies of aquatic plants, no significant difference was found between the levels of genetic diversity in aquatic plants compared to that found in clonal terrestrial plants. Many populations of aquatic plants do however show high levels of subdivision between populations.