Historical museum specimens reveal the loss of genetic and morphological diversity due to local extinctions in the endangered water chestnut Trapa natans L. (Lythraceae) from the southern Alpine lake area

Abstract

Freshwater aquatic plants have increased extinction risks due to strong human pressure and the patchy nature of their habitats. However, their unclear population structure frequently hinders conservation planning. To investigate population subdivision and risks to long-term survival of such species, we combined genetic, morphometric and biogeographical approaches to the example of the endangered water chestnut Trapa natans (Lythraceae) from the southern Alpine lake area (Insubria). Amplified fragment length polymorphism (AFLP) of seven extant local stands revealed similar and intermediate levels of genetic diversity, of which c. 97% was partitioned within lakes. Thus, no signs of strong genetic drift and associated loss of genetic diversity were found, despite a reduction of c. 52% of local populations since the early 19th century. Nuclear ribosomal sequences (ITS1-5.8S rRNA-ITS2) combined with a morphometric study of nuts (based on fresh and historical museum material) revealed the presence of two genetically and morphologically slightly distinct lineages, one of which went extinct during the 20th century after a prolonged period of hyper-eutrophication. Taken together, our results indicate the current presence of one large Insubric Trapa population. To prevent genetic risks to survival associated with small population size and increasing fragmentation due to past extinctions, freshwater managers should preserve the large census sizes still present in many Insubric lakes and reduce eutrophication.