Effects of geographic distance, sea barriers and habitat on the genetic structure and diversity of all-hybrid water frog populations

Abstract

The history of population size and migration patterns leaves
its mark in the genetics of populations. We investigate the
genetic structure of the edible frog, Pelophylax esculentus in the Danish archipelago and adjacent countries. This frog is of particular interest because it is a hybrid that, in this area, forms all-hybrid populations of diploid (LR) and triploid (LLR and LRR) genomotypes with no (or very few) adults of the parental species (LL and RR). This study is the first to cover the entire geographic range of Danish, Swedish and German all-hybrid populations, documenting their extent and providing a broad picture of their diversity of neutral genetic markers and genomotype proportions. With 18 microsatellite markers, we found that genetic diversity declines northwards in agreement with the glacial refuge and central-marginal hypotheses; however, populations on small and mediumsized islands are no less diverse than those on large islands and continental peninsulas. Isolation by distance exists across the
archipelago with limited influence of fragmentation by brackish seawater. The extremely low genetic diversity in all-hybrid populations, compared with adjacent populations, may be responsible for the maintenance of their special breeding system. We also show large variation among ponds in
proportions of LLR, LR and LRR genomotypes, but little
geographic pattern in their distribution. Instead, we found
relationships between the genomotype proportions and some
of 15 habitat parameters monitored. Body size differences
among LLR, LR and LRR further suggest ecological
differences.