Geographical cline in breeding systems and ploidy levels in European populations of Daphnia pulex

Abstract

Ninety populations of Daphnia pulex from sites throughout the UK and Scandinavia were assayed for genetic variation at 11 variable enzyme loci. A latitudinally-related cline in both breeding system and ploidy level was observed. Southern populations (latitudes 52-59°N) comprised only diploid cyclic parthenogens. Northern populations (latitudes 68-71°N) were dominated by polyploid obligate parthenogens and diploid cyclic parthenogens were absent. An assemblage of populations from a site of intermediate latitude (61°N) included both diploid cyclic parthenogens and diploid obligate parthenogens. This cline parallels one previously described from temperate to arctic North America. The obligately parthenogenetic (asexual) clones were generally heterozygous at one or more loci for a common D. pulex allele plus an allele not found in cyclically parthenogenetic (sexual) populations of D. pulex: it is argued that these asexual clones arose polyphyletically following several independent hybridizations between D. pulex and an unrecognized but closely related taxon. Fifteen different asexual clones were detected and 25 of 28 asexual populations were uniclonal: the mean number of asexual clones per population was 1.21. © The Genetical Society of Great Britain.