GEOGRAPHICAL DINE IN BREEDING SYSTEMS AND PLOIDY LEVELS IN EUROPEAN POPULATIONS OF DAPHNIA-PULEX

Ninety populations of Daphnia pulex from sites throughout the UK and S candinavia were assayed for genetic variation at 11 variable enzyme lo ci. A latitudinally-related dine in both breeding system and ploidy le vel was observed. Southern populations (latitudes 52-59 degrees N) com prised only diploid cyclic parthenogens. Northern populations (latitud es 68-71 degrees N) were dominated by polyploid obligate parthenogens and diploid cyclic parthenogens were absent. An assemblage of populati ons from a site of intermediate latitude (61 degrees N) included both diploid cyclic parthenogens and diploid obligate parthenogens. This di ne parallels one previously described from temperate to arctic North A merica. 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 o f D. pulex: it is argued that these asexual clones arose polyphyletica lly following several independent hybridizations between D. pulex and an unrecognized but closely related taxon. Fifteen different asexual c lones were detected and 25 of 28 asexual populations were uniclonal: t he mean number of asexual clones per population was 1.21.