Genotypic diversity and variation in mode of reproduction among populations in the Daphnia pulex group

Abstract

A survey of nine ponds in Illinois and Iowa confirmed the occurrence of both cyclically and obligately parthenogenetic D. pulex and cyclically parthenogenetic D. obtusa. The three taxa co-occurred in two of the ponds and one predominantly cyclical parthenogenetic population of D. pulex also contained an obligately parthenogenetic clone. Three populations consisted solely of obligately parthenogenetic D. pulex, while three consisted solely of cyclically parthenogenetic D. obtusa. Populations of cyclically parthenogenetic individuals, which have a sexual phase, were usually in Hardy–Weinberg equilibrium at individual loci and had a high genotypic diversity. Populations of obligately parthenogenetic individuals deviated from Hardy–Weinberg equilibrium and often consisted of only one or two clones. Mode of reproduction was confirmed by breeding studies on individuals isolated from the populations. Daphnia pulex and D. obtusa were genetically distinct based on variation at six polymorphic loci. Individuals of D. pulex reproducing by the two modes of reproduction were also genetically differentiated. Obligately parthenogenetic individuals were Ldh 13 and Pep 12 heterozygotes while cyclically parthenogenetic individuals were 11 homozygotes at both loci. Seven obligately parthenogenetic clones were found with only one clone occurring in more than one population. The high genotypic diversity and restricted distribution of obligately parthenogenetic clones is consistent with their independent origin from cyclically parthenogenetic individuals.