Morphological evolution and genetic differentiation in Daphnia species complexes

Despite many ecological and evolutionary studies, the history of several sp ecies complexes within the freshwater crustacean genus Daphnia (Branchiopod a, Anomopoda) is poorly understood. In particular, the Daphnia longispina g roup, comprising several large-lake species, is characterized by pronounced phenotypic plasticity, many hybridizing species arid backcrossing. We stud ied clonal assemblages from lakes and ponds comprising daphnids from severa l species complexes. In order to reveal patterns of reticulate evolution an d introgression among species, we analysed three data sets and compared nuc lear, mtDNA and morphological divergence using animals from 158 newly estab lished clonal cultures. By examining 15 nuclear and II mitochondrial (12S/1 6S rDNA) genetic characters (allozymes/restriction enzymes), and 48 morphol ogical traits, we found high clonal diversity and discontinuities in genoty pic and morphological space which allowed us to group clones by cytonuclear differentiation into seven units (outgroup D. pulex). In contrast to six g roups emerging from nuclear divergence (related to three traditional specie s, D. cucullata D. galeata, D. hyalina and three pairwise intermediate hybr ids), a seventh group of clones was clearly resolved by morphological diver gence: distinct mtDNA haplotypes within one nuclear defined cluster, 'D. hy alina', resembled traditional D; hyalina and D. rosea phenotypes, respectiv ely. In other nuclear defined clusters, association between mtDNA haplotype and morphology was low, despite hybridization being bidirectional (recipro cal crosses). Morphological divergence was greatest between young sister sp ecies which are separated on the lake/pond level, suggesting a significant role for divergent selection during speciation along with habitat shifts. P hylogenetic analyses were restricted to four cytonuclear groups of crones r elated to species. mtDNA and nuclear phylogenies were consistent in low gen etic divergence and monophyly of D. hyalina and D. rosea Incongruent patter ns of phylogenies and different levels of genetic differentiation between t raditional species suggest reticulate evolutionary processes.