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Several studies are conducted to explain the molecular characteristics and diversity observed in the Daphnia parasite populations found in the European lakes. The hidden diversity and the parasite sequence variation observed in these populations are also discussed.

Background In natural communities of cyclical parthenogens, rapid response to environmental change is enabled by switching between two reproduction modes. While long periods of asexual reproduction allow some clones to outcompete others, and may result in "clonal erosion", sexual reproduction restores genetic variation in such systems. Moreover, sexual reproduction may result in the formation of interspecific hybrids. These hybrids can then reach high abundances, through asexual clonal reproduction. In the present study, we explored genetic variation in water fleas of the genus Daphnia. The focus was on the short-term dynamics within several clonal assemblages from the hybridizing Daphnia longispina complex and the impact of gene flow at small spatial scales. Results Daphnia individuals belonged either to the parental species D. galeata and D. longispina, or to different hybrid classes, as identified by 15 microsatellite markers. The distribution and genotypic structure of parental species, but not hybrids, corresponded well with the geographical positions of the lakes. Within parental species, the genetic distance among populations of D. galeata was lower than among populations of D. longispina. Moreover, D. galeata dominance was associated with higher phosphorous load. Finally, there was no evidence for clonal erosion. Conclusions Our results suggest that the contemporary structure of hybridizing Daphnia communities from ten nearby lakes is influenced by colonization events from neighbouring habitats as well as by environmental factors. Unlike the parental species, however, there was little evidence for successful dispersal of hybrids, which seem to be produced locally. Finally, in contrast to temporary Daphnia populations, in which a decrease in clonal diversity was sometimes detectable over a single growing season, the high clonal diversity and lack of clonal erosion observed here might result from repeated hatching of sexually produced offspring. Overall, our study provides insights into spatio-temporal dynamics in a hybridizing Daphnia species complex in a recently established lake system, and relates genetic similarities of populations to a scenario of secondary invasion enhanced by environmental factors. [ABSTRACT FROM AUTHOR]

Despite many ecological and evolutionary studies, the history of several species complexes within the freshwater crustacean genus Daphnia (Branchiopoda, Anomopoda) is poorly understood. In particular, the Daphnia longispina group, comprising several large-lake species, is characterized by pronounced phenotypic plasticity, many hybridizing species and backcrossing. We studied clonal assemblages from lakes and ponds comprising daphnids from several species complexes. In order to reveal patterns of reticulate evolution and introgression among species, we analysed three data sets and compared nuclear, mtDNA and morphological divergence using animals from 158 newly established clonal cultures. By examining 15 nuclear and 11 mitochondrial (12S/16S rDNA) genetic characters (allozymes/restriction enzymes), and 48 morphological traits, we found high clonal diversity and discontinuities in genotypic and morphological space which allowed us to group clones by cytonuclear differentiation into seven units (outgroup D. pulex). In contrast to six groups emerging from nuclear divergence (related to three traditional species, D. cucullata, D. galeata, D. hyalina and three pairwise intermediate hybrids), a seventh group of clones was clearly resolved by morphological divergence: distinct mtDNA haplotypes within one nuclear defined cluster, ‘D. hyalina’, resembled traditional D. hyalina and D. rosea phenotypes, respectively. In other nuclear defined clusters, association between mtDNA haplotype and morphology was low, despite hybridization being bidirectional (reciprocal crosses). Morphological divergence was greatest between young sister species which are separated on the lake/pond level, suggesting a significant role for divergent selection during speciation along with habitat shifts. Phylogenetic analyses were restricted to four cytonuclear groups of clones related to species. mtDNA and nuclear phylogenies were consistent in low genetic divergence and monophyly of D. hyalina and D. rosea. Incongruent patterns of phylogenies and different levels of genetic differentiation between traditional species suggest reticulate evolutionary processes. [ABSTRACT FROM AUTHOR]

Recent genomic data suggest that the role of hybridization in evolution might be more important than previously assumed. Here, we examine species-specific differentiation and signatures of reticulation in a multi-species complex of the crustacean genus Daphnia. We did a combined examination of mtDNA, allozymes and ITS1 (a part of the nuclear ribosomal DNA) in the Daphnia longispina group. We focused on the sequence variation of ITS1 in two unambiguous species (D. galeata, D. cucullata) and two ecotypes hyalina and rosea within the recently revised taxon D. longispina (O.F. Müller). We found two ITS1-types (S, L) and intra-individual and intra-specific polymorphisms. Another ITS1-type (XL) was restricted to the outgroup D. umbra. S was present in all taxa but occurred as only two variants. Surprisingly, D. galeata and D. cucullata, which were well differentiated by mtDNA and allozymes, were virtually indistinguishable with respect to S-ITS1 (S(cg)). The two ecotypes of D. longispina shared the second S-ITS1-variant (S(rh)) and were therefore almost indistinguishable for all types of molecular markers surveyed. The L-type differed between D. galeata and D. longispina samples, but L was absent in D. cucullata. Between hyalina and rosea ecotypes, we found some L-differentiation. Combined data suggest that reticulate evolution enabled the spread of one S-ITS1-variant (S(cg)) beyond species boundaries and that S-introgression was species-specific, despite contemporary hybridization between all species. Our data have implications for phylogenetic as well as phylogeographic surveys. Because of the dynamic impact of gene flow in multi-species complexes, misinterpretations of presumed species-specific data should be considered., (2009 Wiley-Liss, Inc.)