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Schwenk, Klaus;Ender, Andrea;Streit, Bruno
doi: 10.1007/BF00031991pmid: N/A
Abstract Despite the wealth of information on the ecology of Daphnia species, the systematics and phylogeny of the genus is still unresolved. The taxonomic uncertainties are based in part on the phenomenon of interspecific hybridization, which has been well documented for species of the D. galeata/cucullata/hyalina complexes. The occurrence of syntopic populations of up to three species and their three hybrids suggest niche differentiation, but very little is known about genetic divergence of hybridizing taxa and the potential consequences of hybridization (i.e. introgression). Since an operational species definition is necessarily based on information on the evolutionary mechanisms that result into the splitting of lineages, ecological and genetic consequences of interspecific hybridization have to be considered. In order to reveal the significance of hybridization and introgression, we combined several aspects of phylogenetic investigations within the D. galeata/cucullata/hyalina complexes. Furthermore we demonstrate how molecular markers contribute to an evaluation of species complexes.
doi: 10.1007/BF00031992pmid: N/A
Abstract The presumption of intercontinental distributions of many species of Anomopoda has been reinforced by their considerable morphological uniformity and ease of passive dispersal via ephippial eggs. To test the validity of this dogma, genetic variation among taxa in the cladoceran genus Simocephalus was examined on a continental scale. Genetic variability (percentage polymorphic loci = 8.8%, individual heterozygosity = 3.4%) was comparable to that determined on a local scale in Simocephalus and somewhat less than for other zooplankton groups. Four species complexes were distinguished allozymically with unique allelic substitutions found for at least two loci between these taxa. Eight species in North America were differentiated within these complexes (Nei s genetic distance, D>0.30), all of which were clearly separated from two European species. Both S. cf. vetulus and S. cf. serrulatus are broadly distributed in North America, and intraspecific divergence is relatively low. In contrast, S. cf. congener and S. cf. exspinosus represent species complexes, both consisting of several species, differentiated allozymically and morphologically. The concept of generalist species with moderate genetic variation throughout a broad range and specialist species, genetically depauperate, with restricted distributions, is explored.
King, Charles E.;Miracle, Maria R.;Vicente, Eduardo
doi: 10.1007/BF00031993pmid: N/A
Abstract The population structure of Daphnia longispina in Lake El Tobar, Spain was studied by measuring variation at the aldehyde oxidase (AO), phosphoglucose isomerase (PGI) and phosphoglucose mutase (PGM) loci in each of 1337 individuals from four collections. In 9 of the 12 comparisons between observed allele frequencies and those expected by Hardy-Weinberg equilibrium there was an excess of heterozygotes. We found 27 of the potential number of 54 composite electromorphs (‘clones’) based on the three allozymes. Clone diversities were rather high in all collections. Three clones reached frequencies of over 25% and different clones were dominant in each of the four collections. Strong temporal variation was found in the genetic structure of this Daphnia population. This variation was driven by changes in the relative frequencies of the component clones in the lake rather than by a recruitment of novel clones into the population. We conclude with a consideration of the role of models relating allele and genotype frequencies in populations of cyclical parthenogens. Because the breeding system of these populations infrequently involves recombination between clones, models such as the Hardy-Weinberg have limited value in providing meaningful measures of population structure.
doi: 10.1007/BF00031994pmid: N/A
Abstract Hybridization is a common phenomenon in Daphnia species complexes. Hybrids often dominate in Daphnia populations; therefore it is worthwhile to look for principal differences between parental and hybrid populations with respect to their genetic structure and clonal differentiation. We studied natural populations of members of the Daphnia galeata/hyalina/cucullata complex in three lakes. In one of these lakes, one parental species (D. galeata) and one hybrid (D. galeata × cucullata) were investigated more intensively. The frequency of sexual reproduction was higher in parental populations, whereas clonal diversity was higher in hybrid populations. Ecological differentiation among clonal groups was more pronounced in the D. galeata × cucullata hybrid compared to D. galeata, whereas selection intensity was weaker. These results are discussed with respect to stability of clonal groups, multiple hybridizations and selective constraints.
doi: 10.1007/BF00031995pmid: N/A
Abstract The life history traits of Daphnia magna were studied in laboratory experiments under freshwater and brackish (5‰ salinity) conditions. The variability of responses within and between populations was examined by comparing 11 clones from a brackish lake and 10 clones from a freshwater pond. Experimental clones were hatched from ephippia collected from the sediment and thus represent random samples of the clone banks of each population. Most clones with a high salinity tolerance were from the population of the brackish habitat, but some were also found in the freshwater population. Thus, freshwater populations appear to have the potential to invade brackish habitats. A proportion of clones from the brackish population had very low fitness (measured as er) under freshwater conditions. This unexpected result means that freshwater adaptation can be lost by the freshwater cladoceran Daphnia magna. The effects of unfavourable conditions on growth and reproduction varied among clones and were not correlated. This clonal variation in growth and reproduction indicates that the environmental sensitivities of these traits are independent. The pattern of fitness reaction norms showed no trade-off between fitness under brackish and under freshwater conditions for either population. Thus, euryhaline generalists should be favoured in habitats with salinity fluctuations between freshwater and brackish conditions.
doi: 10.1007/BF00031996pmid: N/A
Abstract The first mention of the chydorids in the literature was by O.F. Müller in 1769, although he did not use any names for the taxa that he described. The first century after O. F. Müller's first paper was concerned almost exclusively with the cladocerans of Europe. The second century saw European scientists looking at cladocerans from other continents and frequently placing the European names on them. Species had not been described adequately, and the persons looking at the non-European taxa seldom made any detailed comparisons to document their claim of conspecificity. Consequently, the names became distributed everywhere for look-alike taxa, and scientists routinely regarded the chydorid anomopods as cosmopolitan. During the past two decades, scientists are finding through detailed comparisons, that cognate taxa of chydorids on different continents, or even on the same continent, usually are not the same. Attitudes are shifting away from cosmopolitanism (presumably brought about by the passive dispersal of resting eggs), to vicariant events as one of the major mechanisms affecting the evolution of the chydorids and involved in controlling their distributions.
doi: 10.1007/BF00031997pmid: N/A
Abstract The distinctness of the Anomopoda and the polyphyletic nature of the so-called Cladocera are emphasized. An attempt is made to reconstruct the ancestral anomopod, which probably lived in Palaeozoic times. This task is facilitated by the availability of detailed information on extant forms, which includes functional as well as purely morphological considerations and enables us to understand the means whereby complex mechanisms were transformed during evolution. Comparative studies on the ecology and habits of extant forms also throw light on the probable way of life of the ancestral anomopod. Adaptive radiation within the Anomopoda is briefly surveyed and an outline of the suggested phylogeny of the order is indicated.
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