A taxonomic reevaluation of North AmericanDaphnia(Crustacea: Cladocera). III. TheD.catawbacomplex

Despite the importance of Daphnia in freshwater zooplankton assemblages, species boundaries in the genus are unclear. This study verifies the taxonomic validity of D. catawba by establishing its genetic divergence from other species of Daphnia that occur in eastern North America. In addition, it reveals the presence of a second, closely allied species, D. minnehaha, which had previously been placed in synonomy with D. pulex. Daphnia catawba and D. minnehaha share a preference for acidic habitats and are restricted to the deciduous and boreal forest regions of the eastern portion of the continent, where D. catawba is restricted to lakes, while D. minnehaha occurs in ponds. Both species reproduce by cyclic parthenogenesis and, based on the extent of their allozyme differentiation, last had a common ancestor more than 7 million years ago. Populations of D. minnehaha fall into two genetic clades; those from the Great Lakes watershed are morphologically divergent and have much lower levels of genotypic diversity than those from eastern Canada and the New England states.

A taxonomic reevaluation of North American Daphnia (Crustacea: Cladocera). II. New species in the Daphnia pulex group from the south-central United States and Mexico

Although the establishment of species boundaries in the genus Daphnia is complicated by the prevalence of interspecific hybrids and by phenotypic plasticity, genetic studies can resolve these complexities. This investigation employed allozyme analyses to critically assess species boundaries in members of the Daphnia pulex group from the south-central United States and Mexico. These studies demonstrated the occurrence of three common Nearctic species (obtusa, pulex, pulicaria), but also revealed the occurrence of three previously unrecognized taxa (cheraphila, pileata, prolata). All of these newly described species have their distributional centroid in this region of North America and are restricted to clay-water habitats. F1 hybrids were detected between three pairs of species (cheraphila × prolata, obtusa × pileata, pulex × pulicaria), but only the latter hybrids were common. The discovery of daphniid taxa endemic to this region of North America contrasts with the results of a broader survey of sites in Canada, and suggests that additional species await description from other unglaciated regions of North America.

A taxonomic reevaluation of North American Bosminidae

Prior morphological studies have led to the recognition of six species in the genus Bosmina from North America. The present study employed allozymic and morphological analyses to ascertain the taxonomic diversity of bosminids from 72 North American habitats. The results suggest the occurrence of 10 species, including representatives from four recognized subgenera. Populations assignable to Bosmina (Bosmina) longirostris s.l. were rare, but two newly described species in the subgenus Sinobosmina were common in the eastern half of the continent. Four species of the subgenus Eubosmina were detected, one of which is newly described, while a second species new to North America apparently represents a recent invasion from Eurasia. Two species of the subgenus Neobosmina were found in lakes from the northeastern portion of the continent, while a third species was restricted to southern sites.

A taxonomic reevaluation of North American Daphnia (Crustacea: Cladocera). I. The D. similis complex

Allozyme analysis of 28 populations of Daphnia similis s.l. from the western United States and Canada indicated the presence of three different species. One taxon, D. salina, is newly described and a second species, D. exilis, previously synonymized with D. similis, is also shown to be distinct. Species recognition is not solely reliant on allozyme analysis, as morphological differences among the species are conspicuous. Daphnia salina is a halophile, while the other two species occur in habitats of lower but variable salinity. All three taxa appear to reproduce by cyclic parthenogenesis, although levels of genetic diversity were so low in D. salina that determination of its breeding system was tentative. Genetic distance analysis suggested that D. salina diverged from the other two species some 27 million years ago, whereas D. exilis and D. similis separated 9 million years ago.

Patterns of genetic diversity in the sumac gall aphid, Melaphis rhois

In contrast to prior work on aphids, which has focused on pest species, the present study involved the analysis of genetic diversity in Melaphis rhois, a species that employs sumac and moss as alternate hosts. The life cycle involves a spring migration to sumac, with each sexual female producing a single offspring, which elicits a gall and subsequently proliferates clonally within it. Allozyme surveys at 52 sites in Ontario and Quebec established the absence of migration among galls and indicated that this species shares the low level of genetic diversity found in pest aphids. Genotypic frequencies in local populations of M. rhois showed frequent heterozygote deficits, suggesting the prevalence of inbreeding. Gene frequency divergence among populations was exceptionally pronounced, providing further evidence that gene flow was restricted. The unusual genetic attributes of this species suggest the important interaction among host-plant characteristics and the dispersal ability of aphids and their genetic systems.Key words: aphid, Melaphis, parthenogenesis, allozyme, cyclic parthenogen.

Demography and ecological impacts of the invading mollusc Dreissena polymorpha

The bivalve mollusc Dreissena polymorpha colonized Lake St. Clair in 1986 as a result of ballast water discharge. The present study shows that since initiating reproduction in 1988, its population density at all of the sites originally colonized has increased more than a 1000-fold. The species has, as well, expanded its range in Lake St. Clair dramatically. The density and range increase is not yet complete, as populations at most sites remain dominated by juveniles. Densities in excess of 5000 individuals/m2 seem likely to be achieved at most sites in Lake St. Clair within 5 years. It appears likely that D. polymorpha will have a significant impact on composition of the benthos. Shells of individual unionids carry in excess of 10 000 D. polymorpha, and lipid reserves of such unionids are only half those of unaffected individuals. Aside from direct interactions with benthic organisms, the filtering activity of D. polymorpha populations may be sufficient to result in a significant impact on the turbidity of water exiting Lake St. Clair.

Breeding system and genome size of the rhabdocoel turbellarian Mesostoma ehrenbergii

Rhabdocoel turbellarians, such as Mesostoma ehrenbergii, are hermaphrodites which produce both subitaneous and resting eggs. Both genotypic frequencies in natural populations as well as parent–offspring genotypes at a polymorphic allozyme locus confirm that, under natural conditions, M. ehrenbergii produces subitaneous eggs via self-fertilization and resting eggs probably by outcrossing. Chromosome counts show, contrary to an earlier report, that North American populations of the species possess the same chromosome number as European populations. However, other karyotypic differences do exist between Old World and New World populations. DNA quantification studies revealed that M. ehrenbergii possesses one of the largest haploid genome sizes (14.8 pg) yet reported for an invertebrate. Among organisms showing similar genome size, this species appears to be unique for its rapid developmental rate.Key words: Turbellaria, allozyme, breeding system, genome size, cytogenetics, selfing.

Miniature genomes and endopolyploidy in cladoceran crustaceans

The haploid genome sizes (0.37 and 0.47 pg) of two members of the cladoceran crustacean genus Daphnia rank among the smallest known for Crustacea. An examination of cladoceran somatic tissues by scanning microdensitometry revealed abundant endopolyploidy in both species. Although cells in the labrum possessed the highest DNA content (1024C), endopolyploid cells (4–512C) were widely distributed throughout the body. Daphnia pulex and D. magna exhibited similar ploidy levels in most tissues, but differences between the two species were noted in the epidermis and labrum. The prevalence of polyploid nuclei suggests that endopolyploidy is an important process in organisms whose genomes have been miniaturized by nucleotypic selection.Key words: somatic polyploidy, genome size, Daphnia, ploidy shifts, macroevolution.