Identification of general patterns of sex-biased expression in Daphnia, a genus with environmental sex determination

Daphnia reproduce by cyclic-parthenogenesis, where phases of asexual reproduction are intermitted by sexual production of diapause stages. This life cycle, together with environmental sex determination, allow the comparison of gene expression between genetically identical males and females. We investigated gene expression differences between males and females in four genotypes of Daphnia magna and compared the results with published data on sex-biased gene expression in two other Daphnia species, each representing one of the major phylogenetic clades within the genus. We found that 42% of all annotated genes showed sex-biased expression in D. magna. This proportion is similar both to estimates from other Daphnia species as well as from species with genetic sex determination, suggesting that sex-biased expression is not reduced under environmental sex determination. Among 7453 single copy, one-to-one orthologs in the three Daphnia species, 707 consistently showed sex-biased expression and 675 were biased in the same direction in all three species. Hence these genes represent a core-set of genes with consistent sex-differential expression in the genus. A functional analysis identified that several of them are involved in known sex determination pathways. Moreover, 75% were overexpressed in females rather than males, a pattern that appears to be a general feature of sex-biased gene expression in Daphnia.Short summaryIn some species with environmental sex determination, gene expression can be compared between genetically identical males and females. Here, we investigated sex-biased expression in one such species, D. magna, and compared it with data from two congeners. We found that all three species have a common set of 675 genes with consistent differential expression and with a strong bias towards overexpression in females rather than males. Moreover, the proportion of sex-biased genes in each of the three Daphnia species was similar to Drosophila species with genetic sex determination, suggesting that sex-biased expression is not necessarily reduced under environmental sex determination.

Diversity of the genus Daphniopsis in the saline waters of Australia

Although members of the cladoceran genus Daphniopsis form a dominant element of the fauna in the saline inland waters of Australia, their taxonomy has been in flux. In this study allozyme analysis was employed to examine the diversity, distributions, and reproductive biology of species in this genus. The results establish that D. pusilla, a species formerly thought to be widespread, is restricted to Western Australia, while a newly described species, D. truncata, which shares the attribute of producing a one-egged ephippium, is broadly distributed. The results of this study verify the taxonomic validity of the three recognized species of Daphniopsis, which produce two-egged ephippia, but another member of this group, D. wardi, is described from Western Australia. All populations were found to reproduce by cyclic parthenogenesis, except for one obligately asexual population of D. pusilla × D. truncata hybrids. No other case of hybridization was detected, although two species co-occurred in 15% of habitats. The six species of Daphniopsis now known from Australia appear to represent another example of an endemic radiation in the saline lakes of this continent.

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

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). 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.

ENZYME POLYMORPHISM AND CYCLIC PARTHENOGENESIS IN DAPHNIA MAGNA. I. SELECTION AND CLONAL DIVERSITY

ABSTRACT Genotype frequencies and fecundities were recorded over a period of two years for three polymorphic enzyme loci (Est, Mdh and Got) in a parthenogenetic natural population of Daphnia magna Straus (Crustacea: Cladocera) . There was a large excess of heterozygotes at each locus, and some nonrandom association between loci, although 29 different three-locus genotypes were detected. There were small but significant changes in genotype frequencies that did not follow any clear seasonal cycles or overall trends, and the genotypes often differed significantly in fecundity, although the direction of the difference was not constant. These fitness differences were probably not attributable to the specific loci studied.—Models of balancing selection are of two types: segregation-balanced (e.g., heterosis) and competition-balanced (e.g., frequency dependence). Only the latter type can stabilize diversity in a clonal population. The observed selection was not heterotic, but it is not certain that it was stabilizing either. Clonal competition did not lead to victory by a single, fittest clone; genotypic diversity remained high.

ENZYME POLYMORPHISM AND CYCLIC PARTHENOGENESIS IN DAPHNIA MAGNA. II. HETEROSIS FOLLOWING SEXUAL REPRODUCTION

ABSTRACT Cyclical parthenogenesis exaggerates the force of selection relative to recombination and will therefore enhance interlocus effects. Observations of electrophoretic variation in a natural population of Daphnia magna Straus (Crustacea: Cladocera) are interpreted in this light. Sexual reproduction led to Hardy-Weinberg equilibrium, but heterozygote excesses rapidly developed at each of three observed loci during subsequent parthenogenesis. Homozygote fecundity was often lower than that of heterozygotes; this may have been the cause of some of the observed frequency changes. The superior fitness of the enzyme heterozygotes does not imply that selection was necessarily acting on the enzyme loci themselves, since apparent heterosis is the expected result of linkage disequilibrium.