Terrie L. Finston

Macrogeographic patterns of breeding system diversity in the Daphnia pulex group. I: Breeding systems of Canadian populations

Efforts to deduce the evolutionary significance of sexual reproduction from the taxonomic and ecological distributions of asexuals rely on the presumption that reversions of asexual taxa to sexual reproduction are rare. There is growing evidence, however, that reversions may be likely as long as asexuals are diploid, and some sexual populations persist. However, few systems have been studied in sufficient detail to provide insight into the nature of interactions between the sexual and asexual components of a single taxon. The cladoceran crustaceans Daphnia pulex and D. pulicaria both show variation in breeding system with some populations reproducing by cyclic and others by obligate parthenogenesis. The present study used allozymic analysis to characterize the breeding systems of both species across Canada. Although F1 hybrids were prevalent between the species and invariably reproduced by obligate parthenogenesis, each parent species showed regional variation in its breeding system. D. pulex reproduces by obligate parthenogenesis in eastern Canada but switches to cyclic parthenogenesis in the west with an abrupt transition coincident with the forest/prairie ecotone. D. pulicaria shows a divergent pattern, with populations on the priaries reproducing by obligate parthenogenesis, while those in eastern Canada and the far west are cyclic parthenogens. The results make clear that breeding systems in both taxa tend to be uniform over large areas. The discordance in breeding system employed by each species at specific sites suggests that the patterns of breeding system variation are not linked to environmental factors, but are instead a consequence of the interplay between historical factors, such as dispersion patterns from glacial refugia and the sites of origin of mutations promoting the transition to asexuality.

Macrogeographic patterns of breeding system diversity in the Daphnia pulex group from the United States and Mexico

Studies on the biogeographical patterning of reproductive systems promise to extend understanding of the factors which modulate breeding system transitions. Two closely allied cladoceran crustaceans, Daphnia pulex and D. pulicaria, show varied modes of reproduction, with populations reproducing by either cyclic or obligate parthenogenesis. Prior studies have provided a detailed understanding of their breeding system diversity in the polar and cold temperate regions of North America. The present investigation extends this analysis, characterizing breeding systems and clonal diversity at sites throughout the United States and Mexico. Genotypic diversity in these southern areas was high, but only diploids were detected, indicating that polyploids are restricted to the north. F1 hybrids and their two parental species were present in most areas, although their frequencies varied geographically. Hybrids invariably reproduced by obligate asexuality, but both parental taxa showed regional shifts in their breeding system. The complexity of these latter patterns suggests that they reflect the interplay of historical factors and selection.

Genome size variation in aphids

Abstract Genome sizes of invertebrates vary at least 1000-fold, with small genomes most common in species with rapid development. Although few groups have been studied, there is growing evidence that closely related taxa often show quantum shifts in genome size without a co-ordinate shift in chromosome numbers. The present study provides the first detailed information on genome sizes in aphids, a group of insects recognized for their rapid developmental rates. The results of scanning microdensitometry of 34 aphid species show that diploid genome sizes show a 5-fold range (0.36-1.77 pg) which is unassociated with chromosome number. Genome sizes appear to vary in a discontinuous fashion, with quanta corresponding to the basal genome size detected in this study. As genome size is positively associated with body size, quantum shifts in genome size may have led to rapid change in this important life-history character. The small genomes found in some aphid species are among the lowest yet reported for insects, reinforcing the linkage between developmental rate and genome size.

Geographical cline in breeding systems and ploidy levels in European populations of Daphnia pulex

Ninety populations of Daphnia pulex from sites throughout the UK and Scandinavia were assayed for genetic variation at 11 variable enzyme loci. A latitudinally-related cline in both breeding system and ploidy level was observed. Southern populations (latitudes 52–59°N) comprised only diploid cyclic parthenogens. Northern populations (latitudes 68–71°N) were dominated by polyploid obligate parthenogens and diploid cyclic parthenogens were absent. An assemblage of populations from a site of intermediate latitude (61°N) included both diploid cyclic parthenogens and diploid obligate parthenogens. This cline parallels one previously described from temperate to arctic North America. The obligately parthenogenetic (asexual) clones were generally heterozygous at one or more loci for a common D. pulex allele plus an allele not found in cyclically parthenogenetic (sexual) populations of D. pulex: it is argued that these asexual clones arose polyphyletically following several independent hybridizations between D. pulex and an unrecognized but closely related taxon. Fifteen different asexual clones were detected and 25 of 28 asexual populations were uniclonal: the mean number of asexual clones per population was 1.21.

Genetic variation in the onychophoran Plicatoperipatus jamaicensis

The Onychophora are a relic taxon which diverged early in the arthropod radiation and have shown little morphological differentiation for several hundred million years. Jamaica has the richest onychophoran fauna in the Caribbean and although only five species are known, they represent 7 per cent of the global fauna. The present study involved an analysis of allozyme and mtDNA diversity in Plicatoperipatus jamaicensis, an endemic species which is the commonest onychophoran on the island. The work showed that P. jamaicensis includes at least two different species. These taxa are largely allopatric, but co-occur at some sites without interbreeding. Allozyme and mtDNA analyses suggest their divergence in the early Pleistocene, following the Pliocene origin of the Plicatoperipatus lineage from a Macroperipatus ancestor. Allozyme variation in both species was unusually low, but sufficient to confirm that each taxon reproduces sexually. Mitochondrial DNA diversity was abundant in both species and indicated that local aggregations did not consist of kin groups. The mitochondrial genomes of both species were small enough to suggest that their organizations are unusual.

Population structure and gene flow in Stomion : a species swarm of flightless beetles of the Galápagos Islands

Stomion is a swarm of 13 flightless tenebrionid beetle species endemic to the Galápagos Islands. Their distribution is patchy and largely restricted to the littoral and arid zones of the archipelago. Each taxon is found on one or a few geographically close islands. Thirty-five populations representing nine of the taxa were examined at eight polymorphic enzyme loci using cellulose acetate electrophoresis to measure patterns of gene flow and investigate models of dispersal in a relatively young species group exhibiting spatially isolated populations.Genetic subdivision is high, particularly among populations of taxa which inhabit more than one island, revealing restricted gene flow and confirming the high potential for reproductive isolation among subpopulations. The mean FST across taxa was 0.30. The genetic differentiation occurring between spatially isolated populations may explain the exuberant speciation of the genus in the Galápagos. Tests of gene flow models give support to the stepping-stone model of dispersal. Because of their lack of flight wings, interisland dispersal of Stomion probably occurred by oceanic drift as pleuston or on floating debris, with individuals colonizing nearby islands more frequently than ones at a greater distance.