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P D Hebert

Publications and source records attributed to P D Hebert.

30 records · Page 2Linked to original sources
PubMed1994

Mitochondrial DNA diversity in the pea aphid Acyrthosiphon pisum.

The pea aphid Acyrthosiphon pisum has been established in North America for at least a century and occurs on a broad range of host plants. Despite its importance as a crop pest, there is little understanding of the genetic structure of its populations or the extent of genetic divergence associated with different host plant utilization. This study examined the extent of mitochondrial DNA (mtDNA) diversity among 35 clones of pea aphids from alfalfa. Polymorphisms were detected at only 2 of 126 restriction sites, suggesting the same impoverished level of genetic diversity found in prior allozyme studies. However, length variation was common and apparently arose as a result of tandem repeats in two regions of the mtDNA molecule. Region 1 occurred in proximity to the control region of the molecule, while region 2 was close to a cluster of tRNA genes flanking the NAD-dehydrogenase subunit 3 gene on the opposite side of the mtDNA molecule. Each of the aphid clones was homoplasmic for a single length variant in region 1, which varied with respect to the number of copies of a 120-bp repeat. By contrast, one-third of the clones were heteroplasmic at region 2, where they possessed a variable number of copies of a 210-bp repeat. Reanalysis of clones after 30 generations of parthenogenetic reproduction established the stability of length variants over this interval, suggesting their value in studies of the genetic structure of aphid populations.

Animals↗
PubMed1993

Habitat-dependent hybrid parentage and differential introgression between neighboringly sympatric Daphnia species.

Allozyme, morphological, and PCR-generated mtDNA markers were used to analyze hybrid parentage and introgression between the neighboringly sympatric crustaceans Daphnia galeata mendotae and Daphnia rosea. mtDNA analysis of D. galeata mendotae from 15 lakes and of D. rosea from 8 lakes revealed species-specific fragment patterns resulting from TaqI and RsaI digestion. No individuals of one parent species possessed the typical mtDNA of the other parent species, suggesting that mtDNA introgression is rare or nonexistent. Hybrids from 18 lakes possessed either the mtDNA patterns of D. rosea or of D. galeata mendotae, indicating that reciprocal hybridization occurs. The mtDNA genome of the dominant parent species in a lake was overrepresented in the hybrids, suggesting that hybridization most often involves females of the common species and males of the rare species. Such a pattern is consistent with the differing importance of density to the environmental induction of males and sexual eggs in Daphnia. For the assessment of nuclear gene flow, eight sympatric populations of each parental taxon and seven allopatric populations of D. galeata were analyzed for allozyme variation at nine polymorphic loci. Our results provided evidence for asymmetric interspecific gene flow involving alleles at six loci that are unlikely to be convergent or symplesiomorphic. This reticulate evolution accounted for much of the genetic divergence between European and North American populations of D. galeata.

Animals↗
PubMed1991

Cloning and characterization of Daphnia mitochondrial DNA.

The mitochondrial genome of Daphnia pulex (Crustacea, Cladocera) was cloned as a single fragment into the plasmid vector pUC12. The genome size, estimated from restriction endonuclease fragment lengths, is 15,400 +/- 200 base pairs. The GC content, estimated from thermal denaturation studies, is 42%. The positions of 39 cleavage sites were mapped for 14 restriction enzymes. The distribution of these sites within the genome is random (P = 0.44). Heterologous hybridizations with Drosophila sylvestris mitochondrial DNA (mtDNA) probes indicate that gene orders within Daphnia and Drosophila mtDNAs are similar.

Animals↗
PubMed1987

Genotypic characteristics of cyclic parthenogens and their obligately asexual derivatives.

Among extant taxa it appears that cyclic parthenogenesis has evolved on fewer than ten occasions. Evidence suggests that these groups are all derived from bisexual ancestors with genetic systems possessing specific preadaptations for the breeding system shift. Its apparently independent origin in two cecidomyiid lineages suggests that, when such preadaptations exist, the adoption of cyclic parthenogenesis may be a highly probable event. The genetic consequences of this transition are not dramatic; the genotypic characteristics of most cyclic parthenogens are similar to those of bisexuals. Specifically, genotypic frequencies at polymorphic loci are generally in Hardy-Weinberg equilibrium and linkage disequilibrium between variants at different loci is rare. Levels of variation in local populations of cyclic parthenogens are lower than those typical of sexual outcrossers, but the lack of variation appears to be a consequence of founder effects, as regional gene pool diversity is not similarly impoverished. An unusual genotypic structure occasionally develops in populations from habitats in which sustained parthenogenetic reproduction is possible and sexual recruitment is low. Under these circumstances low fitness genotypes are winnowed from the population by natural selection. The truncation of genotypic diversity often produces Hardy-Weinberg deviations and linkage disequilibrium. The depletion of clonal diversity rarely leads to the domination of a habitat by a single clone; an array of ecologically diversified clonal variants remains. The level of clonal variation in such populations is occasionally augmented by sexual recruitment, but the persistence of clonal diversity in local populations of obligate parthenogens makes it clear that clonal coexistence is not a consequence of such recruitment. Cyclic parthenogens have made the transition to obligate asexuality with high frequency, but there is little evidence to support the argument (Williams, 1975) that such shifts result from the relaxation of the short-term selection pressures supposedly necessary to sustain the sexual phase of the life cycle. Maynard Smith (1978) has recognized that in groups such as the aphids, which have never evolved the ability to produce their diapausing eggs asexually, sexuality may simply be retained as a consequence of its role in the formation of these eggs. However, he accepted that short-term selection was required to maintain sexuality in groups such as the cladocerans in which asexual forms have demonstrated their ability to retain life cycle complexity.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1980

Enzyme variability in natural populations of Daphnia carinata king.

Fifteen populations of the cladoceran crustacean, Daphnia carinata, were examined for allozyme variation at sixteen gene loci. Individuals of this cyclic parthenogen were heterozygous at only 2.1 per cent of their loci. Large differences in gene frequencies existed among the populations. Within populations genotypic frequencies were in good agreement with Hardy-Weinberg expectations, and remained stable during annual cycles of population growth. Strong linkage disequilibrium was observed between the allelic variants at two leucine aminopeptidase loci in certain populations, suggesting that the loci are tightly linked and that populations of D. carinata are founded from few individuals.

Alleles↗
PubMed1976

Enzyme variability in natural populations of Daphnia magna. IV. Ecological differentiation and frequency changes of genotypes at Audley End.

Genotypic frequencies were analysed for two years in a permanent population of the cladoceran crustacean, Daphnia magna, which was polymorphic for an esterase and for malate dehydrogenase. Large temporal changes in genotypic frequencies occurred at both loci. There was no evidence of a seasonal pattern in the frequency changes. In most samples, genotypes at the two enzyme loci were non-randomly associated; these associations showed temporal changes. On some occasions marked spatial heterogeneity in genotypic frequencies existed within the population. Genotypic differences in parthenogenetic and sexual egg production were observed. In a primarily parthenogenetically reproducing population, non-random associations between genotypes of structural and regulatory loci will be the rule. The allozyme variants themselves may or may not be under selection. The relevance of these observations to ecological studies on Daphnia is considered.

Animals↗
PubMed1974

Enzyme variability in natural populations of Daphnia magna. II. Genotypic frequencies in permanent populations.

In permanent habitats populations of the cyclical parthenogen, Daphnia magna, reproduce by continued parthenogenesis and are subject to only sporadic sexual recruitment. The genetic effects of this breeding system have been investigated by analyzing allozyme frequencies in thirteen populations of D. magna.-Genotypic frequencies at polymorphic loci characteristically deviated markedly from Hardy-Weinberg proportions and gametic phase imbalance between loci was frequent. Genotypic frequencies were subject to violent, selectively determined oscillations over short periods of time. These observations suggest that permanent populations of D. magna ordinarily consist of a limited number of highly structured genotypes. The adaptational advantages offered by such structuring may have been a major selective factor favoring the evolution of cyclical parthenogenesis.

Animals↗
PubMed1974

Enzyme variability in natural populations of Daphnia magna. 3. Genotypic frequencies in intermittent populations.

In temporary habitats populations of the cyclical parthenogen, Daphnia magna, are re-established each year from sexual eggs and reproduce parthenogenetically for two or at most three generations. The genetic effects of this breeding system have been investigated by analyzing allozyme frequencies in nineteen intermittent populations.-Genotypic frequencies at polymorphic loci were ordinarily found to be in good agreement with Hardy-Weinberg proportions and disequilibria between loci were not observed. Although significant changes in gene frequencies were observed both during and between successive cycles, there was no evidence of the marked instability of genotype frequencies characteristic of permanent populations. The recombinational degradation of genotypes at the end of each annual cycle in temporary habitats effectively prevents the genotypic structuring which develops when continued parthenogenesis is possible.

Animals↗
PubMed1972

Inheritance during parthenogenesis in Daphnia magna.

Natural populations of Daphnia magna have been found which are polymorphic for electrophoretic variants of supernatant malic dehydrogenase, esterase, and alkaline phosphatase. Using these enzyme variants as genetic markers it has been possible to demonstrate the absence of recombination during parthenogenetic reproduction. Genetic uniformity is expected within parthenogenetic clones derived from a single female.

Alkaline Phosphatase↗
PubMedDate not supplied

Biological and physical factors affecting the body burden of organic contaminants in freshwater mussels.

Biological and physical factors affecting the body burdens of hexachlorobenzene, octachlorostyrene, and four polychlorobiphenyl (PCB) congeners in freshwater mussels from Lake St. Clair, Ontario, Canada were investigated. Specimens of Lampsilis radiata (Barnes) and the non-native Elliptio complanata (Lightfoot) were deployed for forty days at four Lake St. Clair sites, to investigate whether the water or the sediment phase represented the primary source of contaminants and to examine the effect of enclosure of the mussels on contaminant uptake. No significant differences in body burdens were detected for any of the compounds between mussels placed in corrals containing uncontaminated sand and those with natural sediment, suggesting the water phase represents the xenobiotic source. Among three enclosure types of varying restriction there were no significant differences in the body burden of each compound indicating the effect of confinement on uptake is minimal. Weak negative correlations between body burden and body length existed for all compounds, although there were no significant differences between the sexes or, with the exception of PCB congener 118, among five indigenous species.

Animals↗