Paul D. N. Hebert

Clonal Coexistence in Daphnia pulex (Leydig): Another Planktonic Paradox

Allozyme variation is common in populations of Daphnia pulex reproducing by obligate parthenogenesis. The genetic diversity within populations results from the coexistence of genetically different clones. Twenty-two clones were recognized in the eleven populations surveyed, of wihich as many as seven werefound in a SCIENCE, VOL. 207, 21 MARCH 1980 0036-8075/80/0321-1363

Ecological and Physiological Differentiation Among Low‐Artic Clones of Daphnia Pulex

00.50/0 Copyright . 1980 AAAS 1363 only the LDH homozygote, while others contained only the LDH heterozygote. Where both LDH genotypes were present, their relative frequencies varied among the individual populations. Genotypic frequencies often deviated markedly from Hardy-Weinberg expectations. Heterozygote excesses were noted at some loci, and deficiencies at others. For instance, all individuals in the Windsor I1 population were heterozygous at the LDH locus, but heterozygotes were lacking at the PGM locus despite the presence of two different homozygotes. When clones were scored for their phenotype at the five polymorphic loci, clear associations of genotypes at different loci were detected (Table 2). In the Windsor I population all clones homozygous at the LDH locus were PGMMM homozygotes, while most of the Table 2. Clonal complements of the eleven populations. These data were obtained by electrophoresis of clones isolated from natural populations. Genotype Pontillation C7lone Freouencv

Clonal diversity in populations of Daphnia pulex reproducing by obligate parthenogenesis

A distributional survey of obligately parthenogenetic clonal groups belonging to the Daphnia pulex complex was conducted on rock bluff and tundra ponds near Churchill, Manitoba. Clonal distributions were spatially heterogeneous. The clonal patterns of mi- crogeographic heterogeneity suggested a strong association of Daphnia clonal type with salinity/conductivity gradients in pond habitats that varied in their proximity to Hudson Bay. Acute salinity tolerance experiments showed significant differences in survivorship of six clonal isolates at salinities encountered in nature. Clones from high-salinity ponds had greater survivorship at high salinities than clones from low-salinity ponds. There were no significant differences in clonal survivorship at low salinities. Clonal differences were also found in hatching success of ephippial eggs at different salinities. Laboratory measurements of hemolymph osmolality from these clones indicated significant differences in osmoreg- ulatory capacity. Coupled with morphological and electrophoretic data, the ecophysio- logical differences among D. pulex clones from Churchill suggest strongly that this Daphnia complex consists of physiologically different ecotypes. The ecological and evolutionary implications of these data are discussed.

Genotypic characteristics of the Cladocera

SummaryAllozyme studies of Daphnia pulex populations in southern Ontario revealed marked Hardy–Weinberg deviations, complete gametic phase imbalance, and high heterozygosity values. These genotypic characteristics reflect the loss of sexual reproduction; individuals reproduce by obligate parthenogenesis. Thirty-nine clones were identified in the twenty-one habitats surveyed; a few of these clones retained the ability to produce male offspring. Several clones were present in many habitats, but most of the thirty-nine clones were found in only a single pond. The large genetic distances between coexisting clones suggested that in situ mutational divergence was not the source of within habitat clonal diversity. Ecological differences between clones were suggested by distributional patterns and by temporal surveys of clone frequencies.

Ecological differences among clones of Daphnia pulex Leydig

Work on the genetics of cladocerans reproducing by cyclic parthenogenesis has indicated that populations usually include a large number of genotypes, whose frequencies are in close approximation to Hardy-Weinberg equilibrium. Less diverse genotypic arrays and pronounced instability in genotype frequencies occur only in permanent populations exposed to limited ephippial recruitment. Genetic diversification among local cladoceran populations is greater than in most other organisms as a consequence of the inefficiency of passive dispersal. Genotypic characteristics of cladocerans reproducing by obligate parthenogenesis are markedly different from those of cyclic parthenogens. Local populations include few clones, but genetic distances between them are often large and accompanied by significant ecological and morphological divergence. When considered over their entire range, cladoceran taxa reproducing by obligate asexuality are the most genotypically diverse asexual organisms known. This diversity has originated from the spread of a sex-limited meiosis suppressor through species that originally reproduced by cyclic parthenogenesis. The confused state of cladoceran taxonomy is partially a consequence of the presence of such obligately asexual groups, but also results from the occurrence of hybridization and sibling species. The genome size of cladocerans is exceptionally small and is associated with a large amount of endopolyploidy. Somatic tissues in adult cladocerans show a range in nuclear DNA content from 2–2048 c. DNA quantification studies have additionally revealed the frequent occurrence of polyploid clones in obligately asexual taxa.

Genotypic diversity and variation in mode of reproduction among populations in the Daphnia pulex group

SummaryNatural populations of Daphnia pulex that reproduce by obligate parthenogenesis include a number of clones. Studies on two common and two rare clones from southwestern Ontario revealed significant differences in their intrinsic rates of increase, competitive abilities, rates of ephippial egg production, and lifespans. Environmental factors such as temperature and food type had large inluences on the rate of increase of each clone. Differences in rates of increase among clones were most pronounced at temperatures higher than those encountered in nature. In general, the covariance of life history traits among clones was high. The outcome of competitive encounters between clones was deterministic and in most cases was unaffected by temperature. Clones with high rates of increase tended to be better competitors than those with low rates of increase.

Genetic consequences of passive dispersal in pond-dwelling copepods

A survey of nine ponds in Illinois and Iowa confirmed the occurrence of both cyclically and obligately parthenogenetic D. pulex and cyclically parthenogenetic D. obtusa. The three taxa co-occurred in two of the ponds and one predominantly cyclical parthenogenetic population of D. pulex also contained an obligately parthenogenetic clone. Three populations consisted solely of obligately parthenogenetic D. pulex, while three consisted solely of cyclically parthenogenetic D. obtusa. Populations of cyclically parthenogenetic individuals, which have a sexual phase, were usually in Hardy–Weinberg equilibrium at individual loci and had a high genotypic diversity. Populations of obligately parthenogenetic individuals deviated from Hardy–Weinberg equilibrium and often consisted of only one or two clones. Mode of reproduction was confirmed by breeding studies on individuals isolated from the populations. Daphnia pulex and D. obtusa were genetically distinct based on variation at six polymorphic loci. Individuals of D. pulex reproducing by the two modes of reproduction were also genetically differentiated. Obligately parthenogenetic individuals were Ldh 13 and Pep 12 heterozygotes while cyclically parthenogenetic individuals were 11 homozygotes at both loci. Seven obligately parthenogenetic clones were found with only one clone occurring in more than one population. The high genotypic diversity and restricted distribution of obligately parthenogenetic clones is consistent with their independent origin from cyclically parthenogenetic individuals.

Distribution of Contaminants in Clams and Sediments from the Huron-Erie Corridor. I–PCBs and Octachlorostyrene

Pond‐dwelling copepods have colonized habitats throughout North America after glaciers have receded. Most species are passively transported via resting eggs into new habitats. Colonists originating in a glacial refugium could lose some of the ancestral genetic diversity when they establish new populations and the attenuation may be substantial in populations far removed from the refugium due to multiple founder events. Genetic variation was measured in Heterocope septentrionalis from 27 populations at arctic sites near potential refugia and those more recently deglaciated to determine the effects of postglacial dispersal on patterns of genetic relatedness and diversity. Some populations were more distant, genetically, from others within the same site than those from other more distant sites. Eleven polymorphic enzyme loci were significantly more variable (F [1,294 df] = 5.94, P < 0.025) among individuals from populations near the Alaskan refuge than those at the eastern limit of their distribution. Because populations are typically extremely large and stable this loss of genetic diversity is attributed primarily to repeated founder events during colonization. This result suggests profound genetic changes may occur on a continental scale in passively dispersed copepods due to founder events alone. Their potential for divergence and speciation is greater than currently recognized.

Cuticular Pigmentation in Arctic Daphnia: Adaptive Diversification of Asexual Lineages?

Abstract Samples of surficial sediments and the clam species Lampsilis radiata siliquoidea were collected from 102 sites covering all of Lake St. Clair and the Canadian shoreline of the St. Clair and Detroit rivers. The distribution patterns of both octachlorostyrene (OCS) and PCBs were mapped throughout this area. The mean level of PCBs in sediments of 3.9 μg kg −1 (Aroclor 1254) was much lower than values for “total PCBs” reported in studies carried out in the early 1970s. This reduction does not appear to reflect a real decrease in PCB levels in the environment, but rather changes that have been made in sampling procedures and analytical techniques. Highest levels of PCBs in both sample types were found along the western shore of Lake St. Clair. Mean levels of OCS in whole clam tissue and surficial sediment (0–10 cm) were 43.0 and 5.1 μg kg −1 , respectively. The distribution pattern of OCS in the Huron-Erie corridor in both clams and sediments suggests that the primary source is in the St. Clair River. The mean chemical concentration factor was 59 for OCS, indicating considerable bioaccumulation in the biota of Huron-Erie corridor.

Patterns of genotypic diversity in Czechoslovakian Daphnia

Daphnia pulex and Daphnia middendorffiana are the most common cladocerans in arctic and alpine ponds of both North America and Eurasia. In his monograph of the genus Brooks (1957) described a number of minor morphological differences between them, and also noted that the dorsal carapace surface of D. middendorffiana from high latitudes was always pigmented, whereas that of D. pulex was not. He additionally observed that D. middendorffiana reproduced by obligate parthenogenesis, while D. pulex was a cyclic parthenogen. Indeed because of its unusual mode of reproduction a d its apparent morphological uniformity, Brooks (1957) hypothesized that a single clone of D. mniddendorffiana h d colonized the entire arctic. Brooks assessment of the morphological distinctiveness of these two species appears to have been optimistic in light of the taxonomic difficulties which more recent workers have encountered (Roen 1962; Meijering 1975; Halvorsen and Gullestad 1976; Bushnell and Byron 1979; Dodson 1981). Specimens often combine morphological features that were regarded as diagnostic of each species. Moreover, it is now known that many D. pulex populations also reproduce by obligate parthenogenesis (Hebert and Crease 1980). We aimed initially to clarify the taxonomic relationship between these two species by analyzing enetic relationships among the clones present at one locality in the Canadian arctic. The allozyme studies indicated that, despite their asexual mode of reproduction, populations of both species are clonally diverse. The genetic data further indicated that cuticular pigmentation i the genus Daphnia has a polyphyletic origin. There is, moreover, good reason to believe that the trait evolved after sexuality was abandoned and that pigmentation confers a fitness advantage in many arctic environments. As such, this study provides tentative evidence of adaptive diversification within asexual lineages.