Steven S. Schwartz

Polyphyletic origins of asexuality in Daphnia pulex. I: Breeding-system variation and levels of clonal diversity

There is growing evidence that transitions from sexual to asexual reproduction are often provoked by internal genetic factors rather than extrinsic selection pressures. In the cladoceran crustacean Daphnia pulex, the shift to asexuality has been linked to sex‐limited meiosis suppression. Most populations of this species reproduce by obligate parthenogenesis, but cyclically parthenogenetic populations persist in the southern portion of its range. The meiosis‐suppressor model predicts that asexuality in D. pulex has polyphyletic origins and that the coexistence of cyclically parthenogenetic lines with male‐producing obligately asexual clones should be unstable.

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.

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

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.

Inter- and intraspecific variation in acute thermal tolerance of Daphnia

The acute thermal tolerances of six Daphnia species were compared at three acclimation temperatures. Of the species tested, all but one showed an increase in thermal tolerance when acclimated at higher temperature. Comparison of the regression relationships between thermal tolerances and acclimation temperature revealed no significant interspecific differences among species in their improvement of thermal tolerance caused by acclimation. However, significant intraspecific differences in thermal tolerance were noted. Daphnia obtusa had the highest tolerance, while D. pulex had the lowest. Among the other species acclimated at 10 C, thermal tolerance declined in a sequence of D. ambigua, D. magna, D. carinata, and D. nivalis. The differences in thermal tolerance noted among species were correlated with maximum temperatures of the environment from which they originated. While comparison of clones of D. pulex from several geographic localities revealed up to 2 C differences in acute thermal tolerance, there was no evidence of clonal variation in thermal tolerance of D. magna.

THE FEEDING ECOLOGY OF HYDRA AND POSSIBLE IMPLICATIONS IN THE STRUCTURING OF POND ZOOPLANKTON COMMUNITIES

Although most invertebrate predators are size-selective, two species of Hydra, H. oligactis and H. pseudoligactis, are not. A marked preference for Daphnia pulex over Simocephalus vetulus, similarly sized prey items, is observed in feeding trials. S. vetulus is virtually ignored and swims among the tentacles of Hydra, whereas D. pulex is rapidly attacked and captured. However, normal feeding responses are induced in the presence of homogenates of both Daphnia and Simocephalus. This suggests that the lack of response to live Simocephalus as prey items may be due to defense mechanisms evolved during the course of long-term coexistence in the shallow, weedy littoral zone of lakes and ponds. Such mechanisms could involve reduced activation of nematocysts, immunity to the toxin, or lack of penetration of nematocysts through the carapace of Simocephalus. The impact on the structure of the zooplankton community of this differential susceptibility to predation by Hydra is discussed.

Prey preference and utilization by Mesostoma lingua (Turbellaria, Rhabdocoela) at a low arctic site

The rhabdocoel Mesostoma lingua is a common inhabitant of rock bluff ponds in the area of Churchill, Manitoba. Unlike most flatworms it has the ability to prey actively on zooplankton. Among the zooplankton, Mesostoma prefers Daphnia pulex, Simocephalus vetulus, and Diaptomus tyrrelli. Larger calanoid copepods are difficult to capture and take much longer to consume. Growth in Mesostoma is maintained when individuals consume 0.5 Daphnia day−1 and is maximal at 4 Daphnia day−1. In comparison, Mesostoma maintained growth only when offered the copepod Diaptomus victoriaensis at the rate of 8–10 individuals-day−1. Mesostoma may be an important agent in structuring pond zooplankton communities in the arctic, producing a shift in dominance away from cladocerans such as Daphnia toward larger calanoid copepods.

The Effect of Hydra on the Outcome of Competition Between Daphnia and Simocephalus

The cladoceran genera Daphnia and Simocephalus often co-occur in nature. In laboratory experiments, populations of the two genera had similar growth rates when grown separately, but when cultured together Daphnia invariably excluded Simocephalus. However, the added presence of the littoral zone predator, Hydra, reversed this trend with Simocephalus remaining after Daphnia had been eliminated. This result was robust in culture vessels as small as 100 ml and as large as 85 l. It is hypothesized that Simocephalus has evolved a suite of energetically expensive traits to deter littoral zone predators, whereas Daphnia, which are planktonic, have not evolved such costly traits and hence have more energy available for reproduction and are able to exclude Simocephalus.

Breeding system of Daphniopsis ephemeralis: adaptations to a transient environment

Populations of the vernal cladoceran Daphniopsis ephemeralis are found in woodland ponds throughout southern Ontario. The species reproduces by cyclic parthenogenesis, and genotype frequencies at allozyme loci are ordinarily in good agreement with Hardy-Weinberg expectations. Occasional heterozygote deficiencies are apparently the consequence of admixture of ephippial hatchlings produced in temporally separated bouts of sexual reproduction. Considerable heterogeneity in genotypic frequencies exists among local populations in southwestern Ontario, indicating that gene flow among populations is restricted. Inbreeding coefficients suggest that populations receive an average of 0.3 migrants per generation. The completion of a sexual life cycle is made possible despite the brief persistence of populations by the emergence of males from ephippial eggs and by the production of equal numbers of male and female progeny in the first parthenogenetic brood.

Geographical Patterns in Genetic Diversity and Parthenogenesis within the Daphnia pulex Group from the Southern United States

-A geographic survey was undertaken to confirm the distribution and breeding system of the Daphnia pulex group in the southern United States. Populations of D. pulex are known to reproduce by either obligate or cyclical parthenogenesis and the breeding system of a population can be inferred from information on its genetic structure. Only two of the seven North American members of this group were present in the 213 ponds sampled in the region. Daphnia pulex was not found during the survey, but D. obtusa occurred in both Georgia and Texas, while D. schodleri was found only in Texas. Electrophoresis of seven allozyme systems in Texas populations of Daphnia obtusa and D. schodleri indicated that the two species showed fixed gene substitutions at Fum and Ldh, permitting their discrimination. The allozyme studies confirmed that the two species do not hybridize and showed that both reproduced by cyclical parthenogenesis. Gene frequency differences were marked among local populations of each species, but populations of D. obtusa from Georgia and Texas shared similar allelic arrays.