David J. Innes

Sexual reproduction of Daphnia pulex in a temporary habitat

Abstract Species of Daphnia (Crustacea: Cladocera) typically reproduce by cyclical parthenogenesis, in which a period of all-female parthenogenetic reproduction is followed by sexual reproduction. Sex in Daphnia is determined by the environment, with factors such as temperature, photoperiod and crowding stimulating the production of males and sexual females. Previous studies on Daphnia pulex from temporary pond habitats demonstrated the coexistence of male-producing and non-male-producing (NMP) females, as determined under crowding in the laboratory. A strong genetic component to this sex allocation variation suggested that sex expression in D. pulex is better described as a result of genotype-environment interaction. The present study examined the switch from parthenogenetic to sexual reproduction in two temporary-pond populations of D. pulex. Both populations showed a very early investment in sexual reproduction, independent of population density, by producing males very soon after the populations were reestablished from resting eggs in the early spring. Approximately 40% of the initial broods were male. Additional evidence for gender specialization was obtained by observing the sex of two or three successive broods for 85 individual females. Fifty-eight females produced successive broods of females, 13 females produced successive broods of males and 14 females produced successive broods which included both male and female broods. Females that produced successive female broods under natural conditions included a higher frequency of NMP females compared to a random sample of females, confirming the existence of NMP females. Sexual females were observed in both populations after the first appearence of males, suggesting that the presence of males may stimulate the production of sexual females. For D. pulex populations in a temporary environment, there appears to be an increased emphasis on sexual reproduction and a decreased influence of the environment on sex determination, compared to Daphnia populations in more permanent habitats.

Genetic variation among populations of Mytilus spp. in eastern Newfoundland

Limited sampling of Mytilus spp. in eastern Newfoundland has suggested that M. trossulus Gould and M. edulis L. are distributed as pure populations or mixtures of both species. In 1989 and 1990 we collected a wider series of samples of Mytilus spp. from 16 sites in eastern Newfoundland in order to determine the genetic structure based on four polymorphic enzyme loci (Est, Pgm, Lap, Ap). Associations among loci (particularly Pgm and Est), and a bimodal distribution of individual principal-component scores (based on fourlocus genotypes) provided evidence for the presence of both M. trossulus and M. edulis. Most of the sites consisted of a mixture of the two species. The relative frequency of each species in the samples varied on both a macro- and microgeographic scale. Two wave-exposed sites had the highest frequency (>90%) of M. trossulus. The distribution of hybrid index scores, based on three partially diagnostic loci (Est, Pgm, Lap), provided no evidence for hybridization.

Sex allocation variation in Daphnia pulex

Daphnia (Crustacea: Cladocera) reproduce by cyclical parthenogenesis in which the sex of offspring is environmentally determined. Although numerous studies have demonstrated that factors such as crowding and short‐day photoperiod stimulate male production, there is limited information on variation in allocation to male and female offspring for any species of Daphnia . The present study assessed the presence or absence of male production in 96 isofemale lines (clones) from each of eight populations of Daphnia pulex . An average of 37% (range 18–51%) of clones failed to produce males under crowded conditions in the laboratory. A subset of 14 of these non‐male‐producing clones also failed to produce males under short‐day photoperiod (8L:16D). Three male‐producing clones were within‐clone mated as well as crossed to three non‐male‐producing clones to study the inheritance of the failure to produce males. The average frequency of non‐male‐producing F 1 progeny was significantly higher (58%, N = 486) among the outcrossed progeny than the inbred progeny (5%, N = 86). In addition, when sixteen of the male‐producing outcrossed progeny were within‐clone mated, only 7% ( N = 106) of the resulting F 2 progeny failed to produce males. These results are consistent with a genetic basis for the absence of male production. Average survival of the progeny from the nine outcrossed matings was more than twice (67%) that of the inbred progeny from the three within‐clone matings (30%), suggesting that within‐clone mating would result in significant inbreeding depression. We present a model that suggests that even low levels of inbreeding could allow non‐male‐producing females to be maintained in a population. The co‐occurrence of non‐male‐producing females and females that produce both males and females in Daphnia pulex bears a similarity to the gynodioecious breeding system found in some plant species.

Gender variation inSilene acaulis (Caryophyllaceae)

Silene acaulis (Caryophyllaceae) is an alpine-arctic plant with a gynodioecious breeding system, but significant variation in sex expression has been reported. In addition, population sex ratio may be modified by the anther-smutMicrobotryum violaceum (Pers.)Deml & Oberwinkler, which sterilizes individuals of both sexes. A survey was undertaken at several sites on Baffin Island, Canada, to determine sex ratio and assess variation in female function among morphologically hermaphroditic individuals. The degree of anthersmut infection was also measured. Six sites had high female frequencies ranging from 72–80% and < 2% smut infection. High female frequencies may indicate cytoplasmic control of male-sterility. A seventh site from a mesic habitat had only 50% females and a higher rate of smut infection (22%). Of the three sites studied in detail, 84% of females set at least one capsule compared to only 25% of the hermaphrodites, indicating reduced female function. Hermaphrodites displayed significant variability in female function. Flowers with short styles (< 4.0mm) had degenerated ovules, and field estimates confirmed that only 5% of these individuals set capsules, but comprised the majority (> 55%) of hermaphrodites. Although hermaphrodites with short-styled flowers functioned solely as males, there was no increase in pollen production compared to long-styled hermaphrodites. Long-styled hermaphrodites produced the same number of ovules as females, and all set at least one capsule but these plants were uncommon (< 11%) at all sites.

D- and L-lactate dehydrogenases during invertebrate evolution

BackgroundThe L-lactate and D-lactate dehydrogenases, which are involved in the reduction of pyruvate to L(-)-lactate and D(+)-lactate, belong to evolutionarily unrelated enzyme families. The genes encoding L-LDH have been used as a model for gene duplication due to the multiple paralogs found in eubacteria, archaebacteria, and eukaryotes. Phylogenetic studies have suggested that several gene duplication events led to the main isozymes of this gene family in chordates, but little is known about the evolution of L-Ldh in invertebrates. While most invertebrates preferentially oxidize L-lactic acid, several species of mollusks, a few arthropods and polychaetes were found to have exclusively D-LDH enzymatic activity. Therefore, it has been suggested that L-LDH and D-LDH are mutually exclusive. However, recent characterization of putative mammalian D-LDH with significant similarity to yeast proteins showing D-LDH activity suggests that at least mammals have the two naturally occurring forms of LDH specific to L- and D-lactate. This study describes the phylogenetic relationships of invertebrate L-LDH and D-LDH with special emphasis on crustaceans, and discusses gene duplication events during the evolution of L-Ldh.ResultsOur phylogenetic analyses of L-LDH in vertebrates are consistent with the general view that the main isozymes (LDH-A, LDH-B and LDH-C) evolved through a series of gene duplications after the vertebrates diverged from tunicates. We report several gene duplication events in the crustacean, Daphnia pulex, and the leech, Helobdella robusta. Several amino acid sequences with strong similarity to putative mammalian D-LDH and to yeast DLD1 with D-LDH activity were found in both vertebrates and invertebrates.ConclusionThe presence of both L-Ldh and D-Ldh genes in several chordates and invertebrates suggests that the two enzymatic forms are not necessarily mutually exclusive. Although, the evolution of L-Ldh has been punctuated by multiple events of gene duplication in both vertebrates and invertebrates, a shared evolutionary history of this gene in the two groups is apparent. Moreover, the high degree of sequence similarity among D-LDH amino acid sequences suggests that they share a common evolutionary history.

The role of hybridization in the origin and spread of asexuality in Daphnia

The molecular mechanisms leading to asexuality remain little understood despite their substantial bearing on why sexual reproduction is dominant in nature. Here, we examine the role of hybridization in the origin and spread of obligate asexuality in Daphnia pulex, arguably the best‐documented case of contagious asexuality. Obligately parthenogenetic (OP) clones of D. pulex have traditionally been separated into ‘hybrid’ (Ldh SF) and ‘nonhybrid’ (Ldh SS) forms because the lactase dehydrogenase (Ldh) locus distinguishes the cyclically parthenogenetic (CP) lake dwelling Daphnia pulicaria (Ldh FF) from its ephemeral pond dwelling sister species D. pulex (Ldh SS). The results of our population genetic analyses based on microsatellite loci suggest that both Ldh SS and SF OP individuals can originate from the crossing of CP female F1 (D. pulex × D. pulicaria) and backcross with males from OP lineages carrying genes that suppress meiosis specifically in female offspring. In previous studies, a suite of diagnostic markers was found to be associated with OP in Ldh SS D. pulex lineages. Our association mapping supports a similar genetic mechanism for the spread of obligate parthenogenesis in Ldh SF OP individuals. Interestingly, our study shows that CP D. pulicaria carry many of the diagnostic microsatellite alleles associated with obligate parthenogenesis. We argue that the assemblage of mutations that suppress meiosis and underlie obligate parthenogenesis in D. pulex originated due to a unique historical hybridization and introgression event between D. pulex and D. pulicaria.

Outcomes of reciprocal invasions between genetically diverse and genetically uniform populations of Daphnia obtusa (Kurz)

Ecological theory predicts that genetic variation produced by sexual reproduction results in niche diversification and provides a competitive advantage both to facilitate invasion into genetically uniform asexual populations and to withstand invasion by asexual competitors. We tested the hypothesis that a large group of diverse clones of Daphnia obtusa has greater competitive advantage when invading into genetically uniform populations of this species than a smaller group with inherently less genetic diversity. We compared competitive outcomes to those of genetically uniform groups of small and large size invading into genetically diverse populations. Genetically diverse invaders of initially large group size increased their representation by more than those of initially small size; in contrast, genetically uniform invaders of initially large group size diminished on average by more than those of initially small size. These results demonstrate an advantage to the genetic variation produced by sexual reproduction, both in invasion and resisting invasion, which we attribute to competitive release experienced by individuals in genetically diverse populations.

Inter-genotype variation in reproductive response to crowding among Daphnia pulex

Crowding is known to have a major influence on reproduction in the freshwater microcrustacean Daphnia pulex. We analyzed reproductive output of six different D. pulex genotypes under two different density regimes in the laboratory. Four of these genotypes reproduce via obligate parthenogenesis, allowing thorough analysis of the life history strategies of some asexual lines. Among 30,109 neonate offspring and 1041 resting egg ephippia collected, several trends were evident. Crowding induced increased resting egg production and reduced neonate offspring production among all genotypes. Offspring sex ratios grew more male-biased with maternal age. The extent, but not direction, of each of these trends varied among genotypes. Offspring sex ratios, and the very direction in which they changed in response to crowding, differed significantly among genotypes with some genotypes producing more and others fewer males in response to crowding. Obligately parthenogenetic genotypes seemed to respond to the crowding stimulus in similar ways as the facultatively parthenogenetic genotypes, as expected from the sexual origins of their genomes. The inter-genotype variation in life-history traits observed in this and other investigations calls into question the common practice of extrapolating results from a single Daphnia genotype to an entire species. Our findings are considered in the context of other research in the field of environmental influences on Daphnia reproduction with a review of representative literature.

Microgeographic genetic variation in the haploid and diploid stages of the moss Polytrichum juniperinum Hedw.

Electrophoretic variability at six enzyme loci was used to examine genotypic variation in the haploid (gametophyte) and diploid (sporophyte) stages of the moss Polytrichum juniperinum Hedw. from a population in eastern Newfoundland. Samples from 16 sites revealed 13 six-locus haploid genotypes and a total of 18 gametophyte genotypes (ten male and eight female) when the genotypic data were combined with sex. Gametophyte genotypic diversity was low within each site and there was marked microgeographic differentiation between sites within the population. There was, however, no relationship between genetic and microgeographic distances among the sites. The observed number of six-locus genotypes and their relative frequency within the population did not differ significantly from the values expected for a sexually reproducing population. This genotypic structure is consistent with sexual reproduction generating the observed variation but vegatative reproduction and limited dispersal maintaining microgeographic genetic heterogeneity within the population. Diploid, six-locus genotypes were determined for 137 sporophytes. The sporophyte genotypic variation suggested that mating was occurring predominantly between female and male gametophytes within the same site. Fewer gametophyte genotypes were observed than were expected to be produced by sporophyte genotypes observed at each site, suggesting that genetical and/or ecological factors may be limiting the recruitment of new haploid gametophyte genotypes into the population.

Quantitative genetic analysis of growth in larval scallops (Placopecten magellanicus)

We estimated the broad-sense heritability of larval size in 20 full-sib families of the giant scallop, Placopecten magellanicus (Gmelin, 1791) grown in laboratory culture in August and September 1991. The goal was to compare scallops with other bivalves which have been shown to have significant heritabilities for larval growth. Secondly, we estimated the lipid content of occytes from female parents, since this is hypothesized to affect larval growth and survival. Finally, we estimated the among-family variation in mortality from 4 to 21 d as a test of possible genetic variation for viability among larval scallops. Significant genetic variation (h2=1.10 to 1.24) was estimated for larval shell length at 4, 14, and 21 d. There was a significant correlation (r=0.66) between larval size at 4 d and lipid content of oocytes, but only when two females with high levels of lipid oocyte−1 were excluded as outliers. There was no correlation between larval size at 14 and 21 d and lipid content of oocytes. Mortality among families from 4 to 21 d was high (69 to 97%), and was significantly different among families. These results indicate that there is significant heritability for larval growth which is largely independent of the lipid content of the oocytes. A high heritability for larval growth may indicate that this trait is only weakly correlated with fitness.