Environmental and genetic control of sexual reproduction in Daphnia

RNA-seq analysis to identify genes related to resting egg production of panarctic Daphnia pulex

BACKGROUND

The genus Daphnia switches its reproductive mode from subitaneous egg production to resting egg production in response to environmental stimuli. Although this life history trait is essential for surviving unsuitable environments, the molecular mechanism of resting egg production is little understood. In this study, we examined genes related to induction of resting egg production using two genotypes of panarctic Daphnia pulex, the JPN1 and JPN2 lineages, which differ genetically in the frequency of resting egg production. We reared these genotypes under high and low food levels. At the high food level, individuals of both genotypes continually produced subitaneous eggs, whereas at the low food level, only the JPN2 genotype produced resting eggs. Then, we performed RNA-seq analysis on specimens of three instars, including before and after egg production.

RESULTS

These results showed that expressed genes differed significantly between individuals grown under high and low food levels and among individuals of different instars and genotypes. Among these differentially expressed genes (DEGs), we found 16 that changed expression level before resting egg production. Some of these genes showed high-level expression only before resting egg production and one gene was an ortholog of bubblegum (bgm), which is reportedly up-regulated before diapause in bumblebees. According to gene ontology (GO) enrichment analysis, one GO term annotated as long-chain fatty acid biosynthetic process was enriched among these 16 genes. In addition, GO terms related to glycometabolism were enriched among down-regulated genes of individuals holding resting eggs, compared to those before resting egg production.

CONCLUSIONS

We found candidate genes highly expressed only before resting egg production. Although functions of candidate genes found in this study have not been reported previously in Daphnia, catabolism of long-chain fatty acids and metabolism of glycerates are related to diapause in other organisms. Thus, it is highly probable that candidate genes identified in this study are related to the molecular mechanism regulating resting egg production in Daphnia.

Partitioning variance in immune traits in a zooplankton host-Fungal parasite system

Host immune traits arise from both genetic and environmental sources of variation. When immune traits have a strong genetic basis, the presence and severity of disease in a population may influence the distribution of those traits. Our study addressed how two immune-related traits (gut penetrability and the hemocyte response) are shaped by genetic and environmental sources of variation, and how the presence of a virulent disease altered the relative frequency of these traits in natural populations. Daphnia dentifera hosts were sampled from five Indiana lakes between June and December 2017 before and during epidemics of their fungal pathogen, Metschnikowia bicuspidata. Collected Daphnia were experimentally exposed to Metschnikowia and assayed for their gut penetrability, hemocyte response, and multi-locus genotype. Mixed-effects models were constructed to partition variance in immune traits between genetic and environmental sources. We then isolated the genetic sources to produce genotype-specific estimates of immune traits for each multi-locus genotype. Finally, we assessed the relative frequency and dynamics of genotypes during epidemics and asked whether genotypes with more robust immune responses increased in frequency during epidemics. Although genotype was an important source of variation for both gut penetrability and the hemocyte response, environmental factors (e.g., resource availability, Metschnikowia prevalence, and co-infection) still explained a large portion of observed variation, suggesting a high degree of flexibility in Daphnia immune traits. Additionally, no significant associations were detected between a genotype's immune traits and its frequency in a population. Our study highlights the power of variance partitioning in understanding the factors driving variation in Daphnia traits and motivates further research on immunological flexibility and the ecological drivers of immune variation.

Inverse Lansing Effect: Maternal Age and Provisioning Affecting Daughters' Longevity and Male Offspring Production

AbstractMaternal age effects on offspring life history are known in a variety of organisms, with offspring of older mothers typically having lower life expectancy (the Lansing effect). However, there is no consensus on the generality and mechanisms of this pattern. We tested predictions of the Lansing effect in several Daphnia magna clones and observed clone-specific magnitude and direction of the maternal age effect on offspring longevity. We also report ambidirectional, genotype-specific effects of maternal age on the propensity of daughters to produce male offspring. Focusing on two clones with contrasting life histories, we demonstrate that maternal age effects can be explained by lipid provisioning of embryos by mothers of different ages. Individuals from a single-generation maternal age reversal treatment showed intermediate life span and intermediate lipid content at birth. In the clone characterized by the "inverse Lansing effect," neonates produced by older mothers showed higher mitochondrial membrane potential in neural tissues than their counterparts born to younger mothers. We conclude that an inverse Lansing effect is possible and hypothesize that it may be caused by age-specific maternal lipid provisioning creating a calorically restricted environment during embryonic development, which in turn reduces fecundity and increases life span in offspring.

Transcriptome analysis of different life-history stages and screening of male-biased genes in Daphnia sinensis

Background

In the life history of Daphnia, the reproductive mode of parthenogenesis and sexual reproduction alternate in aquatic ecosystem, which are often affected by environmental and genetic factors. Recently, the sex-biased genes are of great significance for clarifying the origin and evolution of reproductive transformation and the molecular regulation mechanism of sex determination in Daphnia. Although some genes on reproductive transition of Daphnia had been researched, molecular mechanism on the maintenance of sexually dimorphic phenotypes of Daphnia are still not well known, including differentially expressed genes in different life-history stages.

Results

In this study, four life-history stages of Daphnia sinensis, juvenile female (JF), parthenogenetic female (PF), sexual female (SF) and male (M), were performed for transcriptome, and male-biased genes were screened. A total of 110437 transcripts were obtained and assembled into 22996 unigenes. In the four life-history stages (JF, PF, SF and M), the number of unique unigenes is respectively 2863, 445, 437 and 586, and the number of common unigenes is 9708. The differentially expressed genes (DEGs) between male and other three female stages (M vs JF, M vs PF and M vs SF) were 4570, 4358 and 2855, respectively. GO gene enrichment analysis showed that the up-regulated genes in male were mainly enriched in hydrolase activity and peptidase activity. Thirty-six genes in male were significantly higher expression than in the three female stages, including one Doublesex (Dsx) gene, one laminin gene, five trypsin genes and one serine protease genes, and one chitin synthase gene and two chitinase genes.

Conclusions

Our results showed that thirty-six candidate genes may be as the male-biased genes involving in the maintenance of sexually dimorphic phenotypes. This work will provide a reference for further exploring the functional genes related to sex differentiation in Daphnia species. Moreover, according to previous investigations, we thought that the expression level of functional genes may be related to the life-history stages of organisms, and may be also affected by different Daphnia species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08824-x.

Condition-dependent sexual reproduction is driven by benefits, not costs of sex

Facultative sexual organisms must allocate resources to both asexual and sexual reproduction. Optimal patterns of investment in sex depend on the relative costs and benefits of each reproductive mode, and may consequently be context- and condition-dependent. Two proposed explanations for the observed variation in investment in sex among facultative sexual lineages invoke alternative condition-dependent scenarios. Under the fitness-associated sex hypothesis, sex is predicted when individuals are in poor condition or experience stressful environments. Under the resource-demanding sex hypothesis, sex is only affordable to individuals in good condition experiencing favourable environments. Direct tests of these contrasting hypotheses are rare; moreover, investment in different components of sexual reproduction responds differently to cues promoting sex, and may be subject to different energetic constraints. Using genotypes of facultative sexual Daphnia carinata that differ in their level of investment in sex, we manipulated resource availability while accounting for day length (a seasonal cue for sex) to evaluate these hypotheses. The sexual response to day length depended on resource availability: increased day lengths and reduced food availability increased the production of sexual eggs, and relative investment in males, in a manner consistent with the fitness-associated sex hypothesis. The pattern of condition-dependence was specific to each component of reproductive investment – while male production covaried with asexual fecundity across genotypes, increased sexual egg production was associated with reduced asexual reproduction. Our results suggest that investment in sex is determined largely by its context-dependent advantages, and that this investment is not moderated by immediate costs to asexual reproduction.

Adaptive Divergence of Meiotic Recombination Rate in Ecological Speciation

Theories predict that directional selection during adaptation to a novel habitat results in elevated meiotic recombination rate. Yet the lack of population-level recombination rate data leaves this hypothesis untested in natural populations. Here, we examine the population-level recombination rate variation in two incipient ecological species, the microcrustacean Daphnia pulex (an ephemeral-pond species) and Daphnia pulicaria (a permanent-lake species). The divergence of D. pulicaria from D. pulex involved habitat shifts from pond to lake habitats as well as strong local adaptation due to directional selection. Using a novel single-sperm genotyping approach, we estimated the male-specific recombination rate of two linkage groups in multiple populations of each species in common garden experiments and identified a significantly elevated recombination rate in D. pulicaria. Most importantly, population genetic analyses show that the divergence in recombination rate between these two species is most likely due to divergent selection in distinct ecological habitats rather than neutral evolution.

Speciation in Daphnia

The microcrustacean Daphnia is arguably one of the most studied zooplankton species, having a well understood ecology, life history, and a relatively well studied evolutionary history. Despite this wealth of knowledge, species boundaries within closely related species in this genus often remain elusive and the major evolutionary forces driving the diversity of daphniids remain controversial. This genus contains more than 80 species with multiple cryptic species complexes, with many closely related species able to hybridize. Here, we review speciation research in Daphnia within the framework of current speciation theory. We evaluate the role of geography, ecology, and biology in restricting gene flow and promoting diversification. Of the 253 speciation studies on Daphnia, the majority of studies examine geographic barriers (55%). While evidence shows that geographic barriers play a role in species divergence, ecological barriers are also probably prominent in Daphnia speciation. We assess the contribution of ecological and nonecological reproductive isolating barriers between closely related species of Daphnia and found that none of the reproductive isolating barriers are restricting gene flow completely. Research on reproductive isolating barriers has disproportionally focused on two species complexes, the Daphnia pulex and Daphnia longispina species complexes. Finally, we identify areas of research that remain relatively unexplored and discuss future research directions that build our understanding of speciation in daphniids.

The effect of food, light intensity and tank volume on resting eggs production in Daphnia carinata

Sexual reproduction is an important stage of cyclical parthenogenesis Daphnia, these cyclic parthenogens need to time their shift from asexual to sexual reproduction correctly, based on environmental cues. We investigated the effect of food quality and quantity, light intensity, and culture tank volume on the resting eggs production of a tropical Daphnia carinata clone of Vietnam. The results showed that sexual ephippia production of the D. carinata was strongly induced when fed with green algae Scenedesmus sp., under moderate light intensity condition. The experiments also showed that food limitation is not an obligate requirement of shifting from parthenogenetic reproduction to sexual ephippia reproduction in D. carinata. Beside of that, culture volume seemed to be an important determinant of successfully forming of embryos inside ephippium. These findings here were from cultures in contaminant free condition, thus they are important basics for further studies of additional effects of environment toxicants on D. carinata species of Vietnam.

Local adaptation of sex induction in a facultative sexual crustacean: insights from QTL mapping and natural populations of Daphnia magna

Dormancy is a common adaptation in invertebrates to survive harsh conditions. Triggered by environmental cues, populations produce resting eggs that allow them to survive temporally unsuitable conditions. Daphnia magna is a crustacean that reproduces by cyclical parthenogenesis, alternating between the production of asexual offspring and the sexual reproduction of diapausing eggs (ephippia). Prior to ephippia production, males (necessary to ensure ephippia fertilization) are produced parthenogenetically. Both the production of ephippia and the parthenogenetic production of males are induced by environmental factors. Here, we test the hypothesis that the induction of D. magna resting egg production shows a signature of local adaptation. We postulated that Daphnia from permanent ponds would produce fewer ephippia and males than Daphnia from intermittent ponds and that the frequency and season of habitat deterioration would correlate with the timing and amount of male and ephippia production. To test this, we quantified the production of males and ephippia in clonal D. magna populations in several different controlled environments. We found that the production of both ephippia and males varies strongly among populations in a way that suggests local adaptation. By performing quantitative trait locus mapping with parent clones from contrasting pond environments, we identified nonoverlapping genomic regions associated with male and ephippia production. As the traits are influenced by two different genomic regions, and both are necessary for successful resting egg production, we suggest that the genes for their induction co-evolve.

Induction of male production in clones of Daphnia pulex by the juvenoid hormone methyl farnesoate under short photoperiod

The juvenile hormone, methyl farnesoate (MF), and its analog insecticides have been used successfully to induce the production of males in cladocerans under long-day conditions in the laboratory. However, without hormone addition Daphnia do not usually produce male offspring under long photoperiods, while short photoperiods are a stimulus for the induction of males. We used 21 clones of Daphnia pulex differing in their propensity to produce males under short-day conditions to test whether the treatment with MF would result in an additive effect of shifting the sex ratio towards males. Contrary to our expectations, clones with a high tendency of male production showed a reduced sex ratio in response to MF treatment under short-day conditions, but clones that produced normally few males or did not produce males were stimulated by 700 nM MF to produce up to 40% males. We suggest that the endocrine disruptive effect of MF or juvenile hormone analogs in the field may depend on the clonal composition of the cladoceran population and on the natural photoperiod. This may affect the seasonal occurrence of sexual reproduction and eventually cause a mismatch between the presence of males and ephippial females.

Genomics of environmentally induced phenotypes in 2 extremely plastic arthropods

Understanding how genes and the environment interact to shape phenotypes is of fundamental importance for resolving important issues in adaptive evolution. Yet, for most model species with mature genetics and accessible genomic resources, we know little about the natural environmental factors that shape their evolution. By contrast, animal species with deeply understood ecologies and well characterized responses to environmental cues are rarely subjects of genomic investigations. Here, we preview advances in genomics in aphids and waterfleas that may help transform research on the regulatory mechanisms of phenotypic plasticity. This insect and crustacean duo has the capacity to produce extremely divergent phenotypes in response to environmental stimuli. Sexual fate and reproductive mode are condition-dependent in both groups, which are also capable of altering morphology, physiology and behavior in response to biotic and abiotic cues. Recently, the genome sequences for the pea aphid Acyrthosiphon pisum and the waterflea Daphnia pulex were described by their respective research communities. We propose that an integrative study of genome biology focused on the condition-dependent transcriptional basis of their shared plastic traits and specialized mode of reproduction will provide broad insight into adaptive plasticity and genome by environment interactions. We highlight recent advances in understanding the genome regulation of alternative phenotypes and environmental cue processing, and we propose future research avenues to discover gene networks and epigenetic mechanisms underlying phenotypic plasticity.

Selection of low investment in sex in a cyclically parthenogenetic rotifer

Cyclical parthenogens, which combine asexual and sexual reproduction, are good models for research into the ecological and population processes affecting the evolutionary maintenance of sex. Sex in cyclically parthenogenetic rotifers is necessary for diapausing egg production, which is essential to survive adverse conditions between planktonic growing seasons. However, within a planktonic season sexual reproduction prevents clonal proliferation. Hence, clones with a low propensity for sex should be selected, becoming dominant in the population as the growing season progresses. In this context, we studied the dynamics of the heritable variation in propensity for sexual reproduction among clones of a Brachionus plicatilis rotifer population in a temporary Mediterranean pond during the period the species occurred in plankton. Clonal isolates displayed high heritable variation in their propensity for sex. Moreover, the frequency of clones with low propensity for sex increased during the growing season, which supports the hypothesized short-term selection for low investment in sex within a growing season. These results demonstrate (1) the inherent instability of the cyclical parthenogenetic life cycle, (2) the cost of sexual reproduction in cyclical parthenogens where sex produces diapausing eggs and (3) the role of the association between sexual reproduction and diapause in maintaining sex in these cyclical parthenogens.

Development of resistance to cyfluthrin and naphthalene among Daphnia magna

In this study, Daphnia magna were exposed to a pyrethroid insecticide (cyfluthrin) or a polycyclic aromatic hydrocarbon (naphthalene) for 12 generations to evaluate development of resistance followed by a 12 generation recovery period. Twenty-four hour old D. magna were exposed to concentrations of each chemical resulting in 50-70% mortality to select for the least sensitive individuals. LC50 values, survival, reproductive output, and time to first brood in stressor-exposed and control D. magna were recorded for each generation. Significant changes in LC50 values were observed after 4 generations and then declined after 6-10 generations post-exposure. D. magna were 5 times less sensitive to cyfluthrin and 3 times less sensitive to naphthalene as compared to controls after 12 generations of exposure. There were no differences in survival, time to first brood, or total number of offspring produced between control and either of the resistant F13 D. magna. Cyfluthrin exposed D. magna exhibited cross-resistance to DDT and methyl parathion, and naphthalene resistant D. magna were less sensitive than controls to both pyrene and benz(a)anthracene. When the cytochrome P450 inhibitor piperonyl butoxide was used in conjunction with cyfluthrin and naphthalene the sensitivity of resistant and control D. magna were equal, suggesting P450s were responsible for conveying resistance. This study demonstrates that life history and organisms' capacity to develop resistance is important to consider ensuring accuracy of ecological risk assessments.

Food quality triggers the reproductive mode in the cyclical parthenogen Daphnia (Cladocera)

Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes.

Parasite-mediated selection and the role of sex and diapause in Daphnia

To gain insight into parasite-mediated natural selection, we studied a natural population of the crustacean Daphnia magna during a severe epidemic of the bacterial parasite Pasteuria ramosa. We also investigated the relationship between susceptibility and the production of resting eggs, which are only produced during the sexual phase of reproduction. Live host samples were taken before and after this epidemic and resistance to P. ramosa was examined in the laboratory. Host clones collected after the epidemic were more resistant to P. ramosa than were those collected pre-epidemic, which is consistent with parasite-mediated selection. In our study population, asexually reproducing females were observed across the entire study period, but females carrying resting eggs were observed only prior to the epidemic. For hosts isolated in this pre-epidemic period, we found evidence that those carrying resting eggs (at the time of collection) were more susceptible than those that were reproducing asexually. This was especially apparent for measures of parasite growth, although not all measures of infection success conclusively supported this pattern. Nevertheless, the data suggest that some genotypes invest heavily in diapause at the expense of immunocompetence. Sex could therefore inhibit the evolution of resistance because each spring new genotypes will hatch from resting eggs that are relatively susceptible as they were not exposed to the previous years bout of parasite-mediated selection.

Maternal Effects on Encystment in Crosses Between Two Geographic Strains of Artemia franciscana

Maternal effects can have environmental or genetic causes. A method that can be used to demonstrate the genetic basis of a maternal effect is to look for grandfather effects in a backcross following reciprocal crosses. The absence of a grandfather effect would exclude a chromosomal basis for the maternal effect when the male sex is heterogametic (XX-XY sex determination system). However, in organisms in which the female is heterogametic (ZW-ZZ sex determination system), the absence of a grandfather effect does not rule out a chromosomal basis of the maternal effect, since the genes responsible for that effect can be located in the W chromosome, which is transmitted matrilineally. Conversely, the absence of a grandfather effect would point to a W-chromosome basis for the trait, provided that a maternal effect has been previously demonstrated. Distinguishing between W-located and autosome or Z-located maternal effects is important to understand the evolutionary dynamics of a trait. Here we report on a study of the chromosomal basis of maternal effects on two life-history traits related to encystment in the branchiopod crustacean Artemia franciscana, in which females are heterogametic. We performed crosses of two populations that differ in the number of cysts they produce. The proportion of encysted broods showed a maternal effect and was not affected by the grandfather's genotype, pointing to a W-chromosome basis. The average number of encysted offspring per brood showed a strong paternal effect and also a slight maternal effect. This trait also showed a grandfather effect, which suggests that the geographical variation has an autosomal or Z-chromosomal basis.

Juvenoid hormone methyl farnesoate is a sex determinant in the crustacean Daphnia magna

Daphnids (Daphnia magna) utilize cyclic parthenogenesis as a reproductive strategy. During periods of abundant resources, these organisms reproduce asexually. In response to environmental cues that signal the onset of environmental adversity, daphnids produce males and reproduce sexually. The environmental cues that stimulate the sexual reproductive phase are well known; however, the endocrine signals that transduce these environmental cues remain unknown. The present study was undertaken to test the hypothesis that the crustacean juvenoid hormone, methyl farnesoate, is a male sex determinant in this species. Continuous exposure to aqueous concentrations of methyl farnesoate greater than approximately 30 nM stimulated a concentration-dependent production of male-containing broods of organisms. Short-term exposures to methyl farnesoate during periods of egg and embryo maturation revealed that male sex determination occurred during a specific 12-hour period of ovarian egg development. Exposure of eggs to 400 nM methyl farnesoate during this sensitive developmental period resulted in the production of all-male broods of offspring, while exposure to concentrations as low as 52 nM produced mixed broods of males and females. This active concentration range of methyl farnesoate is consistent with levels measured in the hemolymph of some decapod crustaceans. These results demonstrate that methyl farnesoate is capable of programming daphnid embryos to develop into males and is likely the endocrine factor responsible for initiating the sexual reproductive phase in these organisms.

Temporal and quantitative changes in sexual reproductive cycling of the cladoceran Daphnia magna by a juvenile hormone analog

Cyclic parthenogens, such as the cladoceran, Daphnia magna, utilize both asexual (parthenogenetic) and sexual reproduction in order to maximize population fitness in variable environments. Parthenogenetic reproduction is the default strategy among D. magna, while various environmental cues trigger cycles of sexual reproduction. Experiments were conducted with the juvenile hormone analog methoprene to test the hypothesis that members of the insect juvenile hormone/vertebrate retinoic acid family of transcription factors are involved in the regulation of sexual reproduction in daphnids. Neither methoprene, food reduction, or crowding independently stimulated entry into the sexual reproductive phase of the daphnids. However, the combination of food deprivation and crowding stimulated entry into the sexual reproductive phase characterized by an initial high production of males and the subsequent intermittent production of haploid egg-containing ephippia. Exposure to 160 nM methoprene along with food deprivation and crowding caused a significant reduction in the percentage of males produced during the early phase of the sexual cycle and significantly increased the percentage of males produced during the later stages of the cycle. Methoprene concentrations as low as 6.4 nM significantly reduced the number of resting eggs produced and proportionately increased the production of parthenogenetically-produced neonates. These experiments demonstrate that methoprene uncouples the coordinate production of males and resting eggs during the sexual reproductive period of D. magna. Methoprene stimulates male offspring production and defers their production to latter stages of the sexual reproductive period, while inhibiting the production of resting eggs and promoting the continuance of parthenogenetic reproduction. J. Exp. Zool. 290:148-155, 2001.

Quantitative genetic variation in Daphnia: temporal changes in genetic architecture

Nonadditive genetic variation and genetic disequilibrium are two important factors that influence the evolutionary trajectory of natural populations. We assayed quantitative genetic variation in a temporary-pond-dwelling population of Daphnia pulex over a full season to examine the role of nonadditive genetic variation and genetic disequilibrium in determining the short-term evolutionary trajectory of a cyclic parthenogen. Quantitative traits were influenced by three factors: (1) clonal selection significantly changed the population mean phenotype during the course of the growing season; (2) sexual reproduction and recombination led to significant changes in life-history trait means and the levels of expressed genetic variation, implying the presence of substantial nonadditive genetic variation and genetic disequilibrium; and (3) Egg-bank effects were found to be an important component of the realized year-to-year change. Additionally, we examined the impact of genetic disequilibria induced by clonal selection on the genetic (co)variance structure with a common principal components model. Clonal selection caused significant changes in the (co)variance structure that were eliminated by a single bout of random mating, suggesting that a build-up of disequilibria was the primary source of changes in the (co)variance structure. The results of this study highlight the complexity of natural selection operating on populations that undergo alternating phases of sexual and asexual reproduction.

Avoiding the cost of males in obligately asexual Daphnia pulex (Leydig)

Asexual organisms are thought to gain an advantage by avoiding the cost of producing males. In the cladoceran Daphnia pulex (Leydig), male production is determined by the environment and is independent of the origin of the asexual obligate parthenogens from the sexual cyclical parthenogens. If there is a cost to producing males, successful obligate parthenogens should have reduced or eliminated male production. Field and laboratory observations showed that obligate parthenogens have much-reduced male production compared to cyclical parthenogens. Although the reduction or elimination of males in the obligate parthenogens suggests that the cost of males is avoided, the coexistence of sexual and asexual forms of D. pulex may be partially explained by cyclical parthenogens compensating for the cost of males by having greater fecundity. In addition, the absence of a mating constraint for the obligate parthenogens may favour an increased allocation to asexual diapausing eggs earlier in the season compared to the cyclical parthenogens which require mating with males to produce sexual diapausing eggs. No difference in the production of diapausing eggs was observed, probably because males were abundant in populations of cyclical parthenogens and do not appear to limit the production of sexual diapausing eggs. D. pulex is a useful system for determining the ecological consequences of abandoning sexual reproduction and explaining the coexistence of sexual and asexual forms of a species.

An uncoupling of male and sexual egg production leads to reduced inbreeding in the cyclical parthenogen Daphnia

Cyclical parthenogenesis involves an alternation of parthenogenetic and sexual reproduction. In cyclical parthenogens with a short generation time, seasonal succession of clones switching to sexual reproduction at different periods of the growing season entails the risk of severe inbreeding with associated fitness costs. We show, however, that differences in genotype frequencies between males and sexual females result in a substantial reduction in the probability of selfing in natural Daphnia populations. This suggests that responses to male- and sexual egg-inducing stimuli may be uncoupled at the level of individual clones as a mechanism to avoid severe inbreeding.

Inferring Deleterious-Mutation Parameters in Natural Daphnia Populations

Deng and Lynch (1, 2) proposed to characterize deleterious genomic mutations from changes in the mean and genetic variance of fitness traits upon selfing in outcrossing populations. Such observations can be readily acquired in cyclical parthenogens. Selfing and life-table experiments were performed for two such Daphnia populations. A significant inbreeding depression and an increase of genetic variance for all traits analyzed were observed. Deng and Lynch's (2) procedures were employed to estimate the genomic mutation rate (U), mean dominance coefficient (), mean selection coefficient (), and scaled genomic mutational variance (). On average, and (^ indicates an estimate) are 0.84, 0.30, 0.14 and 4.6E-4 respectively. For the true values, the and are lower bounds, and and upper bounds.