Cloning,expression and cellular localization of Daphnia pulex senescence-associated protein,DpSAP

Daphnia (water fleas) are small crustaceans that undergo an unusual switch from asexual to sexual reproduction that is dependent on environmental conditions. In this study, a senescence-associated protein (SAP) from the common freshwater species Daphnia pulex was cloned using primers based on homologous sequences and rapid amplification of cDNA ends (RACE). Real-time PCR was employed to quantify the expression of D. pulex SAP (DpSAP) in individual organisms. The role of DpSAP in the reproductive transformation was further investigated in both parthenogenetic and sexual females by using digoxin-labeled SAP RNA probes and RNA whole-mount in situ hybridization. DpSAP was more highly expressed in sexual females, indicating a role in growth and reproduction. Cellular localization studies using RNA whole-mount in situ hybridization showed specific expression in the second tentacle joints. These expression patterns suggest an important role for DpSAP in the reproductive transformation of D. pulex.     

Impacts of male and food density on female performance in the brackish cladoceran Daphniopsis australis

The roles of male and food density in regulating female performance were investigated in the brackish cladoceran, Daphniopsis australis. Parthenogenetic females and ephippial females were tested using a 2 × 4 factorial experiment involving the presence and the absence of a male cross-classified with nil, low, medium and high food densities. For parthenogenetic females, the male presence and food density failed to trigger the switch from asexual to sexual reproduction, but the presence of male negatively affected parthenogenesis through egg abortion. Food density affected the animal longevity but depended on the male presence. The reproductive output was favoured by increasing food densities, but the male presence increased egg abortion, suggesting male being an added stress factor to parthenogenetic females. For ephippial females, food densities affected the frequency of switch from sexual to asexual modes in the absence and the presence of a male. However, the male enhanced switch frequency under low and high food densities. Longevity was increased with the male presence but was unaffected by food density. The ephippial females successfully produced diapausing eggs with the male presence. Although, ephippial females could switch to parthenogenesis but the reproductive output of switched ephippial females was inferior to that of parthenogenetic females since birth. The results reveal that the male presence and food density can impact the performance of female D. australis. Hence, this study provides an insight into the understanding of the reproductive biology of cladocerans and a possible alternative explanation for population dynamic of this species and other cladocerans in the field.     

Meiosis genes in Daphnia pulex and the role of parthenogenesis in genome evolution

Background  

Thousands of parthenogenetic animal species have been described and cytogenetic manifestations of this reproductive mode are well known. However, little is understood about the molecular determinants of parthenogenesis. The Daphnia pulex genome must contain the molecular machinery for different reproductive modes: sexual (both male and female meiosis) and parthenogenetic (which is either cyclical or obligate). This feature makes D. pulex an ideal model to investigate the genetic basis of parthenogenesis and its consequences for gene and genome evolution. Here we describe the inventory of meiotic genes and their expression patterns during meiotic and parthenogenetic reproduction to help address whether parthenogenesis uses existing meiotic and mitotic machinery, or whether novel processes may be involved.     

Changes in epibiotic burden during the intermolt of Daphnia determined by hypodermal retraction stages

We calibrated four stages of hypodermal retraction and cuticle regeneration with five stages of parthenogenetic egg development in Daphnia. Using the hypodermal retraction stages, we found that epibiotic burden increased with elapsed intermolt time for juvenile, male, and female Daphnia bearing parthenogenetic or ephippial eggs. The rate of increase of burden was similar for adult females of two Daphnia species and for males and females of D. pulex. Rate of increase of burden was similar between juvenile and adult females of D. galeata mendotae and D. pulex.     

隆线溞孤雌溞和两性雌溞的蛋白质差异表达

本实验提取隆线溞孤雌溞和两性雌溞的可溶性蛋白进行双向电泳和质谱鉴定,分析隆线溞在两种生殖状态下蛋白质组的差异变化。聚丙烯酰胺凝胶SDS-PAGE结果表明:隆线溞在两种生殖状态下存在明显的蛋白质表达差异,孤雌溞的蛋白条带在分子量约50.6kD、36.2kD、32.1kD和25.7kD处表达量较两性雌溞明显;两性雌的蛋白条带在分子量约87.8kD、67.2kD、53.6kD和35.5kD处表达量较孤雌溞明显,其中35.5kD的蛋白条带为两性雌所特有。同时取两个样品的可溶性蛋白进行双向电泳,每个样品重复四次。双向电泳图谱经银染后利用软件分析可知,隆线孤雌平均可检测到约750个蛋白质点,两性雌溞平均可检测到约720个蛋白质点。同时利用软件对凝胶上的蛋白质点进行半定量分析,发现隆线溞从孤雌生殖转化为两性生殖后有18个蛋白质点呈现显著变化,其中14个点表达量明显下降,4个点表达量显著升高。实验结果具有较好的重复性。取4个表达量显著上升的蛋白质点进行质谱分析,得到两个蛋白质点(16号和17号)的测定结果。其中16号点为一类酸性脱氢酶(2I234),它在动物生长发育的各个阶段大量表达,这类蛋白质在隆线溞生殖转化过程中表达量变化尤为显著。本研究结果表明:隆线溞在孤雌生殖和两性生殖状态下存在明显的蛋白质表达差异。     

Sexual reproduction of Daphnia pulex in a temporary habitat

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. Received: 19 February 1996 / Accepted: 20 January 1997     

Cloning,expression and cellular localization of the Doublesex gene in the water flea, Daphnia carinata, during different developmental stages

In this study, one of Doublesex genes from the common freshwater cladoceran Daphnia carinata, designated DapcaDsx1, was cloned using primers based on homologous sequences and rapid amplification of cDNA ends (RACE). qPCR was employed to quantify differences in DapcaDsx1 expression between the different sexual phases, with expression levels being higher in sexual females. The role of DapcaDsx1 in the reproductive transformation was further investigated in parthenogenetic-phase females and sexual-phase females using whole-mount in situ hybridization. This cellular localization study showed specific expression of DapcaDsx1 in the thoracic segments, second antenna and part of the ventral carapace. Higher expression levels were exhibited in sexual females compared to parthenogenetic females. This suggests that the DapcaDsx1 gene plays significant roles in switching modes of reproduction and during sexual differentiation.     

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.     

Single dietary amino acids control resting egg production and affect population growth of a key freshwater herbivore

The enormous success of the genus Daphnia in freshwater ecosystems is at least partially due to their cyclical parthenogenetic life cycle, in which asexual and sexual reproduction alternate periodically. This temporal change between reproductive strategies allows for (1) rapid population growth via subitaneously developing eggs when environmental conditions are appropriate and (2) the maintenance of genetic diversity via sexual reproduction and the production of resting eggs when environmental conditions deteriorate. We show here that dietary amino acids are involved in triggering the switch between reproductive modes in Daphnia pulex. Supplementation experiments demonstrate that specific dietary amino acids, in particular arginine and histidine, avert crowding-induced resting egg production, enhance subitaneous reproduction by increasing algal food quality and, as a combined effect of both processes, increase population growth rates. These findings suggest that the availability of single dietary amino acids potentially affects the seasonal dynamics and long-term persistence of Daphnia populations in the field, which may have consequences for the efficiency of carbon transfer and thus the trophic structure of freshwater food webs.     

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. For the present study, we examined the genotypic structure of D. pulex populations from a region in which there is an abrupt microgeographical shift in breeding system. Populations in Michigan largely reproduce by cyclic parthenogenesis, while those in Ontario are obligately asexual. Allozyme studies on 77 populations from this area revealed 50 obligately asexual clones, divisible into two groups: one derived from a single parent species and the other derived via interspecific hybridization. Although nearly 50% of the clones retained male production, there was, as predicted, no evidence of coexistence between cyclically parthenogenetic populations and male-producing obligately asexual clones. The survey did, however, reveal a low incidence of cyclically parthenogenetic populations in Ontario. The high genotypic diversity of these populations suggests that they are not only resistant to meiosis suppression, but able to rework genetic variation gained from asexual clones into a sexual breeding system.     

The Persistence of Facultative Parthenogenesis in Drosophila albomicans

Parthenogenesis has evolved independently in more than 10 Drosophila species. Most cases are tychoparthenogenesis, which is occasional or accidental parthenogenesis in normally bisexual species with a low hatching rate of eggs produced by virgin females; this form is presumed to be an early stage of parthenogenesis. To address how parthenogenesis and sexual reproduction coexist in Drosophila populations, we investigated several reproductive traits, including the fertility, parthenogenetic capability, diploidization mechanisms, and mating propensity of parthenogenetic D. albomicans. The fertility of mated parthenogenetic females was significantly higher than that of virgin females. The mated females could still produce parthenogenetic offspring but predominantly produced offspring by sexual reproduction. Both mated parthenogenetic females and their parthenogenetic-sexual descendants were capable of parthenogenesis. The alleles responsible for parthenogenesis can be propagated through both parthenogenesis and sexual reproduction. As diploidy is restored predominantly by gamete duplication, heterozygosity would be very low in parthenogenetic individuals. Hence, genetic variation in parthenogenetic genomes would result from sexual reproduction. The mating propensity of females after more than 20 years of isolation from males was decreased. If mutations reducing mating propensities could occur under male-limited conditions in natural populations, decreased mating propensity might accelerate tychoparthenogenesis through a positive feedback mechanism. This process provides an opportunity for the evolution of obligate parthenogenesis. Therefore, the persistence of facultative parthenogenesis may be an adaptive reproductive strategy in Drosophila when a few founders colonize a new niche or when small populations are distributed at the edge of a species'' range, consistent with models of geographical parthenogenesis.     

The influence of changing temperature on the life history of Daphniopsis ephemeralis

We present evidence which suggests that for Daphniopsis ephemeralischanges in water temperature may directly cue changes in sexratio, ephippial and parthenogenetic egg production and indirectlydetermine female body size. Field data suggested that over anannual cycle, a population of D. ephemeralis comprised animalswith three distinct life history phases. The first was madeup of exephippial females which hatched in the fall. They produceda second group made up of large parthenogenetic females whichcould produce more than one overwintering generation. In thespring these parthenogenetic females gave rise to a third lifehistory phase which comprised males and small ephippial females.Field enclosure experiments suggested that the small size ofspring females was not related to predation. Laboratory experimentssuggested that ephippial egg production at small body sizeswas associated with ‘increasing’ temperature. Theseexperiments also suggested that the fall reproductive pattern(parthenogenetic eggs and large body sizes) was associated with‘decreasing’ temperatures. We conclude that directionalityof seasonal temperature change may be the prime factor responsiblefor D. ephemeralis life history changes observed in the field.     

Clonal diversity and turnover in an overwintering Daphnia pulex population,and the effect of fish predation

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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.     

Preadaptation for parthenogenetic colony foundation in subterranean termites Reticulitermes spp. (Isoptera: Rhinotermitidae)

Thelytokous (all-female producing) parthenogenesis, in some cases, involves reproductive advantages against obligate sexual reproduction. However, the completion of parthenogenesis takes multiple steps without the help of males, and thus preadaptation that meets those requirements will be an important factor for the evolution of parthenogenesis. The Japanese subterranean termite, Reticulitermes speratus, is known to have the ability of parthenogenetic colony foundation, where females that failed to mate with males found colonies cooperatively with partner females and reproduce by parthenogenesis. In this study, we compared the parthenogenetic ability and the colony initiation behavior among six Reticulitermes species in Japan. All species other than R. speratus were not able to reproduce parthenogenetically. Nevertheless, females of these species without the parthenogenetic ability performed homosexual female–female colony initiation and produced eggs without fertilization. In addition, in one species without parthenogenetic reproduction, R. kanmonensis, female–female pair initiated founding behavior as quickly as a heterosexual pair. These results suggest that female–female colony initiation and virgin egg-laying are predominant characters among the genus Reticulitermes and provide a preadaptive condition for parthenogenetic colony foundation in R. speratus.     

Food quality triggers the reproductive mode in the cyclical parthenogen <Emphasis Type="Italic">Daphnia</Emphasis> (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. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The cost of males in Daphnia pulex

Sex is paradoxical, because asexuals should replace their sexual ancestors by avoiding the demographic cost of producing males (hereafter referred to as the cost‐of‐males). Despite the large body of theoretical and empirical work dealing with the paradox of sex, the cost‐of‐males assumption has been rarely tested. In the present study, we tested the cost‐of‐males assumption in the cladoceran Daphnia pulex. Populations of this species consist of both cyclically parthenogenetic (i.e. sexuals) and obligately parthenogenetic (i.e. asexuals) lineages. In addition, some of the asexual lineages produce only female offspring, whereas others produce functional males, which can mate with sexual females. We compared the reproductive investment of sexuals, male‐producing asexuals, and non‐male‐producing asexuals when raised separately under various environmental conditions. We also determined the outcome of competition between pair‐wise combinations of these reproductive modes. The cost of males was evident when sexual and asexual females were raised separately: sexuals produced fewer female offspring. However, there was no cost of males when reproductive modes were raised in pairs, as sexuals won the competition with asexuals. Our results directly relate to the field conditions experienced by D. pulex. Sexuals might suffer the cost of males at the beginning of the season, when resource competition is low; but when conditions deteriorate as the population approaches carrying capacity, sexuals seem to be better competitors in spite of male production.     

Density,parasitism, and sexual reproduction are strongly correlated in lake Daphnia populations

Many organisms can reproduce both asexually and sexually. For cyclical parthenogens, periods of asexual reproduction are punctuated by bouts of sexual reproduction, and the shift from asexual to sexual reproduction has large impacts on fitness and population dynamics. We studied populations of Daphnia dentifera to determine the amount of investment in sexual reproduction as well as the factors associated with variation in investment in sex. To do so, we tracked host density, infections by nine different parasites, and sexual reproduction in 15 lake populations of D. dentifera for 3 years. Sexual reproduction was seasonal, with male and ephippial female production beginning as early as late September and generally increasing through November. However, there was substantial variation in the prevalence of sexual individuals across populations, with some populations remaining entirely asexual throughout the study period and others shifting almost entirely to sexual females and males. We found strong relationships between density, prevalence of infection, parasite species richness, and sexual reproduction in these populations. However, strong collinearity between density, parasitism, and sexual reproduction means that further work will be required to disentangle the causal mechanisms underlying these relationships.