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 three 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.

No evidence for genetic sex determination in Daphnia magna

Mechanisms of sex determination (SD) differ widely across the tree of life. In genotypic sex determination (GSD), genetic elements determine whether individuals are male or female, while in environmental sex determination (ESD), external cues control the sex of the offspring. In cyclical parthenogens, females produce mostly asexual daughters, but environmental stimuli such as crowding, temperature or photoperiod may cause them to produce sons. In aphids, sons are induced by ESD, even though GSD is present, with females carrying two X chromosomes and males only one (X0 SD system). By contrast, although ESD exists in Daphnia , the two sexes were suggested to be genetically identical, based on a 1972 study on Daphnia magna (2n=20) that used three allozyme markers. This study cannot, however, rule out an X0 system, as all three markers may be located on autosomes. Motivated by the life cycle similarities of Daphnia and aphids, and the absence of karyotype information for Daphnia males, we tested for GSD (homomorphic sex chromosomes and X0) systems in D. magna using a whole-genome approach by comparing males and females of three genotypes. Our results confirm the absence of haploid chromosomes or haploid genomic regions in D. magna males as well as the absence of sex-linked genomic regions and sex-specific single-nucleotide polymorphisms. Within the limitations of the three studied populations here and the methods used, we suggest that our results make the possibility of genetic differences among sexes in the widely used Daphnia model system very unlikely.

Multiple factors likely explain variation in investment in sexual reproduction by 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 timing of 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, parasite infections, sexual reproduction, temperature, and light attenuation in 15 lake populations of D. dentifera for three years. We monitored infections by nine common parasites; this is notable since most prior studies on investment in sex and parasitism have focused on a single parasite, even though multiparasite communities are the norm in nature. We found substantial variation in investment in sex, with some populations reproducing entirely asexually throughout the study period and others shifting almost entirely to sexual reproduction by late autumn. We found that higher host density and parasitism were associated with greater investment in sex. Temperature and light attenuation were not as predictive of investment in sex, but received some statistical support. While correlational, our results leverage a large time series dataset and suggest multiple factors likely drive variation in sexual reproduction in this dominant member of lake food webs.

Filming of zooplankton: a case study of rotifer males and Daphnia magna

<p>Filming live organisms can give new insights into the hidden life of plankton. Accessibly priced digital cameras are now available for a large range of users. Here, we demonstrate the technical setup and workflow of using a single-lens reflex (DSLR) camera to film the behaviour of males of two rotifer species, <em>Brachionus angularis</em> Gosse (1851) and <em>Keratella cochlearis</em> Gosse (1851), and of the cladoceran <em>Daphnia magna</em> Straus (1820). Rotifers are cyclical parthenogens that produce males only under certain environmental conditions. Thus, knowledge on rotifer males is still limited because of their ephemeral nature and because they are often smaller than females. We filmed males of <em>B. angularis</em> and <em>K. cochlearis</em> with a DSLR camera connected to a compound microscope to better understand their morphology and behaviour in comparison to conspecific females. While written descriptions have their scientific value, seeing is complementary because everyone can verify what has been described. We made our videos publicly accessible through links connected to the paper. Our videos are, to our best knowledge, the first on males of<em> B. angularis</em> and <em>K</em>. <em>cochlearis</em>. Furthermore, we filmed the behavioural response of <em>D. magna</em> to ultraviolet (UV) radiation with a macro lens attached to the DSLR camera. Approaches like this are valuable tools in environmental teaching. To see live organisms with one’s own eyes may contribute to raising public awareness about the value of water resources and their hidden communities. In summary, filming can be a valuable tool to ignite scientific discussion, but the videos need an open-access platform where they can be referenced in a topic-related order.</p>

Evidence for a quiet revolution: seasonal variation in colonies of the specialist tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach) (Hemiptera: Aphididae) studied using microsatellite markers

In cyclical parthenogens, clonal diversity is expected to decrease due to selection and drift during the asexual phase per number of asexual generations. The decrease in diversity may be counteracted by immigration of new genotypes. We analysed temporal variation in clonal diversity in colonies of the monophagous tansy aphid, Macrosiphoniella tanacetaria (Kaltenbach), sampled four times over the course of a growing season. In a related field study, we recorded aphid colony sizes and the occurrence of winged dispersers throughout the season. The number of colonies increased from April, when asexual stem mothers hatched from the sexually produced eggs, to the end of June. The proportion of colonies with winged individuals also increased over this period. After a severe reduction in colony sizes in late summer, a second expansion phase occurred in October when sexuals were produced. At the season's end, the only winged forms were males. A linked genetic study showed that the number of microsatellite multilocus genotypes and genetic variability assessed at three polymorphic loci per colony decreased from June to October. Overall, the relatedness of wingless to winged individuals within colonies was lower than average relatedness among wingless individuals, suggesting that winged forms mainly originated in different colonies. The results demonstrate that patterns of genetic diversity within colonies can be explained by the antagonistic forces of clonal selection, migration and genetic drift (largely due to midsummer population bottlenecks). We further suggest that the males emigrate over comparatively longer distances than winged asexual females.