The transcriptomic signature of cyclical parthenogenesis

Cyclical parthenogenesis, where females can engage in sexual or asexual reproduction depending on environmental conditions, represents a novel reproductive phenotype that emerged during eukaryotic evolution. The fact that environmental conditions can trigger cyclically parthenogens to engage in distinct reproductive modes strongly suggests that gene expression plays a key role in the origin of cyclical parthenogenesis. However, the genetic basis underlying cyclical parthenogenesis remains understudied. In this study we characterize the female transcriptomic signature of sexual vs. asexual reproduction in the cyclically parthenogenetic microcrustacean Daphnia pulex and D. pulicaria. Our analyses of differentially expressed genes, pathway enrichment, and GO term enrichment clearly show that compared to sexual reproduction the asexual reproductive stage is characterized by both the under-regulation of meiosis and cell-cycle genes and the up-regulation of metabolic genes. We suggest that the under-regulation of meiosis and cell-cycle genes is responsible for the origin of parthenogenesis from meiosis, whereas differentially expressed metabolic genes may mediate choice of asexual vs. sexual reproductive pathway. Furthermore, our analyses identify some cases of divergent expression among gene family members (e.g., doublesex, NOTCH2) associated with asexual or sexual reproductive stage, suggesting potential functional divergence among gene family members.

Masculinization of the X-chromosome in aphid soma and gonads

Males and females share essentially the same genome but differ in their optimal values for many phenotypic traits, which can result in intra-locus conflict between the sexes. Aphids display XX/X0 sex chromosomes and combine unusual X chromosome inheritance with cyclical parthenogenesis. Theoretical and empirical works support the hypothesis that the large excess of male-biased genes observed on the aphid X chromosome compared to autosomes has evolved in response to sexual conflicts, by restricting the product of a sexually antagonistic allele to the sex it benefits. However, whether such masculinization of the X affects all tissues (as expected if it evolved in response to sexual conflicts) or reflects tissue specificities (which would contradict the sexual conflict hypothesis) remains an open question. To address it, we measured gene expression in different somatic and gonadic tissues of males, sexual females and parthenogenetic females of the pea aphid. We observed a masculinization of the X at the tissue-level, with male-biased genes being 2.5 to 3.5 more frequent on the X than expected. We also tested the hypothesis that gene duplication can facilitate the attenuation of conflicts by allowing gene copies to neo- or sub-functionalize and reach sex-specific optima. As predicted, X-linked copies of duplicated genes having their other copies on autosomes were more frequently male-biased (40.5% of the genes) than duplicated autosomal genes (6.6%) or X-linked single-copy genes (32.5%). These results highlight a peculiar pattern of expression of X-linked genes in aphids at the tissue level and provides further support for sex-biased expression as a mechanism to attenuate intra-locus sexual conflicts.

Estimation of the propensity for sexual selection in a cyclical parthenogen

Cyclical parthenogenesis is a widespread reproductive strategy in which organisms go through one or multiple rounds of clonal reproduction before sexual reproduction. Because sexual reproduction is typically less common than parthenogenesis in populations of the planktonic cladoceran Daphnia magna, it is not frequently studied. Here we examine the sexual process of D. magna and its relation to sexual selection in Daphnia rockpool populations by observing natural mating in these shallow habitats where sex generally occurs throughout the summer. Although microsatellite markers were found to reveal no evidence of disassortative mating or, thus, of inbreeding avoidance, body length and infection status did reveal assortative mating, suggesting sexual selection to act. When two males mated with a single female, the larger male was observed to remain longer, possibly giving it an advantage in sperm competition. Indirect evidence points at the brood pouch as the likely site of fertilization and thus, sperm competition. Sperm length was as variable within ejaculates as it was among males from different populations. Our data provide firm evidence that sexual selection is present in this species, most likely manifesting itself through a combination of female choice and male–male competition.

Transcriptomic analysis of genes involved in reproduction at different ages in Daphnia pulex (Branchiopoda, Cladocera)

Daphnia pulex is a freshwater microcrustacean that is known for its cyclical parthenogenesis. In D. pulex, parthenogenetic reproduction switches to sexual reproduction when the living conditions worsen. However, this transformation also occurs over age under favourable living conditions. Thus, there might be a relationship between aging and reproductive conversion. We performed Illumina RNA sequencing, generating 51 712 680 and 59 854 588 raw reads from 1 day-old D. pulex and 25 day-old D. pulex, respectively. From these reads, 60 776 transcripts were assembled and 36 569 (60.15%) unigenes were annotated. A number of significantly differentially expressed genes associated with growth, aging, and reproduction were identified and Quantitative real-time PCR for six genes confirmed the transcriptome data. RNA interference (RNAi) of the caspase-3 gene (casp3) that is a key gene for growth, development, aging, and reproduction, was conducted, which achieved an 80% reduction in casp3 mRNA expression. Meanwhile, the mRNA expression of upstream genes jnk and akt, and sir2 (encoding a substrate of casp3) were also detected. The change of jnk mRNA expression before and after RNAi was not significant () and the mRNA levels of sir2 decreased, while akt increased after casp3 RNAi (). The results indicated the interrelationships of some genes in the senescence pathway and helped to identify the molecular mechanism of the aging progress in D. pulex. Overall, the difference in mRNA expression profile during aging of D. pulex forms a basis for further studies aimed at understanding the role of the transcriptional level in regulating aging and reproductive transformation.

Genetic control of male production in Daphnia pulex

Daphnia normally reproduce by cyclical parthenogenesis, with offspring sex being determined by environmental cues. However, some females have lost the ability to produce males. Our results demonstrate that this loss of male-producing ability is controlled by a dominant allele at a single locus. We identified the locus by comparing whole-genome sequences of 67 nonmale-producing (NMP) and 100 male-producing (MP) clones from 5 Daphnia pulex populations, revealing 132 NMP-linked SNPs and 59 NMP-linked indels within a single 1.1-Mb nonrecombining region on chromosome I. These markers include 7 nonsynonymous mutations, all of which are located within one unannotated protein-coding gene (gene 8960). Within this single gene, all of the marker-linked NMP haplotypes from different populations form a monophyletic clade, suggesting a single origin of the NMP phenotype, with the NMP haplotype originating by introgression from a sister species, Daphnia pulicaria. Methyl farnesoate (MF) is the innate juvenile hormone in daphnids, which induces the production of males and whose inhibition results in female-only production. Gene 8960 is sensitive to treatment by MF in MP clones, but such responsiveness is greatly reduced in NMP clones. Thus, we hypothesize that gene 8960 is located downstream of the MF-signaling pathway in D. pulex, with the NMP phenotype being caused by expression change of gene 8960.

Morphology of the nervous system of monogonont rotiferEpiphanes sentawith focus on sexual dimorphism between feeding females and dwarfed males

BackgroundMonogononta is a large clade of rotifers comprised of diverse morphological forms found in a wide range of ecological habitats. Most of the monogonont species display a cyclical parthenogenesis, where generations of asexually reproducing females are interspaced by mixis events when sexual reproduction occurs between mictic females and dwarfed, haploid males. The morphology of monogonont feeding females is relatively well described, however data on male anatomy are very limited. Thus far, male musculature of only two species has been described with confocal laser scanning microscopy (CLSM) and it remained unknown how dwarfism influences neuroanatomy of males.ResultsHere, we provide a CLSM-based description of the nervous system of both sexes ofEpiphanes senta, a freshwater monogonont rotifer. The general nervous system architecture is similar between males and females and shows same level of complexity. However, the nervous system in males is more compact and lacks its stomatogastric part.ConclusionComparison of the neuroanatomy between male and normal-sized feeding females provides better understanding of the nature of male dwarfism in Monogononta. We propose that dwarfism of monogonont non-feeding males is a specific case of progenesis as they, due to their inability to feed, retain a juvenile body size. Reduction of the stomatogastric nervous system in the males correlates with the loss of entire digestive tract and associated morphological structures.

Potential role of the heat shock protein 90 (hsp90) in buffering mutations to favour cyclical parthenogenesis in the peach potato aphid Myzus persicae (Aphididae, Hemiptera)

Heat-shock proteins 90 (hsp90s) are a class of molecules able to stabilize a network of ‘client’ proteins that are involved in several processes. Furthermore, recent studies indicated that mutations in the hsp90-encoding gene induce a wide range of phenotypic abnormalities, which have been interpreted as an increased sensitivity of different developmental pathways to hidden/cryptic mutations. In order to verify the role of hsp90 in aphids, we amplified and sequenced the hsp90 gene in 17 lineages of the peach potato aphid Myzus persicae (Sulzer, 1776) looking for the presence of mutations. In particular, we compared lineages with different reproductive modes (obligate vs. cyclical parthenogenesis), propensity to develop winged females and karyotype stability. Differently from the cyclical parthenogenetic lineages that possessed functional hsp90 genes, the seven analysed asexual lineages showed severe mutations (including frameshift and non-sense mutations). In vivo functional assays with the hsp90-inhibitor geldanamycin showed that some lineages with cyclical parthenogenesis may lose their ability to induce sexuales in the absence of active hsp90 revealing the presence of cryptic mutations in their genomes. As a whole, our data suggest that hsp90 could play in aphids a role in buffering hidden/cryptic mutations that disrupt cyclical parthenogenesis.