Male spiders control offspring sex ratio through greater production of female-determining sperm

Social transmission of bacterial symbionts homogenizes the microbiome within and across generations of group-living spiders

Disentangling modes and fidelity of symbiont transmission are key for understanding host-symbiont associations in wild populations. In group-living animals, social transmission may evolve to ensure high-fidelity transmission of symbionts, since non-reproducing helpers constitute a dead-end for vertical transmission. We investigated symbiont transmission in the social spider Stegodyphus dumicola, which lives in family groups where the majority of females are non-reproducing helpers, females feed offspring by regurgitation, and individuals feed communally on insect prey. Group members share temporally stable microbiomes across generations, while distinct variation in microbiome composition exists between groups. We hypothesized that horizontal transmission of symbionts is enhanced by social interactions, and investigated transmission routes within (horizontal) and across (vertical) generations using bacterial 16S rRNA gene amplicon sequencing in three experiments: (i) individuals were sampled at all life stages to assess at which life stage the microbiome is acquired. (ii) a cross-fostering design was employed to test whether offspring carry the microbiome from their natal nest, or acquire the microbiome of the foster nest via social transmission. (iii) adult spiders with different microbiome compositions were mixed to assess whether social transmission homogenizes microbiome composition among group members. We demonstrate that offspring hatch symbiont-free, and bacterial symbionts are transmitted vertically across generations by social interactions with the onset of regurgitation feeding by (foster)mothers in an early life stage. Social transmission governs horizontal inter-individual mixing and homogenization of microbiome composition among nest mates. We conclude that temporally stable host-symbiont associations in social species can be facilitated and maintained by high-fidelity social transmission.

Temporal and spatial microbiome dynamics across natural populations of the social spider Stegodyphus dumicola

Host symbiont interactions may form obligatory or facultative associations that are context dependent. Long-term studies on microbiome composition from wild populations should assess the temporal and spatial dynamics of host-microbe associations. We characterized the temporal and spatial variation in the bacterial microbiome composition in six populations of the social spider Stegodyphus dumicola for 2.5 years, using 16S rRNA gene amplicon sequencing of whole spiders. Individuals within a nest exhibit highly similar microbiomes, which remain stable over several generations and are not predictably affected by seasonal variation in temperature or humidity. This stability in nest microbiome is likely due to social transmission, whereas drift-like processes during new nest foundations explain variation in host microbiomes between nests. This is supported by the lack of obligate symbionts (i.e. no symbionts are present in all spider individuals). Quantitative PCR analyses showed that the bacterial load of individual spiders is stable in healthy nests but can increase dramatically in perishing nests. These increases are not driven by specific bacterial taxa but likely caused by loss of host immune control under deteriorating conditions. Spider nests show an annual survival rate of approximately 45%, but nest death is not correlated to microbiome composition, and the bacteria found in S. dumicola are not considered to be high virulence pathogens.

Males can adjust offspring sex ratio in an adaptive fashion through different mechanisms

Sex allocation research has primarily focused on offspring sex-ratio adjustment by mothers. Yet, fathers also benefit from producing more of the sex with greater fitness returns. Here, we review the state-of-the art in the study of male-driven sex allocation and, counter to the current paradigm, we propose that males can adaptively influence offspring sex ratio through a wide variety of mechanisms. This includes differential production and motility of X- versus Y-bearing sperms in mammals, variation in seminal fluid composition in haplo-diploid invertebrates, and epigenetic mechanisms in some fish and lizards exhibiting environmental sex determination. Conflicts of interest between mothers and fathers over offspring sex ratios can emerge, although many more studies are needed in this area. While many studies of sex allocation have focused on adaptive explanations with little attention to mechanisms, and vice versa, the integration of these two topics is essential for understanding male-driven sex allocation.

Microbiomes and Specific Symbionts of Social Spiders: Compositional Patterns in Host Species, Populations, and Nests

Social spiders have remarkably low species-wide genetic diversities, potentially increasing the relative importance of microbial symbionts for host fitness. Here we explore the bacterial microbiomes of three species of social Stegodyphus (S. dumicola, S. mimosarum, and S. sarasinorum), within and between populations, using 16S rRNA gene amplicon sequencing. The microbiomes of the three spider species were distinct but shared similarities in membership and structure. This included low overall diversity (Shannon index 0.5–1.7), strong dominance of single symbionts in individual spiders (McNaughton’s dominance index 0.68–0.93), and a core microbiome (>50% prevalence) consisting of 5–7 specific symbionts. The most abundant and prevalent symbionts were classified as Chlamydiales, Borrelia, and Mycoplasma, all representing novel, presumably Stegodyphus-specific lineages. Borrelia- and Mycoplasma-like symbionts were localized by fluorescence in situ hybridization (FISH) in the spider midgut. The microbiomes of individual spiders were highly similar within nests but often very different between nests from the same population, with only the microbiome of S. sarasinorum consistently reflecting host population structure. The weak population pattern in microbiome composition renders microbiome-facilitated local adaptation unlikely. However, the retention of specific symbionts across populations and species may indicate a recurrent acquisition from environmental vectors or an essential symbiotic contribution to spider phenotype.

Sex differences in spiders: from phenotype to genomics

Sexual reproduction is pervasive in animals and has led to the evolution of sexual dimorphism. In most animals, males and females show marked differences in primary and secondary sexual traits. The formation of sex-specific organs and eventually sex-specific behaviors is defined during the development of an organism. Sex determination processes have been extensively studied in a few well-established model organisms. While some key molecular regulators are conserved across animals, the initiation of sex determination is highly diverse. To reveal the mechanisms underlying the development of sexual dimorphism and to identify the evolutionary forces driving the evolution of different sexes, sex determination mechanisms must thus be studied in detail in many different animal species beyond the typical model systems. In this perspective article, we argue that spiders represent an excellent group of animals in which to study sex determination mechanisms. We show that spiders are sexually dimorphic in various morphological, behavioral, and life history traits. The availability of an increasing number of genomic and transcriptomic resources and functional tools provides a great starting point to scrutinize the extensive sexual dimorphism present in spiders on a mechanistic level. We provide an overview of the current knowledge of sex determination in spiders and propose approaches to reveal the molecular and genetic underpinnings of sexual dimorphism in these exciting animals.

Social spider webs harbour largely consistent bacterial communities across broad spatial scales

Social animals that live in domiciles constructed from biomaterials may facilitate microbial growth. Spider webs are one of the most conspicuous biomaterials in nature, yet almost nothing is known about the potential for webs to harbour microbes, even in social spiders that live in dense, long-term aggregations. Here, we tested whether the dominant bacteria present in social spider webs vary across sampling localities and whether the more permanent retreat web harbours compositionally distinct microbes from the more ephemeral capture webs in the desert social spider, Stegodyphus dumicola. We also sampled spider cuticles and prey items in a subset of colonies. We found that spider colonies across large spatial scales harboured similar web-associated bacterial communities. We also found substantial overlap in bacterial community composition between spider cuticle, prey and web samples. These data suggest that social spider webs can harbour characteristic microbial communities and potentially facilitate microbial transmission among individuals, and this study serves as the first step towards understanding the microbial ecology of these peculiar animal societies.

Evidence for Faster X Chromosome Evolution in Spiders

In species with chromosomal sex determination, X chromosomes are predicted to evolve faster than autosomes because of positive selection on recessive alleles or weak purifying selection. We investigated X chromosome evolution in Stegodyphus spiders that differ in mating system, sex ratio, and population dynamics. We assigned scaffolds to X chromosomes and autosomes using a novel method based on flow cytometry of sperm cells and reduced representation sequencing. We estimated coding substitution patterns (dN/dS) in a subsocial outcrossing species (S. africanus) and its social inbreeding and female-biased sister species (S. mimosarum), and found evidence for faster-X evolution in both species. X chromosome-to-autosome diversity (piX/piA) ratios were estimated in multiple populations. The average piX/piA estimates of S. africanus (0.57 [95% CI: 0.55-0.60]) was lower than the neutral expectation of 0.75, consistent with more hitchhiking events on X-linked loci and/or a lower X chromosome mutation rate, and we provide evidence in support of both. The social species S. mimosarum has a significantly higher piX/piA ratio (0.72 [95% CI: 0.65-0.79]) in agreement with its female-biased sex ratio. Stegodyphus mimosarum also have different piX/piA estimates among populations, which we interpret as evidence for recurrent founder events. Simulations show that recurrent founder events are expected to decrease the piX/piA estimates in S. mimosarum, thus underestimating the true effect of female-biased sex ratios. Finally, we found lower synonymous divergence on X chromosomes in both species, and the male-to-female substitution ratio to be higher than 1, indicating a higher mutation rate in males.

Male spiders control offspring sex ratio through greater production of female-determining sperm

Sex allocation theory predicts that when sons and daughters have different reproductive values, parents should adjust offspring sex ratio towards the sex with the higher fitness return. Haplo-diploid species directly control offspring sex ratio, but species with chromosomal sex determination (CSD) were presumed to be constrained by Mendelian segregation. There is now increasing evidence that CSD species can adjust sex ratio strategically, but the underlying mechanism is not well understood. One hypothesis states that adaptive control is more likely to evolve in the heterogametic sex through a bias in gamete production. We investigated this hypothesis in males as the heterogametic sex in two social spider species that consistently show adaptive female-biased sex ratio and in one subsocial species that is characterized by equal sex ratio. We quantified the production of male (0) and female (X) determining sperm cells using flow cytometry, and show that males of social species produce significantly more X-carrying sperm than 0-sperm, on average 70%. This is consistent with the production of more daughters. Males of the subsocial species produced a significantly lower bias of 54% X-carrying sperm. We also investigated whether inter-genomic conflict between hosts and their endosymbionts may explain female bias. Next generation sequencing showed that five common genera of bacterial endosymbionts known to affect sex ratio are largely absent, ruling out that endosymbiont bacteria bias sex ratio in social spiders. Our study provides evidence for paternal control over sex allocation through biased gamete production as a mechanism by which the heterogametic sex in CSD species adaptively adjust offspring sex ratio.

Drivers of sex ratio bias in the eastern bongo: lower inbreeding increases the probability of being born male

Parent sex ratio allocation has consequences for individual fitness, population dynamics, and conservation. Theory predicts that parents should adjust offspring sex ratio when the fitness returns of producing male or female offspring varies. Previous studies have assumed that only mothers are capable of biasing offspring sex ratios, but have neglected fathers, given the expectation of an equal proportion of X- and Y-chromosome-bearing (CBS) sperm in ejaculates due to sex chromosome segregation at meiosis. This assumption has been recently refuted and both paternal fertility and paternal genetic quality have been shown to bias sex ratios. Here, we simultaneously test the relative contribution of paternal, maternal, and individual genetic quality, as measured by inbreeding, on the probability of being born a son or a daughter, using pedigree and lifelong offspring sex ratio data for the eastern bongo ( Tragelaphus eurycerus isaaci). Our models showed first, that surprisingly, as individual inbreeding decreases the probability of being born male increases, second, that paternal genetic effects on sex ratio were stronger than maternal genetic effects (which were absent). Furthermore, paternal effects were opposite in sign to those predicted; father inbreeding increases the probability of having sons. Previous paternal effects have been interpreted as adaptive due to sex-specific inbreeding depression for reproductive traits. We argue that in the eastern bongo, the opposite sign of the paternal effect on sex ratios results from a reversed sex-specific inbreeding depression pattern (present for female but not male reproductive traits). We anticipate that this research will help stimulate research on evolutionary constraints to sex ratios. Finally, the results open a new avenue of research to predict sex ratio allocation in an applied conservation context. Future models of sex ratio allocation should also include the predicted inbreeding level of the offspring and paternal inbreeding levels.

Sexual conflict in action: An antagonistic relationship between maternal and paternal sex allocation in the tammar wallaby, Notamacropus eugenii

Sex ratio biases are often inconsistent, both among and within species and populations. While some of these inconsistencies may be due to experimental design, much of the variation remains inexplicable. Recent research suggests that an exclusive focus on mothers may account for some of the inconsistency, with an increasing number of studies showing variation in sperm sex ratios and seminal fluids. Using fluorescent in-situ hybridization, we show a significant population-level Y-chromosome bias in the spermatozoa of wild tammar wallabies, but with significant intraindividual variation between males. We also show a population-level birth sex ratio trend in the same direction toward male offspring, but a weaning sex ratio that is significantly female-biased, indicating that males are disproportionately lost during lactation. We hypothesize that sexual conflict between parents may cause mothers to adjust offspring sex ratios after birth, through abandonment of male pouch young and reactivation of diapaused embryos. Further research is required in a captive, controlled setting to understand what is driving and mechanistically controlling sperm sex ratio and offspring sex ratio biases and to understand the sexually antagonistic relationship between mothers and fathers over offspring sex. These results extend beyond sex allocation, as they question studies of population processes that assume equal input of sex chromosomes from fathers, and will also assist with future reproduction studies for management and conservation of marsupials.

DNA Methylation Patterns in the Social Spider, Stegodyphus dumicola

Variation in DNA methylation patterns among genes, individuals, and populations appears to be highly variable among taxa, but our understanding of the functional significance of this variation is still incomplete. We here present the first whole genome bisulfite sequencing of a chelicerate species, the social spider Stegodyphus dumicola. We show that DNA methylation occurs mainly in CpG context and is concentrated in genes. This is a pattern also documented in other invertebrates. We present RNA sequence data to investigate the role of DNA methylation in gene regulation and show that, within individuals, methylated genes are more expressed than genes that are not methylated and that methylated genes are more stably expressed across individuals than unmethylated genes. Although no causal association is shown, this lends support for the implication of DNA CpG methylation in regulating gene expression in invertebrates. Differential DNA methylation between populations showed a small but significant correlation with differential gene expression. This is consistent with a possible role of DNA methylation in local adaptation. Based on indirect inference of the presence and pattern of DNA methylation in chelicerate species whose genomes have been sequenced, we performed a comparative phylogenetic analysis. We found strong evidence for exon DNA methylation in the horseshoe crab Limulus polyphemus and in all spider and scorpion species, while most Parasitiformes and Acariformes species seem to have lost DNA methylation.