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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 inmales 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 flowcytometry, and showthatmales 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 throughbiased gamete production as amechanismbywhich the heterogametic sex in CSD species adaptively adjust offspring sex ratio. [ABSTRACT FROM AUTHOR]

Task differentiation in cooperative groups can arise through a wide range of mechanisms. A recent hypothesis, supported by empirical studies of social spiders, suggests that a variation in individual personalities might be one such mechanism. Personality-based task differentiation can either arise as an emergent property of group living based on inherent behavioural variation among individuals, or alternatively it could be an adaptation to permanent sociality favoured by individual fitness benefits arising from increased group efficiency and productivity. We tested whether personality-based task differentiation is present in the temporarily social spider Stegodyphus tentoriicola which represents the ancestral state from which permanent sociality in spiders has evolved. This species is closely related to the permanently social Stegodyphus sarasinorum in which personality-based task differentiation has been documented. We measured individual boldness and investigated individual participation in simulated prey capture events twice per day on 5 consecutive days. We found no evidence for personality-based task differentiation: S. tentoriicola spiders did not exhibit consistent personalities in boldness or consistent task differentiation in prey attack. Our results lend support for the hypothesis that personality-based task differentiation is an adaptation to permanently social living in spiders as their ancestral representative lacks this trait. [ABSTRACT FROM AUTHOR]

Colonies of web building social spiders may persist at a site for several generations; therefore, their placement in the habitat is critical for survival. This study focuses on the development of spatial distribution patterns by means of different dispersal modes in a social spider species (Stegodyphus dumicolaPocock, 1898, Eresidae) in central Namibia. Social spiders disperse varying distances from the parent colony by ballooning or short distances by budding. Using spatial point pattern analyses we aimed to identify the dispersal pattern from source colonies and what drivers affect the resulting distribution patterns. The distribution of suitable vegetation constrained pattern development in the three study plots with mapped colonies. In a plot with isolated large trees, colonies were only aggregated over short distances and the average cluster size was small. First generation colonies were established within the same clusters by solitary ballooning females. In the two study plots with more evenly distributed shrubs, clusters were generally larger and new colonies were formed mainly by budding. In these plots, newly established colonies and older source colonies were either associated in suitable habitat patches or segregated from each other. Possible explanations include high mortality of either older colonies and replacement by newly established colonies or of dispersers in the vicinity of established colonies, or selection of “empty” sites by dispersing individuals based on cues from conspecific colonies. In conclusion, both the distribution of the vegetation and the mode of dispersal may explain the development of spatial patterns inS. dumicola. Our results highlight the importance of spatial pattern analyses for inferring underlying causes of distribution of sedentary organisms. [ABSTRACT FROM PUBLISHER]

Inbreeding mating systems are uncommon because of inbreeding depression. Mating among close relatives can evolve, however, when outcrossing is constrained. Social spiders show obligatory mating among siblings. In combination with a female-biased sex ratio, sib-mating results in small effective populations. In such a system, high genetic homozygosity is expected, and drift may cause population divergence. We tested the effect of outcrossing in the social spider Stegodyphus dumicola. Females were mated to sib-males, to a non-nestmate within the population, or to a male from a distant population, and fitness traits of F1s were compared. We found reduced hatching success of broods from between-population crosses, suggesting the presence of population divergence at a large geographical scale that may result in population incompatibility. However, a lack of a difference in offspring performance between inbred and outbred crosses indicates little genetic variation between populations, and could suggest recent colonization by a common ancestor. This is consistent with population dynamics of frequent colonizations by single sib-mated females of common origin, and extinctions of populations after few generations. Although drift or single mutations can lead to population divergence at a relatively short time scale, it is possible that dynamic population processes homogenize these effects at longer time scales. [ABSTRACT FROM AUTHOR]

Introduction: Sociality has evolved independently multiple times across the spider phylogeny, and despite wide taxonomic and geographical breadth the social species are characterized by a common geographical constrain to tropical and subtropical areas. Here we investigate the environmental factors that drive macro-ecological patterns in social and solitary species in a genus that shows a Mediterranean-Afro-Oriental distribution (Stegodyphus). Both selected drivers (productivity and seasonality) may affect the abundance of potential prey insects, but seasonality may further directly affect survival due to mortality caused by extreme climatic events. Based on a comprehensive dataset including information about the distribution of three independently derived social species and 13 solitary congeners we tested the hypotheses that the distribution of social Stegodyphus species relative to solitary congeners is: (1) restricted to habitats of high vegetation productivity and (2) constrained to areas with a stable climate (low precipitation seasonality). Results: Using spatial logistic regression modelling and information-theoretic model selection, we show that social species occur at higher vegetation productivity than solitary, while precipitation seasonality received limited support as a predictor of social spider occurrence. An analysis of insect biomass data across the Stegodyphus distribution range confirmed that vegetation productivity is positively correlated to potential insect prey biomass. Conclusions: Habitat productivity constrains the distribution of social spiders across continents compared to their solitary congeners, with group-living in spiders being restricted to areas with relatively high vegetation productivity and insect prey biomass. As known for other taxa, permanent sociality likely evolves in response to high predation pressure and imposes within-group competition for resources. Our results suggest that group living is contingent upon productive environmental conditions where elevated prey abundance meet the increased demand for food of social groups. [ABSTRACT FROM AUTHOR]

In animal societies, recognition of group members and relatives is an important trait for the evolution and maintenance of social behavior. In eusocial insects, nest mate recognition is based on cuticular hydrocarbons and allows colony members to reject competitors and parasites. The study of recognition cues in subsocial species can provide insights into evolutionary pathways leading to permanent sociality and kin-selected benefits of cooperation. In subsocial spiders, empirical evidence suggests the existence of both kin recognition and benefits of cooperating with kin, whereas the cues for kin recognition have yet to be identified. However, cuticular hydrocarbons have been proposed to be involved in regulation of tolerance and interattraction in spider sociality. Here, we show that subsocial Stegodyphus lineatus spiderlings have cuticular hydrocarbon profiles that are sibling-group specific, making cuticular hydrocarbons candidates for kin recognition cues. Our behavioral assays indicate that spiderlings can discriminate between cuticular cues from kin and nonkin: In a choice set-up, spiderlings more often chose to reside near cuticular chemical extracts of siblings compared with nonsiblings. Furthermore, we show that cuticular chemical composition changes during development, especially around the stage of dispersal, supporting the hypothesis that cuticular cues are involved in regulating conspecific tolerance levels. Lastly, our results indicate that the potential kin recognition cues might be branched alkanes that are influenced very little by rearing conditions and may be genetically determined. This indicates that a specific group of cuticular chemicals, namely branched alkanes, could have evolved to act as social recognition cues in both insects and spiders. [ABSTRACT FROM PUBLISHER]

Cooperation and group living are extremely rare in spiders and only few species are known to be permanently social. Inbreeding is a key characteristic of social spiders, resulting in high degrees of within-colony relatedness that may foster kin-selected benefits of cooperation. Accordingly, philopatry and regular inbreeding are suggested to play a major role in the repeated independent origins of sociality in spiders. We conducted field observations and laboratory experiments to investigate the mating system of the subsocial spider Stegodyphus tentoriicola. The species is suggested to resemble the ‘missing link’ in the transition from subsociality to permanent sociality in Stegodyphus spiders because its social period is prolonged in comparison to other subsocial species. Individuals in our two study populations were spatially clustered around maternal nests, indicating that clusters consist of family groups as found in the subsocial congener Stegodyphus lineatus. Male mating dispersal was limited and we found no obvious pre-copulatory inbreeding avoidance, suggesting a high likelihood of mating with close kin. Rates of polygamy were low, a pattern ensuring high relatedness within broods. In combination with ecological constraints, such as high costs of dispersal, our findings are consistent with the hypothesis that the extended social period in S. tentoriicola is accompanied with adaptations that facilitate the transition towards permanent sociality. © 2009 The Linnean Society of London, Biological Journal of the Linnean Society, 2009, 98, 851–859. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Deciphering the mechanisms involved in shaping social structure is key to a deeper understanding of the evolutionary processes leading to sociality. Individual specialization within groups can increase colony efficiency and consequently productivity. Here, we test the hypothesis that within-group variation in individual personalities (i.e. boldness and aggression) can shape task differentiation. The social spider Stegodyphus sarasinorum (Eresidae) showed task differentiation (significant unequal participation) in simulated prey capture events across 10-day behavioural assays in the field, independent of developmental stage (level of maturation), eliminating age polyethism. Participation in prey capture was positively associated with level of boldness but not with aggression. Body size positively correlated with being the first spider to emerge from the colony as a response to prey capture but not with being the first to attack, and dispersal distance from experimental colonies correlated with attacking but not with emerging. This suggests that different behavioural responses to prey capture result from a complex set of individual characteristics. Boldness and aggression correlated positively, but neither was associated with body size, developmental stage or dispersal distance. Hence, we show that personalities shape task differentiation in a social spider independent of age and maturation. Our results suggest that personality measures obtained in solitary, standardized laboratory settings can be reliable predictors of behaviour in a social context in the field. Given the wealth of organisms that show consistent individual behavioural differences, animal personality could play a role in social organization in a diversity of animals. [ABSTRACT FROM AUTHOR]