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Evolutionary ecology of intra- and inter-specific interactions in spiders
Spiders are common arthropod predators living in most habitats of the world, and due to their plasticity in exploitation of resources, different species may have wide distribution ranges that cover different habitat types. While most species live solitarily, some spider species have independently evolved a permanently social lifestyle where all group members cooperate in colony tasks such as prey capture and brood care. Spiders harbour microbial communities, which may vary among individual hosts (or host groups), and which could be important for shaping phenotypic responses to the local environment. In this thesis, I study ecological interactions within spider species, and between spiders and their local environment and their microbial communities. The thesis aims to understand adaptive responses that enable persistence in different environments and the occupation of new niches.The first chapter explores the organization of spider groups in relation to task differentiation, especially the link between task differentiation, personality, metabolism and reproductive state. The study found some (but limited) evidence indicating that personality, metabolic rate or reproductive status could explain task differentiation in the social spider Stegodyphus mimosarum.The second chapter investigates the role of intolerance towards conspecifics in dispersal decisions and group cohesion both in a temporary and a permanently social Stegodyphus species. Intolerance towards conspecifics is not the cause of dispersal, but rather develops after dispersal to maintain a solitary lifestyle in the temporary social species Stegodyphus lineatus. Intolerance is lost in the social species Stegodyphus sarasinorum, suggesting that this trait is redundant in permanently social species, because they do not interact with competing groups.The third chapter examined the interaction between social spiders and the host plants on which they build their communal nests. The study suggests that Stegodyphus dumicola spiders build their nests preferentially on specific plant species, including host plants with protective spines, and the preferred host plants are associated with improved survival.The fourth chapter investigated at the mode of transmission of bacterial symbionts in the social spider Stegodyphus dumicola, using a combination of life cycle analysis and experimental cross-fostering of offspring. Bacterial symbionts are transmitted across generations by a mixed mode of transmission involving regurgitation feeding by both mothers and non-reproducing female helpers. In the adult state, social transmission governs homogenization of microbiome composition among group-members.The fifth chapter tested whether ecological drift could cause variation in microbiome composition among groups in a social host species Stegodyphus dumicola. The results suggest that stochastic divergence in host-group microbiomes can arise during the process of group formation by individual founders, which could explain the existence of among-group variation in microbiome composition in the wild.My studies were interrupted by the Covid-19 pandemic, which delayed fieldwork (Chapter 6) and caused me to initiate a new study system (Chapter 7). Chapter 6 explores three-way associations between host plants, the social spider Stegodyphus dumicola and their microbial symbionts, to investigate whether plant host microbiomes or their chemical compounds are beneficial or transmitted to social spiders. This project is ongoing, and preliminary results on the microbiome of host plants and spider nests are presented. In the last chapter, I conducted research on the possible effects of urbanization on microbiome composition and diet of the solitary Garden spider Araneus diadematus, to explore whether the microbiome might be involved in occupation of urban areas. This project is ongoing, and preliminary results are presented.
Spiders are common arthropod predators living in most habitats of the world, and due to their plasticity in exploitation of resources, different species may have wide distribution ranges that cover different habitat types. While most species live solitarily, some spider species have independently evolved a permanently social lifestyle where all group members cooperate in colony tasks such as prey capture and brood care. Spiders harbour microbial communities, which may vary among individual hosts (or host groups), and which could be important for shaping phenotypic responses to the local environment. In this thesis, I study ecological interactions within spider species, and between spiders and their local environment and their microbial communities. The thesis aims to understand adaptive responses that enable persistence in different environments and the occupation of new niches.The first chapter explores the organization of spider groups in relation to task differentiation, especially the link between task differentiation, personality, metabolism and reproductive state. The study found some (but limited) evidence indicating that personality, metabolic rate or reproductive status could explain task differentiation in the social spider Stegodyphus mimosarum.The second chapter investigates the role of intolerance towards conspecifics in dispersal decisions and group cohesion both in a temporary and a permanently social Stegodyphus species. Intolerance towards conspecifics is not the cause of dispersal, but rather develops after dispersal to maintain a solitary lifestyle in the temporary social species Stegodyphus lineatus. Intolerance is lost in the social species Stegodyphus sarasinorum, suggesting that this trait is redundant in permanently social species, because they do not interact with competing groups.The third chapter examined the interaction between social spiders and the host plants on which they build their communal nests. The study suggests that Stegodyphus dumicola spiders build their nests preferentially on specific plant species, including host plants with protective spines, and the preferred host plants are associated with improved survival.The fourth chapter investigated at the mode of transmission of bacterial symbionts in the social spider Stegodyphus dumicola, using a combination of life cycle analysis and experimental cross-fostering of offspring. Bacterial symbionts are transmitted across generations by a mixed mode of transmission involving regurgitation feeding by both mothers and non-reproducing female helpers. In the adult state, social transmission governs homogenization of microbiome composition among group-members.The fifth chapter tested whether ecological drift could cause variation in microbiome composition among groups in a social host species Stegodyphus dumicola. The results suggest that stochastic divergence in host-group microbiomes can arise during the process of group formation by individual founders, which could explain the existence of among-group variation in microbiome composition in the wild.My studies were interrupted by the Covid-19 pandemic, which delayed fieldwork (Chapter 6) and caused me to initiate a new study system (Chapter 7). Chapter 6 explores three-way associations between host plants, the social spider Stegodyphus dumicola and their microbial symbionts, to investigate whether plant host microbiomes or their chemical compounds are beneficial or transmitted to social spiders. This project is ongoing, and preliminary results on the microbiome of host plants and spider nests are presented. In the last chapter, I conducted research on the possible effects of urbanization on microbiome composition and diet of the solitary Garden spider Araneus diadematus, to explore whether the microbiome might be involved in occupation of urban areas. This project is ongoing, and preliminary results are presented.