Host-symbiont relationships exhibit varying degrees of association, from obligate to facultative. Understanding symbiont transmission (vertical – parent to offspring; horizontal – within generation) is crucial for understanding microbiome dynamics in wild populations. While vertical transmission is often associated with stable associations, horizontal transmission can also maintain tight relationships. The timing of symbiont acquisition is also critical, as evidenced by studies on corals, stinkbugs, earthworms, and leaf-cutting ants. Social interactions can mediate symbiont transmission, particularly in group-living animals, where proximity and frequent interactions increase the likelihood of social transmission. Examples across various taxa, including primates, mice, bees, and termites, support this. The social spider *Stegodyphus dumicola* provides a compelling model due to its communal nests, communal feeding, reproductive skew (only about one-third of females reproduce), and allomaternal care (all females provide regurgitation feeding to offspring). These social characteristics may favor horizontal transmission, as non-reproductive females represent dead ends for vertically transmitted symbionts. However, regurgitation and matriphagy also offer pathways for vertical transmission. The researchers hypothesized that endobacteria in *S. dumicola* are maintained by horizontal transmission, partially mediated by social interactions, leading to microbiome homogenization within a nest.

The introduction extensively reviews the existing literature on host-symbiont relationships, focusing on transmission modes and their impact on symbiosis stability. It highlights the importance of considering both vertical and horizontal transmission, and the critical role of timing in symbiont acquisition. The review also emphasizes the role of social interactions in mediating symbiont transmission in various group-living species. Specific examples are drawn from corals, stinkbugs, earthworms, ants, primates, mice, bees, and termites. The literature on social spiders, particularly the *Stegodyphus* genus, is also reviewed, noting the previously observed within-group similarity and between-group variation in microbiome composition. The lack of clarity on the role of social interactions in shaping this pattern is explicitly mentioned, setting the stage for the current study’s investigation.

The study used three main experiments to investigate symbiont transmission in *S. dumicola*. Nests of spiders were collected from Namibia and Botswana. Spiders were maintained under controlled laboratory conditions. **Life Cycle Experiment:** This experiment aimed to determine at which life stage offspring acquire microbial symbionts. Four nests with adult females harboring different bacterial symbionts were sampled across all life stages (eggs, instars). Samples were analyzed using 16S rRNA gene amplicon sequencing and qPCR. **Cross-fostering Experiment:** Seven nests were used in a cross-fostering design to investigate whether offspring acquire their microbiome from their natal or foster nest. The microbiome composition of adult females was analyzed. Offspring were raised either by their natal mothers or foster mothers, and their microbiome composition was compared to that of their biological and foster mothers. **Mixing Experiment:** Spiders from three nests with different microbiome compositions were mixed to investigate whether social transmission homogenizes microbiome composition. The relative abundance of symbionts was tracked over 39 days. DNA extraction was performed using the DNeasy Blood and Tissue Kit. 16S rRNA gene amplicon libraries were prepared following Illumina's 16S Metagenomic Sequencing Library Preparation guide, using Bac341F and Bac805R primers. Sequencing was performed on a MiSeq Desktop sequencer. qPCR was used to determine the bacterial load. Data analysis was performed using R packages including Phyloseq, Microbiome, ggplot2, and Decipher.

The study's key findings are: 1. **Offspring hatch symbiont-free:** No bacterial 16S rRNA genes were detected in eggs or first instar spiderlings. 2. **Vertical transmission through regurgitation feeding:** Bacteria were first detected in instar 2, coinciding with the onset of regurgitation feeding by mothers. The offspring microbiome closely resembled that of their mothers. 3. **Social transmission homogenizes microbiomes:** Cross-fostering experiments showed offspring acquiring microbiomes similar to their foster mothers, indicating horizontal transmission through social interaction. Mixing experiments demonstrated that combining spiders with different microbiomes led to homogenization of symbiotic compositions, particularly with unequal mixing ratios. This homogenization, however, varied depending on the specific symbiont and the presence of other symbionts, suggesting inter-symbiont interactions. 4. **Mixed transmission mode:** The findings suggest a mixed-mode transmission system, combining vertical (through regurgitation) and horizontal (social) transmission. 5. **High fidelity transmission:** Despite the mixed mode, the high similarity in microbiome composition across generations points towards high fidelity transmission, suggesting functional significance of the symbiosis.

The findings demonstrate a mixed-mode transmission system, combining vertical transmission via regurgitation feeding and horizontal transmission facilitated by social interactions. This contradicts the initial hypothesis of predominantly horizontal transmission. The high fidelity of the transmission, evidenced by the temporal stability of the microbiome, highlights the functional importance of the symbiosis. The study also reveals inter-symbiont interactions influencing transmission success, potentially through competition or mutual exclusion. The lack of correlation between host genetics and microbiome composition suggests that host genetics does not primarily drive these symbiont interactions. The results are discussed in the context of the evolution of mixed-mode transmission in social species, highlighting the role of sociality in mitigating the risks associated with vertical transmission to non-reproductive individuals. The study also mentions the relative lack of overlap between the spider microbiome and that of the nest, suggesting the nest itself does not play a significant role in environmental transmission.

This study provides strong evidence for a mixed-mode transmission system, combining vertical and horizontal (social) transmission of bacterial symbionts in the social spider *S. dumicola*. High-fidelity social transmission plays a crucial role in maintaining stable host-symbiont associations. Future research should investigate the mechanisms underlying symbiont compatibility, competition, and the potential influence of social behavior on microbiome assembly and dynamics.

The study was conducted under controlled laboratory conditions, which may not fully reflect the complexity of natural environments. The sample sizes, while sufficient for statistical analysis, could be expanded for a more comprehensive understanding. Further investigation is needed to elucidate the exact mechanisms of inter-symbiont interactions and their impact on transmission dynamics.