Although numerous studies have documented horizontal gene transfer (HGT) from bacteria into insect genomes, they have been heavily biased toward the endosymbiont Wolbachia. In contrast, comparatively few studies have examined HGT from other symbionts and insect-associated bacteria. Moreover, research on these systems has primarily focused on transferred genes that retain function in the insect host, contributing to metabolism, symbiosis, detoxification, or host adaptation. Consequently, relatively few empirical systems have been available to investigate the persistence, degradation, diversification, and vertical inheritance of apparently non-functional HGT-derived sequences over macroevolutionary timescales. The widespread occurrence of putatively Arsenophonus-derived sequences across Cimicidae therefore provides a rare opportunity to extend these investigations beyond the Wolbachia model and to examine the long-term evolutionary fate of non-functional HGT-derived DNA across tens of millions of years of host diversification. We show that Arsenophonus-derived HGTs have multiple independent origins across Cimicidae but that one major HGT lineage has persisted through diversification of the subfamily Cimicinae for at least 50 million years. Phylogenetic and compositional analyses indicate that an ancestral Arsenophonus-derived genomic region has undergone progressive fragmentation, leaving numerous dispersed, apparently non-functional remnants while preserving a clear evolutionary signature. These results extend the study of bacterial HGT beyond the Wolbachia model and demonstrate that non-functional symbiont-derived DNA can persist over macroevolutionary timescales.
Shruti Gupta, J. Martinů, O. Balvín et al.· bioRxiv· 0 citations
Blood-feeding insects typically depend on obligate intracellular bacterial symbionts that provide essential B vitamins absent from vertebrate blood. In contrast, kissing bugs (Triatominae) have long been considered atypical because they rely primarily on extracellular gut-associated bacteria. Recent reports of the genus Symbiopectobacterium in Rhodnius species raise questions about the diversity and evolution of symbiosis in these insects. Here, we investigate the distribution, genome evolution, and tissue localization of Symbiopectobacterium in the genus Rhodnius. Using comparative metagenomics, phylogenomics, fluorescence in situ hybridization, transmission electron microscopy, and hemolymph screening, we characterize a Symbiopectobacterium genome from Rhodnius prolixus and assess its occurrence across publicly available datasets representing multiple Rhodnius species. The R. prolixus strain possesses a large, highly dynamic genome enriched in mobile genetic elements, pseudogenes, and remnants of secretion systems, while retaining biosynthetic pathways for several B vitamins. Comparative analysis revealed variation in genome reduction among Rhodnius-associated strains, suggesting ongoing and potentially independent transitions toward host-restricted symbiosis. Localization analyses detected Symbiopectobacterium intracellularly within posterior midgut epithelial cells and occasionally in the hemolymph, consistent with a facultative intracellular lifestyle. However, no bacteriomes or stable intracellular structures were observed. Together, these findings indicate that Symbiopectobacterium represents an intermediate stage in the transition from environmentally associated bacteria to obligate intracellular mutualists in Triatominae.
Tanisha Moons, S. Mendiola, Hassan Tarabai et al.· bioRxiv· 0 citations
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