Abstract

Biogeographic barriers are well known to structure animal and plant diversity, but whether they are associated with variation in host-associated microbiomes, especially in tropical pollinators, remains poorly understood. Here, we investigated gut bacterial community variation in the stingless bee Tetragonula laeviceps (Apidae: Meliponini) collected from Bali and Lombok, two Indonesian islands situated on opposite sides of the Wallace Line. Using 16S rRNA gene V3–V4 amplicon sequencing, we analyzed 12 colony-level samples collected from forest and agroforestry habitats on both islands. Alpha-diversity analysis revealed variation in bacterial richness and diversity among colonies and sampling groups, with the highest richness observed in the Lombok agroforestry group. The gut microbiota was dominated by Firmicutes, Proteobacteria, Actinobacteriota and Bacteroidota, with several bee-associated genera, including Lactobacillus, Bombilactobacillus, Apilactobacillus, Bombella, Commensalibacter, Snodgrassella, Bifidobacterium and Bartonella, varying in relative abundance among island and habitat groups. Beta-diversity analyses showed significant group-associated differentiation in bacterial community composition (ANOSIM R = 0.444, p = 0.001), indicating that gut microbiome structure differed across the sampled island–habitat groups. Differential-abundance analyses identified candidate bacterial taxa associated with specific groups, suggesting that microbiome divergence was driven mainly by shifts in the relative abundance of shared bacterial lineages rather than complete replacement of dominant taxa. Predicted functional profiling further indicated that the taxonomic variation was accompanied by differences in putative functional potential. These findings suggest that the gut microbiota of T. laeviceps carries a detectable island and habitat-associated signal and highlight host-associated microbial communities as an underexplored dimension of tropical island biogeography. Because island identity and position relative to the Wallace Line are not independent in this design, these results should be interpreted as island associated microbiome variation within a Wallace Line context, not as direct evidence of a causal Wallace Line effect.

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