Ecological drivers of parasite genetic diversity: evidence for dilution effects in a single strongylid species infecting sympatric Bornean primates.

Biodiversity loss and emerging diseases threaten ecosystem and human health. Identifying ecological drivers of host-parasite dynamics in human-altered landscapes is crucial, including at the scale of parasite genetic diversity. We investigated genetic diversity and ecological drivers of gastrointestinal strongylid nematodes infecting a primate community in Sabah, Malaysian Borneo. We surveyed primates in the Lower Kinabatangan Wildlife Sanctuary, and used high-throughput sequencing (HTS) of ITS2 rDNA from 250 fecal samples (five out of ten sympatric primates). Through boat-surveys and remote sensing, we assessed habitat quality, host diversity, and density effects on parasite genetic alpha and beta diversity-i.e., sample amplicon sequence variant (ASV) richness, evenness and composition. Matching previous reports, HTS confirmed Oesophagostomum aculeatum as the sole strongylid species, exhibiting variable ASV diversity. Primate host diversity exerted a negative effect-a.k.a. a dilution effect-on O. aculeatum ASV richness during the wet season, controlling for strong seasonality. Effects of habitat quality and host density were inconsistent on ASV richness. No effect was found on ASV evenness. ASV composition varied by host species, season, and habitat quality, but not primate diversity or density. By demonstrating a local dilution effect at a small spatial and phylogenetic scale, our findings emphasize the importance of integrating ecological and molecular approaches. This study provides a baseline for future research on host-parasite co-evolution in Southeast Asian primates, ecological drivers of parasite genetic diversity, and insights into how phenomena like the diversity-disease relationship can operate across nested scales, with implications for disease emergence risks.