Microhabitat niche partitioning associated with hybridization among sympatric Ixora species in a tropical rainforest

Tropical rainforests exhibit remarkable plant diversity, including many congeneric species that coexist sympatrically within small spatial scales. We investigated whether interspecific hybridization contributes to this diversity by facilitating microhabitat niche shifts within confined geographic ranges. Using five sympatric Ixora species in a 52-hectare rainforest plot in Borneo, we integrated genetic analyses with detailed microhabitat assessments. Hybridization analyses revealed significant past gene flow between Ixora brevicaudata and I. caudata . Microhabitat analysis showed that I. brevicaudata occupies a markedly different niche from its sister species and gene flow partner, I. glomeruliflora and I. caudata , as well as other sympatric congeners within a limited geographic range (~1 km). Specifically, I. brevicaudata thrives in flat, low-elevation areas with calcium- and magnesium-rich soils, whereas other species exhibited niche partitioning along soil pH and phosphorus content. Notably, phylogenies constructed from putatively selected SNPs associated with microhabitat variables grouped I. brevicaudata with I. caudata , contrasting with genome-wide patterns that placed I. brevicaudata with I. glomeruliflora , indicating that introgression involved microhabitat-adaptive loci. These findings support a potential role for hybridization involving microhabitat-adaptive loci in fine-scale species diversity.