Niche divergence of two closely related Carbula species (Insecta: Hemiptera: Pentatomidae) despite the presence of a hybrid zone

1. The extent to which ecologically divergent selection acts to maintain species boundaries in the presence of hybridisation and gene flow is not well understood. Two parapatric taxa, Carbula humerigera (Uhler) and Carbula putoni (Jakovlev), were used to test the extent to which niche differentiation might sustain divergence of related taxa despite ongoing gene flow. 2. Mitochondrial [cytochrome c oxidase subunit I (COI), cytochrome c oxidase subunit II (COII) and cytochrome b (Cytb)] and nuclear [elongation factor 1-alpha (EF-1)] markers were sequenced from 383 individuals. Clusters of morphological variations were estimated and visualised using principal component analysis (PCA). Haplotype networks were constructed using median-joining and neighbour-net algorithm methods. Gene flows were estimated using migrate-n analyses. Suitable habitats for each species and their sympatric distribution were predicted with ecological niche modelling (ENM). Niche comparisons were conducted using Schoener's D and Warren's I. In addition, PCA, multivariate analysis of variance (manova) and discriminant function analysis were used to test ecological differentiation. 3. Morphological clusters and network analysis indicated that samples were generally divided into three groups (C. humerigera, C. putoni and hybrids). Ongoing gene flow was detected among the three groups, with abundant magnitudes in the sympatric region. Niche comparison and statistical analysis showed ecological differentiation between C. humerigera and C. putoni. Two potential hybrid zones were predicted in the ENM reconstruction, located along the Yanshan-Taihang-Qinling and Taishan mountains. 4. These results reveal a geographically delineated hybrid zone between C. humerigera and C. putoni. These two closely related Carbula species still live in different ecological niches despite hybridisation and ongoing gene flow.