Extensive hybridization and introgression between Melastoma candidum and M. sanguineum

Abstract

Natural hybridization can lead to various evolutionary outcomes in plants, including hybrid speciation and interspecific gene transfer. It can also cause taxonomic problems, especially in plant genera containing multiple species. In this study, the hybrid status of Melastoma affine, the most widespread taxon in this genus, and introgression between its putative parental species, M. candidum and M. sanguineum, were assessed on two sites, Hainan and Guangdong, using 13 SSR markers and sequences of a chloroplast intergenic spacer. Bayesian-based STRUCTURE analysis detected two most likely distinct clusters for the three taxa, and 76.0% and 73.9% of the morphologically identified individuals of M. candidum and M. sanguineum were correctly assigned, respectively. 74.5% of the M. affine individuals had a membership coefficient to either parental species between 0.1 and 0.9, suggesting admixture between M. candidum and M. sanguineum. Furthermore, NewHybrids analysis suggested that most individuals of M. affine were F2 hybrids or backcross hybrids to M. candidum, and that there was extensive introgression between M. candidum and M. sanguineum. These SSR data thus provides convincing evidence for hybrid origin of M. affine and extensive introgression between M. candidum and M. sanguineum. Chloroplast DNA results were consistent with this conclusion. Much higher hybrid frequency on the more disturbed Guangdong site suggests that human disturbance might offer suitable habitats for the survival of hybrids, a hypothesis that is in need of further testing.

Figure 1. Morphological illustrations for M. sanguineum, M. affine and M. candidum.

(a) M. sanguineum; (b) M. affine; (c) M. candidum;(d) Leaves of M. sanguineum, M. affine and M. candidum (from left to right), (e) Branchlets of M. sanguineum, M. affine and M. candidum (from left to right) (f) Fruits of M. sanguineum, M. affine and M. candidum (from left to right).

Figure 2. Model-based clustering analysis by STRUCTURE based on SSR markers with K = 2.

Samples from (a) Hainan and Guangdong, (b) Hainan and (c) Guangdong were analyzed, respectively. Vertical bars represent individuals and probabilities of assignment to each cluster. The membership coefficient (qi) for each sample was listed in Table S2, S3 and S4.

Figure 3. Posterior probability distribution by using the NewHybrids program.

All the samples (a, Hainan; b, Guangdong) are represented as a vertical bar partitioned into segments whose length is proportional to the likelihood of belonging to a certain class. Ms, Ma and Mc represent morphologically identified M. sanguineum, M. affine and M. candidum, respectively. HN and GD represent the sampling sites, Hainan and Guangdong, respectively.