Maintenance of plant species cohesion despite ongoing hybridization

The maintenance of species cohesion despite ongoing gene exchange via natural hybridization in plants is a phenomenon attracting increasing research attention. Natural hybridization can create bridges for gene flow, offering a platform for adaptive evolution by introducing variation and novel traits into populations, potentially resulting in introgression and admixture of genotypes. With advances in genetic technologies, researchers have been able to uncover greater complexity within hybrid populations and we can now delve deeper into how interspecific gene exchange can be ongoing despite the presence of strong reproductive barriers.

When species cohesion is maintained despite ongoing natural hybridization, many questions are raised about the evolutionary processes operating in the species complex. A recent study in Annals of Botany examines the extensive natural hybridization between the Australian native shrubs Lomatia myricoides and L. silaifolia (Proteaceae). These species exhibit striking differences in morphology and ecological preferences, exceeding those found in most studies of hybridization to date. The results show that morphological and ecological distinctions between plant species can be maintained despite ongoing gene flow via natural hybridization. Localized gene flow and introgression are expected to be ongoing between L. myricoides and L. silaifolia and their hybrids wherever they occur in sympatry, due to the permeability of this species barrier.

McIntosh, E. J., Rossetto, M., Weston, P. H., & Wardle, G. M. (2014) Maintenance of strong morphological differentiation despite ongoing natural hybridization between sympatric species of Lomatia (Proteaceae). Annals of Botany, 113 (5): 861-872. doi: 10.1093/aob/mct314
Nuclear microsatellite markers (nSSRs), genotyping methods and morphometric analyses were used to uncover patterns of hybridization and the role of gene flow in morphological differentiation between sympatric species.
The complexity of hybridization patterns differed markedly between sites, however, signals of introgression were present at all sites. One site provided evidence of a large hybrid swarm and the likely presence of multiple hybrid generations and backcrosses, another site a handful of early generational hybrids and a third site only traces of admixture from a past hybridization event. The presence of cryptic hybrids and a pattern of morphological bimodality amongst hybrids often disguised the extent of underlying genetic admixture.
Distinct parental habitats and phenotypes are expected to form barriers that contribute to the rapid reversion of hybrid populations to their parental character state, due to limited opportunities for hybrid/intermediate advantage. Furthermore, strong genomic filters may facilitate continued gene flow between species without the danger of assimilation. Stochastic fire events facilitate temporal phenological isolation between species and may partly explain the bi-directional and site-specific patterns of hybridization observed. Furthermore, the findings suggest that F1 hybrids are rare, and backcrosses may occur rapidly following these initial hybridization events.