Maintenance of 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.
Olives have a long and complex history. The origins of the Mediterranean cultivated olive (Olea europaea subsp. europaea) are hotly debated, but it is usually accepted that its domestication started in the Levant based on archaeological, historical and molecular evidence. Multiple local selections of cultivars has been suggested by genetic analyses, followed by secondary diversification of the crop followed the oleiculture diffusion over the whole Mediterranean basin. The contribution of western wild olives in this diversification process remains poorly understood.
A recent paper in Annals of Botany describes patterns of genetic differentiation in Mediterranean and Saharan olives, and tests for admixture between these taxa. Based on the results, the human-meditated diffusion of the oleiculture over the Mediterranean basin and the contribution of O. europaea subsp. laperrinei to the cultivated olive diversification are discussed. Although its genetic contribution is limited, it is clear from this work that Laperrine’s olive has been involved in the diversification of cultivated olives.
Individual mating patterns in mixed oak stands
Individual variation in mating patterns may have significant implications for persistence and adaptation of plant populations, but field data generally focus on population averages. Using a Bayesian approach, Chybicki and Burczyk examine the extent of individual variation of several components of mating patterns in a mixed stand of Quercus robur and Q. petraea. They find that there is a great variation in intra- and inter-specific individual mating preferences, individual pollen immigration rates and heterogeneity of immigrating pollen. They show that trees can mate assortatively, with little respect to spatial proximity. Such selective mating may be a result of variable compatibility among trees due to genetic and/or environmental factors.
Single-copy nuclear genes for palm phylogenetics
Molecular phylogenetic studies of palms (Arecaceae) have not yet provided a fully resolved phylogeny of the family. Ludeña et al. test the value of AGAMOUS 1 and PHYTOCHROME B genes as new nuclear markers to improve phylogenetic resolution in the family, using the subtribe Bactridinae as a case study. The results provide new insights into the intergeneric relationships within Bactridinae and the intrageneric structure of Astrocaryum, and the existence of a monophyletic group sister to Astrocayum, corresponding to the debated genus Hexopetion, is supported. The new markers thus provide additional phylogenetic information within the palm family, and should prove useful in combination with other genes to improve the resolution of palm phylogenies.
Diversification in Hawaiian Dubautia laxa
The Hawaiian silversword alliance (Asteraceae) is one the best examples of a plant adaptive radiation, exhibiting extensive morphological and ecological diversity. Using subspecies of Dubautia laxa as an example, McGlaughlin & Friar show that genetic data demonstrates that members of the species have diverged primarily due to geographic isolation both within and among islands. Despite distinct morphological and ecological traits, subspecies that are distributed allopatrically are determined to have not diverged genetically.
How far can a gene disperse? Historical and contemporary gene dispersal can be estimated from spatial genetic structure and paternity analysis, and Rong et al. (pp. 285–296) find that an estimate of gene flow in Daucus carota ssp. carota based on contemporary pollen dispersal is much larger than an estimate of historical flow. The results have implications for the ease with which transgene flow might occur from cultivated GM carrots to wild carrot populations.