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Island ontogenies, syngameons, and the origins and evolution of genetic diversity in the Canarian endemic flora

Caujapé-Castells, Juli, García-Verdugo, Carlos, Marrero-Rodríguez, Águedo, Fernández-Palacios, José María, Crawford, Daniel J., Mort, Mark E.
Perspectives in plant ecology, evolution and systematics 2017 v.27 pp. 9-22
allopolyploidy, dynamic models, flora, gene flow, genetic variation, genotype, introgression, islands, monophyly, ontogeny, secondary contact
We use the molecular data available for the plant genera currently distributed in the Canaries to propose a hypothetical framework that updates the Surfing Syngameon Hypothesis (SSH) to explain the origins of the high genetic diversity of this flora in connection with the General Dynamic Model of Island Ontogeny and the available knowledge on the geology of the islands. Based on our review, we propose that (i) secondary contact and subsequent gene flow among genotypes previously confined to the mainland or to other insular regions generated syngameons across the archipelago, whose temporal extent and distribution were influenced by the ontogenetic stages of each island; and (ii) the resulting high levels of genetic variation were critical to the successful colonization of many other regions in the Canaries, especially those where more complex geographical and ecological features promoted gene flow cessation, drift, and/or eventual selection processes leading to species radiations. Under the updated SSH, the colonizing contingents of the islands would not necessarily have low genetic variation, because multiple colonizations followed by secondary contact among different genotypes would enhance genetic diversity through introgressive hybridization. As with other theories, the SSH contends that the monophyly detected in a large proportion of its lineages is artifactual. However, the SSH further differs from past hypotheses in (i) suggesting a spatio-temporal sequence of events giving rise to syngameons in different islands and island regions, (ii) showing that the ontogenetic mismatch among the Canarian islands should be largely responsible for similar levels of population genetic diversity in many congeneric endemics distributed in several islands, (iii) hypothesizing a substantial role for both allopolyploid and homoploid hybridization in the origins of the Canarian flora, and (iv) positing that, in many cases, the admixture processes in the syngameons formed at different stages of the islands’ ontogeny made possible the rapid colonization of multiple environments, and the generation of the current floristic diversity.