Apomixis as a facilitator of range expansion and diversification in plants

Abstract

Apomixis, the asexual reproduction via seed, often occurs in huge plant polyploid complexes with large geographical distributions. However, the long-term evolutionary fate of asexuals traditionally was regarded as doomed by extinction. A seven-step evolutionary model is presented to explain the role of sex → apomixis shifts on geographical cytotype distributions, and the potential consequences of reversals apomixis → sex on plant diversity. Accordingly, apomictic polyploid genotypes act as facilitators for range expansions of asexual taxa in agamic complexes by functioning as pioneer explorers of new niches. High intragenomic (allelic) diversity and epigenetic variability may help for rapid adaptation. Therefore, they could rapidly expand the distribution areas of their progenitor sexual populations by occupying new ecological niches and geographical areas. Hence, apomixis would result in divergent patterns of geographic distribution between sexual and asexuals, a pattern described as "geographical parthenogenesis," in which apomicts occupy extensive geographical areas and higher latitudinal zones while sexual relatives are restricted to small refugees. Later on, reversals to complete sexuality would allow for the establishment of new sexual populations in different habitats without the long-term disadvantages of asexuality. The new sexual recombinants will be genetically isolated from the original sexual populations and consequently predisposed to a divergent evolution, and potentially enabled to evolve into new sexual species. The present model stresses a previously unidentified evolutionary significance of the geographical parthenogenesis as a motor for plant diversification.