Hybridization and the colonization of novel habitats by annual sunflowers

Abstract

Although invasive plant species often have a hybrid ancestry, unambiguous evidence that hybridization has stimulated the evolution of invasive behaviors has been difficult to come by. Here, we briefly review how hybridization might contribute to the colonization of novel habitats, range expansions, and invasiveness and then describe work on hybrid sunflowers that forges a direct link between hybridization and ecological divergence. We first discuss the invasion of Texas by the common sunflower and show that the introgression of chromosomal segments from a locally adapted species may have facilitated range expansion. We then present evidence that the colonization of sand dune, desert floor, and salt marsh habitats by three hybrid sunflower species was made possible by selection on extreme or "transgressive" phenotypes generated by hybridization. This body of work corroborates earlier claims regarding the role of hybridization in adaptive evolution and provides an experimental and conceptual framework for ongoing studies in this area.

Plants with double genomes might have had a better chance to survive the Cretaceous–Tertiary extinction event

“…Distributions for other plant species and constraints can be found in SI Text. geous because they can contribute to hybrid vigor and provide variation that might allow fast adaptation to novel conditions (38)(39)(40). Expression pattern instability might offer polyploids a broader phenotypic range compared with their diploid progenitors.…”

Hybridization in contact zone between temperate European pine species

“…This approach has recently been developed in several model plant species demonstrating the role of hybridization and adaptive introgression in the evolution of irises (Iris; Arnold et al 2004), ecological divergence of sunflowers (Helianthus; Rieseberg et al 2007), and the signatures of divergent and balancing selection in campions (Silene; Minder and Widmer 2008) and poplar (Populus; Lexer et al 2010). So far, a few genes involved in adaptation or speciation have been identified in plants including hybrid sterility loci (Lexer and Widmer 2008), determinants of flower color-linked pollinator shifts (Hoballah et al 2007), and genes involved in hybrid necrosis (Bomblies and Weigel 2007).…”

“…For instance, it may cause swamping of the species with the smaller effective population size by gene flow from the more abundant species, integration of genetic material from one species into another through repeated back-crossing (introgression), homoploid hybrid speciation-in which the new hybrid lineages become reproductively isolated from parental populations, or transfer of adaptive traits across species boundaries (Baack and Rieseberg 2007). Well-documented examples show that hybrid genotypes may have equivalent or higher fitness relative to parental species due to environmental selection (Andrew and Rieseberg 2013;Rieseberg et al 2007;Whitney et al 2010). Even where hybrid fertility or viability is reduced in early generations, gene flow can nevertheless allow propagation of hybrids and adaptive divergence (Gross and Rieseberg 2005).…”

Hybridization as a facilitator of species range expansion

“…This introgression has two possible consequences in the focal species: (i) it can result in the acquisition of alleles for key traits that are already adaptive in the new environment [35,36,39,51,52,65,66] and (ii) it could increase standing genetic variation and opportunities for the production of novel In the case where hybridization transfers alleles for already adaptive traits, introgression enables an expanding species to 'adaptively capture' allelic variants that have already been tested, and confer adaptation, in the resident species ( [39,42,63,64,67]; figure 1). Such transfer of key alleles or coadapted sets of genes that code for already adaptive traits means that hybridizing populations can bypass unfavourable intermediate steps in adaptive evolution and thereby jump directly to the adaptive optimum in the new environment [34,39]. This scenario is most likely if key adaptive traits are underlain by major and/or linked loci or if selection on the key traits is strong.…”