Plant speciation across environmental gradients and the occurrence and nature of hybrid zones

Abstract

Environmental gradients are very common and many plant species respond to them through adaptive genetic change. This can be a first step along a continuum of change that leads ultimately to the origin of fully reproductively isolated forms, i.e., 'biological species'. Before complete reproductive isolation is achieved, hybrid zones may form between divergent lineages either through primary intergradation or secondary contact. Here, I review the literature on plant hybrid zones between native species and highlight: mode of origin (primary intergradation versus secondary contact); distribution among plant families, genera and life form; type and genotypic composition related to strength and type of reproductive isolation between parental lineages; nature of prezygotic and postzygotic reproductive barriers; level and direction of gene flow; and the stability of hybrid zones in the face of climate change. The total number of plant hybrid zones detected in a literature search was surprisingly small (137). This was the case even for areas of the world with a long history of research into plant evolution, ecology and systematics. Reasons for this are discussed, including the possibility that plant hybrid zones are naturally rare in the wild. Only for a few hybrid zones have attempts been made to distinguish between formation by primary intergradation or secondary contact, and it is assumed that most hybrid zones originate through secondary contact. From the limited information available, it appears that plant hybrid zones may frequently move in response to climate change, but long-term studies are required to confirm this.

Chloroplast phylogenomics of the New World grape species (Vitis, Vitaceae)

“…) and probable hybrid speciation (see Moore & Wen, ). Future work should specially focus on testing hybrid speciation hypotheses using dense sampling of populations (Abbott, ). Additionally, detailed analyses of Vitis based on nuclear phylogenetic results integrated with niche modeling (also see Zimmer & Wen, , ; Wen et al, ; Callen et al, ) and the existing plastome data may shed insight into the biogeographic and demographic dynamics of Vitis from the Tertiary to the present.…”

Scale and direction of adaptive introgression between black cottonwood (Populus trichocarpa) and balsam poplar (P. balsamifera)

“…Natural hybrid zones provide evolutionary laboratories to identify chromosomal regions that introgress significantly more frequently than expected (Abbott, ; Buerkle & Lexer, ; Lexer, Heinze, Alia, & Rieseberg, ). Forest tree species with extensive natural hybrid zones, large ranges across geographical and climatic clines as well as substantial trait variation (Keller et al., ; McKown et al., ; Savolainen, Pyhäjärvi, & Knürr, ; Soolanayakanahally, Guy, Silim, Drewes, & Schroeder, ) are attractive for the study of adaptive introgression.…”

Nuclear simple sequence repeat markers are superior to DNA barcodes for identification of closely related Rhododendron species on the same mountain

“…Hybridization also results in the sharing of maternally inherited genotypes between closely related species and reduces interspecific divergence of the nuclear ITS sequences through concerted evolution, although the genomic makeup of the current individuals lacks any obvious introgressive signature (Currat et al, ; Du et al, , ; Abbott et al, ; Abbott, ). After excluding all potential hybrids indicated by SSR markers, we found that most cpDNA haplotypes recovered for Rh.…”