Plant hybridization

Abstract

Summary 599 I. Introduction 599 II. Concepts and terminology 600 III. Historical background 600 IV. Studies of experimental hybrids 601 1. Isolating mechanisms 601 2. Prezygotic barriers 602 (a) Gametic barriers to hybridization 602 3. Postzygotic barriers 603 (a) Chromosomal rearrangements 604 (b) Genic sterility or inviability 604 4. Hybrid vigour 605 5. Introgression 606 6. Hybrid speciation 607 V. Experimental manipulations of natural hybrid populations 609 1. Hybrid‐zone formation 610 2. Pollinator‐mediated selection 610 3. Habitat selection 612 VI. The biology of different classes of hybrids 612 1. Character expression 613 (a) Morphological characters 613 (b) Chemical characters 613 (c) Molecular characters 613 2. The fitness of different classes of hybrids 614 (a) The importance of variance 614 (b) Estimating hybrid fitness 615 3. Interactions with parasites and herbivores 616 4. Patterns of mating 617 (a) Outcrossing rate 617 (b) Hybridization frequency 618 (c) Mate choice 618 VII. Conclusions and future research 619 Acknowledgements 620 References 620 Most studies of plant hybridization are concerned with documenting its occurrence in different plant groups. Although these descriptive, historical studies are important, the majority of recent advances in our understanding of the process of hybridization are derived from a growing body of experimental microevolutionary studies. Analyses of artificially synthesized hybrids in the laboratory or glasshouse have demonstrated the importance of gametic selection as a prezygotic isolating barrier; the complex genetic basis of hybrid sterility, inviability and breakdown; and the critical role of fertility selection in hybrid speciation. Experimental manipulations of natural hybrid zones have provided critical information that cannot be obtained in the glasshouse, such as the evolutionary conditions under which hybrid zones are formed and the effects of habitat and pollinator‐mediated selection on hybrid‐zone structure and dynamics. Experimental studies also have contributed to a better understanding of the biology of different classes of hybrids. Analyses of morphological character expression, for example, have revealed transgressive segregation in the majority of later‐generation hybrids. Other studies have documented a high degree of variability in fitness among different hybrid genotypes and the rapid response of such fitness to selection – evidence that hybridization need not be an evolutionary dead end. However, a full accounting of the role of hybridization in adaptive evolution and speciation will probably require the integration of experimental and historical approaches.

Untangling the hybrid origin of the Chinese tea roses: evidence from DNA sequences of single-copy nuclear and chloroplast genes

“…Hybridization has long been considered as an important mechanism in plant speciation (Grant 1981;Abbott 1992;Rieseberg and Wendel 1993;Arnold 1997;Rieseberg 1997;Rieseberg and Carney 1998;Hegarty and Hiscock 2005). The barriers to gene flow among Rosa L. species are weak (Atienza et al 2005), and hybridization has contributed greatly to species diversity of this genus (Rehder 1940;Wissemann and Ritz 2005).…”

Detection of hybrids in nature: application to oaks (Quercus suber and Q. ilex)

“…Natural hybridization and introgression are widespread phenomena in plants, with important evolutionary implications (Rieseberg and Carney, 1998). The movement of genes across species boundaries can promote the appearance of new lineages (Seehausen, 2004), adaptive solutions (Rieseberg et al, 2003) or colonization abilities (Potts and Reid, 1988;Petit et al, 2004).…”

Natural hybridization between cultivated poplars and their wild relatives: evidence and consequences for native poplar populations

“…2). Mechanisms that act to prevent mating are referred to as prezygotic, whereas those that act to reduce the viability or fertility of the hybrid zygote or later-generation hybrid offspring are referred to as postzygotic [77].…”

“…Pollen competition was also studied in detail for Louisiana irises (Iris fulva and Iris hexagona). These studies have focussed on conspecific pollen advantage, including measurements of pollen tube lengths in conspecific and heterospecific styles and examination of hybrid seed production following different types of controlled pollinations (for a review see [77]). In common with introgression studies in irises [77] studies on gametophytic competition in Zea mays [60] and Dianthus chinenesis [59] also suggest that faster growth of conspecific pollen tubes might be acting as a barrier to interspecific hybridization (cf.…”

“…Glossary) in which first generations (F1) hybrids are vigorous (hybrid superiority due to heterosis), robust and fertile, but later generation hybrids are weak or inviable [77]. Poplar hybrids are characterised by reduced fertility relative to parental species, and significant lower pollen production and seed viability in F 1 hybrids [86,94].…”