The origins of reproductive isolation in plants

Abstract

I.II.III.IV.V.VI.References Summary Reproductive isolation in plants occurs through multiple barriers that restrict gene flow between populations, but their origins remain uncertain. Work in the past decade has shown that postpollination barriers, such as the failure to form hybrid seeds or sterility of hybrid offspring, are often less strong than prepollination barriers. Evidence implicates multiple evolutionary forces in the origins of reproductive barriers, including mutation, stochastic processes and natural selection. Although adaptation to different environments is a common element of reproductive isolation, genomic conflicts also play a role, including female meiotic drive. The genetic basis of some reproductive barriers, particularly flower colour influencing pollinator behaviour, is well understood in some species, but the genetic changes underlying many other barriers, especially pollen–stylar interactions, are largely unknown. Postpollination barriers appear to accumulate at a faster rate in annuals compared with perennials, due in part to chromosomal rearrangements. Chromosomal changes can be important isolating barriers in themselves but may also reduce the recombination of genes contributing to isolation. Important questions for the next decade include identifying the evolutionary forces responsible for chromosomal rearrangements, determining how often prezygotic barriers arise due to selection against hybrids, and establishing the relative importance of genomic conflicts in speciation.

Coupling, Reinforcement, and Speciation

“…In other words, the selective sweep of a new, locally advantageous mutation may happen to take a nearby allele to high frequency, and this allele may contribute to a second barrier effect. Hitchhiking of an allele contributing to hybrid inviability with an allele for copper tolerance in Mimulus is a good example of this latter process (Wright et al 2013;Baack et al 2015).…”

No evidence of intrinsic reproductive isolation between two reciprocally non-monophyletic, ecologically differentiated mountain plants at an early stage of speciation

“…Finally, as strength, timing and type of reproductive barriers are highly species-specific (Lowry et al 2008a;Baack et al 2015) further studies are urgently needed to better understand how reproductive isolation arises and finally completes plant speciation (Nosil 2012). In congruence with recent studies revealing prezygotic reproductive isolation based solely on extrinsic, ecological barriers (Westberg et al 2010;Melo et al 2014), our example of H. pusillum and H. veselskyi clearly shows that spatial separation of populations and adaptive phenotypic differentiation may confer a strong reduction in gene flow, which is especially significant at the onset of speciation, when intrinsic barriers are not yet in place.…”

“…Nature and extent of reproductive barriers acting at the onset of speciation vary strongly among plant species (Lowry et al 2008a;Baack et al 2015), which prevents generalisation of the underlying fundamental processes. Isolation can be caused by prezygotically and/or postzygotically acting barriers (Lowry et al 2008a;Baack et al 2015) and may either be intrinsic or extrinsic (Baack et al 2015).…”

“…Nature and extent of reproductive barriers acting at the onset of speciation vary strongly among plant species (Lowry et al 2008a;Baack et al 2015), which prevents generalisation of the underlying fundamental processes. Isolation can be caused by prezygotically and/or postzygotically acting barriers (Lowry et al 2008a;Baack et al 2015) and may either be intrinsic or extrinsic (Baack et al 2015). Prezygotically, intrinsic barriers can for example prevent interspecific pollination and/or pollen tube growth by morphological or physiological incompatibilities (Baack et al 2015), whereas extrinsic isolating barriers may be imposed by spatial isolation of potential mating partners due to ecological preferences (Wang et al 1997;Coyne and Orr 2004).…”

“…Isolation can be caused by prezygotically and/or postzygotically acting barriers (Lowry et al 2008a;Baack et al 2015) and may either be intrinsic or extrinsic (Baack et al 2015). Prezygotically, intrinsic barriers can for example prevent interspecific pollination and/or pollen tube growth by morphological or physiological incompatibilities (Baack et al 2015), whereas extrinsic isolating barriers may be imposed by spatial isolation of potential mating partners due to ecological preferences (Wang et al 1997;Coyne and Orr 2004). Postzygotically, development, viability and/or fertility of hybrids may be impeded by intrinsic developmental and physiological problems (i.e.…”