Review. The genic view of plant speciation: Recent progress and emerging questions

Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond, Surrey TW9 3DS, UK.
Philosophical Transactions of The Royal Society B Biological Sciences (Impact Factor: 6.31). 07/2008; 363(1506):3023-36. DOI: 10.1098/rstb.2008.0078
Source: PubMed

ABSTRACT The genic view of the process of speciation is based on the notion that species isolation may be achieved by a modest number of genes. Although great strides have been made to characterize 'speciation genes' in some groups of animals, little is known about the nature of genic barriers to gene flow in plants. We review recent progress in the characterization of genic species barriers in plants with a focus on five 'model' genera: Mimulus (monkey flowers); Iris (irises); Helianthus (sunflowers); Silene (campions); and Populus (poplars, aspens, cottonwoods). The study species in all five genera are diploid in terms of meiotic behaviour, and chromosomal rearrangements are assumed to play a minor role in species isolation, with the exception of Helianthus for which data on the relative roles of chromosomal and genic isolation factors are available. Our review identifies the following key topics as being of special interest for future research: the role of intraspecific variation in speciation; the detection of balancing versus directional selection in speciation genetic studies; the timing of fixation of alleles of major versus minor effects during plant speciation; the likelihood of adaptive trait introgression; and the identification and characterization of speciation genes and speciation gene networks.

    • "Additionally , in outcrossing plant species, postzygotic barriers seem more prevalent than prezygotic ones. Differences in habitat preference result in divergent selection, making ecological speciation a likely process in groups with weak reproductive barriers (Milne et al. 2003; Lexer and Widmer 2008; Minder and Widmer 2008; Lexer et al. 2010; Abadie et al. 2012). "
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    ABSTRACT: We explored the role of isolation by environment in a white pine species complex: Pinus flexilis, Pinus strobiformis and Pinus ayacahuite distributed from Canada to Central America. We predict that species differentiation would match genetic structure of candidate genes associated with significant differences in climatic niche in the species complex. To test this prediction, we sequenced five candidate genes for drought tolerance and three housekeeping genes, in individuals from across the entire range of each species. We performed neutrality tests, estimated genetic differentiation and performed partial mantel correlations, to test for isolation by environment in the species complex. Our results show that different loci vary in degrees of genetic differentiation within species and contrast in patterns of differentiation among species. This is considered to be a mosaic pattern of genetic differentiation. There was also significant isolation by environment in candidate genes. P. flexilis was genetically differentiated for candidate genes and P. ayacahuite for housekeeping genes. There was also an overall pattern of shared ancestral polymorphism followed by independent evolution. Nonetheless, all loci together recovered groups that correspond to the recognized taxonomy. In conclusion, the pattern of isolation by environment in candidate genes support the idea of ecologically driven differentiation of this species complex, especially in the case of P. flexilis. The observed difference in housekeeping genes between P. strobiformis and P. ayacahuite can be due to limited gene flow. The mosaic pattern of differentiation suggests that speciation is recent and ecological differences could be a factor in the diversification of pines in North America.
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    • "The understanding of mechanisms involved in the evolution of reproductive isolation and speciation is of great current interest in the field of population genomics (The Marie Curie SPECIATION Network 2011; Abbott et al. 2013). Studying the genomics of taxa with Bporous genomes^ provides an opportunity to obtain insights into the genetics of adaptation, reproductive isolation, and speciation (Wu 2001; Lexer and Widmer 2008; Feder et al. 2012). "
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    • "Nascent or recently diverged species pairs may exist almost anywhere along a continuum from complete interbreeding to complete reproductive isolation. Processes responsible for divergence, and the maintenance of species barriers, may vary depending on both the stage of divergence (Nosil and Feder 2012) and the taxa involved (Lexer and Widmer 2008). Moreover, variation in hybrid zone structure for a particular species pair between localities indicates a likely role for ecology in these processes at a local level (e.g., Milne et al. 2003). "
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    ABSTRACT: An increasing number of studies of hybridization in recent years have revealed that complete reproductive isolation between species is frequently not finalized in more or less closely related organisms. Most of these species do, however, seem to retain their phenotypical characteristics despite the implication of gene flow, highlighting the remaining gap in our knowledge of how much of an organism's genome is permeable to gene flow, and which factors promote or prevent hybridization. We used AFLP markers to investigate the genetic composition of three populations involving two interfertile Rhododendron species: two sympatric populations, of which only one contained hybrids, and a further hybrid-dominated population. No fixed differences between the species were found, and only 5.8% of the markers showed some degree of species differentiation. Additionally, 45.5% of highly species-differentiating markers experienced significant transmission distortion in the hybrids, which was most pronounced in F1 hybrids, suggesting that factors conveying incompatibilities are still segregating within the species. Furthermore, the two hybrid populations showed stark contrasting composition of hybrids; one was an asymmetrically backcrossing hybrid swarm, while in the other, backcrosses were absent, thus preventing gene flow.
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