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: 7.06). 07/2008; 363(1506):3023-36. DOI: 10.1098/rstb.2008.0078
Source: PubMed


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.


Available from: Alex Widmer
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    • "Such a time frame may be insufficient for genomewide genetic differentiation among these species to have accumulated, especially at the scale of a relatively small area where interspecific gene flow may have been frequent. Phenotypic novelties, reproductive isolation, and adaptation to environmental conditions do not necessarily depend on large-scale genetic alterations; they can be due to divergence at only a few loci (Wu 2001; Lexer and Widmer 2008; Kane et al. 2009). Finding such relatively small differences within non-model genomes is challenging. "

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    • "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.
    Evolutionary Ecology 08/2015; DOI:10.1007/s10682-015-9785-4 · 2.52 Impact Factor
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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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    ABSTRACT: Resistance genes (R-genes) are essential for long-lived organisms such as forest trees, which are exposed to diverse herbivores and pathogens. In short-lived model species, R-genes have been shown to be involved in species isolation. Here, we studied more than 400 trees from two natural hybrid zones of the European Populus species Populus alba and Populus tremula for microsatellite markers located in three R-gene clusters, including one cluster situated in the incipient sex chromosome region. The results indicate that all three R-gene clusters present extensive linkage disequilibrium (LD). Outlier tests suggest balancing selection as a potential driver of R-gene diversity, but divergent selection was also detected. Nucleotide-binding site leucine-rich repeat (NBS-LRR) type R-gene clusters exhibit low species differentiation and appear to be affected by shared selection pressures between species, recurrent interspecific gene flow, or both. Our results are consistent with a role for R-gene clusters in the evolution of chromosome XIX, recently put forward as an incipient sex chromosome in Populus.
    Tree Genetics & Genomes 08/2015; 11(4):1-12. DOI:10.1007/s11295-015-0904-8 · 2.45 Impact Factor
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