Academic journal article Genetics

The Color Genes of Speciation in Plants

Academic journal article Genetics

The Color Genes of Speciation in Plants

Article excerpt

The genes underlying speciation remain largely undiscovered. An article published in this issue of GENETICS presents results related to "Genetic Dissection of a Major Anthocyanin QTL Contributing to Pollinator-Mediated Reproductive Isolation Between Sister Species of Mimulus." Yuan et al. (2013) provide compelling evidence that the R3 MYB gene causes differences in anthocyanin concentration in the flowers of Mimulus lewisii and M. cardinalis, and is in fact likely responsible for pollinator-mediated reproductive isolation in areas where the two species co-occur. This commentary discusses general implications from the results of Yuan et al. (2013), and frames them in terms of both the genetic basis of color and species evolution.

COLORS in nature bring partners together (Darwin 1871), signal distaste to predators (Bates 1862), and create interactions between radically different organisms (e.g., Bawa 1990). Colors also signal reward (Hamilton and Zuk 1982), or possibly atavistic preferences (Ryan 1998), and drive fundamental evolutionary processes such as sexual selection in animals (Andersson 1994) and pollination preferences in plants (Grant 1949). These processes have major implications for the origin of traits and species both in the animal and plant kingdoms. Although we have made remarkable progress in identifying the genes responsible for color in nature (Wessinger and Rausher 2012), we still know very little about the color genes of speciation (Rieseberg and Blackman 2010). In this issue, Yuan et al. (2013) are helping to fill this gap by identifying some of the genes responsible for flower color variation in a clear case of pollinator-mediated reproductive isolation (RI) in monkeyflowers (Bradshaw et al. 1995; Schemske and Bradshaw 1999; Ramsey and Schemske 2002). Their findings fundamentally advance our knowledge of speciation in plants and further open the doors for understanding the evolution of color and its consequences in nature.

Pollinator isolation, a common prezygotic isolating barrier in flowering plants, reduces gene flow between populations because different pollinators (e.g., birds vs. bees) or the same pollinator (pollinator constancy) transfer pollen predominantly between conspecific and not heterospecific flowers (Bradshaw and Schemske 2003; Hopkins and Rausher 2012). In recent years, a number of studies have demonstrated that shifts in flower color are common in a variety of plant lineages (Rausher 2008) and that some of them cause changes in pollinator behavior leading to pollinator isolation (Hopkins and Rausher 2012). However, very few studies have found the genes responsible for flower-color variation associated with demonstrated cases of plant speciation (Rieseberg and Blackman 2010). For instance, Hopkins and Rausher (2011) identified genetic changes in two genes involved in the anthocyanin biosynthetic pathway (ABP) that cause changes in flower pigment intensity (from violet to dark red) between co-occurring populations (sympatry) of the plants Phlox drumondii and P. cuspidata. Notably, pollinators display visitation fidelity to flowers sharing the color of the first flower they visited, even though the pollinator is able to visit the flowers of both species. Because color shifts do not occur outside areas of sympatry, the evolution of sympatric flower color differences in Phlox likely evolved in response to partial hybrid sterility between the two species and thus reinforced prezygotic reproductive isolation in the system (reviewed in Hopkins 2013).

The discovery of speciation genes requires systems of study with detailed knowledge of the reproductive barriers that separate two species and the ability to perform classical and molecular genetic experiments with them (e.g., QTL mapping and genetic transformation). Traditionally, this has been the domain of model systems, such as Drosophila and Arabidopsis. However, Yuan et al. (2013) demonstrate that Mimulus is joining their ranks, not only by providing these tools of discovery, but also by enabling future genetic experiments in ecological settings. …

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