Evolutionary Paths Underlying Flower Color Variation inAntirrhinum

Abstract

To understand evolutionary paths connecting diverse biological forms, we defined a three-dimensional genotypic space separating two flower color morphs of Antirrhinum. A hybrid zone between morphs showed a steep cline specifically at genes controlling flower color differences, indicating that these loci are under selection. Antirrhinum species with diverse floral phenotypes formed a U-shaped cloud within the genotypic space. We propose that this cloud defines an evolutionary path that allows flower color to evolve while circumventing less-adaptive regions. Hybridization between morphs located in different arms of the U-shaped path yields low-fitness genotypes, accounting for the observed steep clines at hybrid zones.

Antagonistic selection factors induce a continuous population divergence in a polymorphism

“…Clinal variation in morph (allele) frequency could be established by the combination of secondary contact and gene flow (Whibley et al, 2006), the combination of divergent selection and gene flow, the combination of divergent and balancing selection (Endler, 1977) or a complex combination of these three factors. In the Geographic Distance (km) Figure 3 Genetic structure based on six microsatellite loci.…”

“…However, as gene flow is highly effective at relatively small scales in these species, some large-scale clines appear to be difficult to explain by the combination of divergent selection and the effects of gene flow. In practice, a cline induced by gene flow is generally steep (Whibley et al, 2006;Saccheri et al, 2008). Instead, balancing selection, which can maintain genetic polymorphism, has been suggested to induce a large-scale cline in morph frequency (Endler, 1973).…”

“…Alternatively, according to the third mechanism, clines may be established without divergent and balancing selection. In this case, the clinal patterns in morph frequency are observed at hybrid zones, which arise from secondary contact between two historically allopatric populations with different alleles (Latta and Mitton, 1999;Whibley et al, 2006). Thus, the combination of two types of genetic drift, that is, secondary contact and gene flow between adjacent populations, results in a smooth cline in morph frequency.…”

“…Clines in genotype-specific morph/allele frequency are suitable model systems to study the relative importance of selection and historical/ongoing genetic drift in phenotypic population divergence, because the establishment of clines is potentially affected by both selection and historical/ongoing genetic drift (Endler, 1973;Whibley et al, 2006). The mechanisms that establish clines in morph frequency can be roughly classified into three categories (Endler, 1977).…”

Admixture as the basis for genetic mapping

“…Hybridization between invasive and native sculpins (Cottus) in multiple tributaries of the Rhine offers the opportunity to map components of reproductive isolation in a replicated manner [59]. It is likely that variation in flower color, as well as other traits, can be mapped in hybrid populations of Antirrhinum [60] and of Silene [61]. Hybridization between species of Heliconius butterflies with dramatically different wing patterns and coloration offers a final example of a system with pervasive hybridization that might provide the substrate for mapping in natural populations [62].…”