Abstract
We report the investigation of an Aquilegia flavescens × A. formosa population in British Columbia that is disjunct from its parents—the latter species is present locally but ecologically separated, while the former is entirely absent. To confirm hybridity, we used multivariate analysis of floral characters of field-sampled populations to ordinate phenotypes of putative hybrids in relation to those of the parental species. Microsatellite genotypes at 11 loci from 72 parental-type and putative hybrid individuals were analysed to assess evidence for admixture. Maternally inherited plastid sequences were analysed to infer the direction of hybridization and test hypotheses on the origin of the orphan hybrid population. Plants from the orphan hybrid population are on average intermediate between typical A. formosa and A. flavescens for most phenotypes examined and show evidence of genetic admixture. This population lies beyond the range of A. flavescens, but within the range of A. formosa. No pure A. flavescens individuals were observed in the vicinity, nor is this species known to occur within 200 km of the site. The hybrids share a plastid haplotype with local A. formosa populations. Alternative explanations for this pattern are evaluated. While we cannot rule out long-distance pollen dispersal followed by proliferation of hybrid genotypes, we consider the spread of an A. formosa plastid during genetic swamping of a historical A. flavescens population to be more parsimonious.
Highlights
Orphan hybrid populations or lineages are those that occur in the absence of the parental taxa, presenting a challenge for understanding their origins
Geolocations of herbarium specimens confirmed that the ranges of the parental species overlap in montane regions west of the Rocky Mountains Fig. 1C), and that they tend to occur at different altitudes [see Supporting Information—Appendix S2]
Direction of the hybridization event producing the hybrid population Given that the haplotype of the Marble Range hybrid plants was shared by A. formosa both in the adjacent valley and 20 km away along the Pavilion-Clinton Highway, the evidence supports the idea that the hybrids descended from a local A. formosa maternal lineage
Summary
Orphan hybrid populations or lineages are those that occur in the absence of the parental taxa, presenting a challenge for understanding their origins. They may arise through dispersal beyond the range of the parental taxa. In North American Aquilegia (commonly known as columbines), interspecific hybridization is a striking evolutionary phenomenon. Throughout their natural distribution, intergrading floral forms often occur in zones of species range overlap (Payson 1918; Munz 1946; Grant 1952; Whittemore 1997). While naturally occurring hybrids between other Aquilegia members are known, population-level studies of natural hybridization in the genus have far focused nearly exclusively on this species pair, with comparably little attention given to variable outcomes of hybridization among other species (but see Pelton 1957; Miller 1978)
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