Variation in frequency of hybrids and spatial structure among Ipomopsis (Polemoniaceae) contact sites

Abstract

Summary• Rates of hybridization vary among angiosperm taxa. Among-taxon variation in hybridization rate has been used to compare the importance of pre-and postzygotic reproductive isolating mechanisms. Variation in rates of hybridization within a single-species pair would suggest that local conditions also affect reproductive isolation within a single taxonomic context.• In this study, contact sites of Ipomopsis aggregata-Ipomopsis tenuituba were surveyed for variation in frequency of hybrids, and spatial structure. Floral morphology was used to identify parent species and hybrids in seven contact sites in the western Rocky Mountains, USA.• Contact sites varied widely in elevational range, the degree to which morphological variation was clinal rather than mosaic and the frequency of hybrids. Two sites provided a strong contrast between a clinal, unimodal site and a mosaic, bimodal site.• This natural variation among contact sites of the same species pair provides an opportunity to assess the effect of local ecological conditions and spatial structure of parent populations on reproductive isolation, while controlling for between-taxon variation.

Genetic and morphological patterns show variation in frequency of hybrids between Ipomopsis (Polemoniaceae) zones of sympatry

“…Distributions of floral morphology within patches at GR closely match the distribution of morphology in reference populations of the two parent species that lie outside the zone, with only rare individuals outside of the observed range of phenotypic variation for the parent species (histograms in Figure 1a; see Aldridge, 2005 for a detailed description of the histograms). In particular, we have never observed any plants with the characteristic orange-red corolla color of F1 hybrids.…”

“…Two Ipomopsis zones of sympatry in western Colorado, Grizzly Ridge (GR) and Poverty Gulch (PG), display a marked contrast in the frequency of hybrids based on floral characters (Aldridge, 2005 Figure 1a; Aldridge, 2005). Distributions of floral morphology within patches at GR closely match the distribution of morphology in reference populations of the two parent species that lie outside the zone, with only rare individuals outside of the observed range of phenotypic variation for the parent species (histograms in Figure 1a; see Aldridge, 2005 for a detailed description of the histograms).…”

“…Morphometric data and tissue for DNA analysis were collected at both sites every year from 1999 to 2001 from flowering plants only (see Aldridge, 2005 for details on morphometric data). Because plants flower once and die, this sampling scheme prevented any resampling of individuals.…”

“…Studies of one Ipomopsis zone of sympatry have documented strong selection on floral traits, including color (Campbell et al, 1997;Melendez-Ackerman and Campbell, 1998), and environmentally dependent selection during the vegetative stage (Campbell and Waser, 2007), but comparatively little is known about the molecular genetic structure of any of the zones (Wolf et al, 1997;Wu and Campbell, 2005). Variation between sites in frequencies of hybrid individuals has been reported earlier (Aldridge, 2005), but that comparison was made using hybrid index scores derived only from floral morphological traits that may experience selection by pollinators (Campbell et al, 1997). It is of value to compare the genetic structure for such traits with that of neutral genetic markers.…”

Is floral divergence sufficient to maintain species boundaries upon secondary contact in Mediterranean food-deceptive orchids?

“…We used morphological traits to perform a canonical discriminant analysis describing the average floral morphology of each plant in the sympatric populations and to serve as an index of morphological variation (Aldridge, 2005). A stepwise method was applied with an F-value of 3.84 to enter a variable and F-value of 2.71 to remove it.…”