Evolutionary Dynamics of an Ipomopsis Hybrid Zone: Confronting Models with Lifetime Fitness Data

Abstract

Interspecific hybridization is a recurring aspect of the evolution of many plant and animal groups. The temporal dynamics of hybrid zones and the evolutionary consequences of hybridization should depend on fitness of parental and hybrid individuals expressed in different environments. We measured lifetime fitness, including survival and reproduction, of plants of Ipomopsis aggregata, Ipomopsis tenuituba, and their F1 hybrids, in experimental plantings in a natural hybrid zone. Fitness, measured as the finite rate of increase ( lambda ), depended strongly on environment. Each parental species performed well in its home locale and poorly in the locale of the other species. Hybrids performed as well as parents overall but enjoyed their highest fitness in the hybrid site. Furthermore, F1 hybrids with I. tenuituba as maternal parent survived well only at the hybrid site, suggesting a cytonuclearxenvironment interaction. These results support an "environmental cline" model of hybrid zone dynamics, with complexities in the fitness of hybrids consistent also with an "evolutionary novelty" model. Combined with those of earlier studies of pollination, our results suggest that both vegetative adaptation to physical environment and floral adaptation to pollinators contribute to observed patterns of phenotypic expression in this hybrid zone and to persistence of the hybrid zone.

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

“…To obtain estimates of these values, variance components were calculated within each site using the five original morphological traits and employing GLM methods in SPSS, with species as factor. Although our estimates are derived from partitioning phenotypic rather than additive genetic variation, the differences between species in all of the floral traits are retained in common gardens, indicating a strong genetic basis to the differences (Campbell and Waser, 2007). The common garden data for site PG (unavailable for site GR) give rise to similar values of Q ST to those obtained here for that site.…”

“…The values reported are estimates from the full model. Q ST for individual floral traits ranged from 0.3 to 0.9, an order of magnitude higher than any of the F ST values, and similar to values of Q ST based on the same traits when the species were raised in common gardens at PG (range, 0.4-1.0; calculated from data in Campbell and Waser, 2007).…”

“…Though they hybridize readily in sympatry, the species exhibit contrasting suites of floral traits consistent with the hummingbird (I. aggregata) and hawkmoth (I. tenuituba) pollination syndromes (Grant and Grant, 1965;Campbell et al, 1997) even in sympatry. Hybrids in the F1 generation are intermediate for corolla length and width and are most readily identified by a distinct orange-red corolla color (Campbell and Waser, 2007).…”

“…A monocarpic life history, showy flowers and absence of seed bank have made Ipomopsis a frequent model for study of evolutionary processes, including selection in hybrid zones and speciation (Campbell, 2004;Waser and Campbell, 2004). 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).…”

An Integrated Framework for Hybrid Zone Models

“…One of the models, evolutionary novelty, is a special case that can involve selection both against and for hybrids; the direction is dependent on environment and genotype, rather like a mosaic zone, and this model can occur with any type of hybridization event if novel genotypes arise (Campbell and Waser 2007). Because selection based on genotype is the basis of all selection, this is an extreme model that focuses on how generation of genetic diversity via hybridization can occur.…”