Genetic and spatial variation in vegetative and floral traits across a hybrid zone

Abstract

Premise: Genetic variation influences the potential for evolution to rescue populations from impacts of environmental change. Most studies of genetic variation in fitness-related traits focus on either vegetative or floral traits, with few on floral scent. How vegetative and floral traits compare in potential for adaptive evolution is poorly understood.

Methods: We measured variation across source populations, planting sites, and genetic families for vegetative and floral traits in a hybrid zone. Seeds from families of Ipomopsis aggregata, I. tenuituba, and F₁ and F₂ hybrids of the two species were planted into three common gardens. Measured traits included specific leaf area (SLA), trichomes, water-use efficiency (WUE), floral morphology, petal color, nectar, and floral volatiles.

Results: Vegetative traits SLA and WUE varied greatly among planting sites, while showing weak or no genetic variation among source populations. Specific leaf area and trichomes responded plastically to snowmelt date, and SLA exhibited within-population genetic variation. All aspects of floral morphology varied genetically among source populations, and corolla length, corolla width, and sepal width varied genetically within populations. Heritability was not detected for volatiles due to high environmental variation, although one terpene had high evolvability, and high emission of two terpenes, a class of compounds emitted more strongly from the calyx than the corolla, correlated genetically with sepal width. Environmental variation across sites was weak for floral morphology and stronger for volatiles and vegetative traits. The inheritance of three of four volatiles departed from additive.

Conclusions: Results indicate stronger genetic potential for evolutionary responses to selection in floral morphology compared with scent and vegetative traits and suggest potentially adaptive plasticity in some vegetative traits.

Keywords: Polemoniaceae; common garden; floral morphology; floral scent; genetic variation; genotype by environment interaction; heritability; specific leaf area; water-use efficiency.

Figure 1

Trait means and standard errors for four source types (AA, F₁, F₂, TT) planted as seeds at three sites (Ipomopsis aggregata, hybrid, I. tenuituba). AA = I. aggregata × I. aggregata. TT = I. tenuituba × I. tenuituba. F₁ and F₂ are hybrids. (A–C) Floral morphometric traits. (D) Petal color. (E) Nectar production. (F–I) Floral volatiles. (F) The one I. tenuituba plant measured for indole at the I. aggregata site had an unusually high value, but similarly high values have been seen for other I. tenuituba individuals (Campbell et al., 2022a). (J–L) Vegetative traits.

Figure 2

Changes in (A) specific leaf area (SLA) and (B) trichome density with snowmelt date for Ipomopsis plants. Values plotted are least squares means and SE from repeated measures analysis.

Figure 3

Family mean values for (A) pinene and (B) β‐caryophyllene plotted against sepal width. Regression lines are fit separately for each source (AA, F₁, F₂, TT). AA = Ipomopsis aggregata × I. aggregata. TT = I. tenuituba × I. tenuituba. F₁ and F₂ are hybrids. Slopes differed significantly from zero for pinene in the F₂ (P < 0.0001) and caryophyllene in F₂ and TT (P = 0.0108 and 0.0148). The four outbred F₂ families are indicated by red symbols.