Floral scent in natural hybrids of Ipomopsis (Polemoniaceae) and their parental species

Abstract

Background and aims: Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.

Methods: Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography-mass spectrometry.

Key results: The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.

Conclusions: The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.

Keywords: Floral volatiles; Ipomopsis; Polemoniaceae; hawkmoth; hybrid zone; reproductive isolation; scent.

Fig. 1.

Differences in floral scent chemistry between the two Ipomopsis species during day (triangles) and night (circles) separated using canonical discriminant analysis. The first two canonical discriminant functions were defined based on the scent data set for the two parental species only, with I. aggregata in black and I. tenuituba in white. The natural hybrids (grey) were then mapped onto the resulting parental odour space. Small symbols show plants from Grizzly Ridge. Large symbols show plants from Poverty Gulch, where hybrids also were present. Compounds with the largest positive and negative correlations (r) with the discriminant functions are labelled along the axes. The arrow indicates the direction of the loadings on indole.

Fig. 2.

Comparisons of emission rates (mean ± SE) between Ipomopsis aggregata, I. tenuituba and natural hybrids at Poverty Gulch for compounds that differed significantly across plant type. For A–I, significance was based on sequential Bonferroni correction for tests of 47 compounds. Plant types that do not share the same letter differ significantly based on Tukey comparisons.

Fig. 3.

Proportion of floral volatile emissions representing (A) aliphatic compounds, (B) monoterpenes, (C) benzenoids, (D) nitrogenous compounds and (E) sesquiterpenes identified from the flowers of the two parental species Ipomopsis aggregata (black symbols) and I. tenuituba (white) and their natural hybrids (grey), collected during the day (triangles) and the night (circles). Values are means with 95 % confidence intervals, calculated by back-transforming from the arcsin square-root values.