Natural selection maintains a single‐locus leaf shape cline in Ivyleaf morning glory, Ipomoea hederacea

Abstract

Clines in phenotypic traits with an underlying genetic basis potentially implicate natural selection. However, neutral evolutionary processes such as random colonization, spatially restricted gene flow, and genetic drift could also result in similar spatial patterns, especially for single-locus traits because of their susceptibility to stochastic events. One way to distinguish between adaptive and neutral mechanisms is to compare the focal trait to neutral genetic loci to determine whether neutral loci demonstrate clinal variation (consistent with a neutral cline), or not. Ivyleaf morning glory, Ipomoea hederacea, exhibits a latitudinal cline for a Mendelian leaf shape polymorphism in eastern North America, such that lobed genotypes dominate northern populations and heart-shaped genotypes are restricted to southern populations. Here, we evaluate potential evolutionary mechanisms for this cline by first determining the allele frequencies at the leaf shape locus for 77 populations distributed throughout I. hederacea's range and then comparing the geographical pattern at this locus to neutral amplified fragment length polymorphism (AFLP) loci. We detected both significant clinal variation and high genetic differentiation at the leaf shape locus across all populations. In contrast, 99% of the putatively neutral loci do not display clinal variation, and I. hederacea populations show very little overall genetic differentiation, suggesting that there is a moderate level of gene flow. In addition, the leaf shape locus was identified as a major F(ST) outlier experiencing divergent selection, relative to all the AFLP loci. Together, these data strongly suggest that the cline in leaf shape is being maintained by spatially varying natural selection.

Response of Leaf Functional Traits of Cerasus yedoensis (Mats.) Yü Li to Serious Insect Attack

“…Conversely, this negative relationship is not consistent with previous results [32]. As one of the most variable plant characteristics affecting major physiological properties that enable plants to adapt to numerous environmental conditions [9,33], such as defense against pathogens [23] and herbivores [9], leaf shape index was positively correlated with petiole diameter, leaf length, leaf thickness, ratio of leaf length to petiole length, ad single leaf wet and dry weight, but negatively correlated with leaf moisture. Therefore, leaf shape index may be affected by leaf length, leaf thickness, allocated biomass in leaf lamina relative to leaf petiole (the ratio of leaf length to petiole length), a single leaf wet and dry weight (although leaf shape index was calculated as the ratio of leaf length to the corresponding leaf width).…”

Quantitative genetic variance and multivariate clines in the Ivyleaf morning glory, Ipomoea hederacea

“…The dominant trend that emerges from our analysis of quantitative traits in I. hederacea is the absence of absolute constraints on adaptation: we observe significant latitudinal clines in a suite of quantitative traits, despite an absence of genetic structure at AFLPs (electronic supplemental material) or single nuclear polymorphisms [33], significant quantitative genetic variance in individual and composite traits within populations and across the range as a whole, and little evidence that selection has eroded quantitative genetic variance in the traits that show the greatest clinal variation or most population differentiation. Below, we first discuss the implications of our approach of analysing d max , d min and cline max , and then we evaluate the potential ecological mechanisms underlying the clinal variation, population differentiation and quantitative genetic variance we detected.…”

“…We scored phenotypes on F 1 offspring that were the product of one generation of selfing in the greenhouse under common environmental conditions, thus equalizing maternal environmental effects. Population genetic analysis of 173 AFLP from populations in the eastern USA suggests no pattern of isolation by distance or spatial autocorrelation of AFLP allele frequencies, in either a larger sample of 77 populations [18], or in the subset of 20 populations used here (electronic supplementary material, figure S1). Accordingly, we treat and analyse our entire sample as a single group, rather two sub-populations.…”

“…R. Soc. B 369: 20130259 reproductive assurance argument) is difficult because I. hederacea and I. purpurea have largely sympatric, overlapping ranges, and estimated selfing rates in I. hederacea are usually quite high (74-93%; [33,36]). Anther-stigma distance is positively correlated with flowering time at the level inbred line means (r ¼ 0.22, p ¼ 0.002, N ¼ 200), suggesting that it could evolve as a correlated response to selection on flowering phenology, and population means of the two traits are also strongly correlated (r ¼ 0.48, p ¼ 0.03, N ¼ 20), suggesting that the two traits have diverged in concert.…”

“…Ipomoea hederacea inhabits a broad distribution, and in eastern North America can be readily found from Florida to southern Pennsylvania. The species exhibits a latitudinal cline in leaf shape: northern populations are composed primarily of lobed genotypes, while southern populations are polymorphic [18]. Although the molecular basis of the leaf shape polymorphism and the ecological mechanisms behind the cline remain ambiguous, the leaf shape cline appears to be adaptive [33], suggesting the potential for adaptive differentiation across the range in quantitative traits as well.…”