Pollinator preference and the evolution of floral traits in monkeyflowers (Mimulus)

Abstract

A paradigm of evolutionary biology is that adaptation and reproductive isolation are caused by a nearly infinite number of mutations of individually small effect. Here, we test this hypothesis by investigating the genetic basis of pollinator discrimination in two closely related species of monkeyflowers that differ in their major pollinators. This system provides a unique opportunity to investigate the genetic architecture of adaptation and speciation because floral traits that confer pollinator specificity also contribute to premating reproductive isolation. We asked: (i) What floral traits cause pollinator discrimination among plant species? and (ii) What is the genetic basis of these traits? We examined these questions by using data obtained from a large-scale field experiment where genetic markers were employed to determine the genetic basis of pollinator visitation. Observations of F2 hybrids produced by crossing bee-pollinated Mimulus lewisii with hummingbird-pollinated Mimulus cardinalis revealed that bees preferred large flowers low in anthocyanin and carotenoid pigments, whereas hummingbirds favored nectar-rich flowers high in anthocyanins. An allele that increases petal carotenoid concentration reduced bee visitation by 80%, whereas an allele that increases nectar production doubled hummingbird visitation. These results suggest that genes of large effect on pollinator preference have contributed to floral evolution and premating reproductive isolation in these monkeyflowers. This work contributes to growing evidence that adaptation and reproductive isolation may often involve major genes.

Figure 1

M. lewisii (A), an F₁ hybrid (B), M. cardinalis (C), and examples of variation in floral traits found in F₂ hybrids (D–L).

Figure 2

Contribution of floral traits to pollinator visitation, as determined by multiple regression analysis (antho., petal anthocyanin concentration; carot., petal carotenoid concentration; nectar, nectar volume per flower; proj. area, projected area of petals). Bars give the standardized regression coefficients; ★, P < 0.05; **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. n = 228 F₂ plants for all analyses. (A) Multiple regression of floral traits on the proportion of visits by bees (F = 24.2, P < 0.0001, R² = 0.31). (B) Multiple regression of floral traits on the mean daily visitation rates by bees (F = 22.1, P < 0.0001, R² = 0.28) and hummingbirds (F = 13.7, P < 0.0001, R² = 0.20).

Figure 3

Effect of allelic differences at the yup locus on the visitation rate (visits per flower per day) of hummingbirds (A) and bees (B). Heterozygous individuals (LC) or those homozygous for the M. lewisii allele (LL) lack carotenoids in their upper petals and are pink-flowered (n = 165), whereas individuals homozygous for the M. cardinalis allele (CC) have petal carotenoids and vary in color from light orange to red (n = 63). Bars denote the mean + 2 SE. Significance levels were determined by Mann–Whitney U tests.

Figure 4

Effect of marker genotype for the major nectar QTL (RAPD marker L04co; ref. 22) on nectar volume per flower (A), and the visitation rate (visits per flower per day) of hummingbirds (B) and bees (C). Genotypes are: LL, individuals homozygous for the M. lewisii allele (n = 61); LC, heterozygotes (n = 130); CC, individuals homozygous for the M. cardinalis allele (n = 36). Bars denote the mean + 2 SE, and bars with different letters identify means that are significantly different (P < 0.01) based on Mann–Whitney U tests corrected for multiple comparisons (31).