All the Colors of the Rainbow: Diversification of Flower Color and Intraspecific Color Variation in the Genus Iris

Abstract

Floral color plays a key role as visual signaling and is therefore of great importance in shaping plant-pollinator interactions. Iris (Iridaceae), a genus comprising over 300 species and named after the Greek goddess of the colorful rainbow, is famous for its dazzling palette of flower colors and patterns, which vary considerably both within and among species. Despite the large variation of flower color in Iris, little is known about the phylogenetic and ecological contexts of floral color. Here, we seek to resolve the evolution of flower color in the genus Iris in a macroevolutionary framework. We used a phylogenetic analysis to reconstruct the ancestral state of flower color and other pollination-related traits (e.g., the presence of nectar and mating system), and also tracked the evolution of color variation. We further explored weather floral trait transitions are better explained by environmental or pollinator-mediated selection. Our study revealed that the most recent common ancestor likely had monomorphic, purple flowers, with a crest and a spot on the fall. The flowers were likely insect-pollinated, nectar-rewarding, and self-compatible. The diversity of floral traits we see in modern irises, likely represents a trade-off between conflicting selection pressures. Whether shifts in these flower traits result from abiotic or biotic selective agents or are maintained by neutral processes without any selection remains an open question. Our analysis serves as a starting point for future work exploring the genetic and physiological mechanisms controlling flower coloration in the most color-diverse genus Iris.

Keywords: ancestral trait reconstruction; color variation; flower color evolution; mating system; nectar reward; pollination syndrome; pollinator shifts; shelter reward.

FIGURE 1

Flowers of selected Iris species [(A) I. atropurpurea, (B) I. bismarckiana, (C) I. fulva, (D) I. historio, (E) I. loretti, (F) I. lutescens, (G) I. mesopotamica, (H) I. petrana, (I) I. pumila, (J) I. reticulata, (K) I. setosa (L) I. virginica].

FIGURE 2

Maximum likelihood tree inferred from analysis of combined five plastid genomes (matK, trnL, trnK, NADPH and rbcL) and one nuclear internal transcribed spacer (ITS) sequences. The bootstrap values are given along the branches (only values > 50 presented).

FIGURE 3

Maximum-likelihood ancestral state reconstruction of corolla diameter [A] and histogram showing representation of the distribution of the variables [B].

FIGURE 4

Estimation of ancestral states of flower colors among studied Iris taxa calculated using maximum likelihood across the posterior distribution.

FIGURE 5

Estimation of ancestral states of flower pigments among studied Iris taxa calculated using maximum likelihood across the posterior distribution. White and creamy flowers were coded as lacking anthocyanins flavonoids and carotenoids and are represented as white; maroon, purple, pink, or red flowers were coded as having anthocyanins and are represented in purple; yellow or orange flowers were coded as having carotenoids and are represented in yellow.

FIGURE 6

Summarized stochastic mapping of mating system (self-compatibility vs. self-incompatibility), (A) presence or absence of nectar (B), and pollinator type (insect vs. insect and bird) (C) in the genus Iris prepared using All Rates Different model with 1,000 iterations. Pie charts represent the proportion of the iterations showing either presence or absence of nectar at any given node.