Less is more: Independent loss-of-function OCIMENE SYNTHASE alleles parallel pollination syndrome diversification in monkeyflowers (Mimulus).

Abstract

Pollinator-mediated selection on flower phenotypes (e.g., shape, color, scent) is key to understanding the adaptive radiation of angiosperms, many of which have evolved specialized relationships with a particular guild of animal pollinators (e.g., birds, bats, moths, bees). E-β-Ocimene, a monoterpene produced by OCIMENE SYNTHASE (OS) in Mimulus lewisii, is a floral scent important in attracting the species' bumblebee pollinators. The taxa closely related to M. lewisii have evolved several different pollination syndromes, including hummingbird pollination and self pollination (autogamy). We are interested in how floral scent variation contributed to species diversification in this clade. We analyzed variation in E-β-ocimene emission within this Mimulus clade and explored its molecular basis through a combination of DNA sequencing, reverse transcriptase PCR, and enzyme functional analysis in vitro. We found that none of the taxa, other than M. lewisii, emitted E-β-ocimene from flowers. But the molecular basis underlying loss of E-β-ocimene emission is unique in each taxon, including deletion, missense, or frameshift mutations in the OS gene, and potential posttranscriptional downregulation. The molecular evidence suggests that parallel loss-of-function in OS is the best explanation for the observed pattern of E-β-ocimene emission, likely as the result of natural selection.