Abstract
A long-standing question in evolutionary biology asks whether the genetic changes contributing to phenotypic evolution are predictable. Here, we identify a genetic change associated with segregating variation in flower color within a population of Mimulus lewisii. To determine whether these types of changes are predictable, we combined this information with data from other species to investigate whether the spectrum of mutations affecting flower color transitions differs based on the evolutionary time-scale since divergence. We used classic genetic techniques, along with gene expression and population genetic approaches, to identify the putative, loss-of-function mutation that generates rare, white flowers instead of the common, pink color in M. lewisii. We found that a frameshift mutation in an anthocyanin pathway gene is responsible for the white-flowered polymorphism found in this population of M. lewisii. Comparison of our results with data from other species reveals a broader spectrum of flower color mutations segregating within populations relative to those that fix between populations. These results suggest that the genetic basis of fixed differences in flower color may be predictable, but that for segregating variation is not.
Highlights
The past several years have seen an explosion of experimental studies seeking to identify the genetic and molecular bases of adaptive traits [1,2,3,4]
White flowers are due to a recessive allele at a single locus To explore the genetic architecture of this difference in flower color, we conducted classic genetic crosses between white-flowered and pink-flowered plants
Only two discrete flower color classes segregated in the F2 that were similar in appearance to the pink and white parental forms (Figs. 2, 3)
Summary
The past several years have seen an explosion of experimental studies seeking to identify the genetic and molecular bases of adaptive traits [1,2,3,4]. One idea that has been proposed to explain the preferential use of particular types of mutations suggests that the evolutionary time-scale since divergence may shape patterns of predictability [9,12] In their survey of the molecular changes contributing to phenotypic evolution in plants and animals, Stern and Orgogozo [9,12] observed that cis-regulatory changes dominated transitions in morphology between species (what they referred to as ‘long-term’ evolution), but phenotypic variation within species ‘short-term’ evolution) was caused by an elevated frequency of null coding mutations To explain this observation, they argued that alleles that persist between species likely have been exposed to heterogeneous environmental and selective pressures over a longer time-scale than alleles that vary within species. Cis-regulatory mutations with reduced deleterious pleiotropic effects are expected to persist longer than null coding mutations
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