Data for: Genetics of continuous colour variation in a pair of sympatric sulphur butterflies

Continuous colour polymorphisms can serve as a tractable model for the
genetic and developmental architecture of traits, but identification of
the causative genetic loci is complex due to the number of individuals
needed, and the challenges of scoring continuously varying traits. Here we
investigated continuous colour variation in Colias eurytheme and C.
philodice, two sister species of sulphur butterflies that hybridise in
sympatry. Using Quantitative Trait Locus (QTL) analysis of 483 individuals
from interspecific crosses and a high-throughput method of colour
quantification, we found that two interacting large effect loci explain
around 70% of the heritable variation in orange-to-yellow chromaticity.
Knockouts of red Malphighian tubules (red), a candidate gene at the
primary QTL likely involved in endosomal maturation, resulted in
depigmented wing scales showing disorganised pterin granules. The Z sex
chromosome contains a large secondary colour QTL that includes the
transcription factor bric-a-brac (bab), which we show can act as a
modulator of orange pigmentation in addition to its previously-described
role in specifying UV-iridescence. We also describe the QTL architecture
of other continuously varying traits, and that wing size maps to the Z
chromosome, supporting a Large-X effect model where the genetic control of
species-defining traits is enriched on sex chromosomes. This study sheds
light on the genetic architecture of a continuously varying trait and
illustrates the power of using automated measurement to score phenotypes
that are not always conspicuous to the human eye.