DOI: 10.5061/dryad.dz08kprv5
title: Data from: Müllerian mimicry of a quantitative trait despite contrasting levels of genomic divergence and selection

description: Hybrid zones, where distinct populations meet and interbreed, give insight
 into how differences between populations are maintained despite gene flow.
 Studying clines in genetic loci and adaptive traits across hybrid zones is
 a powerful method for understanding how selection drives differentiation
 within a single species, but can also be used to compare parallel
 divergence in different species responding to a common selective pressure.
 Here, we study parallel divergence of wing colouration in the butterflies
 Heliconius erato and H. melpomene, which are distantly related Müllerian
 mimics that show parallel geographic variation in both discrete variation
 in pigmentation, and quantitative variation in structural colour. Using
 geographic cline analysis, we show that clines in these traits are
 positioned in the roughly the same geographic region for both species,
 which is consistent with direct selection for mimicry. However, the width
 of the clines varies markedly between species. This difference is
 explained in part by variation in the strength of selection acting on
 colour traits within each species, but may also be influenced by
 differences in the dispersal rate and total strength of selection against
 hybrids between the species. Genotyping-by-sequencing also revealed weaker
 population structure in H. melpomene, suggesting the hybrid zones may have
 evolved differently in each species; which may also contribute to the
 patterns of phenotypic divergence in this system Overall, we conclude that
 multiple factors are needed to explain patterns of clinal variation within
 and between these species, although mimicry has probably played a central
 role.
description: Digital images
 of butterfly wings were taken with a Nikon D7000 DSLR camera fitted with
 an AF-S DX Micro NIKKOR 40 mm f/2.8G lens (Nikon UK Ltd., Surrey, UK),
 mounted on an adjustable platform. Standardised lighting conditions were
 achieved using two external natural daylight fluorescent lights, mounted
 to illuminate at 45 degrees from incident, to maximise brightness of
 observed iridescent colour. Photographs were taken with a shutter speed of
 1/60 sec and an aperture of f/10. Each sample was photographed with an
 X-Rite colorchecker passport (X-Rite, Inc., MI, USA) in shot. The Nikon
 raw (.NEF) image files were converted to standard raw files (.DNG) using
 Adobe DNG converter (Adobe Systems Inc., USA). The RGB channels in the
 images were then linearized using the neutral grey scale on the
 colorchecker using GNU Image Manipulation Program, v2.8.
 The mean RGB (red, green, blue) values from regions in the discal
 cell on the right forewing and the Cu<sub>2</sub> cell on the
 right hindwing were measured (SI Figure S1A) from photographs. If the
 wings on the right-hand side showed damage, wings on the left-hand side
 were used. Wing regions were selected using the polygon selection tool in
 ImageJ, version 1.50b (Abràmoff, Magalhães, & Ram, 2004), and mean
 RGB scores were measured using the Color Histogram plugin for ImageJ. This
 plugin outputs the mean, mode and standard deviation of each colour
 channel (red, green and blue) in the selected area. 
description: Each row corresponds to an individual. Some individuals have two
 entries, as they were photographed and measured twice to test for
 repeatability of the measurements. WR1 describes the forewing wing region
 which was measured, WR2 described the hindwing region which was measured.
 The mean, mode, and standard deviation of each colour channel (R, G
 & B), in each wing region are reported in separate
 columns.