Selection drives divergence of eye morphology in sympatric Heliconius butterflies

When populations experience different sensory conditions, natural selection may favor sensory system divergence, affecting peripheral structures and/or downstream neural pathways. We characterized the outer eye morphology of sympatric Heliconius species from different forest types and their first-generation reciprocal hybrids to test for adaptive visual system divergence and hybrid disruption. In Panama, Heliconius cydno occurs in closed forests, whereas Heliconius melpomene resides at the forest edge. Among wild individuals, H. cydno has larger eyes than H. melpomene, and there are heritable, habitat-associated differences in the visual brain structures that exceed neutral divergence expectations. Notably, hybrids have intermediate neural phenotypes, suggesting disruption. To test for similar effects in the visual periphery, we reared both species and their hybrids in common garden conditions. We confirm that H. cydno has larger eyes and provide new evidence that this is driven by selection. Hybrid eye morphology is more H. melpomene-like despite body size being intermediate, contrasting with neural trait intermediacy. Overall, our results suggest that eye morphology differences between H. cydno and H. melpomene are adaptive, and that hybrids may suffer fitness costs due to a mismatch between the peripheral visual structures and previously described neural traits that could affect visual performance.

Methods
Butterfly specimens
We collated samples from outbred stocks of Heliconius cydno chioneus (C) and Heliconius melpomene rosina (M), established from wild butterflies caught in Gamboa and the nearby Soberanía National Park, Panama. Most butterflies were sampled in 2015-2017 (9 H. cydno and 20 H. melpomene) or 2018-2019 (16 H. cydno and 10 H. melpomene) versus fewer samples in 2008-2009 (9 H. cydno and 3 H. melpomene). We also exploited previously sampled reciprocal F1 hybrids between H. cydno and H. melpomene, generated by crossing a H. cydno female with a H. melpomene male (CxM) or a H. melpomene female with a H. cydno male (MxC). All hybrids were generated in 2017-2019, involving four CxM crosses and six MxC crosses (unique parents for each cross; see Table S1, Fig. S2). All butterflies were reared under common garden conditions in the Smithsonian Tropical Research Institute insectaries in Gamboa, and all specimens were preserved in DMSO/EDTA/NaCl and stored at -80° C as described in Merrill et al. (2019).

Sample preparation
Samples were prepared following previously published methods (Seymoure et al., 2015; Wright et al., 2023). In brief, we thawed specimens at room temperature, imaged the entire head (minus antennae and proboscis), and dissected out both eyes and the hind legs. The legs were immediately imaged (see below), while the eyes were placed in 20% sodium hydroxide (NaOH) for 18-24 hours to loosen the tissues behind the cuticular cornea. The following day, we cleaned each eye cuticle of excess tissue and mounted it on a microscope slide in Euparal (Carl Roth GmbH). The sample was left to dry overnight before imaging.

Image analysis
We used ImageJ/Fiji (Schindelin et al., 2012) to analyze each mounted cornea for the total number of facets and total corneal area. All slides were imaged at 7.5x on a Leica M80 stereomicroscope fitted with a Leica Flexacam C1 camera and the Leica Application Suite X (LAS X) software. Each image contained a 1mm scale bar for calibration. Facet counts were measured via image thresholding and the Analyze particles function, and corneal surface area was measured with the Freehand selection and Measure options (full protocol provided as supplementary methods). This semi-automated method differs slightly from the approach used by Seymoure et al. (2015) but gives quantitatively similar results (Fig. S1). To account for differences in head and/or body size, we measured the distance between each eye cuticle (inter-ocular width, e.g., (Gaspar et al., 2020; Posnien et al., 2012; Wainwright et al., 2023)) and hind tibia length using the Straight line and Measure options. Finally, we also measured facet diameter as in Seymoure et al. (2015): within each anatomical region of the eye, we measured across ten facets in a row in two separate locations at least ten facets apart (using the Straight line and Measure options). Per facet diameter was calculated by dividing each line segment by ten, and facet diameter per eye region was calculated as the mean of the two locations.