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.