Colias butterflies (the “clouded sulphurs”) often occur in mixed populations where females exhibit two color morphs, yellow/orange or white. White females, known as Alba [A-], reallocate resources from colored pigment synthesis to reproductive and somatic development. Due to this tradeoff Alba females develop faster and have higher fecundity than orange females. However while colored pigments are costly to produce, males preferentially mate with orange females and transfer nutrient rich spermatophores during mating. Thus the wing color morphs represent alternative life history strategies (ALHS) that are female-limited, wherein tradeoffs, due to divergent resource investment, result in distinct phenotypes wi... More
Colias butterflies (the “clouded sulphurs”) often occur in mixed populations where females exhibit two color morphs, yellow/orange or white. White females, known as Alba [A-], reallocate resources from colored pigment synthesis to reproductive and somatic development. Due to this tradeoff Alba females develop faster and have higher fecundity than orange females. However while colored pigments are costly to produce, males preferentially mate with orange females and transfer nutrient rich spermatophores during mating. Thus the wing color morphs represent alternative life history strategies (ALHS) that are female-limited, wherein tradeoffs, due to divergent resource investment, result in distinct phenotypes with associated fitness consequences. Here we map the genetic basis of Alba in Colias croceus to a transposable element insertion downstream of the Colias homolog of BarH-1. We use CRISPR/Cas9 to validate BarH-1’s functional role in the wing color switch and antibody staining confirms expression differences in the scale building cells of pupal wings. We use scanning electron microscopy to determine that BarH-1 expression in the wings causes a reduction in pigment granules within wing scales, and thereby gives rise to the white color. Further lipid and transcriptome analyses reveal additional physiological differences that arise due to the Alba, suggesting pleiotropic effects beyond the wing color. While male-limited ALHS are well documented, comparatively few examples of female-limited ALHS are known. This is either due to biological reality or our lack of understanding of how ALHS manifest in females, highlighting the need for mechanistic insights, as none currently exist. These findings provide, to our knowledge, the first known mechanism for a female ALHS and support an alternative view of color polymorphism as indicative of pleiotropic effects with life history consequences.
