Seasonal polyphenism of wing colors and its influence on sulphur butterfly diversification

Abstract

Seasonal variation of color patterns on butterfly wings are iconic examples of developmentally plastic traits that can influence adaptation and speciation. Yet, there are few examples of such seasonal polyphenisms that have characterized the environmental cues, ecological consequences, or genetic mechanisms involved in generating the plastic variation of wing color. Further, there is a lack of support that such plasticity may impact the adaptive diversification of butterfly wing patterns. Here, we report a case of seasonal polyphenism in pigment and structurally-based color patterns of Zerene cesonia that are strikingly similar to the color pattern divergence seen on the wings of sulphur butterflies. We show that (i) coordinated changes in temperature and photoperiod drive the plasticity, (ii) the plastic color changes impact how fast the butterflies can warm, (iii) identify spalt as likely be involved the genetic coupling of the pigment and structurally-based color plastic response. We further show that this plastic wing changes phenocopy wing pattern divergence between Zerene species, as well as the color pattern differences known to be commonly involved in sexual selection and speciation across sulphur butterflies. Together, our results demonstrate that shared environmental cues and genetic basis for pigment and structural color plasticity may result in conditions that may have facilitated species diversification of sulphur butterflies.

Genetics of continuous colour variation in a pair of sympatric sulphur butterflies

“…The analysis of ventral surfaces revealed a more nuanced picture, because the specific effect of bab mutagenesis on red pigmentation varied across the proximo-distal axis ( Figure 6F-K’ ). In proximal regions, bab mutant clones showed redder pigmentation relative to adjacent WT clones, while more distally, they were comparatively lighter, including in the most distal section of the wing where marginal scales that normally display pink pterins (Fenner et al, 2022) were depigmented. In addition, bab mKOs resulted in an elongation of silver scales and darker melanisation in the ventral discal spot ( Figure S6 ).…”

“…A sexual signalling role of dorsal visible colour (orange vs. yellow) is unlikely in males, and untested in females (Silberglied and Taylor, 1978). While this is also untested, wing chroma differences may have been shaped by ecological adaptation to climatic differences in the native ranges of C. eurytheme vs C. philodice (Hovanitz, 1944), as suggested by the seasonal plasticity of this trait (Fenner et al, 2022;Gerould, 1943). Wing size differences (a proxy for total body size) are likely to reflect the divergent life history of these butterflies, with C. philodice reaching pupation faster than C. eurytheme in spite of identical growth rates on their introduced host plants ( Porter and Levin, 2007).…”