Sexually dimorphic development is responsible for some of the most remarkable phenotypic variation found in nature. Alternative splicing of the transcription factor gene doublesex is a highly conserved developmental switch controlling the expression of sex specific pathways. Here, we leverage sex-specific differences in butterfly wing color pattern to characterize the genetic basis of sexually dimorphic development. We use RNA-seq, immunolocalization, and motif binding site analysis to test specific predictions about the role of Doublesex in the development of structurally-based ultraviolet (UV) wing patterns in Zerene cesonia (Southern Dogface). Unexpectedly, we discover a novel duplication of Doublesex that shows a sex-specific burst of expression associated with the sexually dimorphic UV coloration. The derived copy consists of a single exon that encodes a DNA binding but no protein binding domain, and has experienced rapid amino-acid divergence. We propose the novel dsx paralog may suppress UV scale differentiation in females, which is supported by an excess of Dsx binding sites at cytoskeletal and chitin-related genes with sex-biased expression. These findings illustrate the molecular flexibility of the dsx gene in mediating the differentiation of secondary sexual characteristics.

Sea anemones and associated crustaceans are important components of coral reefs, but their population structure and microhabitat use remain poorly understood. We quantified both types of patterns for two sea anemone and 10 crustacean species at Akumal Bay, Mexico. Rosetip anemones, Condylactis gigantea (Weinland, 1860), occurred as solitary individuals on patch reefs throughout the midbay, while sun anemones, Stichodactyla helianthus (Ellis, 1768), formed dense aggregations on reef patches in the inner bay. Both populations were abundant with exponential size distributions that indicated potentially high recruitment and stable population structure. Individuals of corkscrew anemones, Bartholomea annulata (Le Sueur, 1817), were too rare to analyze their population structure. Crustaceans significantly segregated their microhabitat use among zones on the anemone bodies. Most exhibited exponential size structure, but the chaotic size patterns of some crustaceans suggested recently-sporadic recruitment. We report a previously undocumented association of blue-legged hermit crabs, Clibanarius tricolor (Gibbes, 1850), and anemones; crabs clustered along the anemone columns, and possessed an external, removable coating that protected them from host toxins. We conclude that some anemones and crustacean associates exhibit strong partitioning of habitat and stable population structure on these coral reefs.

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.