Abstract
Sexual dimorphism is a common feature of animals. Sex determination mechanisms vary widely among species and evolve rapidly. Until recently studies have found consistent mechanisms across the body of each individual determine female or male dimorphic body structures. In sexually dimorphic cells throughout the body of Drosophila, the relative dosage of autosomes and X chromosomes leads indirectly to alternatively spliced transcripts from the gene doublesex. The female Dsx isoform interacts with the mediator complex protein encoded by intersex to activate female development in flies. In males the transcription factor encoded by fruitless promotes male-specific behavior. In the milkweed bug Oncopeltus fasciatus, we find a requirement for different combinations of these genes during development of distinct dimorphic structures, within the same sex, suggesting a previously unappreciated level of diversity in sex determination. While intersex and fruitless are structurally conserved, doublesex has a history of duplication and divergence among Paraneoptera. Three doublesex paralogs in O. fasciatus produce multiple transcripts with sex- and tissue-specific expression. intersex and fruitless are expressed across the body, in females and males. RNA interference reveals only one doublesex paralog functions in somatic sex determination. Knockdown of doublesex and fruitless produces intersex phenotypic conditions in two sexually dimorphic structures: genitalia and abdominal sternites. In contrast, intersex is required for dimorphic development of female and male genitalia, but not for sternite dimorphism. These results reveal sex determination roles for intersex and fruitless distinct from their orthologs in other insects. Our results illuminate a novel form of developmental diversity in insect sex determination.
Significance Statement Phenotypic differences between females and males, sexual dimorphisms, are common in animals. Sexual characters are controlled by a rapidly evolving developmental pathway. To explore this diversity we have examined somatic sex determination in the milkweed bug, Oncopeltus fasciatus. We show that (1) the key regulatory gene doublesex has undergone multiple duplications during insect evolution, (2) doublesex isoforms are expressed in sex- and tissue-specific patterns, and (3) doublesex and two other conserved genes, intersex and fruitless, are required for sex determination in females and males. However, different combinations of these genes are required for development of different dimorphic structures within the same sex. Our results illuminate a previously unappreciated form of developmental diversity in insect sex determination.
Competing Interest Statement
The authors have declared no competing interest.
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