Chromosome-level genome and transcriptome analyses provide insights into sexual dimorphism in <i>Bibio rufiventris</i> (Diptrea: Bibionidae)

Abstract Sexual dimorphism is widespread in animals and often reflects evolutionary trade-offs between natural and sexual selection. Understanding the genomic and molecular mechanisms underlying these differences is key to revealing how adaptive traits evolve. Bibio rufiventris, commonly known as the fulvous-female march fly, exhibits striking sexual dimorphism, with males uniformly black and possessing enlarged compound eyes, while females display vivid reddish-orange thoraces and abdomens, making it an ideal model for investigating the genetic and molecular basis of sexually dimorphic traits. Here, we present a high-quality chromosome-level genome assembly of B. rufiventris. Comparative genomics revealed expansion of developmental and metabolic gene families in B. rufiventris, and branch-site analysis identified 265 positively selected genes, some of which are enriched in sensory-system development, including compound eye morphogenesis and multiple sensory perception processes. Transcriptomic profiling showed strong male-biased expression of phototransduction genes (eg ninaE, Arr2, and inaD) in heads, which may explain the enhanced visual processing capability in males. We also find a coordinated regulation pattern across melanin, pteridine, ommochrome, and heme–biliverdin pathways appears to underlie the female-specific coloration. These findings yield novel insights into sexual dimorphism in B. rufiventris in terms of pigment metabolism, visual system specialization, and sensory adaptation. This study also adds to our understanding of the genetic basis and evolutionary diversification of sexual traits in Bibionidae and other Diptera.