Sexual dimorphism arises because of divergent fitness optima between the sexes. Phenotypic divergence between sexes can range from mild to extreme, as observed in various species of bioluminescent beetles - fireflies. Fireflies present various degrees of sexual dimorphism, including female neoteny, posing a unique framework to investigate the evolution of sexually dimorphic traits across species. In this work, we present novel assembled genomes of two firefly species, Lamprohiza splendidula and Luciola italica, species with different degrees of sexual dimorphism. We uncovered high synteny conservation of the X-chromosome across ~150 Mya. We also found full X-chromosome dosage compensation in our two fireflies hinting at common mechanism upregulating the single male X-chromosome. Different degrees of sex-biased expressed genes were found across two body parts showing different proportions of expression congruence between species. Interestingly, we did not find X-chromosome enrichment of sex-biased genes, but retrieved autosomal enrichment of sex-biased genes. We further uncovered higher nucleotide diversity in the intronic regions of sex-biased genes, hinting at a maintenance of heterozygosity through sexual selection. We identified different levels of sex-biased gene expression divergence including sex-biased genes conserved between species. Divergent and conserved sex-biased genes are candidate to test their role in sexually dimorphic traits.
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