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Abstract Sex chromosomes play an outsized role in adaptation and speciation, and thus deserve particular attention in evolutionary genomics. In particular, fusions between sex chromosomes and autosomes can produce neo‐sex chromosomes, which offer important insights into the evolutionary dynamics of sex chromosomes. Here, we investigate the evolutionary origin of the previously reportedDanausneo‐sex chromosome within the tribe Danaini. We assembled and annotated genomes ofTirumala septentrionis(subtribe Danaina),Ideopsis similis(Amaurina),Idea leuconoe(Euploeina) andLycorea halia(Itunina) and identified their Z‐linked scaffolds. We found that theDanausneo‐sex chromosome resulting from the fusion between a Z chromosome and an autosome corresponding to theMelitaea cinxiachromosome (McChr) 21 arose in a common ancestor of Danaina, Amaurina and Euploina. We also identified two additional fusions as the W chromosome further fused with the synteny block McChr31 inI. similisand independent fusion occurred between ancestral Z chromosome and McChr12 inL. halia. We further tested a possible role of sexually antagonistic selection in sex chromosome turnover by analysing the genomic distribution of sex‐biased genes inI. leuconoeandL. halia. The autosomes corresponding to McChr21 and McChr31 involved in the fusions are significantly enriched in female‐ and male‐biased genes, respectively, which could have hypothetically facilitated fixation of the neo‐sex chromosomes. This suggests a role of sexual antagonism in sex chromosome turnover in Lepidoptera. The neo‐Z chromosomes of bothI. leuconoeandL. haliaappear fully compensated in somatic tissues, but the extent of dosage compensation for the ancestral Z varies across tissues and species. 

Abstract Reproduction is a fundamental aspect of life that affects all levels of biology, from genomes and development to population dynamics and diversification. The first Tree of Sex database synthesized a vast diversity of reproductive strategies and their intriguing distribution throughout eukaryotes. A decade on, we are reviving this initiative and greatly expanding its scope to provide the most comprehensive integration of knowledge on eukaryotic reproduction to date. In this perspective, we first identify important gaps in our current knowledge of reproductive strategies across eukaryotes. We then highlight a selection of questions that will benefit most from this new Tree of Sex project, including those related to the evolution of sex, modes of sex determination, sex chromosomes, and the consequences of various reproductive strategies. Finally, we outline our vision for the new Tree of Sex database and the consortium that will create it (treeofsex.org). The new database will cover all Eukaryota and include a wide selection of biological traits. It will also incorporate genomic data types that were scarce or non-existent at the time of the first Tree of Sex initiative. The new database will be publicly accessible, stable, and self-sustaining, thus greatly improving the accessibility of reproductive knowledge to researchers across disciplines for years to come. Lastly, the consortium will persist after the database is created to serve as a collaborative framework for research, prioritizing ethical standards in the collection, use, and sharing of reproductive data. The new Tree of Sex consortium is open, and we encourage all who are interested in this topic to join us.