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Abstract Mitochondrial RNA editing has evolved independently in numerous eukaryotic lineages, where it generally restores conserved sequences and functional reading frames in mRNA transcripts derived from altered or disrupted mitochondrial protein-coding genes. In contrast to this “restorative” RNA editing in mitochondria, most editing of nuclear mRNAs introduces novel sequence variants and diversifies the proteome. This Perspective addresses the hypothesis that these completely opposite effects of mitochondrial vs. nuclear RNA editing arise from the enormous difference in gene number between the respective genomes. Because mitochondria produce a much smaller transcriptome, they likely create less opportunity for off-target editing, which has been supported by recent experimental work expressing mitochondrial RNA editing machinery in foreign contexts. In addition, there is recent evidence that the size and complexity of RNA targets may slow the kinetics and reduce efficiency of on-target RNA editing. These findings suggest that efficient targeting and a low risk of off-target editing have facilitated the repeated emergence of disrupted mitochondrial genes and associated restorative RNA editing systems via (potentially non-adaptive) evolutionary pathways that are not feasible in larger nuclear transcriptomes due to lack of precision. 

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.