Exceptionally Rare Genomic Combinations to Skip Sex: Blending Old Ideas and New Data

The unusual occurrence and developmental diversity of asexual eukaryotes still remain a puzzle. Despite asexual organisms have a theorized two-fold reproductive advantage over sexuals, asexual lineages are rare among multicellular eukaryotes. Justification of such disparity relies on the consequences of a lack of meiosis, which restricts genotype diversity and adaptation to novel conditions while accelerating the genetic degeneration that drives asexual lineages to an early demise. However, evidence indicates asexuals use different strategies to limit negative consequences of ameiosis, and age estimates show some asexual vertebrates and plants are much older than expected. If rapid extinction is not a factor influencing asexuals lifespan, then why asexuals are not more frequent? Here we review traditional ideas and new data and provide a novel unified evolutionary frame to understand the intriguing nature, developmental diversity and maintenance of asexual lineages. As a rule, de novo formation of a functioning asexual genome requires a unique assemblage combining particular sets of genes or gene states to disrupt cellular mechanisms of meiosis and gametogenesis, and affect discrete components of sexuality to produce clonal or hemiclonal offspring. We highlight two usually overlooked but essential conditions to understand the molecular nature of clonal organisms, i.e. a non-recombinant genomic assemblage retaining modifiers of the sexual program, and a complementation between altered reproductive components. These subtle conditions are the basis for physiologically viable and genetically balanced transitions between generations. Genomic and developmental evidence from asexual animals and plants indicates the lack of complementation of molecular changes in the sexual reproductive program is likely the main cause of asexuals' rarity, and can provide an explanatory frame for the developmental diversity and lability of developmental patterns in some asexuals as well as for the discordant time to extinction estimations.