Here a wide distribution of meiotic machinery is shown, indicating the occurrence of sexual processes in all major eukaryotic groups, without exceptions, including the putative “asexuals.” Meiotic machinery has evolved from archaeal DNA repair machinery by means of ancestral gene duplications. Sex is very conserved and widespread in eukaryotes, even though its evolutionary importance is still a matter of debate. The main processes in sex are plasmogamy, followed by karyogamy and meiosis. Meiosis is fundamentally a chromosomal process, which implies recombination (...) and ploidy reduction. Several eukaryotic lineages are proposed to be asexual because their sexual processes are never observed, but presumed asexuality correlates with lack of study. The authors stress the complete lack of meiotic proteins in nucleomorphs and their almost complete loss in the fungus Malassezia. Inversely, complete sets of meiotic proteins are present in fungal groups Glomeromycotina, Trichophyton, and Cryptococcus. Endosymbiont Perkinsela and endoparasitic Microsporidia also present meiotic proteins. (shrink)

Microscopy has revealed tremendous diversity of bacterial and eukaryotic forms. Recent molecular analyses show discordance in estimates of biodiversity between morphological and molecular analyses. Moreover, phylogenetic analyses of the diversity of microbial forms reveal evidence of convergence at scales as deep as interdomain: morphologies shared between bacteria and eukaryotes. Here, we highlight examples of such discordance, focusing on exemplary lineages such as testate amoebae, ciliates, and cyanobacteria. These have long histories of morphological study, enabling deeper analyses on both the molecular (...) and morphological sides. We discuss examples in two main categories: (i) morphologically identical (or highly similar) individuals that are genetically distinct and (ii) morphologically distinct individuals that are genetically the same. We argue that hypotheses about discordance can be tested using the concept of neutral morphologies, or more broadly neutral phenotypes, as a null hypothesis. -/- . (shrink)

The distribution pattern of the meiotic machinery in known eukaryotes is most parsimoniously explained by the hypothesis that all eukaryotes are ancestrally sexual. However, this assumption is questioned by preliminary results, in culture conditions. These suggested that Acanthamoeba, an organism considered to be largely asexual, constitutively expresses meiosis genes nevertheless—at least in the lab. This apparent disconnect between the “meiosis toolkit” and sexual processes in Acanthamoeba led to the conclusion that the eukaryotic ancestor is asexual. In this review, the “meiosis (...) toolkit” is rigorously defended, drawing on numerous research articles. Additionally, the claim of constitutive meiotic gene expression is probed in Acanthamoeba via the same transcriptomics data. The results show that the expression of the meiotic machinery is not constitutive in Acanthamoeba as claimed before. Furthermore, it is argued that this would have no implications for understanding the nature of the eukaryotic ancestor, regardless of the result. (shrink)