Sex is a ubiquitous, ancient, and inherent attribute of eukaryotic life

TLDR

Sexual reproduction and clonality are often viewed as mutually exclusive in eukaryotes, a view that may be biased by a metazoan‑centric focus. The study aims to analyze reproduction across extant eukaryotic diversity, especially protists, and to address the longevity of truly clonal species. The authors examine phylogenetic presence of fusion proteins HAP2 and GEX1 and relate clonal properties to meiotic development derived from prokaryotic repair mechanisms. Their analyses reveal that sex‑related proteins are widespread, suggesting that sexual reproduction originated in the last common eukaryotic ancestor and that most eukaryotes are clonally propagated with episodic sex, while long‑lived clonal species are rare and sex may have evolved as a survival strategy linked to mitochondrial oxidative stress.

Abstract

Sexual reproduction and clonality in eukaryotes are mostly seen as exclusive, the latter being rather exceptional. This view might be biased by focusing almost exclusively on metazoans. We analyze and discuss reproduction in the context of extant eukaryotic diversity, paying special attention to protists. We present results of phylogenetically extended searches for homologs of two proteins functioning in cell and nuclear fusion, respectively (HAP2 and GEX1), providing indirect evidence for these processes in several eukaryotic lineages where sex has not been observed yet. We argue that ( i ) the debate on the relative significance of sex and clonality in eukaryotes is confounded by not appropriately distinguishing multicellular and unicellular organisms; ( ii ) eukaryotic sex is extremely widespread and already present in the last eukaryotic common ancestor; and ( iii ) the general mode of existence of eukaryotes is best described by clonally propagating cell lines with episodic sex triggered by external or internal clues. However, important questions concern the relative longevity of true clonal species (i.e., species not able to return to sexual procreation anymore). Long-lived clonal species seem strikingly rare. We analyze their properties in the light of meiotic sex development from existing prokaryotic repair mechanisms. Based on these considerations, we speculate that eukaryotic sex likely developed as a cellular survival strategy, possibly in the context of internal reactive oxygen species stress generated by a (proto) mitochondrion. Thus, in the context of the symbiogenic model of eukaryotic origin, sex might directly result from the very evolutionary mode by which eukaryotic cells arose.