What can we infer about the origin of sex in early eukaryotes?

Abstract

One contribution of 15 to a theme issue 'Weird sex: the underappreciated diversity of sexual reproduction'. Current analysis shows that the last eukaryotic common ancestor (LECA) was capable of full meiotic sex. The original eukaryotic life cycle can probably be described as clonal, interrupted by episodic sex triggered by external or internal stressors. The cycle could have started in a highly flexible form, with the interruption of either diploid or haploid clonal growth determined by stress signals only. Eukaryotic sex most likely evolved in response to a high mutation rate, arising from the uptake of the endosymbiont, as this ( proto) mitochondrion generated internal reactive oxygen species. This is consistent with the likely development of full meiotic sex from a diverse set of existing archaeal (the host of the endosymbiont) repair and signalling mechanisms. Meiotic sex could thus have been one of the fruits of symbiogenesis at the basis of eukaryotic origins: a product of the merger by which eukaryotic cells arose. Symbiogenesis also explains the large-scale migration of organellar DNA to the nucleus. I also discuss aspects of uniparental mitochondrial inheritance and mitonuclear interactions in the light of the previous analysis.This article is part of the themed issue 'Weird sex: the underappreciated diversity of sexual reproduction'.

All Eukaryotes Are Sexual, unless Proven Otherwise

“…During meiosis, homologous chromosomes are aligned allowing for recombination and silencing of newly translocated selfish elements . As meiosis promotes a large‐scale repair of the nuclear genome, it was suggested that meiosis evolved and was maintained as a way to repair the genome from continuous DNA damage caused by increased amounts of reactive oxygen species generated by mitochondrial metabolism …”

It Takes Two to Evolve Too. The Hypothesis on Three Primary Communication Transitions in Evolution

“…The anisogamy is a result of conflicts inside zygote between mitochondria from cytoplasms of two gamets, which favored the selection for one large ovum exclusively containing mitochondria for future zygote as a gamete for female sex and multiple competing small mitochondria-less sperms as gametes for male sex. Moreover meiosis itself -the evolutionary benefit outweighing the two-fold cost of sex -might have originated as a response to symbiogenesis [76].…”

Weird sex: the underappreciated diversity of sexual reproduction

“…Sex in the classical sense is not the only way to swap genes [3], so why do so many organisms insist on a complex cocktail of the four features of meiotic sex [4]? Sex is very often facultative rather than obligate, yet the organisms most familiar to us are mostly obligatorily sexual [5].…”

“…The topics addressed in this special issue range from very fundamental questions such as the evolutionary benefits [12] and the origin of sex [3,4,13] and mating types [6], to secondary consequences of sexual reproduction, such as sexual selection [5,11] and the increased scope for genetic conflict [8,10,14], and the cost of sex, either due to the production of males in anisogamous species or due to mate finding or mechanisms of meiosis [15].…”

“…Therefore, Lenormand et al [13] start this issue with a review of the evolution of one of the hallmarks of sex, meiosis. However, as pointed out by Speijer [4], for each of the four separate but interlocking mechanisms of complete meiotic sex, prokaryotic factors and/or processes can be identified as evolutionary predecessors. Therefore, Ambur et al [3] discuss the prokaryotic way of mixing of genetic material via transformation, which does not involve meiosis.…”