Mitochondria: The Red Queen lies within (comment on DOI 10.1002/bies.201500057).

Abstract

From a genetic perspective, sex seems like the worst idea in the world. A sexual parent transmits only half of its genes to its offspring, compared with all of its genes for an asexual parent. Yet despite this considerable cost, the vast majority of eukaryotes engage in sex, which makes the evolution of sexual reproduction an enigma in evolutionary biology [1]. So why might sex be worth its cost? Primarily, sex breaks up associations between alleles at different loci. That is why sex is often considered beneficial in the context of host-parasite coevolution, as it breaks up associations between host alleles that may once have conferred resistance to parasites but no longer do so, while at the same time generating novel resistance combinations. This is called the ‘Red Queen’ hypothesis and is one of the major explanations (but not the only one) why sex is so common. However, its generality is debated: for one, selection needs to be strong and outcomes are highly sensitive to the genetic architecture of resistance [1]. In this issue, Havird et al. [2] suggest a novel hypothesis, in which mitochondria drive the evolution of sex. Mitochondria are organelles carrying their own DNA (mtDNA), containing genes indispensable for metabolism. Yet, we know from animal taxa that mtDNA mutates much faster than nuclear DNA (nDNA). Consequently, metabolism is in constant jeopardy of being disrupted by deleterious mutations, which is exacerbated by a limited scope for mtDNA recombination, so that mutations continuously accumulate (Muller’s ratchet).