A Universal Theory of Sex

Sexual reproduction in plants and animals presents an evolutionary enigma. It is genetically risky, time-consuming, and error-prone – possibly leading to reduced fitness. And yet, at least 99% of all plants and animals reproduce sexually. This commonality raises the question of whether universal evolutionary pressures exist to select for and maintain sexual reproduction. 

Elvira Hörandl argues in a review paper in Annals of Botany that the need for DNA repair may be the basis for the near universally of sex in the plant and animal kingdoms. And she uses land plants as a tool to make her case.  

“Land plants are of special interest for this question because, on the one hand, sexual reproduction is predominant, similar to animals; on the other hand, some theories developed for animals are not readily applicable to plants because autotrophic organisms have different physiological constraints.” 

The key process in sexual reproduction is meiosis, defined as the pairing and recombination of corresponding parental chromosomes. During this process, parental DNA is fused to create genetic variation in offspring. The new genetic combinations can be advantageous, neutral, or even detrimental to fitness if well-adapted genes are broken up.  

Hörandl argues that plants don’t need meiosis to achieve advantageous genetic combinations — polyploidy can be used instead. In plants, many species are polyploid, i.e., have multiple genomes, and individual genetic variation is correspondingly high. This built-in genetic diversity confers the necessary ‘phenotypic plasticity’ for plants to respond to environmental stressors such as light, heat, drought, and salt. In this context, asexual reproduction, where plants reproduce without meiosis, can make sense as a reproductive strategy.  

However, while many plants do reproduce asexually, Hörandl shows that many of those species run both sexual and asexual reproductive pathways in parallel in the same plant in a flexible manner and do not rely solely on asexual forms of reproduction. 

And so, the question becomes, if polyploidy and asexual reproduction are enough to create the necessary genetic diversity for fitness, why then do plants bother with sexual reproduction?  

Hörandl suggests that ‘DNA restoration theory’ provides the answer.  DNA restoration theory posits that the major function of meiosis is to repair damaged DNA and remove negative mutations in each generation. In that light, genetic recombination is not the end goal of sexual reproduction but is instead a by-product of a DNA repair mechanism.  

DNA needs repair because it is continuously damaged by mitochondrial respiration as well as by photosynthesis in plants. Hörandl writes that: 

“In the long term, asexual reproduction without any recombination would result in genomic decay owing to the accumulation of deleterious mutations, specifically in small populations, finally leading to extinction of the asexual lineage.” 

Indeed, sexual reproduction, and consequently meiosis, is triggered in plants grown in DNA damage-inducing, stressful environments. Hörandl notes that only 

“…a little bit of sex (a mean of ~6 % recombined offspring in three progenies) is sufficient to avoid accumulation of mutations over generations.” 

Consequently, species may have evolved the ability to reproduce sexually in order to repair and maintain their DNA.

READ THE ARTICLE

Hörandl, E. (2024) “Apomixis and the paradox of sex in plants,” Annals of Botany, p. mcae044. Available at: https://doi.org/10.1093/aob/mcae044.