Rather shockingly, sexual reproduction remains an enigma – despite over a century of study. Theory has identified the costs and benefits of sex, illustrating why almost all* eukaryotes go to the trouble, at least occasionally.
* Even supposedly obligate asexuals have been found to dabble in “cryptic sex” (e.g., Boyer et al. 2021)
While we have a rich literature from which to describe the genetic processes that maintain sexual reproduction, we lack ecological context. What happens in nature? There are abiotic and biotic interactions that result in complex sexual/asexual dynamics in actual populations.
There are some impediments to studying the ‘ecology of sex’. For example, many asexual lineages arise from hybridization events and are polyploid. How to separate diploid vs. polyploid or the effects of hybridization?
Boechera may be an answer.
Rushworth et al. (2021) used a reciprocal transplant experiment with 53 wild-collected diploid genotypes – 24 sexual and 29 asexual. One of the cool things about Boechera is that asexuals may be diploid or polyploid and they arise from either intra- or interspecific outcrossing events. Asexual Boechera are highly heterozygous whereas sexuals are largely homozygous as they are highly selfing. The authors highlight the importance of the identity of sexual partners – genetically, if your genome is similar to your partners genome, it’s much harder to bring together new allelic combinations. As Boechera has an enviably well-described mating system, it presents a pretty powerful system for looking at the ecology of sex.
It’s wild that we don’t know more about the abiotic and biotic factors matter in maintaining sex in the wild. Rushworth et al. (2021) collected genotypes from the wild – these genotypes were produced in nature and survived.
They found that asexuals had higher lifetime fitness and higher survival. For sexuals, inbreeding depression may be the primary cost. Yet, as asexuals are likely continually produced, divergent alleles may actually lead to outbreeding depression.
What was really intriguing was the 73% increase in herbivory in asexuals, especially the hybrids! While Rushworth et al. (2021) didn’t find that this cost overcame the benefits enjoyed by asexuals during the length of time they used, it may be that herbivory may decrease survival such that there is a cost later on in life. Herbivory might just put a stop to the rampant spread of asexuals.
Collectively, [Rushworth et al. (2021) showed] that the complex natural environment and the choice of a sexual partner work in tandem to shape patterns of reproductive variation in the wild.
I’ve read through this paper several times. I need to go back and read it a few times more – likely with it printed out on paper. I have tried to read papers online and even more so that I’m not at my campus desk every day as in the before times – but it just isn’t the same!
I am very envious of the Boechera system and looking forward to the end of yet another remote semester when I can sit down and have time to think about this – particularly incorporating selfing into predictions about sex and asex.