Why Sex?

One of the major questions in evolutionary biology is 'Why Sex?'. This 'queen of problems' puzzled biologists for decades and likewise me for the last few years. The question is, why are the huge majority of organisms mixing genomes through sexual reproduction involving males and females, when there is the straight-forward way of just producing asexual females. And it is not just a little bit of sex every now and then (not meaning individuals, but population scale) but high rates of outcrossing. Sex has many costs. Demographic costs of males, transmission costs, costs of mating (predator exposure and STDs), ... and so on. But albeit these many-fold costs, sex must have some profound advantage compared to completely asexual female populations.

There are a lot of theories for the advantages of sex (see the bookchapter here or ask me to get it), and some were proven to work under lab conditions, but for natural populations, most of them remain elusive. Only recently, with the advance of the 'genomic era', it has become reasonably cheap and easy to produce whole genome data for non-model organisms (but still not easy for some organisms... see the animal systems page). I am mostly interested in the genomic consequences of asexual reproduction in such natural populations. I try to elucidate, if predicted consequences are found in nature and further, if there are general or lineage-specific patterns to be found. My favorite animal systems are those that likely exist longer then predicted extinction times. The so called 'ancient asexuals'. I, together with various other researchers, try to shed some light into the peculiarities of natural populations of (ancient) asexual animals compared to sexuals, to help explaining 'Why Sex?'.

My PhD thesis (in the Scheu group at the University of Goettingen, co-supervised by Ken Kraaijeveld) focused on transposable elements (TEs) as deleterious agents driving mutational meltdown in asexual lineages. Turns out, they seem no problem (why? Check out the resulting main paper and more explanations under projects). As a postdoctoral researcher, I kept working on this topic at the University of Lausanne (in the Schwander group) by following whole-genome TE dynamics in a laboratory setup with yeast to test if TEs evolve to be benign. And actually, asexuality indeed drives the reduction of TE load, consistent with theory (read paper here). Within the Schwander lab, I generated insights into consequences of asexual reproduction in Timema stick insects and oribatid mites, such as identifying deleterious mutations and counteracting forces (including TEs and gene conversion). Turns out that in asexual oribatid mites, purifying selection acts more effectively than in sexuals, breaking with the textbook-knowledge that sex is needed for benefits in the long-term (work with A. Brandt). On the other hand, Timema behave like expected in every aspect we looked at. When studying the consequences of asexuality, to be able to disentangle lineage-specific effects, having good animal systems with independently derived asexuals and sexual relatives matter. This is substantiated by a meta-review/analyzes on all existing asexual genomes I did with Kamil, where we could not detect common features of asexuality.

In the near future, within my research group at the University of Cologne, we generate insights into what is going on with the mites. Why are they so special? How can they survive and diversify over time in the absence of sex? What can we learn from them vise-versa about the existence of sex? I will use modern sequencing technology on single individuals to find out what the genomic substrate for persistence without sex is. How old are asexual oribatid mites really? How does speciation work in these asexuals? What roles do standing genetic variation and population sizes play in this? And finally, why are some mites using sexual reproduction when their asexual sisters seem just fine?

For details see the projects and animal systems pages.