I started this blog in part to get myself to write more in a setting that’s different from scientific writing I do nearly every day (or try to). This month, I’m going to try my hand at research blogging. I hope to write at least four posts reporting on peer-reviewed research in ecology, genetics, and/or evolutionary biology published recently in open-access journals.
First up, a short and sweet study of the genetics of asexual reproduction. It attracted my attention because I’m currently TAing a basic genetics course, and I had been telling my students how rarely anyone does classical breeding experiments anymore!
In this study, Stelzer et al. demonstrate simple Mendelian inheritance of obligate parthenogenesis—that is, asexuality—in the rotifer Brachionus calyciflorus. The rotifer is normally a cyclical parthenogen: it reproduces clonally until it reaches a threshold high density, whereupon it produces sexual females, which then produce haploid males (if their eggs go unfertilized) or diploid, diapausing females. However, some strains of B. calyciflorus only produce clones—never sexual females. The authors wanted to know how this trait is inherited, and modified classic breeding experiments to find out.
The normal rotifer life cycle has some advantages for controlled breeding. First, since they’re normally clonal, you can produce many genetically identical individuals. Second, since males are haploid, they have a subset of their mother’s genes, so that crosses within a clone are equivalent to self-fertilization.
There’s one difficulty, however: obligate parthenogens, obviously, will never mate. So the authors could only look at crosses that produced obligates, and not cross the obligates themselves.
Using clones derived from collections in several locations, the authors conducted self-fertilizations, raised the offspring separately at high densities, and checked whether they switched back to sexual reproduction. In some crosses, all of the offspring because sexual active—no obligate parthenogens were produced. In others, 25% of the offspring were obligate parthenogens. This is consistent with the 3:1 ratio expected when selfing a heterozygote. Thus, the authors hypothesized that obligate parthenogenesis was caused by a recessive allele which they called op. The parent clones were either heterozygous (op/+, where + represents the wild type allele) or homozygous for the wild type (+/+).
To confirm this hypothesis, they selfed the cyclical parthenogenetic offspring of the heterozygous crosses. These are expected to be 1/3 +/+, producing no obligates, and 2/3 op/+, producing 25% obligates. And, indeed, this is what they found. They also did further crosses between heterozygous strains; these also resulted in 25% obligate offspring. (This suggests mutations at the same gene are involved. This is not, as the authors state, complementation. Complementation is when mutations at different genes produce the same phenotype, so that when they’re crossed, the normal phenotype shows up. If the strains had complementation, we’d expect to see no obligates among the offspring.)
The discussion is oddly silent on the consequences of these results, although it’s hinted at in the introduction. Transitions from sexual to asexual reproduction and vice versa could be caused by few large-effect mutations or many mutations of small effect. I’m not sure what is expected, if there is indeed an expectation. But this is a pretty solid result for the former.
The fact that the op allele segregates at such high frequency raises the question of whether it’s adaptive. Without any background in rotifer-ology, I’m going to say “maybe”. Cyclical parthenogenesis with sexual reproduction at high densities is adaptive: start looking for mates when you know there’s enough others to find one. Obligate parthenogens have a fitness advantage in terms of pure numbers, though, so there may be a tug-of-war between sheer numbers and recombination (that is, sexual reproduction allows for new combinations of alleles).
Obligate parthenogenesis in B. calyciflorus is accompanied by much smaller (by ~50%) egg and body size. This makes me wonder if the obligates perhaps never reached their density threshold—even though they were brought to 1 female/mL, it was a much smaller female: maybe biomass rather than numbers is more important for the switch to sexual reproduction. That said, even the density use to test for sexuality in this study is way higher than typical natural densities.
Another thought about body size: In many organisms, we expect a correlation between size and fecundity, with larger females able to produce larger/more eggs. This is not so in B. calyciflorus (the authors note this may be because they produce eggs sequentially rather than in a clutch). Maybe what we’re seeing is a trade-off: obligates make more small eggs while cyclicals make fewer, larger ones. Maybe the two types amount to the same biomass.
Then again, given that you apparently need really unnatural/extreme conditions to induce sexual reproduction in these guys, maybe variation at the OP gene is irrelevant in nature. Perhaps it’s a relic of past evolution. I know so little about rotifer biology that I can’t really say.