One of the main reasons I began working on Macrostomum flatworms is because they are so plastic, dynamically adjusting investment into reproductive traits according to the prevailing social environment. However, we’ve recently discovered that one species we’re keeping in the lab, Macrostomum pusillum, isn’t really plastic at all. That was initially puzzling, but we now think it’s for a very good reason. Rather than preferring to outcross like its close relatives, our recent experiments would seem to suggest that M. pusillum individuals instead prefer to self-fertilise their eggs with their own sperm. That means they don’t really have to plastically respond to their social environment, because that social environment isn’t particularly relevant to how they gain fitness.
Having the option to self is one major advantage of being a hermaphrodite, of course. Even so, it is not straightforward that selfing should evolve, because selfed offspring are often less fit, suffering from inbreeding depression. By forcing worms to self or giving them the option to outcross, we found no evidence for differential inbreeding depression (suggesting the worms may not be outcrossing even when they have the chance), nor did we find any evidence for plasticity in the age that worms begin producing hatchlings, their relative investment in making sperm versus making eggs, or the speed at which sperm are produced (all of which we would have expected under preferred outcrossing, based on what we know from other Macrostomum species). The next step will be to perform direct genetic tests for selfing, but for now our new working hypothesis is that the amazing diversity in Macrostomum sexual biology extends to a novel dimension, namely that some worms prefer to mate with another individual in order to reproduce, whereas others prefer to do it all by themselves.
Congratulations to lab member Athina Giannakara, who performed the study. The paper has just been published in the Journal of Experimental Biology here. (JEB were kind enough to feature my recent work on a hypodermically self-inseminating relative of M. pusillum, so the journal felt like a natural choice.)
Giannakara A, Ramm SA. 2017. Self-fertilization, sex allocation and spermatogenesis kinetics in the hypodermically-inseminating flatworm Macrostomum pusillum Journal of Experimental Biology, doi: 10.1242/jeb.149682
Photo credit: Athina Giannakara