Different stages in the life cycle require different levels of resource allocation. Thus, according to a new paper in The American Naturalist, Competition and the Evolution of Reproductive Restraint in Malaria Parasites, by Pollitt et al., the trade-off in resource allocation between more and less costly life stages is a "key problem for natural selection to solve."
|CDC image, Wikimedia Commons. Parasite life cycle describe here.|
For malaria parasites, in-host replication and between-host transmission are two distinct stages of the life cycle. How, Pollitt et al. wonder, does evolution solve the problem of how to allocate resources between these two stages?
This is analogous to the trade-off between reproduction and maintenance faced by multicellular sexually reproducing organisms. The assumption that reproduction is costly, resulting in tradeoffs between reproduction and survival and between current and future reproductive effort, is a key concept in evolutionary biology.And, they say that, in spite of the toll malarial infection takes around the world, little is known about the investment strategies of malaria parasites, although they seem to invest "remarkably little" in transmission during infection. But, many infections are by a genetically heterogeneous mix of parasites, and in-host competition for resources between the different infecting genotypes seems to lead to reproductive constraint.
Previous studies suggest that when in-host survival is threatened parasites increase investment in between-host transmission at the expense of in-host replication but recent evolutionary theory predicts that the opposite should occur.
The recent discovery that malaria parasites can detect and respond to the presence of unrelated competitors suggests that they could also use this information to decide how much to invest in reproduction. [By the way, see the publication for references; it's chock full.]
First, we used a bank of genotypes to test for genetic variation in patterns of gametocyte investment throughout infections. Second, we monitored three focal genotypes in single and mixed infections with one or more competitors to test whether investment in gametocytes is facultatively reduced in competition. Third, we predicted that if reproductive restraint in mixed infections enables parasites to gain the greatest share of exploitable resources, then the investment decisions of each genotype will be influenced by the availability of these resources.They infected mice with various parasites, and assayed the parasite load and life cycle stages. And, they calculated the 'gametocyte conversion rate', the proportion of parasites that differentiate into sexual stages relative to asexual stages, relative to genetic mix of an infection, which is their measure of the extent of competition a parasite is facing.
They found "significant genetic variation and phenotypic plasticity in the reproductive effort of malaria parasites." And, that, in the context of mixed infections, investment in gametocytes is reduced, and diverted into asexual replication. That is, competition reduces investment in reproduction. Pollitt et al. suggest that they do this by taking the measure of their environment and responding accordingly. But, this brings up the question of how they know that they are in the midst of a heterogeneous infection. What alerts them to the fact that they aren't surrounded only by kin? And if it really does matter to them, are we verging on a 'group selection' scenario? And in that sense, isn't cooperation as important here as possible competition?
Parasites inside you are attacked on their own microscopic level. We described some of this at length in our book The Mermaid's Tale. Big organisms like us have immune systems to recognize invaders by their surface-molecular characteristics, and we do this by generating molecules of all sorts of which one, at least, will 'recognize' the invader, bind to it, and trigger a destructive reaction. But the parasites don't like this, and when we've got too good a hold onto their characteristics, they have gene families coding for cell-surface proteins (called var or avr genes in some species), from which they can randomly pick a different gene to express on their surfaces, which makes them again invisible until the immune system regroups and goes at it anew.
If the results reported in this paper hold up, we think they suggest more about adaptability and facultativeness than any generality about natural selection's problem solving abilities.