Blood-Fluke Monogamy

You know how upsetting it is when a celebrity couple to whom you’ve become attached breaks up?  And you know how it’s the worst when they break up because of infidelity?

Then perhaps you can imagine how distressed I was this week when I learned that Schistosoma mansoni, the endoparasitic blood-fluke, wasn’t really monogamous.

Schistosomes are a genus of class Trematoda, or parasitic flatworms.  They are the cause, in humans, schistosomiasis, which kills tens or hundreds of thousands of people every year (depending on whom you ask), and which is the second-most impactful parasitic infection worldwide after malaria.

Unlike other flatworms, schistosomes are dioecious.  During mating, the male, which is much larger, essentially encloses the female in something called, unsexily, a “gynacophoric canal”, where reproduction will occur and where the female will spend the rest of her life.

Or so we thought!  Turns out, blood-fluke couples split up, or, as scientists kind of preposterously put it, “divorce”.  Schistosome divorce was apparently first reported in 2000 by Pica-Mattoccia et al., in the journal Parasitology, but I don’t read Parasitology (although clearly I need to), and so I missed it.

However, my attention fell this week on a 2009 paper by Beltran et al., ‘Adult sex ratio affects divorce rate in the monogamous endoparasite Schistosoma mansoni’.  The authors of this paper found that they could dramatically increase the blood-fluke divorce rate by flooding the bloodstream of infected animals with male schistosomes, thus increasing the number of unpaired, available males.  However, increasing the number of female schistosomes in the bloodstream, i.e. creating a population of unpaired female blood-flukes, had no effect on divorce rates.

There are two possible explanations for this gendered effect.  The first is that the female schistosome is the choosier sex, and so, when there are more available unpaired males from which to choose, some number of paired females will upgrade.  Supporting this view is the observation that there is greater genetic variety among male blood-flukes than among females; this means that a female schistosome may gain a significant advantage from switching mates, while a male probably will not.  For a male schistosome, one female is very like another, and so when you have one, best just to hold on to her; hence the lack of effect on divorce rate when the number of available females was increased.

The other possible explanation is that the free schistosome males act in a predatory manner, and may physically break-up happily joined couples.  Since the male schistosome is much larger than the female, it makes sense that this would only happen when the number of males was increased: the smaller females would be incapable of separating another blood-fluke couple, even if there were more of them.

Of course, these two hypotheses need not be mutually exclusive: as the authors of the study mention in a slightly victim-blaming aside, “the possibility exists that female schistosomes choose not to resist takeover attempts by rival males if they perceive their higher phenotypic and/or genetic value.”

None of this lightens the emotional blow to those of us who had pinned our hopes on blood-fluke monogamy, of course.  It’s over – it was all a lie.  Love is dead.

Image from National Geographic.

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