Yeerk reproduction: my take
Ever since I listened to the audiobook I Contain Multitudes, a great book about symbiosis with bacteria, I’ve had a headcanon about how Yeerk reproduction works.
First, defining some terms: here when I say sex I mean “a mating type that is fertile with other mating types but not with the same mating type.” Male and female are sexes, but they are not the only sexes out there, even in Earth biology. When I say endosymbiont I mean a symbiotic bacterium that lives inside a eukaryotic cell. Mitochondria and chloroplasts are endosymbionts, though there are also many others.
Also, here’s what we know about Yeerk reproduction in canon: it involves three Yeerks, it is always fatal, and it produces hundreds of grubs.
In the book I Contain Multitudes, there's a super fascinating case of an organism with two types of endosymbionts (that are not mitochondria or chloroplasts) where it's swapped some of its genome with the endosymbionts. So some of their DNA is now in the eukaryotic nucleus, and some of the eukaryote's DNA is in the bacterial endosymbionts. It's not very much that has been swapped, but still: the genome of the organism is now effectively distributed between three organelles.
What if in Yeerks, they also have two bacterial endosymbionts and their genome and that of the endosymbionts is now fairly evenly spread out among the three?
This means each Yeerk in a reproductive trio contributes either the eukaryotic nucleus, one endosymbiont, or the other. And it gets even more interesting, because how do you determine which Yeerk passes on what?
In two-sexed organisms, the egg passes on endosymbionts and the sperm does not. But it gets more complex in many-sexed organisms like slime molds. They effectively have a kind of rock-paper-scissors with sexes. Sex A passes endosymbionts over Sex B which passes on endosymbionts over Sex C until it wraps back around to Sex A.
My proposal: Yeerks, like slime molds, have thousands of sexes. As long as each of the three Yeerks in a reproductive fusion is a different sex, they can reproduce, and the combination of sexes determines which passes on which component of the genome. Technically, if you don't count the endosymbionts, one of the Yeerks is just cloning itself. But then the other Yeerks pass on their endosymbionts with the other components of the genome to the next generation.
I should note that the reason why all these endosymbiont rules are so important is that if both parents pass on the same endosymbiont, then you have two strains of say, mitochondria, which will instantly go to war with each other inside the zygote, which seals its doom. So it's absolutely crucial to survival to make sure that one and only one parent passes on each type of endosymbiont.
Given that Yeerks produce so many offspring when they reproduce, it’s either that they have very high mortality of grubs, or that very few Yeerks reproduce. I tend to believe the latter. It's like social insects, where very few reproduce and their relatives raise offspring communally.
In my series Dæmorphing, reproducers are chosen in a special ceremony like sacrifices. And as with many human sacrifices, it's voluntary and a great honor: a martyr to ensure your pool can go on.
