October 15, 2016 at 01:57AM

Today I Learned: 1) There is an insect with an obligate endosymbiotic bacteria -- that is, a bacteria that lives in the insect's cells, and provides important functions to the insect -- that itself has an obligate endosymbiotic bacteria. More details to follow. 2) Relatedly, there's a well-known nasty effect that happens when one organism becomes a required endosymbiote for another (usually it's a plant or animal that takes on a bacterial endosymbiote) that poses a serious danger to both organisms. See, endosymbiotes have to deal with having an incredibly small effective population size, since a) they can't really get access to outside populations to recombine genetically, and b) they undergo a nasty population bottleneck whenever the host reproduces. It turns out that the effects of genetic drift become much more severe with small populations. If the effective population size is small enough, drift and random mutation will overcome selective pressure, and the symbiote's genome will literally start to degrade on evolutionary timescales. See, population size and mutation rate are intimately linked, and both contribute to the maximum size genome a species can stably maintain through selection. The higher a species' mutation rate, the smaller the biggest genome they can maintain over many generations. Smaller population sizes effectively amplify the effect of mutations on genome degradation. There's some thought that this is basically what determines genome length -- species adapt to have genomes that are sizes their mutation rate can support, basically. That's why RNA viruses, which have insane mutation rates, also have insanely tiny genomes. Probably. In the case of endosymbiotes, the symbiote evolved to cope with a reasonable mutation rate/effective population size, but gets hit rapidly with a much harsher mutation rate/effective population size. The net result is that the symbiote's genome gets quickly whittled down to a small fraction of its original size. An extreme example is the ancestral predecessor of the mitochondria, the genome of which was whittled from a (presumably) normal-sized bacterial genome to a 10-20kb circlet. This puts the host in a real pickle. Now it depends on an endosymbiote, but that endosymbiote is rapidly degenerating. Thus begins a mad scramble to somehow make the symbiote functional before it completely loses any ability to function, killing both symbiote and host. This is presumably why gene transfer from symbiote to host is so common (it's been observed in just about every sufficiently-old endosymbiotic relationship) -- it keeps the endosymbiote functional while reducing its genome size, making it less likely to decay to uselessness. 3) ...what a young termite queen looks like. It's a surprisingly elegant looking insect, at least from afar -- it has long, diaphanous wings, making it look almost like a squat dragonfly, but with a flight pattern more like a moth.