October 21, 2016 at 02:47AM

Today I Learned: 1) Learned a bit about insect immune systems. For one thing, I learned that hemolymph, the goey stuff that insects pump around their body, doesn't carry much oxygen. It mostly seems to be for moving nutrients around... and for immune system function. 90-95% of the cells in the hemolymph are killer cells, which detect, track down, and ingest foreign cells. These are insects' primary line of defense against pathogens. The other major immune system component insects have is a system of anti-microbial peptides, or AMPs. AMPs are a diverse class of small proteins produced across the eukaryotes, but they're particularly important to insects. AMPs act as antibiotics in a number of ways, and are secreted en masse when the insect's immune system detects an invasion. 2) Remember that weird nested symbiosis I mentioned a couple of TILs ago? With the bacteria that lives in a bacteria that lives inside an insect? Today I learned a bit more about that triple symbiosis. The insects (it's actually a whole clade) in question are mealybugs, a clade of tiny little fuzzy sapsuckers, meaning they live on sap sucked from host plants. This isn't a very nutrient-rich diet, so it helps to have symbiotic bacteria that can metabolize unusual carbon sources. The first-level endosymbiont is Tremblaya, a bacteria that has been reduced to almost organelle status -- it has a massively reduced genome, even lacking the standard tRNA synthetases found in, say, mitochondria. The endosymbiont of Tremblaya, in contrast, is much closer to a normal bacteria in genome content. The second-level endosymbiont varies pretty widely across mealyworm species. It's worth noting that having a bacteria living inside a bacteria is extremely unusual. As far as I know, there aren't any known cases outside of insect cells. It's thought that Tremblaya can acquire an endosymbiont because of its unusual cell membrane. See, it's so massively reduced that it no longer has its own genes for synthesizing a membrane and cell wall. It relies on the mealyworm to produce a membrane for it. This means that from the outside, it effectively looks like a eukaryote -- and the symbionts that live in Tremblaya happen to be well-adapted to invading eukaryotic cells. 3) ...about preadaptation. Preadaptation is when a species just happens to have a trait that makes it very well suited for an environment it hasn't encountered before. When the preadapted species *does* encounter that environment, it tends to do quite well. You could consider any successful invasive species an example of preadaptation. Another example is thought to be lobe-finned fishes, which have far, far fewer bones in their fins than ray-finned fishes (most fish are ray-finned). I don't know why they evolved lobs, but it seems to have been really useful to have a small number of finger bones when the first lobe-finned fish started to crawl up onto land -- which is why we have five fingers on a hand instead of fifty (lobe-finned fish have order of ten digits per hand; ray-finned fish have more).