May 28, 2016 at 12:27AM

Today I Learned: 1) The most poisonous tree (not the most poisonous plant) is the Manchineel tree, native to Florida. Its fruit is toxic, and can kill (though usually it just gives a super nasty rash down its eater's throat -- it is, however, apparently sweet and quite tasty). Its sap is caustic, and will produce painful blisters on contact. If you get it in your eye, it will blind you (temporarily). Pretty much every single part of the tree will hurt you if you touch it. One might wonder why a tree would bear a fruit that's toxic. Many plants have fruit that's toxic (holly) or un-tasty (jalapeños) to mammals to keep them from eating it while still being edible to their main seed-spreading vector, birds. Manchineel fruit looks a bit big to be eaten by birds to me, but it might have its seeds spread by something else. Another proposal is that the Manchineel tree's fruits spread by water, but then one would wonder why they would bother making sugar-laden, pulpy fruit. On a side note, Manchineels are endangered. 2) Today I read about a rather remarkable system for tracing the lineages of cells in an organism. First, a word on what that means. Imagine a fertilized fish egg. It's one cell. It's going to eventually grow into a fish with somewhere between tens of millions and trillions of cells. Developmental biologists would really like to know how that cell and its descendents split to to eventually make up the fish. The combined history of all of the cells in the organism is called a lineage tree (or fate map), and the process of figuring out the lineage tree is generally called fate mapping. To fate-map an entire organism is... difficult, to say the least. A lab at Harvard has developed a system called GESTALT (Genome Editing of Synthetic Targetted Arrays for Linage Tracing) for doing just that, at least in a coarse-grained way. It uses... Cas9! Basically, the system uses an array of nine or ten repeated sequences, weakly targetable by a genome-editing version of Cas9 with a guide expressed as part of the construct, plus Cas9 itself. The whole thing is added to the organism's genome at a single-celled stage. As the ogranism grows and divides, Cas9 will occasionally "mark" one of the editing sites with an insertion or deletion. Once it's made an edit in a cell, that cell and *all of its descendents* will have the mutation. Once the organism is fully grown, you can take out single cells, sequence them at the array site, and figure out which ones are related by which mutations they share. Credit on finding out about this goes to Andy Halleran. Apparently he heard about this work while he was interviewing. This is why you should apply to grad schools, kids -- interviews are a fantastic way to travel the country and hear about all the best science before it gets published. * We actually have this for the nematode C. elegans, which has an extremely rigid lineage tree and a fixed number of cells (1031, in the case of the male). It was determined in the early 80s by watching a bunch of developing C. elegans very closely. 3) There are viscoelastic materials that, when compressed, spring back to *more* than their original size. Does bread dough do this? Thanks for teaching me this, Mengsha Gong!