Secular Café: Did arthropods and vertebrates inherit their segmentation?

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Did arthropods and vertebrates inherit their segmentation? May 8th 2012, 05:23 Nice to see PZ blogging about something in his professional specialty, evolutionary developmental biology: How was the vertebrate/arthropod LCA segmented? | Pharyngula Arthropods are well-known for being segmented. For vertebrates, one must look inside, but one can easily find segmented parts. Annelids are also segmented, though PZ did not mention them. Segmentation (biology). So did their segmentation originate in some common ancestor? Or did it originate separately? As PZ himself notes, we still don't know for sure. Vertebrates and arthropods share some genes that are involved in the process, but some of the genes are widely used in other processes. Even worse are differences between vertebrate and arthropod segmentation. Vertebrates grow segments at the rear end with a clock-and-wavefront mechanism, while flies grow them all at once. They start out with some broad layout, then narrow it down to produce the individual segments, all specified individually. They even have an alternation between certain molecules in odd-numbered segments and certain ones in even-numbered ones. Flies? The favorite lab fruit fly, Drosophila melanogaster. However, flies are rather atypical of arthropods for using all-at-once development. It's likely a result of selection for fast growth. So we must look at other arthropods. - PZ blogged on some results of studying the development of segments of the red flour beetle Tribolium castaneum, another common model system. Papers: Choe CP, Miller SC, Brown SJ (2006) A pair-rule gene circuit defines segments sequentially in the short-germ insect Tribolium castaneum. Proc Nat Acad Sci USA 103(17):6560-6564. Sarrazin AF, Peel AD, Averof M (2012) A segmentation clock with two-segment periodicity in insects. Science 336:338-341. It makes new segments on its rear end, as vertebrates do, with a similar sort of mechanism. But some of its mechanisms are more like fruit-fly ones than like vertebrate ones, like its segment alternations. So the puzzle is still not solved. - PZ concluded that we need to look at more species. Flies and beetles are both four-stage insects (Endopterygota), which do most of their growing in a wormlike larval state. Here's where one might look next: Grasshoppers. PZ himself has worked on some of those in the past, though on nervous-system development. They are three-stage insects, with the immature ones looking much like the full-grown ones. Crustaceans, like the brine shrimp Artemia salina: Sea-Monkeys. Crustaceans in general are the closest relatives to the insects. Other arthropods, like arachnids and myriapods. Annelids: polychaete marine worms like Platynereis dumerilii, earthworms, leeches. There's actually been a sizable amount of study of the development of leeches and Platynereis. Much like vertebrates, most arthropods and annelids make segments by rearward growth, as far as can be determined. - There's a big puzzle about the evolution segmentation. Either it evolved more than once or it was lost several times. Protostomia Ecdysozoa Panarthropoda - includes arthropods Cycloneuralia Lophotrochozoa Annelida Mollusca Etc. Deuterostomia Chordata - includes vertebrates Ambulacraria Hemichordata Echinodermata

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