Author Topic: Tardigrades as Disembodied Heads  (Read 154 times)

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Offline lpetrich

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Tardigrades as Disembodied Heads
« on: December 31, 2017, 01:45:46 PM »
PZ Myers has a video on that subject: TardigradeEvoDevo - YouTube  For Christmas, you get a video about tardigrades He summarizes this paper:

Smith FW, Boothby TC, Giovannini I, Rebecchi L, Jockusch EL, Goldstein B (2016) The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region. Curr Biol. 26(2):224-9. doi: 10.1016/j.cub.2015.11.059. Epub 2016 Jan 14.
The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region: Current Biology
The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region - S0960-9822(15)01507-9.pdf

Some phylogeny: (panarthropods: (tardigrades, onychophorans, arthropods), nematodes and the rest of the ecdysozoans)

Tardigrades have a head segment, three body segments with a pair of legs on each one, and a rear-end segment with one pair of legs.

There are two main ways to make segments. The main way across the animal kingdom is for the head end to form first and then for segments to form at the rear end. Some insects, like dipterans (flies and mosquitoes), form all their segments at the same time, and some insects do that with only their front segments, but those states are very likely derived.

Turning to Hox genes, these genes get expressed in regions along the main body axis, and their proteins then control the expression of other genes, thus indicating what each part is to develop as. Their expression patterns are remarkably similar across much of the animal kingdom, indicating that those patterns originated in some ancestor of much of it. Compared to other animals, tardigrades lost a few forward Hox genes and most other ones, though they keep the rear-end one: Abdominal-B. Their expression patterns are still broadly similar, however. So they grow a few segments and then stop.

This makes tardigrades homologous to the heads and rear ends of many other animals, including most of their panarthropod relatives. They are thus much like the nauplius larvae of some crustaceans. These have only a few segments and a few limbs, which they use for swimming. As the animal grows, the nauplius part becomes its head and that part's limbs its antennae.

Thus making tardigrades more-or-less disembodied heads.

Offline Nick

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Re: Tardigrades as Disembodied Heads
« Reply #1 on: December 31, 2017, 05:43:03 PM »
Begging the question..."Why won't God heal disembodied heads?"
Yo, put that in your pipe and smoke it.  Quit ragging on my Lord.

Tide goes in, tide goes out !!!