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
PZ starts off by noting where tardigrades fit into the family tree of life. I'll stick to eubilaterians: animals with bilateral symmetry except for some obscure worms.
- Deuterostomia - (segmented) Chordata, (unsegmented) Echinodermata
- Lophotrochozoa - (segmented) Annelida (includes earthworms and leeches), (mostly unsegmented) Mollusca
- (unsegmented) Cycloneuralia - nematodes and similar worms
- (segmented) Ecdysozoa
- onychophorans (velvet worms)
PZ then gets into how segmentation is formed. The main way across the animal kingdom is for the head end to form first and segments to form at the rear end. Some insects, like dipterans (flies and mosquitoes), form all their segments at the same time, however, but that is most likely derived.
Segmentation is a cheap way of making new features, but it is an effective way.
Then the Hox system. Hox 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 tardigrades are homologous to the heads and rear ends of many other animals, including most of their panarthropod relatives. Making them more-or-less disembodied heads.
When one considers how segments grow, it looks as if tardigrades grow the first few segments, and then stop. This suggests that tardigrades do neoteny, reaching mature form by growing only part of the way to some ancestral mature form.