Growth patterns in Onychophora (velvet worms): lack of a localised posterior proliferation zone.
BACKGROUND
During embryonic development of segmented animals, body segments are thought to arise from the so-called "posterior growth zone" and the occurrence of this "zone" has been used to support the homology of segmentation between arthropods, annelids, and vertebrates. However, the term "posterior growth zone" is used ambiguously in the literature, mostly referring to a region of increased proliferation at the posterior end of the embryo. To determine whether such a localised posterior proliferation zone is an ancestral feature of Panarthropoda (Onychophora + Tardigrada + Arthropoda), we examined cell division patterns in embryos of Onychophora.


RESULTS
Using in vivo incorporation of the DNA replication marker BrdU (5-bromo-2'-deoxyuridine) and anti-phospho-histone H3 immunolabelling, we found that a localised posterior region of proliferating cells does not occur at any developmental stage in onychophoran embryos. This contrasts with a localised pattern of cell divisions at the posterior end of annelid embryos, which we used as a positive control. Based on our data, we present a mathematical model, which challenges the paradigm that a localised posterior proliferation zone is necessary for segment patterning in short germ developing arthropods.


CONCLUSIONS
Our findings suggest that a posterior proliferation zone was absent in the last common ancestor of Onychophora and Arthropoda. By comparing our data from Onychophora with those from annelids, arthropods, and chordates, we suggest that the occurrence of a "posterior growth zone" currently cannot be used to support the homology of segmentation between these three animal groups.
Institute of Biology II, Animal Evolution & Development, University of Leipzig, Talstrasse 33, D-04103 Leipzig, Germany. gmayer@onychophora.com
Georg
G
Mayer
Chiharu
C
Kato
Björn
B
Quast
Rebecca H
RH
Chisholm
Kerry A
KA
Landman
Leonie M
LM
Quinn
D000818:Animals; D002455:Cell Division; D007448:Invertebrates / Q000166:cytology / Q000196:embryology* / Q000254:growth & development*; D008962:Models, Theoretical