A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods.
BACKGROUND
The composition of the arthropod head is one of the most contentious issues in animal evolution. In particular, controversy surrounds the homology and innervation of segmental cephalic appendages by the brain. Onychophora (velvet worms) play a crucial role in understanding the evolution of the arthropod brain, because they are close relatives of arthropods and have apparently changed little since the Early Cambrian. However, the segmental origins of their brain neuropils and the number of cephalic appendages innervated by the brain--key issues in clarifying brain composition in the last common ancestor of Onychophora and Arthropoda--remain unclear.


RESULTS
Using immunolabelling and neuronal tracing techniques in the developing and adult onychophoran brain, we found that the major brain neuropils arise from only the anterior-most body segment, and that two pairs of segmental appendages are innervated by the brain. The region of the central nervous system corresponding to the arthropod tritocerebrum is not differentiated as part of the onychophoran brain but instead belongs to the ventral nerve cords.


CONCLUSIONS
Our results contradict the assumptions of a tripartite (three-segmented) brain in Onychophora and instead confirm the hypothesis of bipartite (two-segmented) brain composition. They suggest that the last common ancestor of Onychophora and Arthropoda possessed a brain consisting of protocerebrum and deutocerebrum whereas the tritocerebrum evolved in arthropods.
Mayer
Georg
G
Institute of Biology II, Animal Evolution & Development, University of Leipzig, Talstrasse 33, D-04103 Leipzig, Germany. gmayer@onychophora.com
Whitington
Paul M
PM
Sunnucks
Paul
P
Pflüger
Hans-Joachim
HJ
D000818:Animals; D001181:Arthropods / Q000033:anatomy & histology* / Q000196:embryology; D005075:Biological Evolution; D001921:Brain / Q000033:anatomy & histology* / Q000196:embryology