Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem

Abstract

Akam, M. (2010) Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem.Development, Genes and Evolution, Onychophoran head segmentation ABSTRACT The arthropod head problem has been a long lasting conundrum which has puzzled arthropodists for more than a century. Onychophorans are the sister group of the arthropods and are a phylum that has for a long time been regarded as the link between a simple worm-like arthropod ancestor and the crown-group arthropods. The arthropod head is a complicated structure with cryptic segment borders because of fusion and migration of segments; hence the long standing debate of the different parts segmental origin. The onychophorans on the other hand have a rather simple head comprising three well defined segments during development, which gives rise to an adult head with three appendages that are specialised for sensory and food capture/manipulative purposes. Based on the expression pattern of the anterior hox-genes; labial, proboscipaedia, hox3 and Deformed, as well as the head patterning genes otd and six3, we show that these three segments and their appendages can be correlated to the segments of the arthropod proto-deuto-and trito-cerebrum and that the onychophoran antenna is an appendage associated with the most anterior territory of the onychophoran head, it is the frontal appendage or primary antenna of stem-group arthropods and its affinities to the arthropod labrum is discussed.

Regressive evolution of the arthropod tritocerebral segment linked to functional divergence of the Hox genelabial

“…The Hox gene labial shows a highly conserved expression pattern in the tritocerebral segment in all arthropods [8] and even in onychophorans [22,23], most probably the arthropod sister group [24,25]. Consistent with this observation, our results identify a highly conserved role of labial in head tissue maintenance that has been known already from functional studies in D. melanogaster and the beetle Tribolium castaneum.…”

The metameric pattern of Hypsibius dujardini(Eutardigrada) and its relationship to that of other panarthropods

“…In this case, expression patterns of Hox genes revealed a conserved pattern of head segmentation across arthropods [40][41][42][57][58][59][60][61][62]. Moreover, expression patterns of Hox genes in onychophorans have revealed the relationship of their head segments to those of arthropods [51,63]. One conclusion of these studies is that the anterior expression boundaries of anterior Hox genes evolved in an ancestor of arthropods + onychophorans and have been highly conserved.…”

“…Thus, although the panarthropod common ancestor is characterized as being homonomously segmented [19,22,23,32,[47][48][49][50], these comparative data suggest that segmental diversity may have evolved prior to the divergence of the three panarthropod lineages. The developmental mechanism responsible for differentiating anterior segments in Onychophora and Arthropoda is conserved [51,52]. The discovery of segmental diversity in tardigrades raises the question of whether this mechanism is even evolutionarily more ancient.…”

A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora

“…By contrast, extant onychophorans occupy a restricted distribution in morphospace (Fig. 5A) that largely reflects their conservative appendage construction-consisting of protocerebral "antennae," deutocerebral jaws, slime papillae, and a variable number of homonomous walking legs-as well as relatively low degrees of variation in other aspects of their nonappendicular morphology (2,30) (Fig. 5 B and D).…”