The expression of the proximodistal axis patterning genes Distal-less and dachshund in the appendages of Glomeris marginata (Myriapoda: Diplopoda) suggests a special role of these genes in patterning the head appendages
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Abstract
The genes Distal-less, dachshund, extradenticle, and homothorax have been shown in
Drosophila to be among the earliest genes that define positional values along the
proximal-distal (PD) axis of the developing legs. In order to study PD axis formation
in the appendages of the pill millipede Glomeris marginata, we have isolated homologues
of these four genes and have studied their expression patterns. In the trunk legs,
there are several differences to Drosophila, but the patterns are nevertheless compatible
with a conserved role in defining positional values along the PD axis. However, their
role in the head appendages is apparently more complex. Distal-less in the mandible
and maxilla is expressed in the forming sensory organs and, thus, does not seem to
be involved in PD axis patterning. We could not identify in the mouthparts components
that are homologous to the distal parts of the trunk legs and antennnae. Interestingly,
there is also a transient premorphogenetic expression of Distal-less in the second
antennal and second maxillary segment, although no appendages are eventually formed
in these segments. The dachshund gene is apparently involved both in PD patterning
as well as in sensory organ development in the antenna, maxilla, and mandible. Strong
dachshund expression is specifically correlated with the tooth-like part of the mandible,
a feature that is shared with other mandibulate arthropods. homothorax is expressed
in the proximal and medial parts of the legs, while extradenticle RNA is only seen
in the proximal region. This overlap of expression corresponds to the functional overlap
between extradenticle and homothorax in Drosophila.