Expression of the Engrailed Gene Reveals Nine Putative Segment-Anlagen in the Embryonic Pleon of the Freshwater Crayfish Cherax destructor (Crustacea, Malacostraca, Decapoda)

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Abstract
Segment formation in the embryonic pleon of the freshwater crayfish Cherax destructor was analyzed by using the monoclonal antibody mAb 4D9 against the product of the segment-polarity gene engrailed. As in other body regions, engrailed is expressed in transverse stripes in the posterior portion of segments in the pleon. Nine engrailed stripes are formed in the pleon. The anterior six stripes correspond to the six pleon segments of adult eumalacostracan crustaceans. The uropods are clearly the appendages of the sixth pleon segment. The seventh engrailed stripe marks the anlage of a seventh ganglion. Stripes eight and nine are transient and disappear before morphogenesis begins. The engrailed stripes seven to nine are interpreted as vestiges of ancestral segments. The seventh segment anlage is thus a recapitulation of the seventh pleonic segment, which is retained in recent adult leptostracans and is considered to be part of the malacostracan ground plan. The stripes eight and nine might point still further back into the phylogeny of crustaceans or even mandibulates. The use of rhodamine-labeled phalloidin reveals that the terminal ganglion of adult crayfish is the fusion product of the anlagen of the sixth and seventh pleonic ganglia and an eighth hemiganglion that is devoid of engrailed expression.

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