Figure 1

Parasegments and limb primordia in the brine shrimp Artemia franciscana. (A) Explanatory drawing of a stage III nauplius larva. The anterior part consists of the ocular region (oc), labrum (lr), first antenna (an1), second antenna (an2) and mandible (md). The part of the trunk that is shown in B, C, and D is boxed in red and magnified. The trunk consist of a large bulge (containing the mesoderm for the presumptive segments of the first and second maxillae (Mx1, Mx2) and first thoracic appendages (Th1)) and a smaller bulge (containing the mesoderm for the second thoracic segment (Th2)). All following body units form from the growth cone. (B) Expression of engrailed in the anterior trunk. The asterisks denote the engrailed stripes of the future first and second maxillary segment. The engrailed stripes just posterior to the arrows are the stripes of the future first and second thoracic segment. The arrows in B-E point to the grooves between the parasegments. (C-D) Expression of wingless in the anterior trunk. The asterisk denotes the wg stripe of the first maxilla. The second maxilla does not (yet?) express wg. The nauplius in D is slightly older than the one in C. Note that the expression of wg is not directly anteriorly adjacent to the groove between large and small bulge in the older nauplius. (E) Expression of Distal-less in the anterior trunk reveals that the circle shaped appendage primordia are located anterior to the grooves. (F) Schematic summary of the model of appendage allocation in annelids and arthropods proposed here. Shown are two hemi-parasegments for each animal group, anterior is to the top. See text for details. Dark grey: en expression; light grey: wg expression; red: Dll/Dlx expression in the appendage primordia.