Figure 3.

Expression patterns of Homeobox-containing genes in Archegozetes demonstrating homology of anterior segments. The embryo is ≈170 μm long in each case. Abbreviations as in Fig. 1. (A) Expression of the Archegozetes engrailed (Alen) homolog in an embryo after the formation of the labrum. Alen is expressed in the posterior portion of each segment with visible limbs. Despite strong staining in these segments, no engrailed expression is evident anterior to the chelicerae. (B) Alpb is expressed in all of the visible walking legs with an anterior boundary at the front of the pedipalp. (C) AlDfd is expressed throughout the posterior of the embryo, excepting the terminal segments, with an anterior boundary at the front of the first walking leg. (D) AlScr is expressed strongly in the third leg and weakly in the second. It appears to be expressed only in the more distal regions of the second leg though the staining is diffuse. (E) Close up of opisthosoma and fourth leg bud. AlAntp expression is strong in the opisthosoma and continues into the rear portion of the fourth leg bud. Because of the small size of the fourth limb bud and its position, tucked in next to the opisthosoma, it has proved very hard to photograph; this expression therefore is schematized in a camera lucida drawing in F. (F) Camera lucida drawing highlighting expression of AlAntp in the opisthosoma and rear of fourth leg bud. (G) Optical mid-sagittal expression showing AlAntp expressed strongly in the opisthosoma with no expression in the chelicerae, pedipalps, or legs 1–3. Leg 4, a small bud, is not in focus in this picture. (H) Alotd is expressed in the ocular lobes and in the ventral midline. The photograph is a sagittal optical section focusing on the expression in the ventral midline. No segmental expression is seen posterior to the ocular segment. (I) Alotd expression is seen in the ocular lobes but not in the chelicerae. View is anterior-ventral with the anterior to the left.