Figure 2

Expression of Nvi-ParaHox genes during formation of the ventral neural system. (a – h) Expression of Nvi-Gsh. (a, b) Early expression during the late trochophore/early metatrochophore stages. (c, d, e) Expression during the middle metotrochophore stage. (e) In some cases Nvi-Gsh positive cells are visible in the lateral parts of the segments. (f, g) At the late metotrochophore stage Nvi-Gsh positive cells gradually disappear from the second and then from the third parapodial segments. (e) In some cases Nvi-Gsh positive cells are visible in the lateral parts of segments (empty arrowheads). (h) Nvi-Gsh expression at the beginning of postlarval segmentation. (i – p) Expression of Nvi-Xlox. Expression of Nvi-Xlox starts later then Nvi-Gsh and Nvi-Cad (i, j). At the early metatrochophore stage Nvi-Xlox positive cells appear in the first (j) and later in the second parapodial segments (k) and during the middle metatrochophore stage Nvi-Xlox positive cells are visible in all parapodial segments in the central part of the forming neuromeres (l, n), above the mesotrochs. In some cases (l, m) Nvi-Xlox positive cells are visible in the lateral part of segments (empty arrowheads). At the late metotrochophore stage (m) short-term Nvi-Xlox expression occurs in the lateral cells of the peristomial segments (white arrowheads). During the nectochaete stage Nvi-Xlox expression gradually decreases (o) and then disappears (p). (q-x) Expression of Nvi-Cad. Expression of Nvi-Cad during the late trochophore and early metatrochophore stages (q, r). This expression disappears by 120 h (s) but in some larvae Nvi-Cad positive cells can be detected during the middle metatrochophore stage (t) at the level of the mesotroch of the third parapodial segment. Later, Nvi-Cad is not expressed in neuroectodermal cells (u-x). During all of larval development very strong Nvi-Cad expression occurs in the posterior of the forming digestive system, in the forming pygidium and in the forming growth zone (see Fig. 4). View from ventral side in all cases except (m), which is a lateral view. Asterisk marks stomodeum. The mesotrochs, which are positional markers for segments, are marked by black arrowheads. Expression of Nvi-Gsh, Nvi-Xlox, and Nvi-Cad in the third parapodial segment (d, l, t) is indicated by black squares.