. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GENETIC REGULATORY NETWORKS 371 Cephalic Domain 1C Segmented Body Trunk. Head Ganglion Segmental Nerve Cord LEFT t RIGHT I 2d Figure 1. Differing symmetry properties in the progeny of different micromere quartets. Schematic diagram of clonal contributions to ectodermal tissues—body wall and central nervous system—at the head end of the Helobdella germinal plate, shown in dorsal view. Four primary micromere clones are shown in shades of orange, with the embryonic mouth or stomadeum situated at the point where the four clones
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GENETIC REGULATORY NETWORKS 371 Cephalic Domain 1C Segmented Body Trunk. Head Ganglion Segmental Nerve Cord LEFT t RIGHT I 2d Figure 1. Differing symmetry properties in the progeny of different micromere quartets. Schematic diagram of clonal contributions to ectodermal tissues—body wall and central nervous system—at the head end of the Helobdella germinal plate, shown in dorsal view. Four primary micromere clones are shown in shades of orange, with the embryonic mouth or stomadeum situated at the point where the four clones meet. Note that the plane of bilateral symmetry falls between the la and Id clones on the left and the Ib and lc clones on the right. The definitive ectoderm of the segmented body trunk (turquoise) derives exclusively from the 'micromere' 2d, otherwise known in leeches as cell DNOPQ. In the trunk, the plane of bilateral symmetry thus bisects the derivatives of the D quadrant. many derivatives of the first quartet of micromeres (7). In contrast, Lox22-Otx is not detectably expressed in the body wall of the segmented trunk, and is expressed in only two pairs of segmentally iterated CNS neurons, similar to what is seen in Drosophila embryos. This finding further supports the idea that head and trunk were genetically distinct body domains by an early stage in the bilaterian radiation (6). One possible evolutionary scenario is that the otd/olx genes were originally involved in the pattern formation of a radially symmetric pre-bilaterian ancestor, in which they were very likely expressed in patterns concentric about the oral-aboral axis. In the Bilateria, the expression of these genes could then have been relegated to the head by the addition of a trunk domain whose developmental patterning relied on the cooptation or expansion of other genetic pathways. If this model is correct, then the radial organization of micromere cell lineages (3) and Lox22- Otx expression (7) that we fin
Size: 1520px × 1644px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
