The organizing role of the D quadrant as revealed through the phenomenon of twinning in the polycheate Chætopterus variopedatus.

The egg of the polychaete Chætopterus, like many spiralian embryos, undergoes unequal cleavage during the first two cell divisions following fertilization. The first cleavage gives rise to a large CD blastomere and a smaller AB blastomere. At second cleavage the CD blastomere divides asymmetrically, forming a large D blastomere and a smaller C blastomere while the AB cell divides to give rise to the two smaller blastomeres, A and B, which are virtually identical in size. As in many of the other spiralians which have been studied, the A, B, C, and D blastomeres give rise to characteristic portions of the larval body plan. It has been shown in these related forms that the D blastomere derivatives are necessary for the production of various structures derived from the remaining three quadrants, as well as in organizing the overall symmetry properties of the embryo. If Chætopterus embryos are compressed underneath a coverslip at the appropriate stage of development prior to first cleavage (the "pear stage") some of the embryos divide to form two equal sized blastomeres. Each of the two cells in equalized embryos appears to develop as a normal CD blastomere often dividing to generate a four-celled embryo with two C and two D blastomeres opposite one another (CDCD embryos). These embryos often grow up to form larvae which have duplicated structures. We investigated the role of the early cleavage program in the production of double larvae by staining individual blastomeres of normal and equalized embryos with Nile blue sulfate. Our results reveal that previous descriptions of the symmetry properties of twinned larvae are incorrect. Twins are not true Janus larvae of the duplicitus cruciata form but are organized along a single axis. The two halves face one another along a plane of mirror symmetry, each one fused to the other at their ventral surface just above a single, common, fused mouth. Furthermore, the median plane of the two fused heads is the same as that of the two trunks. These labeling studies indicate that cells derived from both of the two C blastomeres in compressed embyos contribute to the formation of similar structures in both halves of the double larva. The two C quadrants within the double embryos each can give rise to two eyes, and the plane of mirror symmetry between the two halves of the double embryo as mentioned above corresponds to a plane running through the two C blastomeres in the fourcelled CDCD embryo. These findings indicate that the two D quadrants in these double embryos interact to organize the development of the double embryo in a coordinating manner. Both D quadrants, therefore, appear to be inductively active within the CDCD double embryos. These results are discussed in view of various hypotheses which have been proposed to explain the phenomenon of twinning in spiralians.