Evolutionary Antiquity of the M Teloblasts

As described earlier (Figure 7.3), the PGZ in Helobdella and most clitellates comprises exactly ten teloblasts, one pair of mesoteloblasts (M), and four pairs of ecto- teloblasts (designated N, О/P, О/P, and Q in Helobdella). The M teloblasts are of particular interest evolutionarily, in that they arise from the zygote by an embryonic cell lineage that is conserved across a wide diversity of spiralian taxa, reflecting a pattern of cell divisions and cell fate assignments that was already established as tong as 600 million years ago (Lambert 2008; Peterson et al. 2008). Specifically, the M teloblasts arise from the bilaterally symmetric division of a precursor born at the equivalent cell division (sixth embryonic cleavage within the D quadrant lineage). This precursor is named cell 4d in the standard nomenclature used for describing spiralian development; the corresponding cell is designated blastomere DM" in a specific nomenclature used for Helobdella. Notwithstanding the evolutionary antiquity and extensive conservation of the association between the identity of cell 4d and its fate as precursor of bilateral mesoderm, scientists have discovered a fascinating instance of evolutionary plasticity here as well. In the polychaete annelid Capitella teleta, the left and right longitudinal bands of mesoderm arise from third quartet micromeres in the D and C quadrants, respectively, rather than from the left and right progeny of the 4d cell as in most spiralians (Meyer et al. 2010).

In spiralian embryos generally, the bilateral pair of M teloblasts undergo a stereotyped series of unequal cell divisions that typically contribute individually identifiable sets of mesodermal and germline precursor cells to their respective embryos in a fixed sequence. Despite the apparent homology among spiralian M teloblasts, the conspicuous functioning of paired teloblasts as stem cells in posterior growth is known only from clitellate annelids. Thus, in Helobdella and other clitellates, these unequal divisions are more highly asymmetric in size, more numerous, and more regular than in other spiralian taxa. The first born m blast cells in the left and right mesodermal bandlets arise in contact with one another and this contact is maintained for some time as the distal ends of the bandlets move to the surface of the embryo. As the ectodermal bandlets arise, they seem to move along the underlying m bandlets on each side, thus bringing the distal ends of the left and right germinal bands into contact at the future anterior end of the nascent germinal plate.

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