26.2 Wednesday, Jan. 4 Revaluating spiral-like cell cleavage patterns during the embryonic development of phoronids and brachiopods. SANTAGATA, Scott; Long Island University email@example.com
Embryonic patterns in cell cleavage and blastomere arrangements continue to be used as phylogenetically informative characters. However, cleavage patterns are clearly subject to evolutionary modifications especially among the various phyla that exhibit spiral cleavage. Although the prevailing viewpoint is that phoronids and brachiopods exhibit a form of radial cleavage, aspects of both spiral-like and radial-like cleavage have been observed and vary the most at the eight and 16-cell stages. Here, I use time-lapse video microscopy to investigate the early development of the phoronid, Phoronis pallida from False Bay, WA (USA). The early development of Phoronis pallida deviates from a typical radial pattern at the third, fourth, and fifth cleavages that alternate between dexiotropic and levotropic spiral divisions. These cleavage patterns were also corroborated using confocal microscopy by investigating the angle of the metaphase plate at cell divisions. Review of phoronid and brachiopod developmental patterns along with additional data from the brachiopod, Terebratalia transversa, suggests that both radial-like and spiral-like cell divisions occur within and among species, and that spiral-like patterns may be more prevalent in species with smaller eggs. These data coupled with recent phylogenomic analyses support the hypothesis that both phoronids and brachiopods are derived from an ancestor that exhibited a form of spiral cleavage. Considering allelic differences that underlie patterns of chirality in some gastropod mollusks, further research is needed to test whether cleavage variation in phoronids and brachiopods also reflects genetic variation within and among species.