Meeting Abstract

S9.3  Wednesday, Jan. 6  Paxβ: a lophotrochozoan gene family implicated in spiral cleavage SHANKLAND, M.*; SCHMERER, M.W.; NULL, R.W.; U. of Texas at Austin hastypig@mail.utexas.edu

The Paxβ gene family is restricted to the bilaterian superphylum Lophotrochozoa. A conserved intron:exon organization suggests that the ancestral Paxβ arose by duplication and divergence of a Pax2/5/8 gene. This founding gene duplication appears to have occurred near the base of the lophotrochozoan radiation, prior to the separation of platyhelminths, molluscs, annelids, nemerteans, and brachiopods. Spiral cleavage is also unique to the lophotrochozoan clade. Functional studies in the leech Helobdella indicate that Hau-Paxβ1 regulates a critical step in spiral cleavage, raising the possibility that the Paxβ family may have been involved in the evolution of this cleavage program. During normal spiralian development the ectodermal and mesodermal blastomeres of the D quadrant undergo symmetric cleavages that presage the bilateral symmetry of the adult body plan. But these symmetric divisions follow a series of asymmetric spiral cleavages that lead to the production of micromeres. Interfering with Paxβ1 in the Helobdella embryo prevents this developmental transition: i.e. the ectodermal and mesodermal blastomeres continue to cleave at the appropriate time, but rather than adopting bilateral symmetry they produce additional micromeres of alternating chirality. Genetic and pharmacological data suggest that maternal Hau-Paxβ1 protein is acting as a transcriptional activator during these developmental events. We hypothesize that Hau-Paxβ1 is activating the expression of one or more zygotic gene products, which are responsible in turn for repositioning the mitotic spindle to the median plane at the symmetric divisions that terminate spiral cleavage. Ongoing studies are directed at identifying these putative Hau-Paxβ1 target genes.