THE LOCATION OF THE 3RD CLEAVAGE PLANE OF XENOPUS EMBRYOS PARTITIONS MORPHOGENETIC INFORMATION IN ANIMAL QUARTETS

Analysis of the developmental potential of animal quartets (the set of four animal blastomeres isolated from the 8-cell stage Xenopus embryo ) provided insight into the manner in which morphogenetic information is distributed along the animal-vegetal axis. Gravity treatments were employed to alter the partitioning plane. Animal quartets isolated fro m embryos exposed to simulated weightlessness had larger animal blasto meres, and they formed structures such as a groove and a protrusion mo re often than 1g-control animal quartets. Animal quartets with an unus ual non-horizontal third cleavage plane were also found to have a high er frequency of protrusion formation than animal quartets with a typic al horizontal cleavage plane. The increase in the frequency seen in si mulated weightlessness animal quartets was not due to their increased size. Fusing two animal quartets isolated from hypergravity (3g) expos ed embryos (small blastomeres and low incidence of protrusions) did no t affect the frequency of protrusion formation. Molecular analyses rev ealed that a partial induction was associated with the protrusion form ation. Transcripts of the dorsal lip specific homeobox gene, goosecoid , and alpha-cardiac actin were detectable by PCR amplification in the animal quartet with a protrusion, and alpha-cardiac actin mRNA was fou nd by whole-mount in situ hybridization to be localized in the protrus ion. Taken together, all these results are consistent with the notion that both animal and vegetal information is necessary for normal devel opment and the partitioning of morphogenetic information into animal q uartets results in gravity-dependent differential morphogenesis and ge ne regulation.