The role of parental positional information in the determination of antero-posterior polarity during palleal budding in ascidians

Identification of a novel gene involved in asexual organogenesis in the budding ascidian Botryllus schlosseri

Development via regeneration or budding shares some known genetic pathways with embryogenesis, but no concerted effort has been made to identify genes unique to asexual development. We have identified a novel gene that plays a role in cyclical bud formation and asexual organogenesis in the colonial ascidian Botryllus schlosseri. Athena mRNA is transcribed at high levels during the 24- to 36-hr interval of programmed cell death and new bud initiation at the conclusion of the budding cycle (takeover). Knockdown of Athena by RNAi and antisense morpholinos induced defects in the development of new buds ranging from retardation in growth and abnormal organogenesis to hollow buds lacking organs. As genetic intervention in this organism has not been possible, this study establishes the use of RNAi and morpholinos in Botryllus as well as describing the knockdown phenotype of a new gene.

Reversal of cellular polarity and early cell-cell interaction in the embryos of Caenorhabditis elegans

During early embryogenesis of Caenorhabditis elegans the serial stem cell-like cleavages of the germ line cells P0-P3 generate a number of somatic founder cells with different developmental potentials. Observations on partial embryos show that in the first two of these unequal divisions in the germ line the somatic daughter cell comes to lie anterior to the new germ line cell. In the following two, however, the somatic daughter cell comes to lie posterior to the new germ line cell, suggesting a reversal of polarity in the germ line. By the use of a laser microbeam, egg fragments can be extruded from young embryos; the fragments often cleave like partial twins. Depending on whether the fragment is derived from the posterior region of the uncleaved zygote P0 or its daughter P1, the mirror image duplications that are generated are joined at their larger soma-like cells or at their smaller germ line-like cells, respectively. This result is best explained as a reversal of polarity taking place in the germ line cell P2. This notion is strengthened by the finding that partial embryos derived from the posterior region of the P2 cell in late interphase do not undergo stem cell-like (i.e., unequal) cleavages in contrast to those derived from P0 or P1. Finally, an apparent early cell-cell interaction is described which is inconsistent with the classical notion of "mosaic" nematode development: removal of the germline cell P2 results in an altered developmental pattern of its somatic sister cell EMS. A working model is presented linking reversal of polarity and cell-cell interaction and offers an explanation for the unique behavior of the EMS cell in normal development.

Morphogenetic tissue interactions during posterior commitment in palleal buds of the polystyelid ascidian, Polyandrocarpa misakiensis

The development of chimeric double-anterior and double-posterior buds in the polystyelid ascidian, Polyandrocarpa misakiensis, was studied. Bud pieces used were allowed to develop for various time periods before the fusion operation. One-day double-half buds developed into complete zooids with normal anteroposterior polarity. In these cases, the bud half whose site of origin on the parent was higher (more posterior) than the partner formed the posterior organ, the digestive tract, of a resultant zooid. Two-day double-posterior buds developed into biposterior zooids that had two digestive tracts, whereas 2-day double-anterior buds still underwent pattern regulation in the same manner as 1-day buds (23-24 degrees C). Bianterior zooids could only be induced from 3-day double-anterior buds, suggesting that the anterior determination occurs later than the posterior determination. Next, a 3-day posterior half was combined with a 1-day posterior half from a higher site. Though "high-level" tissue has a high potential for posterior differentiation, the 1-day bud half failed to form the digestive tract. This result suggests that the 3-day bud half has acquired the ability to inhibit the formation of additional posterior structures. The kinetics of posterior-forming activity of developing bud pieces was measured using a 12-hr bud piece of the highest level as a probe. The activity increased with increasing developmental time, and the increase was accompanied by posterior inhibition activity. Buds of different positional levels showed different time courses of these activities. The results of this study show that the posterior end is the dominant region of developing ascidian buds.