DAVIDSON, B.J.*; MOODY, W.; SWALLA, B.: Tunicate Out of Body Experiences: Extra-Somatic Cell Migration and Other Insights and Observations on Urochordate Metamorphosis

We are investigating the morphological changes occurring during urochordate metamorphosis and the hormonal/transcriptional signals which coordinate this process. Initially, we have described a series of discrete morphological stages in the tunicate Boltenia villosa during the period of juvenile organ differentiation (the first two weeks after settlement.) These include a clear pattern of body-wall muscular differentiation which has been visualized through actin staining. This post-settlement pattern of differentiation may be coordinated by thyroid hormones, as TH blockers arrest juveniles early in development and TH levels may rise at the time when juvenile differentiation is initiated. We have also used subtractive hybridizations of mRNA expressed at times surrounding metamorphosis to investigate the differential transcription of genes which may coordinate metamorphic events. This technique has led to the cloning of a variety of immunological genes which seem to be transcribed around the time of settlement. The previously described migration of blood cells across the epidermis into the heamocoel at settlement may be related to this immune response. Careful observations of this migration has led to the detection of a group of mesenchymal cells which migrate through a tube connecting the anterior epidermis to the outside of the juvenile tunic. The function of this extra-somatic migration is currently under investigation. Through the detailed investigation of tunicate metamorphosis we are hoping to lay the groundwork for a better understanding of life-cycle evolution among the urochordates as well as how tunicate metamorphosis and post larval development is related to vertebrate embryogenesis.