Meeting Abstract

S2-1.6  Thursday, Jan. 3  Head organization and the head/trunk relationship in protochordates: problems and prospects LACALLI, T.C.; Univ. of Victoria, Victoria, B.C. lacalli@uvic.ca

The fossil record is an invaluable aid for reconstructing the major events of vertebrate evolution. Because there is no comparable record for invertebrate chordates, we have no way of directly testing hypotheses regarding the morphology, habits and mode of life of the invertebrate ancestors of vertebrates. A simple thought experiment will serve to illustrate how serious an impediment this is. The only available alternative is inference based on an interpretation of living protochordates, but this is fraught with problems, not least being our own biases of what we think an ancestral chordate ought to look like. Relevant to the present symposium is the problem of head/trunk relationships and whether both of the latter are primitively segmented, or only one. I will review what is now known of tunicate and amphioxus neural development and innervation patterns in relation to Romer�s somaticovisceral concept of the vertebrate body to show how little progress has yet been made to either prove or disprove this idea. There are, in contrast, surprisingly good prospects for resolving some other, quite puzzling evolutionary issues. Currently, dorsoventral inversion provides the best example. Here a consensus is emerging, based on molecular data from hemichordates, that casts head asymmetry in amphioxus in a new light. One consequence is that the differential growth that reestablishes symmetry in amphioxus can now be seen, at least in part, as a recapitulation of past evolutionary events, which is not something that would have been predicted. Further, there are important implications here concerning the origin and basic organization of the brain.