New developments in vertebrate cytotaxonomy VI. Cytotaxonomy and evolution of lower chordates

A correlation between evolutionary trends and karyotype has been recognized in tunicates. In the ambit of lower chordates and related species, Hemichordata, Cephalochordata and, in the Tunicata, some ascidians seem to maintain a primitive organization of chromosomes: haploid numbers around 20, absence of sex chromosomes, and rod-shaped bivalents. Within the class Ascidiacea, at the karyological level Clavelina depadiformis seems to be very specialized and the family Perophoridae (suborder Phlebobranchiata) very primitive. Thaliacea and Larvacea appear specialized in comparison with the primitive ascidians.This is the sixth in a series of articles, by different authors, to appear in this journal under the editorship of Professor Chiarelli, supplementing the data published in Cytotaxonomy and Vertebrate Evolution (A. B. Chiarelli & E. Capanna, eds, Academic Press, London & New York)-Eds.