MOLECULAR BIOLOGY OF LANCELETS: INSIGHTS INTO DEVELOPMENT AND EVOLUTION

ABSTRACT

It is only during the past five to six years that the powerful techniques of gene cloning and DNA sequencing have been applied to lancelets (subphylum Cephalochordata). The impact has already been significant. Here I review the short history of lancelet molecular biology, and tabulate all lancelet genes published to the end of 1995 (plus many unpublished genes). Much attention has focused on the homeobox gene family, which includes the HOX genes involved in distinguishing axial positions along the anteroposterior axis of animal embryos. Like other invertebrates, the lancelet Branchiostoma floridae has a single cluster of HOX genes (vertebrates have multiple); however, genomic organization of the genes is very similar between lancelets and vertebrates. HOX gene expression patterns help reveal homologous axial positions between lancelet and vertebrate embryos; the comparison suggests that the vertebrate hindbrain is homologous to an extensive region of the lancelet neural tube. A sufficient diversity of genes have now been analyzed to allow a broad comparison between lancelet and vertebrate genomes, leading to an important general conclusion. The data suggest that after their divergence from a common ancestor, the lineage leading to vertebrates underwent an increase in the total number of genes, while the lineage leading to lancelets retained a largely archetypal condition. These data are used to argue that the lancelet body plan may also be predominantly archetypal; it is not degenerate from a vertebrate-like organization although it may have diverged in relatively minor ways from the body form of vertebrate ancestors.