OF MICE AND GENES - EVOLUTION OF VERTEBRATE BRAIN-DEVELOPMENT

In this review the current understanding of genetic and molecular evol ution of development, in particular the formation of the major axis of bilateral animals, is critically evaluated, and the parry pattern for mation in the hindbrain is related as much as possible to these proces ses. On the genetic level it is proposed that the exuberant multiplica tion of regulatory genes compared to that of structural genes relates to the increased flexibility of early vertebrate development. In compa risons to fruit flies, many conserved genes are found to be expressed very differently, while many others seem to reflect a comparable patte rn and thus suggest a conservation of function. Even genes with a larg ely conserved pattern of expression may change the level at which they are expressed and the mechanisms by which they are regulated in their expression. Evolution and development of hindbrain motoneurons is rev iewed, and it is concluded that both comparative data as well as more recent experimental data suggest a limited importance for the rhombome res. Clearly, many cell fare-specifying processes work below the level of rhombomeres or in the absence of rhombomeres. It is suggested that more comparative developmental data are needed to establish firmly th e relationship between homeobox genes and rhombomere specification in vertebrates other than a few model species.