Of Mice and Genes: Evolution of Vertebrate Brain Development

Abstract

In this review the current understanding of genetic and molecular evolution of development, in particular the formation of the major axis of bilateral animals, is critically evaluated, and the early pattern formation in the hindbrain is related as much as possible to these processes. On the genetic level it is proposed that the exuberant multiplication of regulatory genes compared to that of structural genes relates to the increased flexibility of early vertebrate development. In comparisons to fruit flies, many conserved genes are found to be expressed very differently, while many others seem to reflect a comparable pattern and thus suggest a conservation of function. Even genes with a largely 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 reviewed, and it is concluded that both comparative data as well as more recent experimental data suggest a limited importance for the rhombomeres. Clearly, many cell fate-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 the relationship between homeobox genes and rhombomere specification in vertebrates other than a few model species.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

“…Motor neurons in the hindbrain include somitic motor neurons, which innervate somitically derived muscle, much as in the spinal cord; in addition, there is a population of branchiomeric motor neurons that innervate muscles derived from the branchial arches (reviewed in Ref. 189). The branchiomeric trigeminal motor neurons derive from r2, facial motor neurons from r4.…”

Evolutionary Patterns of Cranial Nerve Efferent Nuclei in Vertebrates

“…Lampreys have an apparent homolog of the mandibular nerve, but in addition, a larger 'maxillary' division that innervates rostral muscles via the apical and basilar nerves [Hardisty and Rovainen, 1982]. Accordingly, the trigeminal motor nucleus in larval and adult lamprey is organized quite differently than in gnathostomes [Homma, 1978;Fritzsch, 1998b;Kuratani et al, 2004;Murakami et al, 2004]. The V motor nucleus in a late larval lamprey ( fi g. 1 B) occupies a large area in the H 25); Mouse, E9.5-11.5, except ipsilateral IV, from adult mammal.…”

Turning heads: Development of vertebrate branchiomotor neurons

“…These data collectively provide us tremendous insights into the devel-opment and evolution of the vertebrate brainstem and its resident neurons (reviewed in Fritzsch, 1998;Glover, 2001). Here, I will focus on the organization of BMNs in zebrafish, chick, and mouse embryos ( Fig.…”