Cranial and Spinal Nerves of Fishes: Evolution of the Craniate Pattern

Abstract

The vertebrates’ peripheral nervous system, including its sensory systems, shows limited reorganizations, allowing one to establish homology of cells and their underlying molecular developmental pathways across vertebrates. In contrast, whereas certain molecular and even cellular homology can be found with non-craniate deuterostomes, the dissimilarities at the level of organs abound and defy easy recognition of homologies with craniates. The molecular and anatomical data indicate that the deuterostome ancestors had a cutaneous nerve net resembling that of modern diploblasts and some basal triploblasts. Such an organism with a nerve net had none of the major cranial senses but should have possessed certain molecular and cellular similarities as recently revealed for eye, ear, antero-posterior, and dorso-ventral patterning transcription factors in the central nervous system. However, such homologies rest only on a limited set of genes and single cells not organized into organs. This suggests that molecular pathways underlying cell fate decision making, cell fate patterning and organ formation, including the cellular composition and distribution of peripheral nerves, evolved rapidly through the co-option of additional genes into an increasingly sophisticated genetic network. Close examination shows that evolution alters even clearly homologous organs and peripheral nerves through loss and gain of specific components for which the molecular basis is not well understood.