Genoarchitecture of the rostral hindbrain of a shark: basis for understanding the emergence of the cerebellum at the agnathan-gnathostome transition.

The cerebellum is present in all extant gnathostomes or jawed vertebrates, of which cartilaginous fishes represent the most ancient radiation. Since the isthmic organizer induces the formation of the cerebellum, comparative genoarchitectonic analysis on the meso-isthmo-cerebellar region of cartilaginous fishes with respect to that of jawless vertebrates could reveal why the isthmic organizer acquires the ability to induce the formation of the cerebellum in gnathostomes. In the present work we analyzed the expression pattern of a variety of genes related to the cerebellar formation and patterning (ScOtx2, ScGbx2, ScFgf8, ScLmx1b, ScIrx1, ScIrx3, ScEn2, ScPax6 and ScLhx9) by in situ hybridization, and the distribution of Pax6 protein in the developing hindbrain of the shark Scyliorhinus canicula. The genoarchitectonic code in this species revealed high degree of conservation with respect to that of other gnathostomes. This resemblance may reveal the features of the ancestral condition of the gene network operating for specification of the rostral hindbrain patterning. Accordingly, the main subdivisions of the rostral hindbrain of S. canicula could be recognized. Our results support the existence of a rhombomere 0, identified as the ScFgf8/ScGbx2/ScEn2-positive and mainly negative ScIrx3 domain just caudal to the midbrain ScIrx1/ScOtx2/ScLmx1b-positive domain. The differential ScEn2 and Pax6 expression in the rhombomere 1 revealed anterior and posterior subdivisions. Interestingly, dissimilarities between S. canicula and lampreys (jawless vertebrates) were noted in the expression of Irx, Lhx and Pax genes, which could be part of significant gene network changes through evolution that caused the emergence of the cerebellum.