Evolution of bilaterian central nervous systems: a single origin?

Table 1 Gene expression in nervous tissues of - Saccoglossus kowalevskii , Branchiostoma floridae, Ciona intestinalis and vertebrates

Gene	S. kowalevskii		B. floridae		C. intestinalis		Vertebrate		References
Gene	s.e.	CNC^a	s.e.	CNS	Ectoderm	CNS	Placode	CNS
SoxB	+	+	+	+	+	+	+	+	[131]
Hu/Elav	+	+	+	+	?	?	+	+
Nrp/Musashi	+	+	+	+	?	+	+	+	[141]
Vax	+	–	?	?	gene	lost	–	retina, forebrain	[142]
Rx	+	–	–	+	–	+	–	+	[69, 76]
Six3	+	+/−	+	+	–	+	–	+	[43, 131, 143]
Nkx2.1	+	–	–	+	?	?	–	+	[140, 144]
Bf-1	+	+/−	+	+	+	?	+	+	[131, 145, 146]
Dlx	+	+	+	+	+	+	+	+	[77]
Pax6	+	+	+	+	–	+	+	+	[75]
Tll/Tlx	+	+/−	+	+	?	?	+	+	[147, 148]
BarH	+	+	?	?	?	?	+	+	[149]
Emx	+	+	?	?	?	?	+	+	[150]
Otp	+	?	?	?	+	+	?	+	[131, 151]
Dbx	+	+	?	?	?	?	–	+	[152]
Lim1/5	+	+	+	+	?	?	+	+	[153]
Irx	+	+	+	+	?	?	+	+	[154]
Otx	+	+	–	+	+	+	+	+	[78, 137]
En	+	+	+	+	–	+	+ lamprey	+	[78, 155]
Gbx	+	+	+	+	gene	lost	+	+	[142]
Hox1	+	+	+	+	+	+	+	+	[156–158]
Hox3	+	–	+	+	+	+	+	+	[134, 156, 158]
Hox4	+	?	+	+	–	–	–	+	[156]
Hox7/8	+	?	?	?	gene	Lost	–	+	[156]
Hox11/13	+	–	?	?	Hox12+	Hox12+	Hox11+	+	[156, 159]

Gene expression in ectoderm and nervous tissue of the hemichordate Saccoglossus kowalevskii, the cephalochordate Branchiostoma floridae, the tunicate Ciona intestinalis and vertebrates. Note: En expression in placodes only documented for lamprey among the vertebrates. CNC, collar nerve cord; CNS, central nervous system; s.e., surface ectoderm. ^aIn the absence of sections, which would distinguish between expression in the surface ectoderm and in the collar nerve cord, all genes with ectodermal expression in the region of the dorsal portion of the collar are listed.

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