Meeting Abstract

S2-1.2  Thursday, Jan. 3  Amphioxus and the Evolution of Head Segmentation HOLLAND, L. Z.*; RASMUSSEN, S. L. K.; BEASTER-JONES, L.; KOOP, D.; Univ. of California at San Diego; Univ. of California at San Diego; Univ. Calif. San Diego, Augustana College; Univ. of California at san Diego lzholland@ucsd.edu

In the basal chordate amphioxus (Branchiostoma), all three germ layers are regionalized along the anterior/posterior (A/P) axis. The central nervous system is divided into a forebrain/midbrain and a hindbrain/spinal cord, the mesoderm is segmented into muscular somites that extend the entire length of the body, and the endoderm is divided into a pharynx with segmental gill slits, a midgut and a hindgut. Starting at the mid-gastrula stage, nested expression of Hox genes and the Hox-related gene Gbx specifies A/P position in all germ layers. As in vertebrates, the midbrain/hindbrain boundary (MHB) is established where the domains of Otx in the forebrain/midbrain and Gbx in the hindbrain/spinal cord abut. To this fundamental organization, vertebrates have added the genetic machinery that confers organizer properties upon the MHB. The head mesoderm begins to be partitioned into somites at the late gastrula stage by a suite of segmentation genes. Of these, only engrailed expression is restricted to the head somites; all others are also expressed during segmentation of the trunk/tail somites, which occurs, as in vertebrates, from the tailbud. Although adult vertebrates lack head somites, mesodermal head segments have been described in the embryos of several vertebrates. Expression of engrailed in the anterior somites of amphioxus, the velar muscles of the lamprey, which derive from the wall of the mandibular head cavity, and in certain jaw muscles of gnathostomes is consistent with the idea that the vertebrate head is fundamentally segmented and that in the ancestral vertebrate, as in amphioxus, segmented mesoderm extended the entire length of the body.