Evidence that Hoxa expression domains are evolutionarily transposed in spinal ganglia, and are established by forward spreading in paraxial mesoderm

Transposition of anatomical structures along the anteroposterior axis has b een a commonly used mechanism for changing body proportions during the cour se of evolutionary time. Earlier work (Gaunt, S.J., 1994. Conservation in t he Hox code during morphological evolution. Int. J. Dev. Biol. 38, 549-552; Burke, A.C., Nelson, C.E., Morgan, B.A., Tabin, C., 1995. Hox genes and th e evolution of vertebrate axial morphology. Development 121, 333-346) showe d how transposition in mesodermal derivatives (vertebrae) could be attribut ed to transposition in the expression of Hox genes along the axial series o f somites. We now show how transposition in the segmental arrangement of th e spinal nerves can also be correlated with shifts in the expression domain s of Hox genes. Specifically, we show how the expression domains of Hoxa-7, a-9 and a-10 in spinal ganglia correspond similarly in both mouse and chic k with the positions of the brachial and lumbosacral plexuses, and that thi s is true even though the brachial plexus of chick is shifted posteriorly, relative to mouse, by seven segmental units. In spite of these marked speci es differences in the boundaries of Hoxa-7 expression, cis regulatory eleme nts located up to 5 kb upstream of the chick Hoxa-7 gene showed much functi onal and structural conservation with those described in the mouse (Puschel , A.W., Bailing, R., Gruss, P., 1991. Separate elements cause lineage restr iction and specify boundaries of Hox-1.1 expression. Development 112, 279-2 87; Knittel, T., Kessel, M., Kim, M.H., Gruss, P., 1995. A conserved enhanc er of the human and murine Hoxa-7 gene specifies the anterior boundary of e xpression during embryonal development. Development 121, 1077-1088). We als o show that chick Hoxa-7 and a-10 expression domains spread forward into re gions of somites that are initially negative for the expression of these ge nes. We discuss this as evidence that Hox expression in paraxial mesoderm s preads forward, as earlier found for neurectoderm and lateral plate mesoder m, in a process that occurs independently of cell movement. (C) 1999 Elsevi er Science Ireland Ltd. All rights reserved.