Two homeobox-containing genes that belong to different homeodomain cla
sses have been isolated from a sea urchin genomic library. One, P1Hbox
11, is the sea urchin homologue of the human and mouse Hox B3 gene, th
e other, P1Hbox12, shows about 55% identity with paired class genes. E
xpression profile analysis of the two sea urchin Hbox genes suggests t
hat they play different roles during embryogenesis. In fact, P1Hbox11
transcripts are rare and are detected only in the pluteus larva and in
the Aristotle's lantern and intestine of the adult. The P1Hbox12 gene
is, on the contrary, transiently expressed in the very early embryo a
lready at the four cell stage; it accumulates at the 64 cell stage and
disappears at later stages of development. In situ hybridization expe
riments to 16 and 32 cell stage whole mount embryos showed localizatio
n of the P1Hbox12 mRNA to part of the mesomere-macromere region of the
early cleavage embryo. These observations suggest a possible role of
this gene in early events of cell specification.