In St12m we observed zygotic Cad expression in the NE of PS15 (Macdonald and Struhl, 1986), where it overlaps with the expression domain of Abd-B.r, but it is also expressed posterior to this (Sánchez-Herrero and Crosby, 1988).
Our results, along with those on Otx- and Hox-gene expression, demonstrate that the cephalopod condition is similar to that in mouse and fruit fly, with Otx being expressed in the anterior-most brain region (except for the vertical lobe) and a Pax2/5/8 expression domain separating the Otx-domain from a Hox-gene expressing posterior brain region.
We show that genes expressed posterior to the first somite level, which define cells that give rise to the pancreas and small intestine, absolutely require RA but are not shifted anteriorly by exogenous RA.
In the wild-type embryos, Fgf8 is induced broadly in the rhombomere 1 but gets restricted to the most anterior rhombomere 1 next to the Wnt expressing posterior midbrain by E9.0.
As CPA6 was expressed posterior to the eye during this same time period (24 68 hpf, Fig. 5) and morpholinos were found to result in >95% knockdown at 48 hpf (Fig. S1), knockdown of CPA6 was expected to result in a defect in VIth nerve pathfinding if CPA6 was involved.
In the juvenile, ClantHox is expressed posterior to the statocyst, extending to the end of the body but not at the posterior tip.
This was expected, considering that Abd-B.m is not expressed posterior to PS13 and that in Abd-B null mutants (m-r) the Eg pattern is only affected in PS14 and PS15.
Thus, genes at the 3' end of the clusters, such as HOXD1, are expressed early in the anterior and proximal regions, whereas genes at the 5' end, such as HOXD13, are expressed later in posterior and distal regions [ 20].
HOX genes that are positioned closer to the 5′ end of the cluster, such as HOXC9, are expressed later and more posterior during early development.
Temporal colinearity refers to a situation in which anterior Hox genes are expressed earlier and posterior genes are expressed later [ 9].
