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Evodevo
2014 Dec 22;51:46. doi: 10.1186/2041-9139-5-46.
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Echinoderm conundrums: Hox genes, heterochrony, and an excess of mouths.
Lacalli T
.
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Two issues relating to the translocation of anterior Hox genes in echinoderms to the 5'' end of the Hox cluster are discussed: i) that developmental changes associated with fixation to the substratum have led to an acceleration of mesodermal development relative to that of ectoderm, resulting in a mismatch of anteroposterior registry between the two tissues and a larger role for mesoderm in patterning control, and ii) whether this helps explain the ability of some echinoderms to form separate mouths at different locations, one for the larva and one for the juvenile rudiment. Freeing the mesoderm from ectodermal influences may have encouraged morphogenetic innovation, paralleling the situation in tunicates, where an early genomic (or genomic and developmental) change has allowed the body to evolve in novel ways.
Figure 1.
Settlement and metamorphosis in the crinoid
Oxycomanthus japonicus, modified from [[9]]. (A) A swimming, non-feeding doliolaria larva near settlement, with the A/P axis indicated. At this stage the vestibular depression that defines the future oral surface lies at a roughly 90° angle from this axis with the preoral adhesive organ (arrow, in red) located just above it. At settlement (B), the adhesive organ attaches to the substratum and subsequently acts as the attachment disc, and the preoral region begins to narrow and elongate to form the stalk. The vestibule has closed over internalizing the oral surface and the future site of mouth formation. (C) Late cystidean stage, with the visceral organs (coeloms and gut) rotated so the vestibular sac, now open to expose the mouth, points upward. Colors indicate the developing gut (yellow), axocoel and hydrocoel (light blue, the former is the smaller), and the left and right somatocoels (green, the latter is darker). A lighter red is used to trace the expected extent of the preoral larval ectoderm during rotation and stalk elongation, assuming its expansion occurs without distortion or significant cell rearrangement, though precisely how early ectodermal domains map to later stages is not known. Nevertheless, it does seem that the ectoderm of the stalk comes largely from preoral larval ectoderm, whereas, in contrast, internal structures, including all or parts of the chambered organ, and possibly the ligaments, appear to derive from the right somatocoel, as indicated by the green rectangle. The query is a reminder that these are all suppositions based on the morphology that have yet to be verified experimentally.
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