|
| ||
|
|
||
Chordate Body Plan (chordate + body_plan)
Selected AbstractsThe emergence of the chordate body plan: some puzzles and problemsACTA ZOOLOGICA, Issue 1 2010Thurston C. Lacalli Abstract Lacalli, T.C. 2010. The emergence of the chordate body plan: some puzzles and problems. ,Acta Zoologica (Stockholm) 91: 4,10 Rather than being sessile filter feeders, ancestral chordates are now thought to have evolved from more active benthic animals, possibly hemichordate-like, that took to swimming, to generate something resembling modern amphioxus. This general picture conceals a number of specific problems that underline how little we understand the transition in detail. I will address three. First, and closest to resolution is the issue of dorsoventral inversion, which has implications for understanding how an internalized brain evolved. This is because the mouth, dorsal after inversion, has first to be moved out of the way. Its migration down the left side of the head during amphioxus development may be a recapitulation of this event. Two other puzzles, both further from resolution are: (1) the significance, if any, of the neurenteric canal, which may be telling us something important about the true nature of deuterostomy, specifically whether hemichordates and echinoderms are deuterostomes for a different reason than chordates, and (2) whether the functional digestive tract of chordates is a secondary replacement of an earlier structure whose fate remains unexplained. Resolving these latter two issues will require a better understanding of molecular level events during development in protochordates and their immediate invertebrate relatives. [source] Is retinoic acid genetic machinery a chordate innovation?EVOLUTION AND DEVELOPMENT, Issue 5 2006Cristian Caņestro SUMMARY Development of many chordate features depends on retinoic acid (RA). Because the action of RA during development seems to be restricted to chordates, it had been previously proposed that the "invention" of RA genetic machinery, including RA-binding nuclear hormone receptors (Rars), and the RA-synthesizing and RA-degrading enzymes Aldh1a (Raldh) and Cyp26, respectively, was an important step for the origin of developmental mechanisms leading to the chordate body plan. We tested this hypothesis by conducting an exhaustive survey of the RA machinery in genomic databases for twelve deuterostomes. We reconstructed the evolution of these genes in deuterostomes and showed for the first time that RA genetic machinery,that is Aldh1a, Cyp26, and Rar orthologs,is present in nonchordate deuterostomes. This finding implies that RA genetic machinery was already present during early deuterostome evolution, and therefore, is not a chordate innovation. This new evolutionary viewpoint argues against the hypothesis that the acquisition of gene families underlying RA metabolism and signaling was a key event for the origin of chordates. We propose a new hypothesis in which lineage-specific duplication and loss of RA machinery genes could be related to the morphological radiation of deuterostomes. [source] Organizing chordates with an organizerBIOESSAYS, Issue 7 2007Jordi Garcia-Fernāndez Understanding how the chordate body plan originated and evolved is still controversial. The discovery by Spemann and Mangold in 1924 of the vertebrate organizer and its inductive properties in patterning the AP and DV axis was followed by a long gap until the 1960s when scientists started characterizing the molecular events responsible for such inductions. However, the evolutionary origin of the organizer itself remained obscure until very recently; did it appear together with the origin and radiation of vertebrates, or was it a chordate affair? A recent study by Yu and collaborators,1 which analyses the expression of several organizer-specific genes in amphioxus together with recent phylogenetic data that reversed the position of invertebrate extant chordates (e.g. urochordates and cephalochordates), indicates that the organizer probably appeared in early chordates. It likely had separate signalling centres generating BMP and Wnt signalling gradients along the DV and AP axis. The organizer was then lost in the urochordate lineage, most probably as an adaptation to a rapid and determinate development. BioEssays 29:619,624, 2007. Š 2007 Wiley Periodicals, Inc. [source] | ||||||||||