The Fall and Rise of Orders of Symmetry
From Palaeos.org
One feature of the deuterostomes deserves a closer look - the widespread occurrence across the clade of deviations from the bilateral symmetry found in most Bilateria. Almost all the deuterostomes are asymmetrical to some degree, and in the vast majority of echinoderms bilateral symmetry has been lost entirely in the adult and replaced by pentaradial symmetry.
In vertebrates, left-right asymmetry is mostly found in the viscera - we're probably all aware that our heart and digestive organs are not positioned directly down the midline, but are coiled and directed to one side or another depending on the organ. Other features, such as the skeleton, are more or less symmetrical.
The immediate outgroup of craniates, the Cephalochordata (e.g., amphioxus), show strong asymmetry during development. From the appearance of the fifth somite, the anterior borders of the somites move visibly out of register. Minguillon & Garcia-Fernandez (2002); Boorman & Shimeld (2002). After neurulation, paired cavities called Hatschek's diverticula (possible pituitary homologue) form in the head endoderm. The left cavity moves posteriorly to form the pre-oral pit, while the right expands to fill the vacated space in the head. The larval head becomes significantly asymmetrical, with the mouth appearing on the left side and the gill slits (originating on the ventral midline) spreading up to right side only. At metamorphosis, the mouth moves to the front, the pre-existing gill slits move from the right side to the left, and new gill slits open on the right. The adult therefore regains pharyngeal symmetry. Other asymmetries remain, such as the displaced muscle blocks, the position of Hatschek's pit (an organ derived from the pre-oral pit, and probably homologous to the pituitary gland) on the left side, and a blind gut diverticulum on the right (Boorman & Shimeld, 2002).
Urochordata, the next group out, show little asymmetry in the larva, except in the positions of the two sensory pigment spots and adjoining regions of the brain. In the adult, the gut becomes asymmetrically folded.
The hemichordates are the most symmetrical of the deuterostomes. The gut is coiled asymmetrically but unlike other deuterostomes the direction of asymmetry differs between individuals. Directional asymmetries are few and relatively small - the protocoel pore is typically on the left. Boorman & Shimeld (2002). Rhabdopleura is something of an exception - in this genus, the right feeding arm is usually longer than the left, the single gonad is on the right, the mouth is on the left and the anus is on the right. Jefferies (1986).
In echinoderms the larva is initially bilaterally symmetrical, only to be replaced by a whole new order of symmetry in the adult. In sea urchins the pentaradial adult body plan is laid down in a structure called the adult rudiment, which lies on the left within the larva. Boorman & Shimeld (2002); Arenas-Mena et al. (2000). In starfish and crinoids, the left somatocoel becomes oral and the right one aboral. In starfish, only the left hydrocoel of the larva develops into the adult water-vascular system, while the right hydrocoel disappears. Jefferies (1986). A lot of debate still occurs about the appropriate interpretation of echinoderm axes, and how (if at all) they can be compared to axes in other Bilateria. It is uncertain, for instance, if the anterior-posterior axis in other animals corresponds to the oral-aboral axis of echinoderms or the central axes of the five rays. The fossil lineage of echinoderms suggests some very odd evolutionary stages, with homalozoans representing a strongly asymmetrical stage in echinoderm ancestry (if they are indeed stem echinoderms).
The obvious question then becomes what this all means. Why are deuterostomes such individualists? Jefferies (e.g., 1986) has suggested that the asymmetry of chordates and echinoderms is homologous, and evidence that they form a monophylum to the exclusion of the more symmetrical hemichordates. This clade he named Dexiothetica after his suggested explanation for its origin - that the ancestor of the echinoderm-chordate clade underwent a 90 degree rotation so that it came to lie on its right side with the left side becoming dorsal (Greek dexios = right, thetikos = suitable for lying down). In chordates, the outer body subsequently re-oriented itself over time to become bilaterally symmetrical, but the viscera remained dexiothetic.
Jefferies' theory was strongly dependent on his interpretation of the fossil homalozoans (which he referred to as calcichordates). On this page, we take the more orthodox view that homalozoans are stem echinoderms, as suggested by their possession of a calcite skeleton with a similar ultrastructure to modern echinoderms. Jefferies held this skeleton to be ancestral for Dexiothetica, and subsequently independently lost in each of the three chordate subphyla. However, this seems to require a lot of convergence, expecially if, as molecular phylogenies suggest, the hemichordates are actually closer to echinoderms. A stem echinoderm position for homalozoans seems much more parsimonious.
Could dexiothetism have still been ancestral to deuterostomes as a whole? The theory does have a certain appeal in relation to echinoderms - as mentioned before, left and right do become dorsal and ventral in echinoderm development, and the unpaired nature of the gill structures in Vetulocystidae and Cornuta at the base of the echinoderm stem offers further support (if they are correctly interpreted as such). In the case of Chordata, however, if the homalozoans are not regarded as relevant to the chordate stem, the support for dexiothetism is much lower. Asymmetries in the viscera may have been an adaptation for greater complexity (a coiled heart has greater pumping ability, while a coiled gut allows for more surface area in a smaller volume). Cooke (2004). The strong asymmetry in amphioxus may be an autapomorphy of that particular lineage. Evidence is therefore equivocal as to whether dexiothetism can be regarded as ancestral for Deuterostomia, or whether chordate and echinoderm asymmetries arose independently. Among fossils that may lie along the stem group of deuterostomes, the Vetulicolia have not been recorded as showing any sign of asymmetry. However, the mostly lateral, two-dimensional preservation of vetulicolians means that this is not necessarily informative.
Studies of developmental gene expressions and their relation to deuterostome asymmetry are still at a fairly early stage. The Nodal-Ptx pathway has been identified as a major factor in establishing the left-right axis in chordates. Ptx, for instance, is expressed predominantly on the left-hand side of the embryo in all three chordate subphyla. However, neither Ptx or Nodal have yet been identified in echinoderms, though once again the extremely derived body plan of echinoderms makes the identification of gene functional homologies difficult. Wray & Lowe (2000). Preliminary analysis suggests Ptx expression is symmetrical in hemichordates, but this requires confirmation. Boorman & Shimeld (2002).
In conclusion, while asymmetry is common in deuterostomes, there does not appear to be any strong uniting features supporting a common origin of the various asymmetries. Asymmetry in vertebrates may reflect greater complexity, while in urochordates it may be a consequence of changing from a free-living to a sessile mode of life, with the oral and anal openings both directed more or less upwards. Rhabdopleura has probably become asymmetrical as a result of reduction in size. Dexiothetism may still have occurred in the echinoderm stem, but is not likely to be ancestral for deuterostomes as a whole.
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