Draft:Asterozoa Body Patterning

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  Starfish

Asterozoa Clade and Their Body Patterning

Asteroidea refers to both sea stars and starfish. Ophiuroidea refers to brittle stars. Both of these are part of the Echinodermata phylum, and the Asterozoa subphylum or subgroup. The connections between Asteroidea and Ophiuroidea are what create their clade, the Asterozoa.

Life Cycle

Like all other echinodermata sea stars and brittle stars have pluteus larvae, transparent, calcareous, and bilateral. This embryonic stage that includes the pluteus larvae is a connecting mark for echinoderms.. They develop a calcareous skeleton, and leave their transparency behind when moving out of the embryonic stage of development. Adult Asterozoa have pentaradial symmetry at the center of their body mass. That five-sided center is where their 5 arms reach out. Even though their center mass is pentaradial their arms are bilaterally symmetrical.

Body Plan

The main identifying markings of the Asterozoa are the flattened stellate body plan, unique to the subphylum. Within this body plan is the Echinodermata ambulacra, but specific to Asterozoa is how it is uniform and radiating from the center ring canal, where the madreporite is, the mouth of both sea stars and brittle stars. Also specific to the Asteroidea class is the ventral side being on the bottom side of the stars, where their mouth, the madreporite, sits. And that location of the madreporite is the same in brittle stars, making the location another feature of the Asterozoa subgroup. The oral shields of the madreporite system vary slightly from Asteroidea, the other interradii. Although this location doesn’t help differentiate among the different species within the classes. The dorsal side of the starfish is the side that faces up, for sea stars it is covered by a spiny epidermis. The last part of the body plan that is generally specific to the Asterozoa subphylum is the tube feet that are associated with the ambulacral ossicles. The five arms are also called both rays, and tails. Since they found that there is a lack of trunk patterning genes in sea stars the rays are generally tails, and they are patterning like such. In sea stars, the arms exhibit both anterior and posterior signaling, with the anterior signaling being at the most center part of the ray, the radial canal, and the posterior signaling coming from the lateral canals that extend out from the radial canal. Anterior and posterior signaling are the cell signals that tell the organism which side of the body the cells are front and which are the back. A lack of Hox gene expression was found when comparing different gene expressions in sea stars and other echinoderms . This was done by L. Formerly and others by taking the arms of juvenile sea stars and performing tests to see if they have the transcription factors involved in certain patterning in chordates and hemichordates. These transcription factors included Hox, Wnt, and Pax6. They found that the only Hox genes were expressed in the digestive track and even then there were not enough to consider it a trunk. In all members of the Echinodermata the Hox genes are different from those in other phylums, they have incredibly reorganized changing the signaling of the organisms. Each subphylum of Echinodermata has a very different reorganization, not just the phylum itself. This spatial reorganization of the Hox genes results in different body plans, because Hox codes for the trunk of an organism, the reorganization would result in incredibly different phenotypes of the trunks, or even the loss of a trunk entirely, like in sea stars. Since the Hox gene split in Echinodermata there has been even more reorganization within the subphylums creating even more variation in looks with the phylum. The divergence of the Hox genes in correlated with the torsion of the mouth in sea stars, likely brittle stars as well, the similar position of the madreporite in sea stars and brittle stars could suggest that the torsion of the mouth in Asterozoa is what separated both clades from the overall Echinodermata. The lack of torsion in the mouth of sea urchins matches with the less reorganization of the Hox genes

Skeletogenesis

A difference between Asteroidea class and other Echinoderms that hasn’t been deeply studied in Ophiuroidea class is the skeletogenesis pathway, or TOR gene activation pathway. The specific pathway in sea urchins of activating skeletogenesis is absent in sea stars . But most of the other pathways of growth in the embryonic stage stay the same. Since this specific pathway hasn’t been tested in brittle stars we cannot assume either, but it is likely that the pathway is more similar to Asteroidea because of the similarities in shape, although the similar phenotype could be misleading.

Systematics

While there is still some uncertainty about where Asterozoa fits in the echinodermata tree, most are confident in the idea that sea stars and brittle stars share more recent common ancestor with each other than they do with sea urchins and sea cucumbers. . This is most likely the Somasteroidea, an extinct class of the Asterozoa. One of the proposals for a tree with Asterozoa creates this subphylum by focusing on the 5 rayed body plan, the specifically pentaradial center with radial canals extending from the ring canal. There are multiple possibilities for a phyletic tree of Echinodermata, and all have Asteroidea and Ophiuroidea sharing a most recent common ancestor .