Hemichordate
| Hemichordate | |
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| Acorn worm, a hemichordate. | |
| Scientific classification | |
| Kingdom: | |
| Subkingdom: | |
| Superphylum: | |
| Phylum: | Hemichordata Bateson, 1885
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| Classes | |
Hemichordata is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They date back to the Lower or Middle Cambrian and include an important class of fossils called graptolites, most of which became extinct in the Carboniferous.
Structure
The bodies of Hemichordates are divided into three parts, proboscis, collar and trunk. They have open circulatory systems and a complete digestive tract but the musculature in their gut is very poorly developed, and food is mostly transported through it by using the cilia that cover its inside surface.
They have a diverticulum of the foregut called a stomochord, previously thought to be related with the chordate notochord, but this is most likely the result of convergent evolution rather than homology . A hollow neural tube exists among some species (at least in early life), probably a primitive trait they share with the common ancestor of chordata and the rest of the deuterostomes.
Development
The hemichordates give us the closest extant phylogenetic relative between the chordates and other invertebrates. Thus these marine worms, described to be the sister group of such animals as sea urchins, are of great importance to the scientific community interested in knowing the origins of chordate development. There are several species classified as hemichordates and there exists a moderate diversity of embryological development between these species. Hemichordates are classically known to develop in two ways both directly and indirectly .[1] Hemichordates are a phylum composed of two classes the enteropneusts and the pterobranchs, both are forms of marine worm. The enteropneusts have two developmental strategies direct and indirect development. The indirect way of development is known to end in an extended pelagic plankotrophic tornaria larval stage, which means that this hemichordate exists in a larval stage that feeds on plankton before turning into an adult worm[2]. Those species that are direct developing bypass this prolonged larval stage and develop directly into an adult worm. The following details the development of two popularly studied species of the hemichordata phylum Saccoglossus kowalevskii and Ptychodera flava. Saccoglossus kowalevskii is a direct developer and Ptychodera flava is an indirect developer. Most of what has been detailed in Hemichordate development has come from hemichordates that develop directly.
Ptychodera flava
P.Flava’s early cleavage pattern is similar to that of S.kowalevskii. The first and second cleavages from the single cell zygote of P.flava are equal cleavages, are orthogonal to each other and both include the animal and vegetal poles of the embryo. The third cleavage is equal and equatorial so that the embryo has four blastomeres both in the vegetal and the animal pole .The fourth division occurs mainly in blastomeres in the animal pole, which divide transversally as well as equally to make eight blastomeres. The four vegetal blastomeres divide equatorially but unequally and they give rise to four big macromere’s and four smaller micromeres. Once this fourth division has occurred the embryo has reached a 16 cell stage. P.flava has a 16 cell embryo with four vegetal micromeres, eight animal mesomeres and 4 larger macromeres. Further divisions occur until P.flava finishes the blastula stage and goes on to gastrulation. The animal mesomeres of P.flava go on to give rise to the larva’s ectoderm, animal blastomeres also appear to give rise to these structures though the exact contribution varies from embryo to embryo. The macromeres give rise to the posterior larval ectoderm and the vegetal micromeres give rise to the internal endomesodermal tissues[3]. Studies done on the potential of the embryo at different stages have shown that at both the two and four cell stage of development P.flava blastomeres can go on to give rise to a tornaria larvae, so fates of these embryonic cells don’t seem to be established till after this stage[4].
Saccoglossus kowalevskii
Eggs of S. kowalevskii are oval in shape and become spherical in shape after fertilization. The first cleavage occurs from the animal to the vegetal pole and usually is equal though very often can also be unequal. The second cleavage to reach the embryos four cell stage also occurs from the animal to the vegetal pole in an approximately equal fashion though like the first cleavage it’s possible to have an unequal division. The eight cell stage cleavage is latitudinal; so that each cell from the four cell stage goes on to make two cells. The fourth division occurs first in the cells of the animal pole which end up making 8 blastomeres(mesomeres) that are not radially symmetric, then the four vegetal pole blastomeres divide to make a level of four large blastomeres (macromeres) and four very small blastomeres (micromeres). The fifth cleavage occurs first in the animal cells and then in the vegetal cells to give a 32 cell blastomere. The sixth cleavage occurs in a similar order and completes a 64 cell stage, finally the seventh cleavage marks the end of the cleavage stage with a blastula with 128 blastomeres. This structure goes on to go thru gastrulation movements which will determine the body plan of the resulting gill slit larva, this larva will ultimately give rise to the marine acorn worm [5][6]
Genetic control of dorsal ventral hemichordate patterning
Much of the genetic work done on hemichordates has been done to make comparison with chordates, so it is obvious that many of the genetic markers identified in this group are also found in chordates or are homologous to chordates in some way. Studies of this nature have been done particularly on S. kowalevskii, and like chordates S. kowalevskii has dorsalizing bmp-like factors such as bmp 2/4 which is homologous to Drosophila’s decapentaplegic dpp. The expression of bmp2/4 begins at the onset of gastrulation on the ectodermal side of the embryo, and as grastulation progresses its expression is narrowed down to the dorsal midline but is not expressed in the post anal tail. The bmp antagonist chordin is also expressed in the endoderm of gastrulating S. kowalevskii. Besides these well known dorsalizing factors, further molecules known to be involved in dorsal ventral patterning are also present in S. kowalevskii, such as a netrin that groups with netrin gene class 1 and 2 [7]. Netrin is important in patterning of the neural system in chordates, as well as is the molecule Shh, but S. kowalevskii was only found to have one hh gene and it appears to be expressed in a region that is uncommon to where it is usually expressed in developing chordates along the ventral midline.
Classification
Hemichordata are divided into two classes: the Enteropneusta,[8] commonly called acorn worms, and the Pterobranchia, which may include the graptolites. A third class, Planctosphaeroidea, is proposed based on a single species known only from larvae. The phylum contains about 100 living species. Hemichordata appears to be sister to the Echinodermata as Ambulacraria; Xenoturbellida may be basal to that grouping. Pterobranchia may be derived from within Enteropneusta, making Enteropneusta paraphyletic.
References
- ^ Hemichordate embryos: Procurement, Culture and Basic Methods, Lowe C., Tagawa K, Humphreys T, Kirschner M, Gerhart J, Methods in Cell Biology 74:171-94 2004.PMID: 15575607
- ^ The Spawning and Early Development of the Hawaiian Acorn worm (Hemichordate) , Ptycodhera flava, Tagawa K., Nishino A, Humphreys T, Satoh N., Zoological Science 15: 85-91 1998.PMID: 18429670
- ^ Deuterostome evolution: early development in the enteropneust hemichordate, Ptychodera flava, Henry J, Tagawa K, Martindale M, Evolution and Development 3: 375-390 2001 PMID: 11806633
- ^ The developmental capacities of separated early blastomeres of an enteropneust, Saccoglossus Kowalevskii., Colwin A and Colwin L, Journal Experimental Zoology 155: 263-296 1950.
- ^ Relationships between the egg and larva of Saccoglossus kowalevskii (Enteropneusta): axes and planes; general prospective significance of the early blastomeres. Colwin A and Colwin L, Journal Experimental Zoology 117: 111-138 1951.
- ^ The normal embryology of Saccoglossus kowalevskii (Enteropneusta). Colwin A and Colwin L, Journal of Morphology 92: 401-453 1953.
- ^ Dorsoventral Patterning in Hemichordates: Insights into Early Chordate Evolution. Lowe C, Terasaki M, Wu M, Freeman Jr. R, Runft L, Kwan K, Saori H, Aronowicz J, Lander E, Chris G, Smith M, Kirschner M, Gerhart J, PLOS Biology 4: 1603-1618 2006.
- ^ Attention: This template ({{cite pmid}}) is deprecated. To cite the publication identified by PMID 19348951, please use {{cite journal}} with
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Cameron, C.B. 2005. A phylogeny of the hemichordates based on morphological characters. Canadian Journal of Zoology. 83(1): 196-2
Winchell, C.J., J. Sullivan, C.B. Cameron, B.J. Swalla, and J. Mallatt. 2002. Evaluating hypotheses of deuterostome phylogeny and chordate evolution with new LSU and SSU ribosomal DNA data. Molecular Biology and Evolution. 19(5): 762-776.
Cameron, C.B., B.J. Swalla and J.R. Garey. 2000. Evolution of the chordate body plan: New insights from phylogenetic analysis of deuterostome phyla. Proceedings of the National Academy of Sciences (USA) 97(9): 4469-4474.
External links
- Cameron, C. B. Evolution and classification of Hemichordata
- Dr. Billie J. Swalla on the WEB "Development and Evolution of Hemichordates"
- Dr. Ken Halanych "Phylogeny and Evolution of Hemichordates"
- Dr. Chris Lowe "Genomics and Development of Saccoglossus kowalevskii"
- At the Tree of Life
