Development Hemichordate

1 development

1.1 ptychodera flava
1.2 saccoglossus kowalevskii
1.3 genetic control of dorsal-ventral hemichordate patterning

development

together echinoderms, hemichordates form ambulacraria, closest extant phylogenetic relatives of chordates among invertebrates. these marine worms of great interest study of origins of chordate development. there several species of hemichordates, moderate diversity of embryological development among these species. hemichordates classically known develop in 2 ways, both directly , indirectly. hemichordates phylum composed of 2 classes, enteropneusts , pterobranchs, both being forms of marine worm.

the enteropneusts have 2 developmental strategies: direct , indirect development. indirect developmental strategy includes extended pelagic plankotrophic tornaria larval stage, means hemichordate exists in larval stage feeds on plankton before turning adult worm. pterobranch genus extensively studied rhabdopleura plymouth, england , bermuda.

the following details development of 2 popularly studied species of hemichordata phylum saccoglossus kowalevskii , ptychodera flava. saccoglossus kowalevskii direct developer , ptychodera flava indirect developer. of has been detailed in hemichordate development has come hemichordates develop directly.

schematic of embryonic cleavage , development in p.flava , s.kowalevskii


ptychodera flava

p. flava’s cleavage pattern similar of s. kowalevskii. first , second cleavages single cell zygote of p. flava equal cleavages, orthogonal each other , both include animal , vegetal poles of embryo. third cleavage equal , equatorial embryo has 4 blastomeres both in vegetal , animal pole. fourth division occurs in blastomeres in animal pole, divide transversally equally make 8 blastomeres. 4 vegetal blastomeres divide equatorially unequally , give rise 4 big macromeres , 4 smaller micromeres. once fourth division has occurred, embryo has reached 16 cell stage. p. flava has 16 cell embryo 4 vegetal micromeres, 8 animal mesomeres , 4 larger macromeres. further divisions occur until p. flava finishes blastula stage , goes on gastrulation. animal mesomeres of p. flava go on give rise larva’s ectoderm, animal blastomeres appear give rise these structures though exact contribution varies embryo embryo. macromeres give rise posterior larval ectoderm , vegetal micromeres give rise internal endomesodermal tissues. studies done on potential of embryo @ different stages have shown @ both 2 , 4 cell stage of development p. flava blastomeres can go on give rise tornaria larvae, fates of these embryonic cells don’t seem established till after stage.

saccoglossus kowalevskii

eggs of s. kowalevskii oval in shape , become spherical in shape after fertilization. first cleavage occurs animal vegetal pole , equal though can unequal. second cleavage reach embryos 4 cell stage occurs animal vegetal pole in approximately equal fashion though first cleavage it’s possible have unequal division. 8 cell stage cleavage latitudinal; each cell 4 cell stage goes on make 2 cells. fourth division occurs first in cells of animal pole, end making 8 blastomeres (mesomeres) not radially symmetric, 4 vegetal pole blastomeres divide make level of 4 large blastomeres (macromeres) , 4 small blastomeres (micromeres). fifth cleavage occurs first in animal cells , in vegetal cells give 32 cell blastomere. sixth cleavage occurs in similar order , completes 64 cell stage, seventh cleavage marks end of cleavage stage blastula 128 blastomeres. structure goes on go thru gastrulation movements determine body plan of resulting gill slit larva, larva give rise marine acorn worm

genetic control of dorsal-ventral hemichordate patterning

much of genetic work done on hemichordates has been done make comparison chordates, obvious many of genetic markers identified in group found in chordates or homologous chordates in way. studies of nature have been done particularly on s. kowalevskii, , chordates s. kowalevskii has dorsalizing bmp-like factors such bmp 2/4, homologous drosophila’s decapentaplegic dpp. expression of bmp2/4 begins @ onset of gastrulation on ectodermal side of embryo, , grastulation progresses expression narrowed down dorsal midline not expressed in post anal tail. bmp antagonist chordin expressed in endoderm of gastrulating s. kowalevskii. besides these known dorsalizing factors, further molecules known involved in dorsal ventral patterning present in s. kowalevskii, such netrin groups netrin gene class 1 , 2. netrin important in patterning of neural system in chordates, molecule shh, s. kowalevskii found have 1 hh gene , appears expressed in region uncommon expressed in developing chordates along ventral midline.