Annelids on a roll : Part III
NK homeobox genes in Annelid Platynereis dumerilii:
So far in Annelids series we talked about three articles which were published recently in major scientific journals,in the form of two posts ( Annelids on a roll Part : I and Part II ) and today we will end this series by discussing a article from Guillaume Balavoine’s lab dealing about Homeobox NK genes in annelid Platynereis.
We talked already about NK genes couple of times but just to remind that NK genes are clustered and involved in patterning various mesodermal derivatives during embryo genesis.Nk genes were first discovered in fruit fly Drosophila as a cluster of genes involved in patterning mesoderm,including the dorsal vessel(Heart), the visceral mesoderm and various somatic muscles. It was believed that Nk cluster might represent an part of ancient Homeobox cluster (involving ectodermal Hox genes and endodermal para Hox genes) which patterns mesoderm and this property might be an ancestral property of NK genes. So in order to test this hyposthesis Guillaume’s lab looked into the expresion pattern of NK cluster in Platynereis. They started by cloning orthologs of seven NK genes Msx/drop, NK4/tinman, NK3/bagpipe, Lbx/ladybird, Tlx/C15/Clawless, NK1/Slouch,S59 and NK5/Hmx from platynereis and analysed their expression pattern.
Credit 
hoto by Kristin Tessmar-Raible,(EMBL)
The phylogenetic position of Platynereis makes it a crucial model to understand better the evolutionary history of NK genes.The formation of trunk segments (and therefore organogenesis as well) occurs in two phases in Platynereis.The three first anterior segments are formed during trochophore development and all the other segments are produced by a posterior growth zone during most of the life of the worm.This was reason they checked the expression patterns of Nk genes in both embryonic/larval development and juvenile posterior growth.
The NK genes of Platynereis seem to be involved in a diversity of functions in the epidermal and neural ectoderm,mesoderm and digestive tract. The expression of the homologue of NK4/tinman in the pulsatile dorsal vessel of Platynereis strongly supports the hypothesis that the vertebrate heart derived from a dorsal vessel.The most surprising result was the expression of five NK NK4, Lbx, Msx, Tlx and NK1 orthologues in complimentary stripes the ectoderm and/or mesoderm of forming segments, suggesting their role in segment formation.The ancient role of NK genes in segment formation was something not expected from the studies in Drosophila and vertebrates.Authors suggest that more work in other bilaterians especially non-insect arthropods and onychophorans might unravel potential involvement of NK genes in segment formation.This role of segment formation by NK Homeobox cluster in Platynereis is a annelid innovation or a legacy carried on from Urbilateria is something to be investigated.
Reference :
Saudemont A, Dray N, Hudry B, Le Gouar M, Vervoort M, Balavoine G.
Complementary striped expression patterns of NK homeobox genes during segment formation in the annelid Platynereis.
Dev Biol. 2008 May 15;317(2):430-43.
World of NK genes on Hoxful Monsters :
NK Genes Introduction
Nk Genes in most basal organism on Earth
Nk genes in Sponges
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