Homeobox genes in Ctenophores

By | May 6, 2021
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Organisms of phyla Ctenophora failed to generate considerable interest in evo devo in spite of displaying remarkable features which pose important questions in evo devo .These features are outlined below:

1) Well developed nerve net, Gut with openings on two sides (only bilaterians have fully formed gut with two openings in mouth and anus) 3) Some parts show bilateral symmetry 4) the most interesting properties of ctenophores is their true triploblastic condition (But the mesoderm of ctenophores may not be homologous to the ones in Bilaterians and molecular phylogenies classify them without any doubt as nonbilaterians) 5) Presence of true muscle cells (originates from mesoderm in bilaterians).

The most shocking thing came out in April 2008 was that an animal with such complexities is placed at the bottom of tree of animals meaning they have branched off first in metazoans tree of evolution.This makes understanding of gene composition and their expression in Ctenophores very vital for evolutionary developmental biology.

Ctenophore development has been pretty well worked out ( Refer Martindale and Henry 1999) but little is known for developmental gene families in this phyla. Research in last couple of years identified few members of some important developmental gene families in ctenophores like forkhead,Sox,Fox ,pax, T-Box and Homeobox.

Kevin pang and Mark Mrtindale describes expression pattern of seven Homeobox genes in ctenophore Mnemiopsis leidy,four belongs to Antennapedia class (NK like genes) and three falls into paired class.Homeobox genes are well known for their role in axis formation and cell specifications in wide range of organisms like Fungi,plants and metazoans(animals).The main homeobox classes are ANTP, PRD, POU, LIM, SINE, and TALE class of protein families.

Four ANTP class genes MlBar, MlBsh ,MlTlx-like, MlNKL1 discovered were placed into NK group(NKLike) based upon phylogeny analysis and MlPrd1, MlPrd2, MlPrd3 belong to the PRD class.These paired class genes posses paired like homeodomain but lack paired domain(Normally paired class od proteins have two DNA binding domains Homeodoman and paired domain.)However No Hox or ParaHox genes were discovered,but its better to wait till ctenophore genome is completely sequenced to confirm lack of Hox and para hox genes.

MlNKL1:
MlNKL1 shows greatest similarity to Dlx orthologs (50% identity and 69% similarity to Drosophila Distal-less homeodomain).But phylogenic analysis were not conclusive enough to assign a clade.MlNKL1 is expressed first expressed during gastrulation along the tentacular plane in two areas of the blastopore and later in cells of ectoderm which later invaginate to form parts of pharynx.

MlBsh:
MlBsh is closely related to Bsh or the brain-specific class of homeoboxes and also possess EH1 motif upstream of homeodomain (characteristic of all NK genes).MlBsh expression starts at gastrulation, at a similar time as MlNKL1, in two rows of aboral ectodermal cells along the tentacular axis.

MlBar
MlBar is closely related to brain homeobox family, BarH-like/BarX.Their expression comes up before MlNKL1 and MlBsh in development,which comes up highly in micromeres which form mesoderm.

MlTlx-like

MlTlx-like contains an EH1 repression domain (KLSFSIDQILGL) at the amino terminal that is present in other Tlx proteins.Expressed in ectoderm throughout development.

Most of these genes start to express at gastrulation meaning specification occurs early in development,except MlPrd2 which is expressed ubiquitously in the egg, all other described in this article (six others) are not transcribed untill gastrulation,about 3 to 4 h post fertilization.

The study of pang and Martindale showing expression pattern of seven Homebox genes is a great step forward in understanding ctenophore development as it happens to be the first animal group to branch off and in this way form sister group to all the metazoans. So understanding Ctenophores genes and development might reveal more secrets of metazoan evolution.

Reference:
Pang K, Martindale MQ.
Developmental expression of homeobox genes in the ctenophore Mnemiopsis leidyi.
Dev Genes Evol. 2008 Jun;218(6):307-19

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