In the previous two posts related to tunicates , we saw a brief introduction to them and also their position in the tree of life. Today we take a look into some aspects of Hox genes in these amazing animals. We live on a wonderful planet which is full of life and there exists enormous diversity when it comes to body plan of animals.Its more amazing to know that all animals are made up of essentially almost similar set of genes but only thing that differs is the combination’s of the same stuff. This is responsible for generating diversity we see among various life forms and homeotic genes forms a very vital part of this set.
As you know homeotic genes or hox genes encode transcription factors, which play an important role in giving a unique identity to each segment and thereby setting up morphogenesis along the anterior posterior axis. Interestingly it was discovered that these genes were actually arranged on the chromosome in clusters and in the same order that they’re expressed in along the axis of the embryo. That means that the gene that’s expressed in the most anterior part of the organism, up in its head, is located at one end of cluster and the posterior most gene in the cluster expresses in tail region.
Previous work in the field have shown that variation in Hox genes number, expression pattern and Hox protein activities have played a major role in the evolution of animal body plan.Hox genes are involved in establishing morphological identities along the anteroposterior axis of bilaterians and cnidarians. This goes on to indicate that despite their remarkable conservation, Hox genes are responsible for the dramatic developmental differences in body plans within and between phyla. So this forms a very valid reason to understand the cluster organization and expression pattern of Hox genes in different species, especially in animals occupying unique evolutionary positions.
Hox genes in Ascidian, Ciona intestinalis:
Japanese scientists working on Ciona have shown through in situ hybridization and by studying genomic organization that , seven Hox genes out of total nine Hox genes are located on one chromosome. In the article published in PNAS , 2004 issue ,authors have examined the organization of the Hox gene cluster and the expression pattern, spanning from the egg to juvenile of all of the known Hox genes in the ascidian C.
intestinalis by using fluorescence in situ hybridization (FISH) and whole-mount in situ hybridization (WISH).
The extended Hox genes , Evx and Mox ( called so because these genes are closely associated to Hox genes on chromosomes and also exhibit close similarity to nuclotide sequence of Hox genes) were also examined by authors and shown that Evx is in fact present along with seven hox genes on a single chromosome, but the organization of the member genes was quite unusual in the spacing and order of the genes. Based on these findings authors propose that the cluster structure of Hox genes is under dispersing conditions, and, hence the colinearity is under disintegrating conditions in C. intestinalis.
Hox genes in Oikopleura dioica:
In the same year of 2004 ,three different groups came together to work out the Hox complement in another tunicate,Oikopleura dioica(appendicularian). Like Ciona , Oikopleura also have 9 Hox genes ,but unlike Ciona it lacks all central hox genes. Oikopleura has three anterior Hox genes (Hox1, Hox2 and Hox4) and six posterior Hox genes (Hox9A, Hox9B, Hox10, Hox11, Hox12 and Hox13). They also looked at the expression patterns of the Hox genes , which occurs mostly in the tail, with some tissue preference, and a strong partition of expression domains in the nerve cord, in the notochord and in the muscle.Unlike ascidians, appendicularians conserve a chordate tail complex during the entire short life cycle.
Authors performed genome walking on about 250kb on either side of hox genes ,but failed to detect linkage between any hox genes. So this makes Hox genes in Oikopleura more dispersed in the genome than in any other organism studied so far.
I suggest you to go through these two paper completely to gain more insights into role of hox genes in diversity of animal body plan.
1) Ciona intestinalis Hox gene cluster: Its dispersed structure and residual colinear expression in development.
Ikuta T, Yoshida N, Satoh N, Saiga H.
Proc Natl Acad Sci U S A. 2004 Oct 19;101(42):15118-23.
2) Hox cluster disintegration with persistent anteroposterior order of expression in Oikopleura dioica.
Seo HC, Edvardsen RB, Maeland AD, Bjordal M, Jensen MF, Hansen A, Flaat M, Weissenbach J, Lehrach H, Wincker P, Reinhardt R, Chourrout D.
Nature. 2004 Sep 2;431(7004):67-71.