الرد على الشبهة التطورية synteny... قنابل بالجملة

Rather surprisingly, it has been found that the sequence of homeotic genes in fruit flies, known as the Hox genes, are lined up in exactly the same order as the part of the fly they affect. That is to say, the first gene affects the mouth, the second the face, the third the top of the head and so on up until the eighth and final gene that affects the abdomen
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The phenotypes of the mutant embryos indicate that the process of segmentation involves at least three levels of spatial organization: the entire egg as developmental unit, a repeat unit with the length of two segments, and the individual segment.

Vertebrate and Drosophila Iroquois genes are organized in clusters of 3 genes sharing blocks of conserved regulatory sequences. Here, we report a 3-gene cluster in the basal, preduplicative chordate amphioxus. Surprisingly, however, the origin of the amphioxus cluster is independent of those in vertebrates and drosophilids. Investigation of genomic organization of Iroquois genes in other 17 metazoan genomes revealed a fourth independent 3-gene cluster organization in polychaetes, as well as additional 2- and 4-gene clusters in other clades, in one of the most striking examples of convergence in genomic organization described so far. The recurrent independent evolution of Iroquois clusters suggests a functional importance of this organization for these genes, perhaps related to the sharing of regulatory elements. Consistent with this, comparative analysis of genomic regions flanking the 3 amphioxus Irx genes revealed several blocks of sequences, conserved for at least 100 Myr. Finally, we discuss the possible causes and implications of the convergent evolution of this genomic and regulatory organization throughout metazoans

In the case of CG10632, this linkage has been
maintained in members of all major metazoans groups, encompassing
more than 600 million years of evolution, in
one of the most striking examples of conserved synteny between
2 phylogenetically unrelated genes.

In summary, the results presented here show that theorganization of Iroquois genes in gene complexes has evolved independently several times in metazoan evolution especially generating clusters of 3 genes. The number of convergent clusterization events thus seems to exceed what would be expected merely by chance. If so, it is tempting to speculate that tandem duplication and neighborhood organization for Iroquois genes are fixed in evolution at a high rate, maybe related to the functional importance of shared regulatory elements. Intriguingly, our results would suggest that gene tandem duplication is more widespread than previously thought, being only fixed at high rates in cases in which a cluster organization bear a functional relevance, as it seems the case for the Iroquois complexes. An alternative explanation is that some genomic regions are more prone to undergo tandem duplication; however, there is currently no evidence that specific groups of genes or types of sequences are more likely to duplicate than others. Whatever the case, the study of Iroquois gene organization will help to understand how and why functional architectures evolve in metazoans genomes. Understanding the causes and mechanisms involved in the evolution of these genes will help to understand the flexibility, constraints, and evolvability of genome

organization

http://mbe.oxfordjournals.org/conten.../1521.full.pdf.