Washington: Researchers compared structure and behavior of clusters of Hoxa and Hoxd genes, possessed by both fish and land animals, in embryos from mice and zebrafish and discovered similar 3-dimensional DNA organization of the fish and mouse clusters.
The clusters are necessary for both fin and limb formation during embryonic development.
A new study led by Denis Duboule`s team, at the UNIGE and the EPFL, Switzerland, indicated that the main mechanism used to pattern tetrapod limbs was already present in fish.
However, when inserted into transgenic mouse embryos, the fish Hox genes were only active in the mouse arm but not in the digits, showing that the fish DNA lacks essential genetic elements for digit formation.
The study thus concluded that, although the digital part of the limbs evolved as a novelty in land animals, this happened by elaborating on an ancestral, pre-existing DNA infrastructure.
In animals, the Hox genes, often referred to as `architect genes`, are responsible for organizing the body structures during embryonic development.
The team showed that, during mammalian development, Hoxd genes depend on a `bimodal` 3-dimensional DNA structure to direct the development of the characteristic subdivision of the limbs into arm and paw, a division which is absent from fish fins.
Surprisingly, the researchers found a similar bimodal 3-dimensional chromatin architecture in the Hoxd gene region in zebrafish embryos.
These findings indicated that the regulatory mechanism used to pattern tetrapod limbs probably predates the divergence between fish and tetrapods.
The study was published in journal PLoS Biology.