Spare ribs result of faulty gene
American scientists have isolated the genes that stop ribs from forming all the way down the spine, answering questions about mammalian evolution.
Two groups of homeobox (Hox) genes play a crucial role in the formation of the spine and legs in mammals, according to geneticist Professor Mario Capecchi and colleagues at the University of Utah. Homeobox genes orchestrate the activity of other genes in the developing embryo.
Having a ribcage that only emanates from part of the spine, leaving the lower back free was a major step forward in the flexibility and agility of vertebrates, like mammals. These changes would have also allowed the evolution of mammalian reproduction, where newborns are delivered through the pelvis.
More ancient vertebrates, like frogs, dinosaurs and primitive fish, have ribs along the whole length of their spine, from neck to tail, allowing only limited movement in the body.
The study helps to explain why different vertebrates have different skeletal structures. "For example, a pigeon has five pairs of ribs. We have twelve pairs of ribs. The question is how do you make different body plans and how did the ability to have different body plans evolve?" questioned Capecchi.
All mammals have Hox genes, which evolved from a single gene in ancient spineless creatures. As the genes multiplied and mutated over the past 500 million years, they acquired new functions. "Hox genes have been utilised to modify the basic body plan to give rise to the different vertebrate body plans," said Capecchi.
The study looked at two groups of Hox genes, Hox10 and Hox11, each containing three alleles (or forms). The aim was to find out what the genes' role is on skeletal development in the mouse, and by implication in humans and other mammals.
Capecchi is renowned for having developed 'gene targeting' in mice, a process where a gene of interest can be disabled, allowing scientists to work out what it normally does. By 'switching off' two different families of Hox genes in mice, Capecchi's team were able to see what role they play in skeletal development.
Using the knockout mice the researchers discovered that, without any Hox10 alleles, ribs were formed along the whole length of the spine, like a snake's, and the bones in the upper leg were abnormally short. The results appear in this week's issue of the journal Science.
When the Hox11 set of genes were switched off, the lower leg bones were shortened, and vertebrae in the hip region did not form properly, so that the pelvis could not attach to the spine.
The researchers found that the alleles have 'considerable functional redundancy' which means that if any one of them functions properly the defects would be minimal. Gross abnormalities only occur when all six alleles fail to function.