Why the most evolved snakes don’t have limbs while the primitive species like python have poorly developed hind limbs (They have pelvic girdle and incompletely developed femur) or to put it in simple words : why snakes don’t have legs???? Imagine a Snake with fore and hind limbs.True it doesn’t give a good picture and moreover makes life difficult for snake.That could be the reason why Nature has selected out limbs from tetrapods and which might have led to evolution of present day snakes.
Snakes are evolved from lizards and are supposed to have lost their limbs during evolution.The fossil and embryological data available suggest that some primitive snakes had hind limbs but no fore limbs–meaning snakes lost their fore limbs first then the hind limbs.python with poorly developed hind limbs serves a good example.
The expression of Hox genes in somites determine the type of vertebrae formed ( Regarding this topic) whether thoracic vertebrae forms or cervical (Neck). The main difference between two is presence of ribs on thoracic vertebrae.Snakes have undergone a drastic change in their vertebrae body plan.
Some wonderful work done by Martin J .Cohn & Cheryll Tickle,which published in Nature for the first showed the developmental basis for loss of limbs in snakes. They show that broaden expression of hox genes in the python embryo can account for both loss of fore limbs and expansion of thoracic identity in axial skeleton.
Python have more than 300 vertebrae and all vertebrae anterior to hind limbs have ribs (characteristic of Thoracic) except one (Atlas).One important thing to note in anterior vertebrae is that they have both characters of cervical and thoracic indicating information required for thoracic formation have extended into neck.Hence the whole trunk resembles as one extended thorax.As mentioned earlier Hox genes are involved in specifying type of vertebrae( Thoracic or cervical or lumbar) and also in regionalization lateral plate mesoderm into fore limbs ,flank and hind limbs to specify limb position.
For the study they looked into the expression of three hox genes HOXC6 ,HOXC8 known for their role in formation of thoracic vertebrae in other tetrapods and HOX B5 ,which is expresses till first cervical vertebrae.Normal anterior boundary expression of these genes extended till fore limb in tetrapods and fishes where they are required for forming fore limbs and shoulder.But when it comes to Python embryos these are expressed throughout the trunk starting from hind limbs till first cervical vertebrae,telling the vertebrae to make ribs throughout most of the body.A sharp boundary of HoxC8 was detected near future hind limb.Expansion of HOXC6 and HOXC8 expression in python embryos can account for extended throrax and loss of fore limbs.
The specification of hindlimb position and initiation of budding appears to be normal in python embryos. The outgrowth of vertebrate limb buds depends on the apical ectodermal ridge (AER). Lack of AER and expression of limb associated genes like distalless could account for poorly formed hind limbs.In absence of AER sonic hedgehog (which is required for anterior posterior patterning of limb) siganlling is also affected ,but grafting experiments in chicken showed that limb bud in python retains the power to express SHH.Finally in the article they come out with a model that Hox genes expression expansion along the AP axis could account for major morphological changes in snake evoution.Such higher order genetic changes could have resulted in sudden anatomical transformations (example of HOPEFUL MONSTER of Richard Goldschmidt), rather than gradual changes, during snake evolution,a hypothesis which can be tested by the fossil record.In highly advanced snakes even more homogenous expression of Hox genes can be seen than python.
Developmental basis of limblessness and axial patterning in snakes
Martin J .Cohn & Cheryll Tickle
NATURE |VOL 399 | 3 JUNE 1999
Image credit: Pixabay