How Snakes Lost Their Legs

Have you ever wondered why snakes have no limbs? Research in the U.K. may be able to shed some light on this age old question. Evidence taken from the fossil record has long shown that the ancestor of snakes possessed limbs like modern lizards. Limb loss in snakes is thought to have been caused by changes in expression of several “toolkit” genes during development.  Alterations in these gene networks prevent formation of the forelimb and hindlimbs during development in snake embryos. One of the families of genes affected are Hox genes. In almost all animals, the overall body plan is based on a pattern of repeated segmentation along the anterior/posterior axis. Just think of vertebrae in chordates or body segments in insects. Combinations of Hox genes expressed along the anterior/posterior axis determine the identity of the different segments. For example, Hox gene expression boundaries determine the placement of the wing segments, antenna segments, and abdominal segments in fruit flies. Consequently, changing the expression patterns of Hox genes can cause aberrations, such as a fruit fly growing legs out of it’s head.

The researchers in this study looked at the expression patterns of the Hox genes HoxC6, HoxC8, and HoxB5 in python embryos. In other tetrapods and fish, the anterior expression boundaries of these proteins specify placement of forelimb developmental fields. In snakes, the anterior expression boundaries of HoxC8 and HoxB5 are shifted more anterior as compared to other tetrapods, which seems to completely eliminate the python’s ability to generate forelimbs. In contrast, these python embryos had normal posterior expression boundaries of these genes as compared to other tetrapods. This is consistent with existing morphological information. In ancient species of snakes such as pythons and boa constrictors, elimination of the hindlimb developmental field is not as complete as the forelimb field. Pythons actually do posses small outgrowths near the end of their bodies that are vestigial structures left over from when they possessed fully formed hindlimbs (Fig.1). This indicates that separate mechanisms are responsible for loss of the hindlimbs as compared to the forelimbs, and maybe even that the developmental programs necessary to specify hindlimb outgrowth in pythons are still present. The researchers set out to test this. Outgrowth of limbs depends on a layer of embryonic tissue known as the apical ectodermal ridge (AER) and a set of genes including fgf2 and msx that are normally expressed in it. Upon examination, the researchers found no evidence of an AER or the associated genes in python embryos. This could explain the lack of hindlimb development in pythons. In other organisms, fibroblast growth factors (fgfs) are responsible for formation of an AER. The researchers tested whether they could induce AER formation in python embryos by grafting fgf2 laced beads into the embryos. One day after grafting, this resulted in a 31% increase in hindlimb bud outgrowth in these python embryos, indicating that fgf signaling can induce hindlimb formation in pythons.

                                         Fig.1: spurs on python tail, vestigial structures left over from hindlimbs

The findings of these researchers illustrate a common trend that is emerging in the new field of evolutionary developmental biology, or evo-devo, which seeks to determine the developmental genetic mechanisms that underlie evolutionary change. Large changes in organismic morphology can occur through changes in a small number of developmentally important genes. This principle is shown by the ability to partially rescue growth of hindlimbs in python embryos by introduction of a single gene. Another important finding in this field is that the developmental programs necessary to generate novel morphological structures are often present in organisms that don’t even yet have those structures. For example, the pythons in this study maintained all of the genetic machinery necessary to develop hindlimbs. All they needed was the right genetic switch at the right place and time in developmental to kick off the whole program. And just like that, you get a snake with legs.

Posted by Sean McDougall (2)