Biologists can heave a sigh of relief as the century-old mystery relating to birds' three different digits is finally resolved, say scientists at Yale University.
While paleontological evidence suggests birds' three digits are developed from digit position 1, 2 and 3 (thumb, index and middle finger) similar to that of the theropod dinosaurs that roamed Earth two hundred million years ago, the embryological evidence suggests digits 2, 3 and 4 (index, middle and ring finger). All these conflicting evidences gave rise to many unanswered questions on the tri-part structure of the bird wing.
The recent study published online Sept. 4 in the journal Nature, however, claimed to solve the puzzle by applying genomic analysis. However, studies of embryo development stressed the fact that index-middle-ring scenario is more likely in birds' tri-part scaffolding.
According to Yale scientists, the evolutionary changes of the birds are orchestrated in the embryo. One mystery, however, has still remained unanswered: Which digits are they? A thumb with index and middle fingers, or the index, middle and ring fingers?
The study revealed the stem cells that produce first digit of the birds die off in the early stage of embryonic development and the cells that are supposed to produce the index digit, instead produce a thumb-like appendage.
In Vertebrates with five digits, the thumb comes from the precursor stem cells labeled pa. The pa precursor cells become extinct during development and do not produce another digit in adults. As birds have a digit that looks like thumb, that forced the scientists to think whether the precursor cells in pb can make a thumb-like digit.
The genomic analysis has revealed the answer the scientists were looking for so many years. It is a yes -- even though the first bird digit develops where the index finger on a five-finger vertebrae should be. Thus, the thumb in a bird wing is indeed a thumb but that is developed from the pb precursor stem cells which is supposed to form an index finger.
A group of Yale scientists led by Dr Gunter Wagner used a technique called gene expression profiling on chickens to solve the digital mystery.
We used a new technology called transcriptome sequencing. It has been around for a few years and we happened to be first to use it for this question, said Wagner.
With this study, another mystery on homology between the other two digits buried in the bird wing and those found in the foot was found, the scientists believe.