How the squirrel got its stripes
Understanding what gives rise to the patterns on the coats of rodents arise has moved a step closer, says S.Ananthanarayanan.
Indian epics and folklore say squirrels participated by carrying pebbles when Lord Rama built a bridge across the sea to Lanka. In appreciation, Lord Rama stroked them on the back, and hence the stripes that they carry since then. Evolutionary biologists,
however, have sought an answer more in terms of the cellular, developmental and molecular processes that lead to the periodic appearance of hair pigmentation, even if it were Lord Rama who set those processes off, to start with.
Ricardo Mallarino, Corneliu Henegar, Mercedes Mirasierra, Marie Manceau, Carsten Schradin, Mario Vallejo, Slobodan Beronja, Gregory S. Barsh and Hopi E. Hoekstra, from Institutes in Harvard, Massachusetts, Stanford, Seattle, Madrid, Paris, Strasbourg and
Johannesburg report in the journal, Nature, that they have identified the agent that causes differences in the action of pigment-creating cells, and hence the variations in the colouring of animal hair. This is a finding that could help understand how individual,
visible forms of organisms evolve, the study says.
The genes that give rise to cells that produce melanin, a complex, chained molecule that gives skin and hair its colour, and the mechanisms that regulate the balance between dark and light pigmentation, are part of existing knowledge through research based
on animals, like laboratory mice. How these processes contribute to the array of pigment patterns seen in wild animals, however, is not understood, the study says. The researchers hence took up the naturally occurring coat pattern of the African striped mouse,
to gain insight into the processes that bring about stripes, “a striking and characteristic pattern that has evolved independently” in a large number of animals, the study says.
While the melanocytes, or the cells that produce melanin, are present in many parts of the skin tissue, it is the presence of agents that allow the transport of specific parts of the genetic code to pigment creating centres that get the cells to become
active. The study discovers that it is a protein called Alx3 that plays the role of regulator, by suppressing the action of Mitf, another protein that is implicated in pigment production by melanocytes. The researches examined traces of these regulators,
which are called transcription agents because they allow parts of the DNA to be copied as templates for the assembly of proteins, in the tissue of different parts of the animal body and where fur of different shades arose, and also at different times, starting
from the early embryonic stages of growth, till adulthood.
A remarkable thing about the striped pattern that arises in animals is that similar patterns are seen in species which seem to have evolved along different genealogical lines. The last common ancestor of the mouse and the chipmunk, for instance, the paper
says, dates back to 70 million years, or the time of the dinosaurs. While the African striped mouse has one light coloured stripe, sandwiched between two darker lines, on either side of its spine, the chipmunk and the squirrel display similar but distinct
patterns. Analysis of the proteins present in skin biopsies revealed that the colour distribution arose in the animals through similar pathways, of expression of the Alx3 gene and the Alx3 transcription agent, which regulates the rate of pigment production,
and also the ASIP and Edn3 genes, which bring about changes in the pigments produced.
The discovery by the group writing in Nature is an advance in understanding the specific mechanics of the development of colours in a large class of animals. The computer scientists-mathematician Alan Turing had developed a theory of how color or form-yielding
agents, called morphogens, could spread or diffuse at different speeds, and interfere, like light waves, to clump together or to negate each other, and create bands or spots of different colours. Although without speaking of what the morphogens may be, Turing’s
theory of ‘reaction and diffusion’ was able to explain the black and white patches on a breed of cows. The theory was carried forward by others and the stripes that appear on the back of the tiger are now understood as a pattern of pigmentation brought about
by periodic waves of diffusion of chemicals in the tiger embryo.
There has also been some work to identify possible morphogens that lead to shapes, like ridges on the roof of the mouth of the common mouse, or the shape of some cacti. This approach is also used in modeling why some stripes are vertical while others maybe
horizontal, or even the reason for organs, like the fingers, to grow in particular directions. The current work, on the African striped mouse, however, is the first time that the very agent that results in colour distribution has been identified. The reason
why the variation in colours is in the form of stripes, or even the different numbers of stripes is yet to be understood, but the action of Alx3 has now been identified and tracked down to the early embryonic stage when the spinal chord is just forming.
An editorial in the journal, Nature says that the genes that affect the colour of the skin also affect other things. At the embryonic stage, the tissue from which the spinal chord is formed also spreads out and leads to the formation of the hair and skin,
the bones of the face, the nerves in the intestines, parts of the heart and the adrenal glands, crucial parts of the sense organs and many body structures that are unique to vertebrates, the editorial says. Unusual skin pigmentation is thus seen to signal
other ailments. Deficiency in Alx3 leads to the spinal chord or brain not forming correctly, a condition that can be moderated by doses of folic acid and deficiency of which in human mothers leads to spinal chord defects in human babies, the editorial says.
It is again mutations in the Alx3 gene that causes facial deformities and failure of the facial bones to knit properly. Understanding how the stripes come about on the backs of rodents is hence more than skin deep, the editorial says.
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