Texas A&M Professor Leads Study on Domestic Animals as Models for Evolutionary Processes
Posted April 07, 2017
Dr. Leif Andersson
COLLEGE STATION, TX—In a new study published in PLoS Genetics,
an international team of researchers report that two independent
mutations are required to explain the development of the sex-linked
barring pattern in chickens. Both mutations affect the function of
CDKN2A, a tumor suppressor gene associated with melanoma in
Leif Andersson, Uppsala University, Swedish University of
Agricultural Sciences and Texas A&M University, led the study
that illustrates how useful domestic animals are as models for
evolutionary processes in nature. Andersson argues that a similar
evolution of gene variants comprising multiple genetic changes
affecting the function of a single gene is the rule rather than the
exception in natural populations.
Research in pigmentation biology has made major advances the
last 20 years in identifying genes controlling variation in
pigmentation in mammals and birds; however, the most challenging
question is still how color patterns are genetically controlled.
Birds are outstanding as regards the diversity and complexity in
color patterning, according to Andersson.
The study published April 7 has revealed the genetic basis for the
striped feather characteristic of sex-linked barring. Sex-linked
barring refers to the alternating of pigmented (usually red or
black) and apigmented (white) stripes that occur on certain breeds
One example is the French breed Coucou de Rennes, the name of
which refers to the fact that its plumage color resembles the
barring patterns present in the common cuckoo (Cuculus
canorus). The sex-linked barring locus is on the Z chromosome.
In chickens, as well as in other birds, the male has chromosomes ZZ
while females have ZW.
Coucou de Rennes, a French breed with the characteristic
sex-linked barring phenotype. Photo: Hervé Ronné, Ecomusée du
pays de Rennes.
“Our data show that sex-linked barring is caused by two
independent mutations that act together. One is a regulatory
mutation that increases the expression of CDKN2A. The other changes
the protein sequence and makes the protein less functionally
active,” Andersson said. “We are sure that both mutations
contribute to the sex-linked barring pattern because we have also
studied chicken that only carry the regulatory mutation and they
show a very pale plumage with only weak dark stripes. Thus, this
represents an evolutionary process in which the regulatory mutation
occurred first followed by the mutation affecting the protein
structure. The combined effect of the two mutations causes an even
more appealing phenotype for the human eye.
Anderson also believes the most important reason for the
extensive color variation among the domestic animals is that we
appreciate its diversity, as long as the mutations underlying the
variation are not causing health issues for the animals.
CDKN2A is a well-studied tumor suppressor gene that takes part
in the regulation of cell division and cell survival. Mutations
that inactivate CDKN2A are the most common explanation for familiar
forms of melanomas in humans. However, the great majority of
melanoma cases are not associated with a strong genetic risk
“The gene variant underlying sex-linked barring has an opposite
effect compared with the mutations causing melanoma in humans.
Sex-linked barring is associated with a gene variant that makes
CDKN2A more active, leading to a cyclic deficit of pigment cells
and causing the white stripes during the development of an
individual feather. It appears that pigment cells are particularly
susceptible to changes in the function of CDKN2A as inactivating
mutations in humans are associated with melanoma but rarely other
cancer forms and activating mutations cause sex-linked barring in
chickens but no other side effects are known,” said Doreen
Schwochow Thalmann, PhD student and first author of the paper.
“It is fascinating that a large proportion of chickens used for
egg and meat production around the world carry these mutations in a
tumor suppressor gene. An example of such a breed is White Leghorn,
which is one of the most prominent breeds used for egg production,
but sex-linked barring is not apparent in these breeds because they
also carry the dominant white color that eliminates all pigment
production and masks the effect of sex-linked barring,” Andersson
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