University of Illinois animal sciences professor Harris Lewin, and fellow researchers recently released their study which delves into the role of “breaking points” in evolutionary diversity. These “breaking points” are the points in a chromosome that literally break in processes such as ‘crossing over’ where genes are swapped between chromosomes in the prophase stage of meiosis. Meiosis is the process where sperm and eggs (gametes) are formed; having the process of crossing over occur allows for greater genetic variability in offspring.
Mr. Lewin said the study’s results demonstrated that ‘chromosome breakage in evolution is non-random and that the breakpoint regions and the more stable regions of chromosomes are evolving in distinctly different ways’. Though crossing over has been know to be a driver of genetic variation for many years, previously scientists have been unable to explain how these breakages aided in evolutionary changes in the genome. This was due to the fact that sometime crossovers can disrupt a gene families or genes that are regulated together on the chromosome, in some cases the wrong chromosomes recombine, segments can become inverted or complete breakages occur. These changes can cause subtle or dramatic changes to the functioning of these genes and, in some cases, to the organism itself in the form of a new species or a genetic disease.
Whilst previously thought to occur randomly in the chromosome, in 2003 a team from the University of California at San Diego and the Llewin laboratory found that these breaks were more likely to occur in some regions than others. The Llewin laboratory went on to discover that these regions are more highly dense in genes than other part of the chromosome, as opposed to the genetic ‘deserts’ previously thought to facilitate breaks, and that they also contain higher numbers of segmental duplications, a mutation that increases the number of copies of genes and the sequences that border them.
This new study involved the comparison of the chromosome of nine mammals and a chicken; the results showed that the breakpoint regions contained many more copy number variant, insertions and deletions in their sequences compared to other regions of the chromosome. According to Lewin, these characteristics make these regions more susceptible to breakages. These regions were also found to only contain certain types of genes as well. While other more stable areas of the chromosome held the genes vital for an organism to survive for example those involved in developement, the breakpoint regions contain genes such as those for immunity and muscle contraction which may be liable to natural selection.
The study and it's researchers expressed the importance of these breakpoint areas in the evolution of organisms and in our undestanding of evolution through natural selection.
Based on the article:
http://www.geneticarchaeology.com/research/Chromosome_breakpoints_contribute_to_genetic_variation.asp
For more information:
http://www.sciencemag.org/cgi/content/abstract/309/5734/613
By Jessica Whyte
