Austen Ganley
Ph.D. 2000, Massey University, New Zealand
"Experimental studies of concerted evolution in Saccharomyces cerevisiae"
May 2000 - April 2002
Currently: post-doc at National Instititue for Basic Biology, Okazaki, Japan
Experimental studies of concerted evolution in Saccharomyces cerevisiae
Concerted evolution is the pattern seen in multigene families where all the repeats in a repeat array are kept very similar in sequence, but the underlying mechanism that brings this about is not well understood. Two aspects of this process were investigated. (1) The rate at concerted evolution occurs. To do this, yeast strains containing a single "tagged" ribosomal RNA gene unit were constructed. This "mutant unit" is expected to be eliminate in some cells, as well as increase in copy number in others. Both these outcomes were observed. Further, concerted evolution seems to occur rapidly, maybe at a rate as high as an event every three cell divisions. (2) The equilibrium between homogenization and mutation. Homogenisation is the process underlying concerted evolution, and counters the effect of mutation. Although calculating the level of polymorphism in an gene array has previously been difficult, whole genome sequences allow one to examine all the ribosomal repeats along with the rest of the genome. Data from three fungal genomes (Ashbya gossypii, Cryptococcus neoformans and Phanerochaete crysosporium) were used to estimate polymorphism in the ribosomal RNA gene array in each species. The level of polymorphism is extremely low - only 5 - 40 polymorphics sites in repeat arrays containing 50-100 repeats of size 8-9 kb (therefore over 500-800 kb of DNA), implying a rapid rate of homogenisation relative to mutation.