| The Major Repeat Sequence (MRS) is found at least once on all but one chromosome in Candida albicans, but as yet it has no known relation to the phenotype. The MRS affects karyotypic variation by serving as a common location for chromosome translocation and by expanding and contracting internal repeats, thereby changing chromosome length. Thus, MRSs on different chromosomes and those on chromosome homologues can differ in size. I proposed that the MRS's unique repeat structure and more specifically, the size of the MRS could also affect karyotypic variation by altering the frequency of mitotic nondisjunction. Strains with chromosome 5 homologues that differ in the size of the naturally occurring MRSs show a preferential loss of the homologue with the larger MRS on sorbose, indicating that a larger MRS leads to a higher risk of mitotic nondisjunction for that homologue. While deletion of an MRS has no deleterious effect on the deletion chromosome under normal growth conditions and leads to no obvious phenotype, strains that have the MRS deleted from one chromosome 5 homologue lose preferentially the homologue with the MRS remaining. This effect on chromosome segregation is the first demonstration of a phenotype associated with the MRS. I also proposed that the MRS may influence nearby mitotic recombination events between two chromosome homologues due to its proposed role in chromosome translocation. My examination of mitotic recombination frequencies between two chromosome 5 homologues shows that the MRS has no enhancing effect on mitotic recombination frequency under normal growth conditions. This analysis of mitotic recombination frequencies across half of chromosome 5 is the first large scale analysis of mitotic recombination done in C. albicans and indicates that mitotic recombination frequencies are similar to those found in other eukaryotic organisms. |
