9629462 QUINN Evidence that DNA or protein sequences change at a steady rate through time was presented in the early 1960s, and now comprises the rationale for using amounts of sequence divergence to estimate how long ago species pairs diverged from each other. This has become known as the "molecular clock". Just how steady that rate is has been a matter of considerable debate. In particular, there has been recent interest in what the "motor" is that determines the rate at which the clock "ticks". One hypothesis is that DNA mutation occurs primarily during replication and that the rate is determined by number of rounds of replication encountered by germ line cell lineages per unit of time. This may explain why, for instance, mice evolve more quickly than humans at the sequence level. If this hypothesis is correct, then DNA within chromosomes that are passed on primarily through male or primarily through female vertebrate lineages should evolve at different rates because females use fewer numbers of rounds of replication in the production of gametes than do males. While this trend has been observed in mammals, other interpretations of the meaning of this result are possible. In this study, Thomas W. Quinn will compare rates of DNA sequence change within the W, Z and autosomal chromosomes in birds. Molecular techniques will be used to identify and sequence comparable DNA sequences found on the W and at least one other chromosome. The rate of change in these sequences will be measured and compared with the predicted result, that sequences on the W chromosome will show fewer changes than comparable sequences on other chromosomes. This is an important addition to mammalian studies because there is a female-specific chromosome in birds rather than a male-specific chromosome as in mammals. The bird study system presents an opportunity to test the hypothesis of the dependence of DNA mutation rate on frequency of DNA replication, in a case where rate differences between various chromosomes are predicted to be different in direction and magnitude from those found on mammalian chromosomes.
