MEIOTIC GENE CONVERSION TRACT LENGTH DISTRIBUTION WITHIN THE ROSY LOCUS OF DROSOPHILA-MELANOGASTER

Employing extensive co-conversion data for selected and unselected sit es of known molecular location in the rosy locus of Drosophila melanog aster, we determine the parameters of meiotic gene conversion tract le ngth distribution. The tract length distribution for gene conversion e vents can be approximated by the equation P(L greater than or equal to n)=phi(n) where P is the probability that tract length (L) is greater than or equal to a specified number of nucleotides (n). From the co-c onversion data, a maximum likelihood estimate with standard error for phi is 0.99717 +/- 0.00026, corresponding to a mean conversion tract l ength of 352 base pairs. (Thus, gene conversion tract lengths are suff iciently small to allow for extensive shuffling of DNA sequence polymo rphisms within a gene.) For selected site conversions there is a bias towards recovery of longer tracts. The distribution of conversion trac t lengths associated with selected sites can be approximated by the eq uation P(L greater than or equal to n\selected)=phi(n)(1 - n + n/phi), where P is now the probability that a selected site tract length (L) is greater than or equal to a specified number of nucleotides (n). For the optimal value of phi determined from the co-conversion analysis, the mean conversion tract length for selected sites is 706 base pairs. We discuss, in the light of this and other studies, the relationship between meiotic gene conversion and P element excision induced gap rep air and determine that they are distinct processes defined by differen t parameters and, possibly, mechanisms.