Abstract
The negative correlation between transmission ratio (k) and male recombination frequency (θ), which was found in the presence of the “major” male recombination element (Mr) in the pr+ region of the T-007 chromosome, was examined for hypothesized “minor” male recombination elements associated with T-007. The data indicate that many minor male recombination elements are distributed along the arms of the T-007 chromosome. They generally cause a reduction in k and an increase in θ, and their expression seems to be constant among males within each line.
More interestingly, the presence of another type of male recombination element is also suggested by the present observations. It appears that a “Mr-related mutational change” (analogous to paramutation) occurs in the pr region (therefore, the Mr+ region) of previously non-male recombination second chromosomes. This change, designated Mr', enables those chromosomes which carry it to induce some male recombination and a distorted transmission ratio. In this case, the expression of Mr' seems to vary among males within each line.
It is suggested that the relationship between k and θ among males within the same line is related to the position of the elements in the chromosome. Specifically, minor elements, which locate in largely euchromatic regions of the chromosome, appear to be “stable” in their expression, while the Mr and Mr' elements, which locate in or adjacent to the centric heterochromatin, appear to be “unstable”. The expression of these elements may be variable among males within each line such that the activities of Mr or Mr' are stronger (lower k, higher θ) for some males, while they are weaker (higher k, lower θ) for other males.
Finally, it is estimated that the rate of the “Mr-related mutational change”, Mr+→Mr', is roughly 0.50.
