Evolution of hybrid dysgenesis determinants in Drosophila melanogaster.

Hybrid dysgenesis is manifested as a group of correlated aberrant genetic traits such as sterility, increased mutation rate, and male recombination. Previous work has shown that it appears when males of strains carrying either of two independent families of transposable elements called I and P factors are hybridized with females of susceptible strains called R and M, respectively. Here the results of an extensive survey for dysgenic potential in Drosophila melanogaster strains are reported. Striking temporal trends in the distribution of strains were observed with respect to the two transposable element systems; in particular, the frequency of R and M strains is positively correlated with laboratory age. In recent tests of strain samples, those collected from nature about 50 years ago were the earliest observed to possess I characteristics. The I type was increasingly frequent in samples from strains more recently originating in the wild. This type is apparently ubiquitous in present day natural populations. the P type was not found in strain samples collected before 1950, and collections made subsequently showed increasing frequencies of P-factor activity with decreasing laboratory age. Marked geographical patterns are documented in the contemporary worldwide distribution of variant strains within the P-M system. M strains are currently fairly common in natural populations from various parts of the world, except on the American continent where they are rare. The degree and distribution of quantitative variation within M and P strain categories is related to their time of origin in the wild. The implications of these results are discussed in relation to the hypothesis that hybrid dysgenesis determinants have evolved recently in natural populations and to an alternative hypothesis of laboratory evolution.