Studies of linkage in populations. XIII. A unique cause of linkage disequilibrium in natural populations of Drosophila robusta.

Natural populations of Drosophila robusta are polymorphic for chromosomal gene arrangements in most of its range, the deciduous forests of North America east of the Rocky Mountains. Many of the gene arrangements are the result of paracentric inversions on both arms of the metacentric second chromosome. They are frequently in linkage disequilibrium, determined in many areas largely, or entirely, by differing frequencies of cis and trans forms of the double heterokaryotypes and their component linkage combinations. Presence and degree of linkage disequilibrium in these populations varies with locality and gender, with males from southern localities exhibiting the largest deviation from equilibrium. Analysis of the extensive karyotype data encompassing the entire species range collected over the past 50 years shows that natural selection is primarily responsible for maintaining these complex polymorphisms.