Genetic load and coadaptation of chromosomal inversions. II. O-chromosomesin Drosophila subobscura populations

We have analysed the inversion polymorphism and genetic load of O-chromosom es in three populations of D. subobscura from southeastern Europe. As expec ted for a central populations the inversion polymorphism was extensive. In a like fashion, the genetic load, in particular the frequency of lethals, w as heavy in all three populations. There were significant differences in th e frequency of moderately deleterious genes. These differences in viability can be attributed to balancing selection. A comparison of these two kinds of genetic polymorphism indicates that there are differences in mean viabil ity among different gene arrangements of O-chromosomes in the three populat ions. The differences observed are due to an unequal distribution of variou s viability classes among O-chromosome gene arrangements. We here show for the first time a specific distribution of lethal genes among these arrangem ents within the Palearctic distribution area of D. subobscura. The lethal a llelism test showed lethals are non-randomly associated with the O-st gene arrangement. The amount of genetic load is heavy in gene arrangements with a high frequency, in comparison with the ones with a low frequency. Lethal genes may be protected in combinations of low and moderate frequency gene a rrangements that harbor more lethal genes, as the O-st in the one populatio n. Some arrangements that are less protected against recombination have a h igher load than ones that are more protected against recombination. This ca n be taken as evidence for coadaptation.