X and Y chromosome inheritance and mixtures of rDNA intergenic spacer regions in Anopheles gambiae

We observed Anopheles gambiae sensu stricto stocks that contained both Mopti (M) and Savanna (S) rDNA intergenic spacers (IGS). ASEMBO1 male IGS sequences consistently had a mixture. A diagnostic M and S Hha I restriction enzyme site in these fragments was concordant with two SNPs associated with M and S. Standard M and S stocks demonstrated X-chromosome-only inheritance of the rDNA form, but the ASEMBO1 males showed X and Y chromosome linkage of mixed rDNA. The metaphase Y chromosomes of ASEMBO1 contained a significantly larger amount of DNA relative to the X than a standard S stock. Analysis of wild A. gambiae males from the putative location of origin of the ASEMBO1 stock did not detect the same pattern of polymorphism observed in the laboratory stock but did detect heterogeneous arrays including some missing a diagnostic Hha I restriction site. These results demonstrate that M and S IGS types can occur within the rDNA arrays of a chromatid in laboratory A. gambiae stocks, and some A. gambiae s.s. have rDNA on the Y chromosome.