Indirect evidence of alteration in the expression of the rDNA genes in interspecific hybrids betweenDrosophila melanogaster andDrosophila simulans

Crosses betweenDrosophila melanogaster females andD. simulans males produce viable hybrid females, while males are lethal. These males are rescued if they carry theD. simulans Lhr gene. This paper reports that females of the wild-typeD. melanogaster population Staket do not produce viable hybrid males when crossed withD. simulans Lhr males, a phenomenon which we designate as the Staket phenotype. The agent responsible for this phenomenon was found to be the StaketX chromosome (X^mel,Stk). Analysis of the Staket phenotype showed that it is suppressed by extra copies ofD. melanogaster rDNA genes and that theX^mel,Stk chromosome manifests a weak bobbed phenotype inD. melanogaster X^mel,Stk/0 males. The numbers of functional rDNA genes inX^mel,Stk andX^mel,y w (control) chromosomes were found not to differ significantly. Thus a reduction in rDNA gene number cannot account for the weak bobbedX^mel,Stk phenotype let alone the Staket phenotype. The rRNA precursor molecules transcribed from theX^mel,Stk rDNA genes seem to be correctly processed in both intraspecific (melanogaster) and interspecific (melanogaster-simulans) conditions. It is therefore suggested that theX^mel,Stk rDNA genes are inefficiently transcribed in themelanogaster-simulans hybrids.