Abstract
The D. simulans SimES strain is characterized by the presence of individuals of S phenotype (malformation and/or suppression of thoracic bristles, mainly the dorsocentral and scutellar ones), with a variable frequency ranging approximately from 50 p. 100 to 60 p. 100. Since this phenotype is transmitted maternally, the presence of a microorganism responsible for that phenotype can be suspected. The presence of 2 pathogenic microorganisms in the SimES strain, a microsporidia (Nosema kingi) and a Picornavirus (Dro.sophila C virus) was detected. In flies free of both microorganisms, the S phenotype maintains the same properties, and therefore, it can be concluded that they play no part in the S phenotype determination. However, several experimental results, i.e. effects of temperature, infectious action of SimES extracts, and no effect of antibiotics, have led us to formulate the hypothesis that S phenotype is due to action of slightly pathogenic virus transovarially transmitted.