CO₂ Sensitivity in Drosophila (sigma factor)

Most Drosophila flies, can be subjected to a contact with pure CO₂ for long hours, without injury. L'Heritier and Teisser (1958), however, discovered a true breeding strain of Drosophila which was sensitive to CO₂. The sensitive flies, when exposed briefly to CO₂ for a short period, become unconscious in a characteristic way, with their legs becoming paralysed. When reciprocal crosses were made between CO₂ sensitive and normal strains, it could be shown that the trait was inherited only from female parent. In other words, while sensitive mothers always give sensitive progeny, sensitive fathers only rarely give sensitive progeny if the mother is normal. In rare transmission through male also, sensitivity is quickly lost after first generation. It has also been shown that if an extract obtained by crushing sensitive flies, is injected into the body of wild (normal) flies, sensitivity can be induced. It was also shown that this sensitivity can be attributed to a virus like particle called sigma found in cytoplasm of the cells of a sensitive fly.

Sigma factor is transmitted through egg cytoplasm and its reproduction depends on initial supply and on suitable temperature of 20°C, because it is heat labile at high temperature. It does not need a specific gene and may be found associated with different genotypes. A sensitive fly retains its sensitive trait, even when all its chromosomes are replaced by those of normal fly, suggesting that sigma factor or particle is located in cytoplasm and has properties of non-chromosomal genes or plasma genes. Some physical characteristics of sigma are also known. It seems to be a particle of 0.07 micron in diameter, and contains DNA responsible for its hereditary nature.