Ectopic pairing responsible for a constantly found «loop» configuration in polytene chromosomes of Drosophila

SUMMARY

In homosequential members of the Drosophila nasuta subgroup, division 53 at the proximal end of polytene chromosome arm 2R always displays a «loop» configuration of 55 chromosome bands. Homologous configurations are constantly found in the high polytenized salivary gland chromosomes as well as in the low polytenized gut chromosomes. Light and electron microscopic studies of Surface Spread Polytene (SSP) chromosomes in Drosophila nasuta albomicans revealed that the loop configuration is formed by multiple ectopic pairing among bands of subdivisions 53A, 53E, 54A and 55A. The constant presence at homologous loci in different tissues of D. n. albomicans and its conservation in the members of the D. nasuta subgroup distinguish this configuration from other occasionally found ectopic pairing configurations in Drosophila. By this, division 53 offers an unique material for cytogenetic and molecular genetic studies of this special phenomenon of somatic pairing.