Intercalary Heterochromatin in the Pupal Foot Pad Polytene Chromosomes of Flesh Fly Parasarcophaga ruficornis Fab. (Sarcophagidae: Diptera)

Abstract

Summary Polytene complement of Parasarcophaga ruficornis (Sarcophagidae: Diptera) has been analysed to provide data on intercalary heterochromatin (IH). The location of weak points and ectopic contacts has been attributed to the presence of IH in the polytene complement.The spontaneously occurring weak points appear to be distributed on all the 5 polytene chromosomes. The telomeric end of right arm of chromosome II often turns back to form an ectopic contact with the region 5B/C of the same chromosome, thus, resulting in the formation of a loop.Key words Intercalary heterochromatin, Polytene chromosomes, Ectopic contacts, Weak points.The term Intercalary Heterochromatin (IH) was introduced by Kaufmann (1939) in his studies on polytene chromosomes of Drosophila melanogaster. Since then numerous reports on the distribution of break and weak points on polytene chromosomes in Drosophila spp. have established that breaks occur much more frequently in IH than in other chromosome regions (see review Ananiev et al. 1978, Mukhina et al. 1981, Zhimulev et al. 1982, Bolshakov et al. 1985, Zhimulev 1998. Zhimulev et al. (1982) distinguished several types of weak points on polytene chromosomes of D. melanogaster viz. i) constrictions i.e. local attenuations of material between bands, ii) breaks, iii) semibreaks i.e., cracks along the bands forming V shape and iv) shifts i.e. the broken parts moved relative to each other. Moshkin et al. (2001) while examining the different aspects of IH in the region 89E1-4, the locus of the Bithorax complex, in salivary gland polytene chromosomes of Drosophila melanogaster described a similar variety of morphologies.Besides this, an infrequent non-homologus association between different chromosome arms in the salivary gland chromosomes of D. melanogaster has been reported by several workers (Bridges 1935, Kaufmann 1939, 1944, 1946, Kaufmann et al. 1947, 1948, and, referred to as "ectopic pairing" by Slizynski (1945) and Kaufmann and Iddles (1963). Many of the ectopically paired regions behave as 'weak points' and appear as constrictions (Prokofieva-Belgovaskaja 1941, Kaufmann and Iddles 1963, Zhimulev 1998. Thus, ectopic pairing is not at random and the sites that pair ectopically correlate well with the sites that are frequently the weak points. It is established that the identification of IH regions is well manifested by 2 criteria i) the presence of breaks or weak points and ii) ectopic pairing.In the present communication the occurrence of intercalary heterochromatin expressed as weak points and ectopic contacts in the pupal foot pad polytene complement of Parasarcophaga ruficornis has been analysed.