This was revealed by Giemsa C-banding of the F : hybrids and progeny of certain backcrosses between two closely related species, Ae. The expression of the intercalary C-band on the sex chromosome in a particular species varies as a function of the genetic background in which it is placed. The constitutive and facultative heterochromatin replicate at different times in the cell cycle ( Marchi and Rai, 1986). aegypti lacks even the centromeric heterochromatin ( FigureВ 1.2). The male-determining chromosome (M) in Ae. The fact that these species belong to three different subgenera suggests that heterochromatinization of particular segments is species-specific. triseriatus lack the intercalary band ( FigureВ 1.3). annandalei and in telomeric position on both the male- and female-determining chromosomes in Ae. The intercalary band is located proximal to the centromere in Ae. The former is present around the centromere region of all three chromosome pairs and the latter in an interstitial position on one of the arms of the female-determining (m) chromosome in most Aedes species ( FigureВ 1.2). Motara and Rai (1977, 1978) reported two distinct types, constitutive and facultative heterochromatin, in Aedes mosquitoes. The organization of heterochromatin is markedly different in the two homologues of the sex chromosome pair in most Aedes species as well as between anopheline and culicine mosquitoes. atroparvus ( Fraccaro et al., 1976) possess telomeric C-bands also. bahamensis ( Rao and Rai, 1987a) and the long arms of the sex chromosomes in An. In addition to centrometric bands, the autosomes in species such as Ae. Using different banding techniques, three types of heterochromatin were identified on the basis of staining characteristics in the pericentromeric regions in the Culicini species Culiseta longiareolata ( Mezzanotte et al., 1979 Marchi and Mezzanotte, 1988). All species showed the presence of heterochromatin around the centromeres of the autosomes, although there are often large inter-and intraspecific differences in amounts of the same. These studies established that the distribution of heterochromatin is markedly different in anopheline and culicine mosquitoes, particularly in the heteromorphic sex chromosomes. C-banding patterns were also studied in representative species of Tipulidae, Dixidae, and Chaoboridae in order to examine how chromosomes have evolved in these families ( Rao and Rai, 1987a). Studies have been completed in 36 species belonging to seven-genera of Culicinae ( Aedes, Mansonia, Culiseta, Armigeres, Sdbethes, Wyeomyia, and Toxorhynchites) including 28 Aedes species ( Motara and Rai, 1977, 1978 Rao and Rai, 1987a), three species of Culex ( Motara 1982), and several Anopheles species ( Gatti et al., 1977 Baimai, 1988 Baimai et al., 1993a, b, 1995, 1996 Marchi and Mezzanotte, 1990). The application of Giemsa C-banding and other banding procedures to somatic and meiotic chromosomes has provided important insights into linear differentiation and evolution of chromosomes in Culicidae. Black IV, in Advances in Genetics, 1999 VII HETEROCHROMATIN: LOCALIZATION, VARIATION, AND EXPRESSION

Stained the band