CYTOLOGICAL MARKERS ## 1: Chromosome 1 See X in this subsection. # 2: Chromosome 2 In mitotic figures, chromosome 2 is less than twice the length of the X and sightly smaller than chromosome 3. It is a V-shaped element with two centrally-located heterochromatic segments presumably separated by the centric constriction; the heterochromatic segments are late replicating, according to thymidine incorporation studies (Barigozzi, Dolfini, Fraccaro, Raimondi, and Tiepolo, 1966, Exptl. Cell Res. 43: 231-34). In early prophase, there is often a long achromatic gap separating the euchromatic portion of one arm from the heterochromatin. Kaufmann (1934, J. Morphol. 46: 125-55) reported the gap to be in 2L. Hinton (1942, Genetics 27: 119-27) stated that both the constriction and the centromere are located in the region between the chromosome 2 breakpoints of T(1;2)N[264-59] = T(1;2)3C8-9;40F and T(1;2)N[264-23] = T(1;2)3C8-9;41A, a segment containing one or two bands but comprising about 15% of the metaphase length of 2. # 2L: Left arm of chromosome 2 See 2 in this subsection. # 2R: Right arm of chromosome 2 See 2 in this subsection. # 3: Chromosome 3 A V-shaped element in mitotic figures that is slightly larger than chromosome 2 but less than twice the length of the X chromosome. In prophase, there are two medial heterochromatic segments separated by a constriction that presumably marks the position of the centromere; these segments comprise about the proximal one-fifth of each arm at metaphase, and according to tritiated thymidine incorporation studies (Barigozzi, Dolfini, Fraccaro, Raimondi, and Tiepolo, 1966, Exptl. Cell Res. 43: 231-34), they are late replicating. # 3L: Left arm of chromosome 3 See 3 in this subsection. # 3R: Right arm of chromosome 3 See 3 in this subsection. # 4: Chromosome 4 In mitotic configurations, chromosome 4 is a dot-like element that is separated into two segments of grossly unequal size by a sometimes-visible centric constriction. Claimed to lack heterochromatic material but can be involved in rearrangements that produce variegated position effect [e.g., T(1;4)w[m5] = T(1;4)3C3-4;101F1-2] and shows incorporation of tritiated thymidine in cells in which only heterochromatic regions of the other chromosomes are labeled (Barigozzi, Dolfini, Fraccaro, Raimondi, and Tiepolo, 1966, Exptl. Cell Res. 43: 231-34). In salivary gland chromosomes, the longer right arm is associated with the chromocenter. The shorter left arm is occasionally discernible within the chromocenter; it has been demonstrated genetically by translocations between it and the X chromosome [e.g., T(1;4)w[m11] (Panshin and Khvostova, 1938, Biol. Zh. (Moscow) 7: 359-80) and T(1;4)w[mA] (Griffen and Stone, 1940, Texas Univ. Publ. 4032: 201-7)]. # 4L: Left arm of chromosome 4 See 4 in this subsection. # 4L: Right arm of chromosome 4 See 4 in this subsection. # 4-sim: Chromosome 4 from Drosophila simulans Chromosome 4 of D. simulans was introduced into an otherwise D. melanogaster genome by Muller and Pontecorvo (1940, Nature 146: 199-200). Phenotypic effects of 4-sim were described by Muller and Pontecorvo (1942, Genetics 27: 157) and Pontecorvo (1943, Proc. Roy. Soc. Edinburg B 61: 385-97; 1943, J. Genet. 45: 51-66). 4 sim/4 apparently normal in phenotype; 4-sim/ci has occasional wing vein interruption; 4-sim/ci[W] has more extreme ci phenotype than 4/ci[W] (Uphoff, 1949, Genetics 34: 351-27); no dominance of spa[Cat] in 4-sim/spa[Cat]. Homozygous 4-sim/4-sim has fair viability with slight morphological peculiarities (e.g., body flattened, trident heavy, and eyes reduced). Male genitalia said to be a little like those of D. simulans. Homozygous female fertile and male sterile. Testes well developed; meiosis occurs, but no motile sperm are produced. 4-sim/M(4) is Minute and male sterile; 4-sim/+ is male sterile but is not Minute. Muller and Pontecorvo (1942) suggest a gene necessary for male fertility of D. melanogaster is deleted from Df(4)M and absent from 4-sim. Comparisons of melanogaster and simulans salivary chromosomes were published by Kerkis (1936, Am. Naturalist 70: 81-86), Horton (1939, Genetics 24: 234-43), and Patau (1935, Naturwissenschaften 23: 537-43). According to Horton (1939), 4-sim differs from 4 by a relatively long inversion, which includes at least from 102B1-2 through 102E1-2 and probably through 102E3, 4, and 5. A darkly-staining terminal ring is at the tip of the chromosome. Slizynski (1941, Proc. Roy. Soc. Edinburg B 61: 95-106) identified the short left arm of 4-sim; it lacks a dark band present in the middle of the arm in D. melanogaster. In melanogaster-simulans hybrids, the fourth chromosomes do not pair in salivary gland cells; however, Slizynski found one nucleus of sim-4/4 in a melanogaster background in which the inverted segment was paired. In triplo-4 larvae with one 4-sim, the melanogaster 4's are paired and the 4-sim is unpaired. 4 tends to have the tip of 4R stuck into the chromocenter; 4-sim, however, always has its tip free of the chromocenter. # BkA: Block A Muller, Raffel, Gershenson, and Prokofyeva-Belgovskaya (1937, Genetics 22: 87-93) defined it as the distal and major portion of the segment of proximal heterochromatin of the X chromosome located between the right breakpoints of In(1)sc[L8] and In(1)sc[8]. This region was considered to be a unit or the product of a single genetic unit not subject to X-ray-induced breakage. Subsequent work by Kaufmann [1954, In Radiation Biology (A. Hollaender, ed.). McGraw-Hill, Inc., Vol. 1, pp. 627-711] and, in particular, that of Cooper and Krivshenko with Dp(1;f) derivatives suggests that breakability in Xh is distributed uniformly over its mitotic length; therefore, the block concept of heterochromatic structure no longer seems valid. # BkB: Block B Muller, Raffel, Gershenson, and Prokofyeva-Belgovskaya (1937, Genetics 22: 87-93) claimed it to be proximal to BkA in the region of Xh between the right breakpoints of In(1)sc[L8] and In(1)sc[8]. Gershenson [1940, Vid. Akad. Nauk SSSR., (Kiev) 3-116] defined it as the segment of the proximal heterochromatin of XL to the right of the right breakpoint of In(1)sc[8] (claimed to comprise 20-30% of Xh); however, according to Cooper (1959, Chromosoma 10: 535-88), a much smaller proportion of Xh lies between the right breakpoint of In(1)sc[8] and the centromere. Existence of BkB subject to same doubts as that of BkA. # hA The proximalmost of four segments discernible in the proximal heterochromatin of XL (see X in this subsection). # hB The second from the centromere of four segments discernible in the proximal heterochromatin of XL (see X in this subsection. # hC The third from the centromere of four segments discernible in the proximal heterochromatin of XL (see X in this subsection. # hD The distalmost of four segments discernible in the proximal heterochromatin of XL (see X in this subsection). # LA The proximalmost of three discernible segments of Y[L] (see Y in this subsection). # LB The middle of three discernible segments of Y[L] (see Y in this subsection). # LC The distalmost of three discernible segments of Y[L] (see Y in this subsection). # NO: Nucleolus Organizer The region in the proximal heterochromatin of the X and the short arm of the Y chromosome where the nucleolus is organized. The nucleolus is visible in interphase, and its relation to the NO may be seen in early prophase; it may also be seen associated with the chromocenter in salivary gland preparations. The nucleolus is formed at the constriction between hB and hC in the proximal heterochromatin of the X chromosome and at the constriction one-third the way from the centromere along the short arm of the Y. Homozygous deficiency for the nucleolus organizer is lethal. Ritossa and Spiegelman (1965, Proc. Natl. Acad. Sci. U.S. 53: 737-45) showed that the amount of DNA complementary to ribosomal RNA in a cell is directly related to the number of nucleolus-organizing regions present; they believe that the NO is the chromosomal site of ribosomal RNA synthesis. The ribosomal RNA-complementary DNA comprises 0.27% of the total DNA of a haploid genome; on the basis of the amount of DNA per cell and the molecular weight of ribosomal RNA in Drosophila, they have calculated that the amount of ribosomal RNA-complementary DNA in a haploid genome is sufficient to synthesize 130 molecules each of 18S and 28S ribosomal RNA simultaneously. Ritossa, Atwood, and Spiegelman (1966, Genetics 54: 819-34) postulated that the NO is the cytological counterpart of the bb locus on the basis of the demonstration that replacing a normal X with an X carrying bb reduces the amount of ribosomal-RNA-complementary DNA per cell. # Puffs Localized swellings in polytene chromosomes marking regions of metabolic activity. They are found in specific regions of the polytene complement, and each puff has characteristic times of appearance and disappearance during development which have been described by Becker (1959, Chromosoma 10: 654-78) for the puffs in 3L and the distal part of XL in the salivary glands. Studies with other diptera show that the puffing patterns in the polytene chromosomes of different tissues are not identical; Becker (1959) describes one puff in region 15BC of Drosophila melanogaster that is present in the anterior but not the posterior portion of the salivary gland. The puffing pattern responds to changes in cellular environment, e.g., changes in hormonal concentration (Becker, 1962, Chromosoma 13: 341-84). Becker (1959) and Schultz catalogued the regions of the salivary gland chromosomes of Drosophila melanogaster in which puffs may be seen (see following tabulation). Becker Schultz 1C 1C 2B5-6 2B 2B13-17 2EF 2F 3AB 3A 3C11-12 3E 3DE 4EF 7AB 8B 9B 9EF 10 10EF 11B 11BC 12E 12-13 13B 14EF 15BC 15C 16BC 16A 16DE 16F 21F 21DE 22B 22A 22C 22CD 23C 23BC 23DE 25A-C 25BC 25D 26B 27-28 29-30 33B 33B 33E 33E 34A 34A 34C 35AB 36F 36EF 37 37B 38B 38F 39B 40B 42A 42AB 43E 43-44 44E 46F 47A 47AB 47BC 48B 48A 49EF 50C 50CD 50D 51DE 52BC 53-54 55B 55E 56D 56DE 58BC 58A 58F 58DE 59F 60A 60B 60B 62E 62B-E 63E 63C 63F 64B 66B 66B 66DE 67B 67E 68C 68BC 68-69 70C 70C 71C-E 71DE 72CD 72CD 73C 74EF 74EF 75B 75AB 76A 76D 78D 78DE 79DE 82CD 82BC 82EF 82EF 83AB 83C 83EF 84BC 85B 85CD 85D 85EF 85EF 86-87 88D 88D 88EF 88EF 89B 89BC 90BC 90C 92A 93B 93D 94 95F 96E 97BC 97B 98B 98B 98F 99B 99EF # SA The proximal and smaller of two discernible segments of Y[S] (see Y in this subsection). # SB The distal and larger of two discernible segments of Y[S] (see Y in this subsection). # X: X chromosome Also known as chromosome 1. Present in one dose in male and two doses in female. In mitotic figures, the X is virtually a rod-shaped element with a quasiterminal centromere and a minute second arm designated as the right arm, XR. The left arm, XL, is divided into a distal euchromatic or isopycnotic region, Xe, in which the chromatids are usually separated and into a proximal heterochromatic of heteropycnotic region, Xh, in which the chromatids are not separated. The relative lengths of these subdivisions of XL vary according to mitotic stage: in early prophase, the isopycnotic region is longer; by metaphase, the two regions are of about equal length. Tritiated thymidine incorporation studies (Barigozzi, Dolfini, Fraccaro, Raimondi, and Tiepolo, 1966, Exptl. Cell Res. 43: 231-34) demonstrate that the heterochromatic region is late replicating. The proximal heterochromatin of XL is subdivided by constrictions into four segments of about equal length; these segments are designated from proximal to distal hA, hB, hC, and hD (Cooper, 1959, Chromosoma 10: 535-88). The constriction between hB and hC marks the position of the nucleolus organizer, NO; in early prophase, it may be a very long gap occupied by the nucleolus. The polytene X consists of just over 1000 bands of which 25-30 correspond to the region that is heteropycnotic in the mitotic X. The length of Xh is large compared with that of Xe when measured at metaphase or estimated by relative frequency of involvement in X-ray-induced rearrangements and is small when measured in salivary-gland chromosomes or by crossing over. Most sex-linked genes are in Xe, only the locus of bb and possibly that of su(f) being in Xh. # Xe: euchromatin of X chromosome See X in this subsection. # Xh: heterochromatin of X chromosome See X in this subsection. # XL: Left arm of X chromosome See X in this subsection. # XR: Right arm of X chromosome See X in this subsection. # Y: Y chromosome In miotitc figures, the Y chromosome appears as an entirely heterochromatic element; tritiated thymidine studies (Barigozzi, Dolfini, Fraccaro, Raimondi, and Tiepolo, 1966, Exptl. Cell Res. 43: 231-34) show it to be late replicating. The Y is a two-armed chromosome, Y[S], being about two-thirds the length of Y[L]. At metaphase, the Y is usually somewhat shorter than the X chromosome. The position of the centromere is indicated by a constriction. The short arm is subdivided by the constriction associated with the nucleolus organizer into a distal segment, SB, about two-thirds and a proximal segment, SA, one-third the length of Y[S]. In early prophase, the nucleolus may sometimes separate SA from SB by a considerable distance. Y[L] is divided into three segments of about equal size by a pair of constrictions; the more distal of which is the more constant landmark. From the centromere, the segments are designated LA, LB, and LC (Cooper, 1959, Chromosoma 10: 535-88). In salivary gland preparations, Prokofyeva-Belgovskaya (1937, Genetics 22: 94-103) observed a small collection of bands that she attributed to the Y. Nicoletti and Lindsley (1960, Genetics 45: 1705-22) found no evidence of bands attributable to the Y chromosome in a study of T(1;Y)'s. A series of complex structural elements in primary spermatocyte nuclei, whose formation depends on the presence of the Y, have been postulated to represent a highly modified state of the Y chromosome (Meyer, Hess, and Beerman, 1961, Chromsoma 12: 676-716). The Y chromosome carries a normal allele of bb and two complexes of factors essential to male fertility: KL is on Y[L] and composed of five cistrons, kl1-kl5; KS is on Y[S] and composed of two, ks1 and ks2. Brosseau (1960, Genetics 45: 257-74) showed the order of genetically-demonstrable factors on the Y to be kl5 kl4 kl3 kl2 kl1.bb[+] ks1 ks2. Addition of Y's to a normal chromosome complement suppresses variegated position effects, and removal of the Y from the male enhances variegation (1933, Gowen and Gay, Proc. Natl. Acad. Sci. U.S. 19: 122-26). Two or more Y's added to the normal complement cause variegation of otherwise self-colored eyes (Cooper, 1956, Genetics 41: 242-64). # Y[L]: Long arm of Y chromosome See Y in this subsection. # Y[S]: Short arm of Y chromosome See Y in this subsection. #