Lindsley & Grell, 1972, p. 26
location: 1-66.0 (Bridges).
discoverer: Sturtevant, 20b.
synonym: bb5. What is now referred to as bb was derived from fifth finding of bb. First allele found was lost and is here omitted from consideration.
phenotype: Bristles of homozygous females decreased in both length and thickness. Tergites etched at sides. Considerable veriability of bristle character, with some overlapping; abdominal character extremely erratic. X/0 male has phenotype similar to but more extreme than homozygous female. X/Y male is wild type, owing to presence of normal allele of bb in YS; X/X/Y female similarly normal in phenotype. bb/bb1 is extreme bb in phenotype. Viability variable. Ritossa, Atwood, and Spiegelman (1966, Genetics 54: 819-34) showed that bb contains about half as much ribosomal RNA-complementary DNA as bb+. They conclude that the bb locus is the site of ribosomal RNA synthesis. On the basis of calculations that suggest that there is enough DNA in bb+ to specify approximately 130 molecules each of 28S and 18S ribosomal RNA, these authors view the bb locus as highly redundant and perhaps composed of a very large series of tandem duplications. They interpret bb mutations as partial deletions of the locus. They postulate that in bb flies the rate of protein synthesis is limited by the amount of ribosomal RNA and the bb phenotype results in part because normal bristle production represents maximum protein synthesis on the part of the trichogen cells during a particular interval in development. RK2.
cytology: Judged to be in 20C2 (or 20C1) by Cooper (1959, Chromosoma 10: 535-88) based on extensive consideration of published cytology of base of X chromosome. The bb locus lies in proximal heterochromatin of X, probably proximal to and very close to nucleolus organizer in heterochromatic region hB (Cooper, 1959). Ritossa, Atwood, and Spiegelman (1966), on the other hand, postulate that the nucleolus organizer is the cytological counterpart of the bb locus. Presence of a normal allele of bb on Y chromosome postulated by Burlingame and demonstrated by Stern [1927, Z. Induktive Abstammungs- Vererbungslehre 44: 187-231 (fig.)]. This bb+ allele almost certainly in YS (see Cooper, 1959).
other information: bb stocks show marked tendency to accumulate modifiers that suppress the phenotype. Outcrossing generally brings about return of bb phenotype. Ritossa, Atwood, and Spiegelman (1966), however, doubt that this is the case and postulate that the level of tandem redundancy of the locus is subject to frequent stepwise increases or decreases by unequal crossover types of events. Many laboratory stocks can be shown by crossing to bb1 to carry bb alleles of unknown origin.
color figure: P. A. Otto (original)
p & b figure: an extreme bobbed. Edith M. Wallace, unpublished