Drosophila eye pigments
information is available, belongs in the subgenus So- phophora. See Appendix C, 1926, Chetverikov; 1936, Sturtevant and Dobzhansky; 1952, Patterson and Stone; 1985, Carson; Drosoph- ila virilis, Hawaiian Drosophilidae. Drosophila databases See Appendix E. Drosophila eye pigments the ommatidia of the dull red compound eyes of Drosophila contain two classes of pigments, one brown (the ommochromes) and one bright red (the drosopterins).
Studies of the precursor compounds isolated from eye color mu- tants played an important role in the development of the one gene-one enzyme concept (see Appendix C, 1935, Beadle and Ephrussi). An example of an ommochrome is xanthommatin. Hydroxykynure- nine, a compound biosynthesized from tryptophan, serves as a precursor of xanthommatin. Flies lacking the plus allele of the cinnabar gene are unable to synthesize hydroxykynurenine, and therefore this is sometimes called the cn+ substance. Drosopterins are pteridine derivatives. Sepiapterin is a precursor of drosopterin that accumulates in sepia mutants. It also gives the Drosophilia testis its yellow color.
kynurenine. Drosophila immune peptides antimicrobial pep- tides produced by Drosophila in response to infec- tions. Drosomycin, a potent antifungal agent, is syn- thesized under the control of Toll diptericin, an antibacterial agent, is synthesized un- der the control of the immune deficiency gene See Appendix C, 1996, Lemaitre et al. Drosophila Information Service a yearly bulletin that lists all publications concerning Drosophila that year, the stock lists of major laboratories, the ad- dresses of all Drosophila workers, descriptions of new mutants and genetic techniques, research notes, and new teaching exercises. See Appendix D.
Drosophila melanogaster commonly called the ”fruit fly,” this species is a model organism for the study of specific genes in multicellular development and behavior. Its haploid genome contains about 176 million nucleotide pairs. Of these, about 110 million base pairs are unique sequences, present in the euchromatin (q.v.). The diagram on page 134
shows the relative lengths of the sex chromosomes (X and Y), the major autosomes (2 and 3), and the microchromosome (4) as they appear at the meta- phase stage of mitosis. The numbers give the amounts of DNA in megabases for the adjacent segments. About 13,000 genes are located in the euchromatin, and about 20% of these have been defined chemi- cally. The average gene contains four exons, and the average transcript is made up of 3,060 nucleotides.
Many Drosophila genes show base sequence similari- ties to human genes. For example, comparative studies of 290 human genes that increase suscepti- bility to cancer showed that 60% have Drosophila or- thologs (q.v.).
See Appendix A, Arthropoda, Insecta, Diptera; Appendix C, 1910, 1911, 1912, 1919, Mor- gan; 1921,1923,1925,1935, Bridges; 1916, 1918, 1927, Muller; 1933, Painter; 1935, Beadle and Ephrussi; 1966, Ritossa et al.; 1972, Pardue et al.; 1973, Gar- cia-Bellido et al.; 1974, Tissiers et al.; 1975, McKen- zie et al.; 1978, Lewis; 1980, Nu¨sslein-Volhard and Wieschaus; 1982, Bingham et al., Spradling and Ru- bin; 1983, Scott et al., Bender et al.; 1984, Bargiello and Young; 1987, Nu¨sslein-Volhard et al.; 1988, MacDonald and Struhl; 1990, Milicki et al.; 1993, Maroni; 1994, Tully et al., Orr and Sohal; 1995, Halder et al., Zhao, Hart, and Laemmli, Kerrebrock et al.; 1996, Dubnau and Struhl, Rivera-Pomar et al.; 1998, Lim, Serounde, and Benzer; 2000, Adams et al., Rubin et al.; Appendix E, Individual Databases; centromere, Drosophila targeted gene expression technique, heterochromatin, shotgun sequencing.
Drosophila salivary gland chromosomes the most extensively studied polytene chromosomes. During larval development, the cells of the salivary gland undergo 9 or 10 cycles of endomitotic DNA replications to produce chromosomes that contain 1,000-2,000 times the haploid amount of DNA. The cytological map of the chromosomes of D. mel- anogaster contains slightly over 5,000 bands. It is di-
vided into 102 divisions, distributed as illustrated. The solid circles represent the centromeres. Each di- vision is subdivided into subdivisions lettered A-F, and the subdivisions contain varying numbers of bands. Genes have been localized within these bands by studying overlapping deficiencies and, more re- cently, by in situ hybridization with labeled probes.
Since the number of genes in the euchromatin of Drosophila is known to be 13,000, the average band in a giant chromosome must contain two or three genes. The insertion of a transposable element (q.v.) can generate new bands and interbands in the sali- vary chromosomes.
The cells of the larval salivary gland are in interphase, and within each nucleus the chromosomes show a typical orientation. The telo- meres tend to be on the surface of the nuclear enve- lope, opposite the portion of the envelope nearest to the nucleolus, where all the centromeres are located. The arms of each autosome remain close together, whereas the relative positions of the arms vary.
Dif- ferent chromosomes are never entangled. The domi- nant chromosome folding motif is a right-handed coil. See Appendix C, 1933, Painter; 1935, Bridges; 1968, Semeshin et al.; 1988, Sorsa; biotinylated DNA, chromosomal puff, deficiency loop, Drosophila virilis, heterochromatin, insulator DNAs, Rabl orienta- tion, salivary gland chromosomes.