Molecular characterization of the Drosophila genome.

Abstract

A remarkable feature of evolution in the genus Drosophila is the conservation of the basic karyotype. Postulation of Robertsonian centric fusions (ROBERTSON 1916) is sufficient to explain most of the karyotypic rearrangements found in the various species. The fixation of large duplications of genetic material during the evolution of this genus appears to have been rare (STONE 1955). On the other hand, amino acid sequence data for proteins of other organisms indicate that many cistrons have very similar nucleotide sequences, and are presumably related by prior gene duplication (JUKES 1966). Physical studies of DNA of various eucaryotes also suggest that these genomes contain very large families of related cistrons (BRITTEN and KOHNE 1968). The size of such families is often in excess of that inferred from existing protein data (BRITTEN and KOHNE 1968; MCCARTHY 1967; WETMUR and DAVIDSON 1968). This may in some cases reflect the presence of cistrons with similar nucleotide sequences which specify proteins of overtly different functions. It is also likely that some of these families of genes have functions other than the determination of amino acid sequences. In keeping with the cytological studies, the reassociation kinetics of denatured Drosophila melanogaster DNA imply that the genome of this organism is much less complex, in terms of intragenome nucleotide sequence homologies, than those of vertebrates (LAIRD and MCCARTHY 1968b). Estimates made from these measurements suggest that perhaps 90% of D. melanogaster nucleotide sequences are not closely related to others elsewhere in the genome. In light of the extensive diversity within the genus Drosophila (LAIRD and MCCARTHY 1968a) , a detailed analysis of the intragenome homologies of representative species is expected to provide information about the evolution of genomes. In particular, our results indicate that genome simplicity extends to other Drosophila species and to a representative of another Dipteran family. In addition, there are variations among Diptera in genome size as inferred from the informational content of nucleotide sequences. MATERIALS AND METHODS Details of labeling and isolation of DNA and RNA from Drosophila, preparation of filterbound DNA, hybridization and duplex formation procedures, and analytical CsCl pycnography