THE ORGANIZATION OF THE ALPHA-TUBULIN GENE FAMILY IN THE DROSOPHILA-MONTIUM SUBGROUP OF THE MELANOGASTER SPECIES GROUP

The alpha 1-, alpha 2-, alpha 3-, and alpha 4-tubulin genes have been mapped by in situ hybridization to the polytene chromosomes of five sp ecies representative of the Drosophila montium subgroup geographical d istribution. A lambda phage clone containing al-tubulin specific seque nces was isolated from a genomic DNA library of Drosophila auraria and its restriction endonuclease pattern is presented. Both well-characte rized heterologous and homologous probes were used to assess orthogona lity of gene members between species groups. The in situ hybridization pattern observed in all species studied is consistent with that of Dr osophila melanogaster, since alpha 1-, a2-, and alpha 3-tubulin genes are located on the same polytene arm, and the a4-tubulin gene is found on a different arm. Cross-hybridization was observed among alpha 1-, alpha 2-, and alpha 3-tubulin specific sequences in all species studie d, using either heterologous or homologous probes. However, unlike D, melanogaster, in all montium species studied, both alpha 1- and alpha 3-tubulin specific probes hybridize to the same polytene band, indicat ing a clustered organization of the above genes. The chromosomal organ ization of this gene family would suggest that taxa within the montium subgroup are closer to their common ancestor than are the taxa in the melanogaster species group. A mode of evolution for this gene family in Drosophila is proposed.