No abstract

The mitotic chromosome complement of D. ananassae consists of four structurally distinguishable submetacentric pairs and all four have been identified with their linkage groups. For the polytene chromosome complement of six arms representing the X, second and third chromosomes, an improved reference map has been constructed and used to describe selected cytogenetically useful rearrangements. In meiotic prophase of spermatocytes, chromosomes 2 and 3 form pachytene-diplotene bivalents whose arms may be associated by chiasmata in postdiplotene stages, but the X, Y and fourth chromosomes participate in a complex multivalent. No correlation was detected between meiotic chromosome behavior and specific genes that regulate crossing over in males. In male inversion heterozygotes having high levels of genetically monitored crossing over, no unequivocal evidence was found for formation of either pachytene inversion loops or anaphase bridges and fragments.

No abstract

A stock exhibiting hypermutability with respect to visible mutants (Om) affecting optic morphology was subjected to genetic analysis. The production of Om mutants, independently recovered with a frequency of two per 104, is restricted to females and depends primarily on homozygosity of their X chromosomes; in heterozygotes, Ommutability is stimulated by the presence of either one of two extrachromosomally replicating elements previously identified in other stocks having cryptic mutability systems. The semidominant and nonpleiotropic Om mutants are not associated with gross rearrangements and they map to at least 15 loci. Most of the loci defined by mapping are represented by two or more Ommutants which, despite considerable interlocus mimicry, sometimes display locus-specific phenotypes. Om mutants are moderately unstable, and they are subject to dominant suppressors that arise spontaneously at either of two X-linked loci. An interpretation of these observations invokes an X-linked transposable element (tom) that specifically inserts into control sequences shared by a set of structural genes involved in eye morphogenesis.

No abstract