M. M. Green

Unequal crossing over and the genetical organization of the white locus ofDrosophila melanogaster

SummaryUnequal crossing over occurs in females heterozygouswa/wa4such that a deficiency and duplication can be recovered regularly. A triplication was also found following crossing over in the homozygous duplication.Genetic analysis of the duplication showed that of the several knownw loci only theweandsp-w loci are duplicated.By means of crossing overweandsp-w were introduced separately into the left half of the duplication andweinto the right half. The phenotypic consequences of such substitutions are interpreted in terms of the loci which are included in specific functional units or genes. They suggest thatweandsp-w are in distinctive functional units.The fact that these duplications and deficiencies are without obvious affect on salivary gland band morphology suggests that the gene loci constitute only a small portion of the band structure.

A cytogenetic analysis of the lozenge pseudoalleles in Drosophila.

Summary1. A cytogenetic analysis of 18 independent lz mutants is presented.2. On a phenotypic basis the mutants can be separated into three classes.3. The results of a recombination analysis show that each of the 18 mutants can be assigned to one of three loci. No case where a lz mutant involved more than one locus was found.4. Both phenotypic and recombination evidence place the amx mutant outside the lz pseudoallelic system.5. Cytologically the lz pseudoalleles have been localized to section 8D of the salivary X chromosome.6. The relevance of the data presented to the nature of pseudoallelism is discussed.

Putative non-reciprocal crossing over in Drosophila melanogaster

Summary1.From ⧫⧫ heterozygous for the mutantswa/wa4white-eyed progeny are recovered in association with crossing over. On the basis of the marker gene distribution to the white-eyed exceptions, the left segment of theirw locus comes from thewa4chromosome, the right segment from thewachromosome.2.No exceptions reciprocal to the white-eyed exceptions were found.3.Evidence is presented which demonstrates that the white-eyed exceptions are associated with a cytologically undetectable loss.4.The occurrence of the white-eyed exceptions is dependent not on thewamutant but rather on tightly linked factors to the right ofwa.5.It is suggested that the white-eyed exceptions occur as a consequence of crossing over within not strictly homologusly paired component segments of thewhite region of the chromosome.

Some genetic properties of intrachromosomal recombination

SummaryIntrachromosomal recombination was estimated by the occurrence of unequal crossing over without marker gene exchange in three different, independent tandem duplicaltions in Drosophila melanogaster. Each tandem duplication gave rise to intrachromosomal recombinants. The frequency of intrachromosomal recombination is independent of the genetic length of the tandem duplication. Further, intrachromosomal recombinants were recovered as frequently in ring as in rod X chromosomes implying that the recombination event is not equatable to a single interchromosomal crossover.

Conversion as a possible mechanism of high coincidence values in the centromere region of Drosophila.

SummaryCrossing over has been studied with mutants which flank the centromere of chromosome 3 in Drosophila melanogaster. Coefficients of coincidence greater than one were obtained. Putative double crossovers were recovered for genetic intervals whose genetic length is less than one centimorgan. It is suggested that gene conversion rather than conventional double crossing over accounts for the negative interference observed.

Genetic instability in Drosophila melanogaster: The effect of male recombination (MR) chromosomes in females

SummaryWe demonstrate that male recombination (MR) second chromosomes, known to act as mutators and recombination inducers in males, function in an equivalent manner in females. We suggest such chromosomes are more properly designated as mutation-recombination chromosomes.

Molecular characterization of spontaneous mutations at the scarlet locus of Drosophila melanogaster

Six spontaneous mutations of the scarlet (st) locus of Drosophila melanogaster have been studied at the molecular level. Two of the mutants (st1 and stsp) arose in laboratory populations, while the other four (stcob, stct89, stdct and stdv) were isolated from natural populations. In five of these there is a DNA insertion within the st region and in four cases the insertion has been identified as being a transposable element; these include the retrotransposons 412 and B104/roo, and also jockey a member of the LINE family. In the other case (stdct), the insertion appears to consist of partially duplicated st sequences. In two of the mutants (st1 and stdv) the same transposable element (412) has inserted in the same orientation at exactly the same site within the st gene. The transposable element insertions are found in intron and exon regions of the st gene and also in the putative upstream regulatory region; insertions located in introns or exons result in the production of truncated st transcripts. The results show that the same types of transposable elements that cause spontaneous mutation in laboratory stocks of D. melanogaster also cause mutation in the wild.

Genetic instability in Drosophila melanogaster: Transpositions of the white gene and their role in the phenotypic expression of the zeste gene*

SummarySeven independent transpositions of the w+ gene have been recovered as derivatives of two separate direct tandem duplications of the white locus. The transpositions map to discrete sites on both major autosomes. Five transpositions were employed to study the role of w+ gene dos-age on zeste (z) gene expression. Each transposition generates a unique zeste phenotype; one transposition is not predictive for another. A functional allele of zeste, z77h, responds to w+ gene dosage contrary to the z response.

The beadex locus in Drosophila melanogaster: genetic analysis of the mutant Bxr49k.

Summary1.A genetic analysis of the mutant Bxr49k (herein designated as Bxr2) in D. melanogaster is reported.2.Crossing-over occurs between Bxr2 and Bx1. From the analysis of the crossover types recovered, it is concluded that Bxr2 is associated with a tandem duplication in which the Bx+ locus as well as unknown gene loci to the right and left of Bx+ are duplicated. The sy+ and fu+ loci are not duplicated in Bxr2 as previously reported for Bxr.3.The duplication nature of Bxr2 has been confirmed by a salivary gland chromosome analysis.4.In ♀ ♀ homozygous Bxr2 unequal crossing over occurs by which means triplications of the Bx+ locus have been recovered. This occurrence of unequal crossing over is in sharp contrast to the absence of unequal crossing over previously reported for ♀ ♀ homozygous Bxr.5.Unequal crossing over also occurs in ♀ ♀ homozygous for the Bx+ triplication and as a consequence quadruplications of the Bx+ locus have been recovered.6.It is concluded that the phenotypes associated with duplication, triplication and quadruplication of the Bx+ locus are the result of position effect.

THE MOLECULAR ANALYSIS OF BROWN EYE COLOR MUTATIONS ISOLATED FROM GEOGRAPHICALLY DISCRETE POPULATIONS OF DROSOPHILA MELANOGASTER

A large proportion of spontaneous mutations inDrosophila melanogaster strains of laboratory origin are associated with insertions of mobile DNA elements. As a first step toward determining whether spontaneous laboratory mutations are predictive for mutational events occurring in the wild, recessivebrown (bw) eye color mutants were isolated. By inbreeding the progeny of wild-caughtDrosophila melanogaster females,bw mutations were isolated from seven separate geographic sites distributed among Japan, California, Siberia and Hungary. Among a total of 14 mutations studied, no case of transposon mutagenesis was found. At least 4 mutations are associated with small deletions in thebw gene. The remainder are inseparable from wild-typebw by Southern analysis and are presumed to be basepair changes or very small indels. Although only two spontaneousbw mutants of laboratory origin have been analyzed molecularly, one is a mobile element insertion.