Nucleocytoplasmic Relations in a Mutator-Suppressor System of DROSOPHILA ANANASSAE

A partially characterized mutator-suppressor system, previously identified in the ca; stw stock of Drosophila ananassae, was shown to exist in the ca ancestral stock; it consists of a clastogenic mutator of sperm chromosomes and a supressor that functions in the oöcyte soon after fertilization. Transmission of these components was monitored by Minute mutation frequencies produced by the progeny of recurrently backcrossed hybrid females derived from reciprocal outcrosses of the ca stock. In this way, the mutator was shown to be an extrachromosomally transmitted element whose propagation depends upon nuclear genes. Suppressivity was found to be determined by nuclear genes, some of which are expressed only after a delay of several generations. Neither the mutator nor its suppressor appear to be infectious. Measurement of dominant lethal frequencies showed that the suppressor is completely effective in repair of premutational lesions induced by the mutator. The properties of this mutator-suppressor system were compared with those of hybrid dysgenesis in Drosophila melanogaster.     

Relations between Factors Controlling Crossing over and Mutability in Males of DROSOPHILA ANANASSAE

Several stocks, selected because they carried previously identified factors governing either crossing over in males or mutability, were examined to determine whether the effects of these factors are restricted to one or the other process. Neither of two dominant enhancers of male crossing over had detectable effects on Minute mutation frequencies among progenies of assayed F₁ males. Genetically equivalent F₁ males monitored for crossing over showed no unequivocal effect of either of three mutators (two dominant and one extrachromosomal) or of a suppressor of mutability. However, one combination of a dominant crossover enhancer with a dominant mutator showed synergistic increases in both crossover and Minute frequencies, and the possibility exists that a single extrachromosomally transmitted element suppresses both male crossing over and mutability. This suppressor element (or elements) had been previously diagnosed in the pc stock which, in this study, was discovered to have also a dominant enhancer of male crossing over and a dominant mutator occupying separable loci in the third chromosome. The pc enhancer of male crossing over differs from the dominant enhancer in another stock with respect to the regional distribution of crossovers, and the pc mutator is distinguished from another 3-linked mutator by its preferential induction of mutations at the Delta locus.     

OM Mutations in DROSOPHILA ANANASSAE Are Linked to Insertions of a Transposable Element

It has been hypothesized that Om mutability in Drosophila ananassae (involving spontaneous mutation at 20 loci, resulting in semidominant, nonpleiotropic eye morphology defects) was due to insertion of a transposable element, tom. One particularly unstable X-linked Om allele produced several derivatives, one of which has a more extreme Om phenotype and was accompanied by a singed bristle mutant, sn ^9g. DNA probes from the sn locus of D. melanogaster were used to clone the homologous region of D. ananassae. Analysis of sn^9g DNA detected a 6.5-kb insert. Genomic Southern blotting and in situ hybridization techniques showed that this insert is repetitive and dispersed. The existence of the tom element is supported by genetic mapping that established homology between the 6.5-kb sn^9g insert and Om mutants at the four X-linked loci tested.     

Genic Heterogeneity at Two Alcohol Dehydrogenase Loci in DROSOPHILA PSEUDOOBSCURA and DROSOPHILA PERSIMILIS

A sequential electrophoretic survey of the second chromosome loci, alcohol dehydrogenase-6 (Adh-6) and octanol dehydrogenase ( Odh), was performed on 147 isochromosomal lines of Drosophila pseudoobscura and 60 lines of its sibling species, D. persimilis. Gels run with a variety of acrylamide concentrations and buffer pH's revealed the presence of 18 alleles of Adh-6 in the two species, where only eight had been previously detected by conventional electrophoretic methods. Only two alleles were added with our techniques to the previous total of nine in both species at the largely monomorphic Odh locus. Both enzymes show a predominance of one allele, with the other variants being fairly rare. There was no evidence of increased genetic divergence between the two species, but we found a striking increase in differentiation of Adh-6 alleles between the main body of D. pseudoobscura populations and the conspecific isolate from Bogotá, Colombia. These results are compared with our previous surveys of xanthine dehydrogenase in these species and discussed in reference to theories of genic polymorphism.     

Formal Relations between Om Mutants and Their Suppressors in Drosophila Ananassae

Optic morphology (Om) mutants associated with insertions of the tom transposable element at each of three tested loci are neomorphs as defined by the phenotypic equivalence of +/+/Om with +/Om and of +/Om/Om with Om/Om. Mutants behaving as suppressors of Om mutants and mapping to at least six loci are recovered from the same source and in similar frequency as Om mutants. The semidominant and nonpleiotropic suppressors at four of the six loci display defective eye phenes themselves, and the phenotypically normal mutants at a fifth locus are suspected alleles of a gene represented by recessive furrowed eye mutants. These and other properties imply that the suppressors, like suppressible Om mutants, are neomorphic due to insertion of the tom element into a hypothetical sequence they share with other members of a set of genes involved in development of the eye. Concurrently premature expression of both the suppressor and suppressed mutants would allow interaction of their products just as in normal development.     

Dominant Mutators in ESCHERICHIA COLI

In this paper we report on the isolation and genetic analysis of a series of strong mutators mapping at five minutes on the E. coli chromosome. These mutations are dominant and show no evidence of interaction in merodiploids. Cultures grown in broth medium exhibit mutant frequencies five to six orders of magnitude higher than mut⁺ strains. Cultures propagated in minimal salts media mutate at rates one to three orders higher than wild-type. Three-factor crosses have been used to order these mutators relative to metD, proA, and a Tn10 insertion near five minutes.     

Polymorphism for Y-Linked Suppressors of Sex-Ratio in Two Natural Populations of Drosophila Mediopunctata

In several Drosophila species there is a trait known as ``sex-ratio': males carrying certain X chromosomes (called ``SR') produce female biased progenies due to X-Y meiotic drive. In Drosophila mediopunctata this trait has a variable expression due to Y-linked suppressors of sex-ratio expression, among other factors. There are two types of Y chromosomes (suppressor and nonsuppressor) and two types of SR chromosomes (suppressible and unsuppressible). Sex-ratio expression is suppressed in males with the SR(suppressible)/Y(suppressor) genotype, whereas the remaining three genotypes produce female biased progenies. Now we have found that ~10-20% of the Y chromosomes from two natural populations 1500 km apart are suppressors of sex-ratio expression. Preliminary estimates indicate that Y(suppressor) has a meiotic drive advantage of 6% over Y(nonsuppressor). This Y polymorphism for a nonneutral trait is unexpected under current population genetics theory. We propose that this polymorphism is stabilized by an equilibrium between meiotic drive and natural selection, resulting from interactions in the population dynamics of X and Y alleles. Numerical simulations showed that this mechanism may stabilize nonneutral Y polymorphisms such as we have found in D. mediopunctata.     

Population Genetics of DROSOPHILA NASUTA NASUTA, DROSOPHILA NASUTA ALBOMICANA and Their Hybrids. I. Karyotypic Mosaicism in the Hybrid Populations

D. n. nasuta and D. n. albomicana constitute a pair of chromosomal races with 2n=8 and 2n=6, respectively. The F₁ of these has 2n=7 and it is fertile. There exists a state of karyotypic mosaicism as evidenced by the presence of 26 types of chromosome combinations in F₂, F₃ and F₁₀ populations. In the midst of this karyotypic noise, the karyotype similar to that of F₁ reached 51% of the population. Implications of these findings are discussed.