Sex chromosome loss induced by the "sex-ratio" trait in Drosophila pseudoobscura males.

Frequencies of sex chromosome aneuploid sperm were measured for standard and "sex-ratio" Drosophila pseudoobscura males with identical genetic backgrounds. The "sex-ratio" males produced a 15.6-fold higher frequency of nullo-XY sperm than the standard males produced, but diplo-XY sperm were produced at the same rate in the two types of males. "Sex-ratio" males with the same SR chromosome but with different chromosome backgrounds were found to produce different frequencies of sterile sons. These results indicate that strains of D. pseudoobscura differ at autosomal or Y-linked modifiers of sr. Such modifiers are similar to the msr genes found in the L116 strain of this species.     

Nonrecombining genes in a recombination environment: the Drosophila "dot" chromosome

Rate of recombination is a powerful variable affecting several aspects of molecular variation and evolution. A nonrecombining portion of the genome of most Drosophila species, the "dot" chromosome or F element, exhibits very low levels of variation and unusual codon usage. One lineage of Drosophila, the willistoni/saltans groups, has the F element fused to a normally recombining E element. Here, we present polymorphism data for genes on the F element in two Drosophila willistoni and one D. insularis populations, genes previously studied in D. melanogaster. The D. willistoni populations were known to be very low in inversion polymorphism, thus minimizing the recombination suppression effect of inversions. We first confirmed, by in situ hybridization, that D. insularis has the same E + F fusion as D. willistoni, implying this was a monophyletic event. A clear gradient in codon usage exists along the willistoni F element, from the centromere distally to the fusion with E; estimates of recombination rates parallel this gradient and also indicate D. insularis has greater recombination than D. willistoni. In contrast to D. melanogaster, genes on the F element exhibit moderate levels of nucleotide polymorphism not distinguishable from two genes elsewhere in the genome. Although some linkage disequilibrium (LD) was detected between polymorphic sites within genes (generally <500 bp apart), no long-range LD between F element loci exists in the two willistoni group species. In general, the distribution of allele frequencies of F element genes display the typical pattern of expectations of neutral variation at equilibrium. These results are consistent with the hypothesis that recombination allows the accumulation of nucleotide variation as well as allows selection to act on synonymous codon usage. It is estimated that the fusion occurred ～20 Mya and while the F element in the willistoni lineage has evolved "normal" levels and patterns of nucleotide variation, equilibrium may not have been reached for codon usage.     

DNA-protein interactions in the Drosophila virilis mitochondrial chromosome.

The location of proteins on the mitochondrial DNA (mtDNA) of Drosophila virilis was investigated by Me3 psoralen photoreaction of mitochondria isolated from embryos. After photoreaction the mtDNA was purified and the pattern of DNA cross-linking was determined by electron microscopy of the DNA under totally denaturing conditions. The transcribed regions of the mtDNA molecule contained some uncross-linked regions, but such regions were infrequent and randomly distributed. In contrast, the A + T-rich region around the origin of replication of the mtDNA was usually protected from psoralen cross-linking. The data were best fit by two protected sites, each approximately 400 base pairs, compared to the four 400 base pair sites observed in the equivalent region of D. melanogaster mtDNA [Potter et al. (1980) Proc. Nat. Acad. Sci. USA 77, 4118-4122]. Thus this region of the mtDNA appears to be involved in a DNA-protein structure that is highly conserved even though the DNA sequence has diverged rapidly relative to protein-coding sequences.     

HeT-A, a transposable element specifically involved in "healing" broken chromosome ends in Drosophila melanogaster.

Eight terminally deleted Drosophila melanogaster chromosomes have now been found to be "healed." In each case, the healed chromosome end had acquired sequence from the HeT DNA family, a complex family of repeated sequences found only in telomeric and pericentric heterochromatin. The sequences were apparently added by transposition events involving no sequence homology. We now report that the sequences transposed in healing these chromosomes identify a novel transposable element, HeT-A, which makes up a subset of the HeT DNA family. Addition of HeT-A elements to broken chromosome ends appears to be polar. The proximal junction between each element and the broken chromosome end is an oligo(A) tract beginning 54 nucleotides downstream from a conserved AATAAA sequence on the strand running 5' to 3' from the chromosome end. The distal (telomeric) ends of HeT-A elements are variably truncated; however, we have not yet been able to determine the extreme distal sequence of a complete element. Our analysis covers approximately 2,600 nucleotides of the HeT-A element, beginning with the oligo(A) tract at one end. Sequence homology is strong (greater than 75% between all elements studied). Sequence may be conserved for DNA structure rather than for protein coding; even the most recently transposed HeT-A elements lack significant open reading frames in the region studied. Instead, the elements exhibit conserved short-range sequence repeats and periodic long-range variation in base composition. These conserved features suggest that HeT-A elements, although transposable elements, may have a structural role in telomere organization or maintenance.     

X chromosome DNA variation in Drosophila virilis.

Comparisons of polymorphism patterns between distantly related species are essential in order to determine their generality. However, most work on the genus Drosophila has been done only with species of the subgenus Sophophora. In the present work, we have sequenced one intron and surrounding coding sequences of 6 X-linked genes (chorion protein s36, elav, fused, runt, suppressor of sable and zeste) from 21 strains of wild-type Drosophila virilis (subgenus Drosophila). From these data, we have estimated the average level of DNA polymorphism, inferred the effective population size and population structure of this species, and compared the results with those obtained for other Drosophila species. There is no reduction in variation at two loci close to the centromeric heterochromatin, in contrast to Drosophila melanogaster.     

Addition of telomere-associated HeT DNA sequences "heals" broken chromosome ends in Drosophila

Stocks of D. melanogaster X chromosomes carrying terminal deletions (RT chromosomes) have been maintained for several years. Some of the chromosomes are slowly losing DNA from the broken ends (as expected if replication is incomplete) and show no telomere-associated DNA added to the receding ends. Two stocks carry chromosomes that have become "healed" and are no longer losing DNA. In both stocks the broken chromosome end has acquired a segment of HeT DNA, a family of complex repeats found only at telomeres and in pericentric heterochromatin. Although the HeT family is complex, the HeT sequence joined to the broken chromosome end is the same in both stocks. In contrast, the two chromosomes are broken in different places and have no detectable sequence similarity at the junction with the new DNA. Sequence analysis suggests that the new telomere sequences have been added by a specific mechanism that does not involve homologous recombination.     

Correlations between chromosome segments and fitness in Drosophila melanogaster. IV. Fecundity, viability and the third chromosome

Unmarked segments within the third chromosome of three different Drosophila melanogaster lines were assessed for their effects on egg production and egg viability. By making a series of crosses among original and derived recombinant lines, it was possible to estimate parameters representing additive, dominance (for egg production), and interaction effects of the segments. Each segment influences both traits, but to extend which are dependent on the genetic background provided by an adjacent segment. There is no clear pattern, however, with respect to the segments' joint effects on the two characters. Unlike in the previous study involving the X chromosome, the majority of the derived recombinant lines were superior in fitness to their original lines. The agricultural implications of this result with respect to the manipulation of chromosomal segments in order to achieve higher yields are discussed.     

Aneuploidy in Drosophila, I. Genetic test systems in the female Drosophila melanogaster for the rapid detection of chemically induced chromosome gain and chromosome loss

An account is provided of two genetic schemes in the Drosophila melanogaster female designed as rapid detectors of chemically induced aneuploidy, including both chromosome gain and chromosome loss. One scheme is referred to as FIX, in which the female carried free (heterozygously) inverted X (chromosomes) and the other, ZESTE, where females do not carry inversions and the X-linked sexually dimorphic zeste mutation plays the key role in the detection of aneuploid offspring. The principle attribute of the FIX system is that all euploid offspring are wild-type for body and eye color whereas aneuploid females have a yellow body and aneuploid males white eyes; int he ZESTE system all euploid individuals are wild-type for eye color, aneuploid females possess zeste-colored eyes and aneuploid males white eyes. In addition induced polyploidies (2X:2A gametes) appear as yellow and zeste male intersexes in the FIX and ZESTE systems, respectively. In this way all aneuploids are recognized immediately. Consequently, detection of compounds with weak effects requiring large sample sizes may be made in a fraction of the time associated with more traditional schemes for aneuploidy detection in Drosophila.     

Two separated nucleolus organizers on the Drosophila hydei Y chromosome.

By genetical, cytological, and filter saturation hybridization methods it is shown that the Y chromosome of Drosophila hydei contains two separate nucleolus organizers, one on the short arm, the second near the tip of the long arm.     

Cytogenetic analysis of chromosome segments containing radiosensitivity genes in Drosophila. Radiation mutagenesis in the 44-45 region of chromosome 2 of Drosophila melanogaster

The first step of cytogenetic analysis of Drosophila melanogaster chromosome 2 44F-45D containing the radiosensitivity gene rad(2)201 is described. Using various mutation selection systems as well as lines of different origin and two kinds of ionizing radiation--gamma-rays and neutrons--the mutagenesis in the region of interest is characterized at the cytogenetic level. 85 gamma-induced mutations affecting viability were isolated in the 44F 2-4; 45C6-7 interval, 27% of mutations being chromosomal aberrations. 15 radiation-induced aberrations were obtained by selecting mutations at the white gene inserted into the 45D region by P-mediated transformation. The 44F-45D region is characterized by relatively low frequency of deficiency formation and by significant predomination of heterochromatic aberrations in the spectrum of rearrangements. In these regions, the existence of hot spots for heterochromatic aberrations was discovered. As low deletion frequency is not connected with the presence of haplolethal and haplosterile loci in the region studied, the unusual character of radiation mutagenesis reflects possibly the peculiarities in sequence organization of the chromosomal region mentioned or the packaging in the sperm nuclei.     

Is the Y chromosome of Drosophila an evolved supernumerary chromosome?

The Y chromosomes of most Drosophila species are necessary for male fertility but they are not involved in sex determination. They have many puzzling properties that resemble the effects caused by B chromosomes. Classical genetic and molecular studies reveal substantial affinities between Y and B chromosomes and suggest that the Y chromosomes of Drosophila are not degenerated homologues of the X chromosomes, but rather that their Y chromosomes evolved as specialized supernumeraries similar to classical B chromosomes.     

Genetic organization of the ci-M-pan region on chromosome IV in Drosophila melanogaster.

The genes cubitus interruptus (ci), ribosomal protein S3A (RpS3A), and pangolin (pan) are localized within 73 kb in the cytological region 101F-102A on chromosome IV in Drosophila melanogaster. A region of 13 kb harbours the regulatory regions of both ci and pan, transcribed in opposite directions, and a 1.1-kb gene encoding RpS3A. This dense clustering gives rise to very complicated complementation patterns between different alleles in these loci. We investigated this region genetically and molecularly by use of an enhancer trap line (IA5), where the P-element was found to be inserted into the first intron of pan. Screens for imprecise excisions of the P-element were performed, and complementations between new and old established mutant lines were investigated. We found that when mutated or deleted the RpS3A gene gives rise to a Minute phenotype, and we conclude that M(4)101 encodes the ribosomal protein S3A.     

Temperature-sensitive mutations in Drosophila melanogaster. XI. Male sterile mutants of the Y chromosome

Eight temperature-sensitive (ts) male sterile mutations have been induced by ethyl methanesulfonate treatment of Y chromosomes derived from a selected temperature-resistant Amherst wild-type stock of Drosophila melanogaster. Males carrying such mutated Y chromosomes (Y^ts) are sterile when raised at 29°C but fertile when reared at 22°C. Complementation tests of the mutants with Y chromosome fragments, deletions, and inter se localized all eight to the long arm of the chromosome in four different complementation groups.When Y^ts-bearing males, reared to adulthood at 22°C, were subjected to a 48-hr regimen at 29°C and mated to fresh virgin females daily, a significant reduction in fertility resulted 5 days after initiation of 29°C treatments. This period of sterility was transient (48–72-hr duration) and corresponded to a temperature-sensitive period (TSP) of spermatogenesis during the primary spermatocyte stage. A more precise definition of the TSP utilized exposure of subadult males to 29°C at selected developmental periods during which only certain germ cell stages are present. Upon eclosion adult males were subjected to a similar schedule of consecutive matings of 12-hr duration in order to detect any delay in the appearance of fertility. Different ts males could be distinguished by the resultant pattern of sterility, and the TSP of different mutations thus localized to either primary spermatocyte or immediately post-meiotic stage.Associated with Y^ts-mediated sterility, spermiogenesis is defective at restrictive temperature as evidenced by the production of nonmotile sperm and a failure to transfer such sperm to the female during copulation. In addition, electron microscopy detected a variety of ultrastructural abnormalities, including defects of axoneme formation, irregularities of Nebenkern derivative development, and failures of separation from the syncitial state or mature cyst with subsequent degeneration.     

Reduced sequence variability on the Neo-Y chromosome of Drosophila americana americana.

Sex chromosomes are generally morphologically and functionally distinct, but the evolutionary forces that cause this differentiation are poorly understood. Drosophila americana americana was used in this study to examine one aspect of sex chromosome evolution, the degeneration of nonrecombining Y chromosomes. The primary X chromosome of D. a. americana is fused with a chromosomal element that was ancestrally an autosome, causing this homologous chromosomal pair to segregate with the sex chromosomes. Sequence variation at the Alcohol Dehydrogenase (Adh) gene was used to determine the pattern of nucleotide variation on the neo-sex chromosomes in natural populations. Sequences of Adh were obtained for neo-X and neo-Y chromosomes of D. a. americana, and for Adh of D. a. texana, in which it is autosomal. No significant sequence differentiation is present between the neo-X and neo-Y chromosomes of D. a. americana or the autosomes of D. a. texana. There is a significantly lower level of sequence diversity on the neo-Y chromosome relative to the neo-X in D. a. americana. This reduction in variability on the neo-Y does not appear to have resulted from a selective sweep. Coalescent simulations of the evolutionary transition of an autosome into a Y chromosome indicate there may be a low level of recombination between the neo-X and neo-Y alleles of Adh and that the effective population size of this chromosome may have been reduced below the expected value of 25% of the autosomal effective size, possibly because of the effects of background selection or sexual selection.