Spontaneous Chromosome Breakage at Male Meiosis Associated with Male Recombination in DROSOPHILA MELANOGASTER

An inbred line (OK1) of Drosophila melanogaster , recently derived from a natural population in Oklahoma, has been found by Woodruff and Thompson to exhibit a low frequency of spontaneous male recombination when outcrossed to marker stocks. There is also a reciprocal-cross effect, such that recombination is found only if OK1 males are used in the initial cross. When OK1 females are used, however, male recombination is again found if their male progeny are used for a subsequent cross.-In the present cytological analysis, chromosome behavior at male meiosis was studied in reciprocal crosses between the OK1 line and both a marker gene stock and an inversion stock. If the recombination events were "conventional" and premeiotic (gonial) in origin, no chromosome aberrations would be expected during meiosis. If they were "conventional" and meiotic, some dicentric bridges with free fragments would be expected in the inversion heterozygote, but none should be present in the marker gene cross.-The results demonstrated that the occurrence of recombination in males is most likely a meiotic event, though the occurrence of some limited premeiotic recombination can not be disproven. Meiosis was found to be perfectly normal in all crosses lacking male recombination. In all of the inversion stock and noninversion marker stock crosses that showed male recombination, however, anaphase bridges were found at both first and second meiotic divisions. These were often accompanied by more than the single fragment expected from a conventional inversion bridge and fragment situation. In extreme cases, almost complete pulverization of one or more autosomes was found.-All metaphase I stages were perfectly normal, suggesting that no comparable breakage occurs in premeiotic gonial mitoses. The form of chromosome damage is similar in many ways to that produced by some DNA synthesis inhibitors, or by some viral or mycoplasma infections. This possibility is discussed, and some of the evolutionary implications of the system are briefly considered.     

Suppressor Systems of Segregation Distorter (SD) Chromosomes in Natural Populations of DROSOPHILA MELANOGASTER

Several natural populations of D. melanogaster were investigated for the presence (or absence) of the Segregation Distorter ( SD) chromosomes and their suppressor systems. The SD chromosomes were found, at frequencies of a few percent, in two independent samples taken in different years from a Raleigh, North Carolina, population, whereas no SD chromosomes were found in samples collected from several populations in Texas. The populations in these localities were found to contain suppressor X chromosomes in high frequencies (75% or higher). They also contained relatively low frequencies of partial suppressor or insensitive second chromosomes of varying degrees, but completely insensitive second chromosomes were practically absent in all populations examined. The frequencies of suppressor X chromosomes, as well as those of the partially insensitive or suppressor second chromosomes, were the same among the populations investigated. This suggests the possibility that the development of a suppressor system of SD in a population could be independent of the presence of an SD chromosome. Segregation distortion appeared to be occurring in natural genetic backgrounds, but the degree of distortion varied among males of different genotypes. There were many instances in which the SD chromosomes showed transmission frequencies from their heterozygous male parents that were smaller than 0.6 and, in several cases, even smaller than 0.5. The presence of a recessive suppressor, or suppressors, of SD in natural populations was suggested.     

Meiosis in Male DROSOPHILA MELANOGASTER I. Isolation and Characterization of Meiotic Mutants Affecting Second Chromosome Disjunction

Two second chromosome, EMS-induced, meiotic mutants which cause an increase in second chromosome nondisjunction are described. The first mutant is recessive and causes an increase in second chromosome nondisjunction in both males and females. It causes no increase in nondisjunction of the sex chromosomes in either sex, nor of the third chromosome in females. No haplo-4-progeny were recovered from either sex. Thus, it appears that this mutant, which is localized to the second chromosome, affects only second chromosome disjunction and acts in both sexes.-The other mutant affects chromosome disjunction in males and has no effect in females. Nondisjunction occurs at the first meiotic division. Sex chromosome disjunction in the presence of this mutant is similar to that of sc(4)sc(8), with an excess of X and nullo-XY sperm relative to Y and XY sperm. In some lines, there is an excess of nullo-2 sperm relative to diplo-2 sperm, which appears to be regulated, in part, by the Y chromosome. A normal Y chromosome causes an increase in nullo-2 sperm, where B(s)Y does not. There is also a high correlation between second and sex chromosome nondisjunction. Nearly half of the second chromosome exceptions are also nondisjunctional for the sex chromosomes. Among the double exceptions, there is an excess of XY nullo-2 and nullo-XY diplo-2 gametes. Meiotic drive, chromosome loss and nonhomologous pairing are considered as possible explanations for the double exceptions.     

Selection for Male Recombination in DROSOPHILA MELANOGASTER

Two-way selection for male recombination over seven intervals of the third chromosome in Drosophila melanogaster was practiced for nine generations followed by relaxed selection for five generations. Significant responses in both directions were observed but these mainly occurred in early generations in the low line and in later generations in the high line. Divergence of male recombination frequencies between the two selection lines was not restricted to any specific region but occurred in every measured interval of the chromosome. However, right-arm intervals showed a more pronounced response than either left-arm intervals or the centromeric region. Correlated responses in sterility and distortion of transmission ratios occurred as a result of selection for male recombination. Cluster distributions of male recombinants suggested a mixture of meiotic and late gonial events but relative map distances more closely resembled those of the salivary chromosome than standard meiotic or mitotic distances. Patterns of male recombination over time in both second and third chromosomes strongly suggested a major effect associated with the presence of third chromosomes from the Harwich strain. Evidence was also found for modifiers with relatively small effects located in other regions of the genome. The overall results are interpreted in terms of a two-component model of hybrid dysgenesis.     

On the Components of Segregation Distortion in DROSOPHILA MELANOGASTER

The segregation distorter (SD) complex is a naturally occurring meiotic drive system with the property that males heterozygous for an SD-bearing chromosome 2 and an SD⁺-bearing homolog transmit the SD-bearing chromosome almost exclusively. This distorted segregation is the consequence of an induced dysfunction of those sperm that receive the SD⁺ homolog. From previous studies, two loci have been implicated in this phenomenon: the Sd locus which is required to produce distortion, and the Responder (Rsp) locus that is the site at which Sd acts. There are two allelic alternatives of Rsp—sensitive (Rsp^sens) and insensitive (Rsp^ins); a chromosome carrying Rsp^ins is not distorted by SD. In the present study, the function and location of each of these elements was examined by a genetic and cytological characterization of X-ray-induced mutations at each locus. The results indicate the following: (1) the Rsp locus is located in the proximal heterochromatin of 2R; (2) a deletion for the Rsp locus renders a chromosome insensitive to distortion; (3) the Sd locus is located to the left of pr (2-54.5), in the region from 37D2-D7 to 38A6-B2 of the salivary chromosome map; (4) an SD chromosome deleted for Sd loses its ability to distort; (5) there is another important component of the SD system, E(SD), in or near the proximal heterochromatin of 2L, that behaves as a strong enhancer of distortion. The results of these studies allow a reinterpretation of results from earlier analyses of the SD system and serve to limit the possible mechanisms to account for segregation distortion.     

On the Models of Segregation Distortion in DROSOPHILA MELANOGASTER

The Segregation Distorter system of Drosophila melanogaster consists of two major elements, Sd and Rsp. There are two allelic alternatives of Rsp-sensitive (Rsp(s)) and insensitive (Rsp(i)); a chromosome carrying Rsp(i) is not distorted. According to the model proposed by Hartl (1973), these two elements interact to cause segregation distortion. For a sperm to complete the maturation process, it is assumed that the Rsp locus has to be complexed with the product of the Sd locus. This product is assumed to be a multimetric regulatory protein. Three kinds of regulatory multimers may be distinguished: Sd(+)/Sd(+), which is assumed to complex with both Rsp(s) and Rsp(i); Sd(+)/Sd heteromultimers, which complex preferentially with Rsp(i); and Sd/Sd homomultimers, which complex with neither Rsp(s) nor Rsp(i). Most of the regulatory protein in the Sd(+)/Sd heterozygous male is assumed to be the Sd(+)/Sd heteromultimer.--Some modifications of Hartl's model were made by Ganetzky (1977). Rather than the binding of a product of Sd at the Rsp locus being a necessary condition for normal spermigenesis, this binding causes sperm dysfunction. It is assumed that the product of Sd complexes more readily with Rsp(s) than with Rsp(i) and that the amount of Sd product is limited with respect to the number of binding sites available. No function is ascribed to the Sd(+) locus. In order to explain reduced male fertility of some genotypes, Ganetzky further assumes that the Sd product, when not competed for by an Rsp(s) locus, can bind to an Rsp(i) locus.--Two consequences of these models were critically examined: according to these models (1) an Sd Rsp(s)/Sd(+)Rsp(s) male should not show any segregation distortion, and (2) an Sd Rsp(s)/Sd Rsp(s) male should show much reduced fertility, if not complete sterility.--The results of the present study bear on these two points. (1) Rsp(s) locus seems to consist of multiple alleles, each having a different degree of ability to interact with the product of the Sd locus. An Sd Rsp(s)/Sd(+)Rsp(s) male shows a certain degree of segregation distortion when the two Rsp(s) alleles are different, but it shows a normal Mendelian segregation ratio when the Rsp(s) alleles are homozygous. The first prediction of the models is supported by actual observation when the two Rsp(s) alleles are the same. (2) There is a suggestion of slight reduction in fertility, but generally Sd Rsp(s)/Sd Rsp(s) males are quite fertile. Thus, the second prediction is not supported by actual observation. The mechanism of segregation distortion is still open for future studies.     

High Mutability in Male Hybrids of DROSOPHILA MELANOGASTER

The frequencies of sex-linked lethal mutations arising in hybrid male offspring from various crosses and in nonhybrid controls were determined. The hybrids were produced by crossing representative strains of the P-M system of hybrid dysgenesis in all possible combinations. Males from the cross of P males x M females had a mutation rate about 15 times higher than that of nonhybrid males from the P strain. Genetically identical males from the reciprocal cross had a mutation rate 3 to 4 times that of the nonhybrids. For crosses involving a Q strain, a significant increase in the mutation rate was detected in males produced by matings of Q males with M females. No increase was observed in genetically identical males from the reciprocal mating. Crosses between P and Q strains gave male hybrids with mutation rates not different from those of nonhybrids. Many of the lethals that occurred in hybrids from the cross of P males x M females appeared to be unstable; fewer lethals that arose in hybrids from the cross of Q males x M females were unstable. The relationship between P and Q strains is discussed with respect to a model of mutation induction in dysgenic hybrids.     

Heterozygous Effects on Fitness of Ems-Treated Chromosomes in DROSOPHILA MELANOGASTER

The heterozygous effects on fitness of second chromosomes carrying mutants induced with different doses of EMS were ascertained by monitoring changes in chromosome frequencies over time. These changes were observed in populations in which the treated chromosomes, as well as untreated competitors, remained heterozygous in males generation after generation. This situation was achieved by using a translocation which links the second chromosome to the X chromosome; however, only untranslocated second chromosomes were mutagenized. Chromosomes were classified according to their effects on viability in homozygous condition. A preliminary homozygosis identified completely lethal chromosomes; secondary tests distinguished between drastic (viability index < 0.1) and nondrastic chromosomes. Chromosomes that were nondrastic after treatment were found to reduce the fitness of their heterozygous carriers by 3-5%. The data show that flies homozygous for these chromosomes were about 2.7% less viable per treatment with 1 mm EMS than flies homozygous for untreated chromosomes. By comparing the fitness-depressing effects of nondrastic EMS-induced mutants in heterozygous condition with the corresponding viability-depressing effects measured by Temin, it is apparent that the total fitness effects are several times larger than the viability effects alone. Completely lethal chromosomes derived from the most heavily treated material reduced fitness by 11% in heterozygous condition; approximately half of this reduction was due to the lethal mutations themselves.     

Elements Causing Male Crossing over in DROSOPHILA MELANOGASTER

A second chromosome line of Drosophila melanogaster (Symbol: T-007) has previously been shown to be responsible for the induction of male recombination. In the present investigation, the genetic elements responsible for this phenomenon have been partially identified and mapped. A major element (Symbol: Mr, for Male recombination) locates on the second chromosome between the pr (2L-54.4) and c (2R-75.5) loci and is responsible for the large majority of male recombination. In addition, there appear to be "secondary elements" present which have the ability to induce male recombination in much reduced frequencies and which are diluted out through successive backcross generations when Mr is removed by recombination. The possible nature of these "secondary elements" is discussed.     

Heterozygous Effects of Irradiated Chromosomes on Viability in DROSOPHILA MELANOGASTER

Two large experiments were conducted in order to evaluate the heterozygous effects of irradiated chromosomes on viability. Mutations were accumulated on several hundred second chromosomes by delivering doses of 2,500r over either two or four generations for total X-ray exposures of 5,000r or 10,000r. Chromosomes treated with 5,000r were screened for lethals after the first treatment, and surviving nonlethals were used to generate families of fully treated chromosomes. The members of these families shared the effects of the first irradiation, but differed with respect to those of the second. The chromosomes treated with 10,000r were not grouped into families since mutations were accumulated independently on each chromosome in that experiment. Heterozygous effects on viability of the irradiated chromosomes were tested in both isogenic (homozygous) and nonisogenic (heterozygous) genetic backgrounds. In conjunction with these tests, homozygous viabilities were determined by the marked-inversion technique. This permitted a separation of the irradiated chromosomes into those which were drastic when made homozygous and those which were not. The results indicate that drastic chromosomes have deleterious effects in heterozygous condition, since viability was reduced by 2–4% in tests performed with the 10,000r chromosomes, and by 1% in those involving the 5,000r material. Within a series of tests, the effects were more pronounced when the genetic background was homozygous. Nondrastic irradiated chromosomes did not show detectable heterozygous effects. They also showed no homozygous effects when compared to a sample of untreated controls. In addition, there was no evidence for an induced genetic component of variance with respect to viability in these chromosomes. These results suggest that the mutants induced by high doses of X-rays are principally drastic ones which show deleterious effects on viability in heterozygous condition.     

Population Dynamics of the Segregation Distorter Polymorphism of DROSOPHILA MELANOGASTER

Two two-locus models of the population dynamics of the segregation distortion (SD) polymorphism of Drosophila melanogaster are described. One model is appropriate for understanding the population genetics of SD in nature, whereas the other is a special case appropriate for understanding an artificial population that has been extensively analysed. The models incorporate the general features of the Sd and Rsp loci which form the core of the SD system. It is shown that the SD polymorphism can be established only when there is sufficiently tight linkage between Sd and Rsp. An approximate treatment, valid for tight linkage, is given of all the equilibria of the system and their stabilities. It is shown that the observed composition of natural and artificial populations with respect to the Sd and Rsp loci is predicted well by the model, provided that restrictions are imposed on the fertilities of certain genotypes. Highly oscillatory paths towards equilibrium are usually to be expected on the basis of this model. The selection pressures on inversions introduced into this system are also investigated.