Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Sterility Resulting from Gonadal Dysgenesis in the P-M System

Crosses between two types of strains, called P and M, characteristically give high frequencies of F(1) sterility and other aberrant traits. Previous studies indicated that, in addition to the direction of the parental cross, many factors influence the manifestation of this phenomenon known as "hybrid dysgenesis."-The present study is concerned with the characteristics of GD (gonadal dysgenesis) sterility associated with the P-M system and its temperature dependence. Female sterility is accompanied by a complete absence of egg-laying, and this is not attributable to an inability to mate. Thus, it seems likely that sterility results from a defect in gametogenesis of hybrid individuals. This conclusion is supported by the morphological and cytological observations presented in an accompanying paper (Schaefer, Kidwell and Fausto-Sterling 1979).-A narrow, critical, developmental temperature range was found in which both female and male sterility rose sharply from a low level to a high maximum. The critical range was 27 to 29 degrees for males, slightly higher than the range of 24 to 26 degrees for females. Two other dysgenic traits, male recombination and transmission ratio distortion, were affected by developmental temperature, but temperature response curves were quite different from those for sterility. The temperature-sensitive stage for female sterility occurs during embryonic and early larval development.-GD sterility is compared and contrasted with SF sterility, another specific type of non-Mendelian sterility resulting from a different interstrain dysgenic interaction.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: A Syndrome of Aberrant Traits Including Mutation, Sterility and Male Recombination

A syndrome of associated aberrant traits is described in Drosophila melanogaster. Six of these traits, mutation, sterility, male recombination, transmission ratio distortion, chromosomal aberrations and local increases in female recombination, have previously been reported. A seventh trait, nondisjunction, is described for the first time. All of the traits we have examined are found nonreciprocally in F(1) hybrids. We present evidence that at least four of the traits are not found in nonhybrids. Therefore we have proposed the name hybrid dysgenesis to describe this syndrome.-A partition of tested strains into two types, designated P and M, was made according to the paternal or maternal contribution required to produce hybrid dysgenesis. This classification seems to hold for crosses of strains from within the United States and Australia, as well as for crosses between strains from the two countries. Strains collected recently from natural populations are typically of the P type and those having a long laboratory history are generally of the M type. However, a group of six strains collected from the wild in the 1960's are unambiguously divided equally between the P and M types. The dichotomy of this latter group raises interesting questions concerning possible implications for speciation.-Temperature often has a critical effect on the manifestation of hybrid dysgenesis. High F(1 ) developmental temperatures tend to increase the expression of sterility, sometimes to extreme levels. Conversely, low developmental temperatures tend to inhibit the expression of some dysgenic traits.-There are potentially important practical implications of hybrid dysgenesis for laboratory experimentation. The results suggest that care should be exercised in planning experiments involving strain crosses.     

Sterility and Hypermutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Inbred wild strains of Drosophila melanogaster derived from the central and eastern United States were used to make dysgenic hybrids in the P-M system. These strains possessed P elements and the P cytotype, the condition that represses P element transposition. Their hybrids were studied for the mutability of the P element insertion mutation, snw, and for the incidence of gonadal dysgenesis (GD) sterility. All the strains tested were able to induce hybrid dysgenesis by one or both of these assays; however, high levels of dysgenesis were rare. Sets of X chromosomes and autosomes from the inbred wild strains were more effective at inducing GD sterility than were sets of Y chromosomes and autosomes. In two separate analyses, GD sterility was positively correlated with snw mutability, suggesting a linear relationship. However, one strain appeared to induce too much GD sterility for its level of snw destabilization, indicating an uncoupling of these two manifestations of hybrid dysgenesis.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Morphological and Cytological Studies of Ovarian Dysgenesis

A type of intraspecific hybrid sterility, between two strains of Drosophila melanogaster, referred to as GD (gonadal dysgenesis) sterility, is observed when females from a type of strain called M are crossed with males from a second type called P. Absence of egg-laying is characteristic of female GD sterility and its manifestation is conditional on high developmental temperatures. Morphological and cytological studies of GD sterile females are described. These individuals were normal in body size and external appearance. No defects in sperm storage were observed. Both adult and larval ovaries were drastically reduced in size in comparison with control ovaries. This ovarian dysgenesis was sometimes unilateral, but more frequently it was bilateral, particularly in females developing at the highest test temperature. The ovarioles of dysgenic ovaries contained no vitellaria; the germaria lacked any cells resembling the cystocyte clusters of normal ovaries. It is concluded that sterility results from an early blockage in ovarian development, rather than from atrophy of previously developed structures. Possible mechanisms for this developmental arrest are discussed.     

Bisexual Hybrid Sterility in DROSOPHILA MELANOGASTER

A new type of hybrid sterility was investigated in D. melanogaster . Matings between strain 27 males from Para Wirra, South Australia, and Canton-S females produce 70–80% fully sterile male and female progeny. Strain 27 males produce sterile progeny when crossed to females of other geographic origins, but produce fertile progeny when crossed to a second sympatric strain. The sterility is avoided by lower rearing temperatures. Heat shock and tetracycline produce no improvement in the fertility of the hybrids. Normal flies produce sterile progeny when injected with, or fed, homogenates of sterile flies. A combination of maternal and paternal factors may interact to produce sterile hybrids by inhibiting gonad development.     

The Absence of Somatic Effects of P-M Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

The wings and abdomens of dysgenic and nondysgenic control flies were scored for the presence of clones of cells mutant for first and third chromosome markers. These exceptional clones can arise from mitotic recombination, de novo mutation or deletion, and P-M hybrid dysgenesis has been shown to increase the frequency of parallel processes occurring in germ-line cells. Particular attention was given to careful genetic and molecular characterization of all stocks and to providing adequate and appropriate controls so that even very small increases in somatic clone frequency due to P-M hybrid dysgenesis would be detected. No difference was found in the frequency, size distribution or anatomical distribution of mutant somatic clones correlated to hybrid dysgenesis, confirming previous indications. The potential adaptive significance of a germ-line restriction of P-M hybrid dysgenesis is discussed.     

Components of Hybrid Dysgenesis in a Wild Population of DROSOPHILA MELANOGASTER

Hybrid dysgenesis is a condition found in certain interstrain hybrids of Drosophila melanogaster caused by the interaction of chromosomal and cytoplasmic factors. Germ-line abnormalities, including sterility, high mutability and male recombination, appear in the affected individuals. There are at least two distinct systems of hybrid dysgenesis. We examined a Wisconsin wild population in two consecutive years to determine the distribution of the chromosomal P factor and the extrachromosomal M cytotype that together cause one kind of hybrid dysgenic sterility. The P factor was found to be very common in the population, with all three major chromosomes being polymorphic for it. This polymorphism was strongly correlated with variability for male recombination elements, suggesting that these two traits are part of the same system of hybrid dysgenesis. There was a slight tendency for the P factor to be lost in lines taken from this population and inbred in the laboratory for many generations. A large-scale search for the M cytotype, which causes susceptibility to the P factor, showed that it is present in the population at only very low frequencies. Further evidence that the population is mostly immune to the action of the P factor was our finding of a general lack of dysgenic sterility in the wild flies themselves. However, we were able to isolate several wild strains that consistently showed the M cytotype. In some cases, the frequency of the M cytotype could be maintained in these lines, but it could not usually be increased by artificial selection. Some possible consequences of hybrid dysgenesis for the evolutionary biology of Drosophila are suggested.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Nature and Inheritance of P Element Regulation

The genetic determination of the control of resistance or susceptibility to germ line changes mediated by P elements was studied in two strains and in derivatives of crosses between them. One strain, characterized as true M, completely lacked P elements. The second strain, pseudo-M (M'), carried a number of P elements, but these did not have the potential to induce the gonadal sterility that is associated with P-M hybrid dysgenesis. Individuals from the true M strain were invariably unable to suppress P factor activity (i.e., all daughters of outcrosses of M females and P males were sterile). In contrast, individuals from the M' strain showed variable degrees of suppression that were manifested in a wide range of gonadal sterility frequencies in standard tests. This continuous distribution pattern was reproducible for more than 25 generations.--The results of the genetic analysis indicate that a strain with a variable degree of suppression of gonadal dysgenesis is not necessarily in a transient state between the extreme conditions of P and M cytotype. A large variance in the ability to suppress gonadal dysgenesis with a mean value intermediate between the extremes of P and M cytotype may be a relatively stable strain characteristic. No reciprocal cross effect was observed in the suppression of sterility of F1 females from M X M' matings. Thus, the existence of M' strains indicates a Mendelian component in P element regulation and suggests that cytotype, which has an extrachromosomal aspect, may be only one of perhaps several mechanisms involved in regulation. Analysis of the effects of individual chromosomes from the M' strain showed that each chromosome contributed to the reduction of gonadal dysgenesis in the progeny of test matings. The results are consistent with a one-component titration model for P element regulation.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: The Genetics of Cytotype Determination in a Neutral Strain

The genetic determination of resistance to the sterility-producing genetic elements called P factors was studied in a strain characterized as neutral (Q) in the P-M system of hybrid dysgenesis. Sixteen lines were synthesized, representing all possible homozygous combinations of the three major chromosomes and differing maternal cytoplasms of an original resistant (Q) and susceptible (M) strain.—The results provide a detailed genetic analysis of the determination of cytotype (which mediates resistance or susceptibility to P factors) in the absence of the P-M dysgenic interaction. They extend the findings of Engels (1979) by providing specific information on both the location and relative magnitude of effect of cytotype-determining chromosomal factors and their interaction over time with maternally transmitted cytoplasm.—Cytotype was found to be primarily controlled by the genotype, but the maternal cytoplasm, under some circumstances, has an important short-term effect. Major cytotype-determining chromosomal factors are localized to the distal half of the X chromosome. However, there was also evidence for minor factors located on the major autosomes, particularly chromosome 3. Under certain circumstances, cytotypic switches in either direction can be produced in a single generation by the substitution of an X chromosome carrying a major cytotype determinant. This may provide an explanation of why reciprocal differences have sometimes been interpreted as direct effects of X-chromosome suppressors. However, slow but systematic changes of M to P cytotype were observed in five synthesized lines of mixed origin over twenty generations with no chromosomal substitution. Alternative explanations of these changes in terms of delayed effects of minor autosomal factors or of the transposability of cytotype determinants are discussed.     

Analysis of Y-Linked Mutations to Male Sterility in DROSOPHILA MELANOGASTER

Mating type in haploid cells of the yeast Saccharomyces cerevisiae is determined by a pair of alleles MATa and MAT alpha. Under various conditions haploid mating types can be interconverted. It has been proposed that transpositions of silent cassettes of mating-type information from HML OR HMR to MAT are the source of mating type conversions. A mutation described in this work, designated AON1, has the following properties. (1) MAT alpha cells carring AON1 are defective in mating. (2) AON1 allows MAT alpha/MAT alpha but not MATa/MATa diploids to sporulate; thus, AON1 mimics the MATa requirement for sporulation. (3) mata-1 cells that carry AON1 are MATa phenocopies, i.e., MAT alpha/mata-1 AON1 diploids behave as standard MAT alpha/MATa cells; therefore, AON1 suppresses the defect of mata-1. (4) AON1 maps at or near HMRa. (5) Same-site revertants from AON1 lose the ability to convert mating type to MATa, indicating that reversion is associated with the loss of a functional HMRa locus. In addition, AON1 is a dominant mutation. We conclude that AON1 is a regulatory mutation, probably cis-acting, that leads to the constitutive expression of silent a mating-type information located at HMRa.     

Hybrid Dysgenesis in DROSOPHILA MELANOGASTER: Factors Affecting Chromosomal Contamination in the P-M System

The two interacting components of the P-M system of hybrid dysgenesis are chromosomally associated elements called P factors and a susceptible cytoplasmic state referred to as M cytotype. Previous experiments have indicated that P factors are a family of multiple-copy transposable genetic elements dispersed throughout the genome of P strains but absent in long-established M strains.—Evidence is presented that the sterility and male recombination-inducing potential of P elements may be acquired by X chromosomes, derived from M strains, through nonhomologous association with P strain autosomes, a process referred to as "chromosomal contamination." The frequencies of chromosomal contamination of X chromosomes by P strain autosomes were highly variable and depended on a number of factors. M cytotype (as opposed to P cytotype) was essential for high frequencies of P factor contamination. There were large differences in contamination potential among individual female families, and a weak negative correlation existed between family size and contamination frequency. Chromosomal contamination in the P-M system was shown to be independent of that in the I-R system.—Frequency distributions suggested that the relationship between sterility production and P factor insertion is complex. The majority of P element transpositions, identified by in situ hybridization in one X chromosome, were not associated with gonadal sterility. However, high sterility potential was found to be associated with the presence of at least one P element inserted into the X chromosome. This potential was lost at a rate of about one-sixth per generation in M cytotype but was stabilized in P cytotype. Various hypotheses concerning the relationship between transposition and chromosomal contamination are discussed.     

Chromosomal Effects on Mutability in the P-M System of Hybrid Dysgenesis in DROSOPHILA MELANOGASTER

Two manifestations of hybrid dysgenesis were studied in flies with chromosomes derived from two different P strains. In one set of experiments, the occurrence of recessive X-linked lethal mutations in the germ cells of dysgenic males was monitored. In the other, the behavior of an X-linked P-element insertion mutation, snw, was studied in dysgenic males and also in dysgenic females. The chromosomes of one P strain were more proficient at causing dysgenesis in both sets of experiments. However, there was variation among the chromosomes of each strain in regard to the ability to induce lethals or to destabilize snw. The X chromosome, especially when it came from the stronger P strain, had a pronounced effect on both measures of dysgenesis, but in combination with the major autosomes, these effects were reduced. For the stronger P strain, the autosomes by themselves contributed significantly to the production of X-linked lethals and also had large effects on the behavior of snw, but they did not act additively on these two characters. For this strain, the effects of the autosomes on the X-linked lethal mutation rate suggest that only 1/100 P element transpositions causes a recessive lethal mutation. For the weaker P strain, the autosomes had only slight effects on the behavior of snw and appeared to have negligible effects on the X-linked lethal mutation rate. Combinations of chromosomes from either the strong or the weak P strain affected both aspects of dysgenesis in a nonadditive fashion, suggesting that the P elements on these chromosomes competed with each other for transposase, the P-encoded function that triggers P element activity. Age and sex also influenced the ability of chromosomes and combinations of chromosomes to cause dysgenesis.     

Non-Mendelian Female Sterility in DROSOPHILA MELANOGASTER: Characterization of the Noninducer Chromosomes of Inducer Strains

In relation to non-Mendelian female sterility, Drosophila melanogaster strains can be divided into two main classes, inducer and reactive. The genetic element responsible for the inducer condition (I factor) is chromosomal and may be linked to any inducer-strain chromosome. Each chromosome carrying the I factor (i(+) chromosome) can, when introduced by the paternal gamete into a reactive oocyte, give rise to females (denoted SF) showing more-or-less reduced fertility. As long as i(+) chromosomes are transmitted through heterozygous males with reactive originating chromosomes (r chromosomes), I factor follows Mendelian segregation patterns. In contrast, in heterozygous i(+)/r females, a varying proportion of r chromosomes may irreversibly acquire I factor, independently of classical genetic recombination, by a process called chromosomal contamination. The contaminated reactive chromosomes behave as i(+) chromosomes.-In the present paper, evidence is given that the Luminy inducer strain displays a polymorphism for two kinds of second chromosomes. Some of them are i(+), while others, denoted i(o), are unable to induce any SF sterility when introduced by paternal gametes into reactive oocytes. They are also unable to induce contamination of r chromosomes, but, like r chromosomes, they may be contaminated by i(+) chromosomes in SF or RSF females. The study of the segregation of i(+) and i(o) second chromosomes in the progeny of heterozygous Luminy males and females leads to the conclusion that on chromosome 2 of the Luminy stock the I factor is at a single locus. -X, second and third i(o) chromosomes have been found in several inducer strains. Since these chromosomes can be maintained with i(+) chromosomes in inducer strains in spite of their ability to be contaminated in RSF females, it can be concluded that chromosomal contamination does not take place in females of inducer strains. This implies that contamination occurs only in cells having cytoplasm in a reactive state.     

Gonadal Dysgenesis Determinants in a Natural Population of DROSOPHILA MELANOGASTER

Three populations of Drosophila melanogaster from northern California were surveyed for the ability to produce and resist gonadal dysgenesis in the P-M system of hybrid dysgenesis. Males from all three populations produced low to moderate levels of gonadal dysgenesis in crosses to Oregon-R M females. Most females had the P cytotype, but the M cytotype occurred occasionally. The three populations could not be statistically differentiated from one another, but were easily distinguished from populations from Australia and Wisconsin on the basis of gonadal dysgenesis potential. The California populations had higher levels of M cytotype than did the Wisconsin population. Thirteen X chromosomes and 11 pairs of autosomes were extracted from one of the California populations, using a modification of the standard balancer chromosome technique to suppress hybrid dysgenesis during extraction. All lines produced strongly skewed sterility distributions in crosses to M-strain females, and mean levels of sterility were less than 50%. There was evidence of nonadditive interactions between the autosomes. Most extraction lines had the P cytotype, but M and intermediate cytotypes were observed. Some of the intermediate cytotypes were stable over time. Lines were tested at two different times after extraction. Some lines evolved higher sterility potential as they were kept in the laboratory, even in the presence of P cytotype. The results point out a number of deficiencies in current genetic and population genetic models of hybrid dysgenesis and imply that gonadal dysgenesis is unlikely to be an important evolutionary force in this population.     

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.     

Non-Mendelian Female Sterility in DROSOPHILA MELANOGASTER: Principal Characteristics of Chromosomes from Inducer and Reactive Origin after Chromosomal Contamination

Strains of Drosophila melanogaster can be divided into two main classes, inducer and reactive, in relation to non-Mendelian female sterility. The genetic element responsible for the inducer condition (I factor) is chromosomal and may be linked to any inducer-strain chromosome. Each chromosome carrying the I factor (i(+) chromosome) can produce females showing more-or-less reduced fertility when it is introduced by paternal gametes into a reactive oocyte. As long as i(+) chromosomes are transmitted through heterozygous males with reactive originating chromosomes (r chromosomes), I factor strictly follows Mendelian segregation. In contrast, in heterozygous i(+)/r females, a varying proportion of r chromosomes may acquire I factor independently of classical genetic recombination, by a process called chromosomal contamination. This paper reports investigation of the characteristics of the three kinds of chromosomes produced by females in which contamination occurs. It appears that the contaminated reactive chromosomes have irreversibly acquired I factor and behave like i(+) chromosomes, while the i(+) chromosomes used as contaminating elements and the reactive originating chromosomes that have not been contaminated have not undergone any change.     

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.