Rapid unidirectional change of hybrid dysgenesis potential in Drosophila

Har-P, a short P-element variant, weaponizes P-transposase to severely impair Drosophila development

Without transposon-silencing Piwi-interacting RNAs (piRNAs), transposition causes an ovarian atrophy syndrome in Drosophila called gonadal dysgenesis (GD). Harwich (Har) strains with P-elements cause severe GD in F1 daughters when Har fathers mate with mothers lacking P-element-piRNAs (i.e. ISO1 strain). To address the mystery of why Har induces severe GD, we bred hybrid Drosophila with Har genomic fragments into the ISO1 background to create HISR-D or HISR-N lines that still cause Dysgenesis or are Non-dysgenic, respectively. In these lines, we discovered a highly truncated P-element variant we named ‘Har-P’ as the most frequent de novo insertion. Although HISR-D lines still contain full-length P-elements, HISR-N lines lost functional P-transposase but retained Har-P’s that when crossed back to P-transposase restores GD induction. Finally, we uncovered P-element-piRNA-directed repression on Har-P’s transmitted paternally to suppress somatic transposition. The Drosophila short Har-P’s and full-length P-elements relationship parallels the MITEs/DNA-transposase in plants and SINEs/LINEs in mammals.

DNA provides the instructions needed for life, a role that relies on it being a very stable and organized molecule. However, some sections of DNA are able to move from one place in the genome to another. When these “mobile genetic elements” move they may disrupt other genes and cause disease. For example, a mobile section of DNA known as the P-element causes a condition called gonadal dysgenesis in female fruit flies, leading to infertility.

Only certain strains of fruit flies carry P-elements and the severity of gonadal dysgenesis in their daughters varies. For example, when male fruit flies of a strain known as Harwich (or Har for short) is crossed with female fruit flies that do not contain P-elements, all of their daughters develop severe gonadal dysgenesis and are infertile. However, if the cross is done the other way around, and female Har flies mate with males that do not contain P-elements, the daughters are fertile because the Har mothers provide their daughters with protective molecules that silence the P-elements. But it was a mystery as to why the P-elements from the Har fathers always caused such severe gonadal dysgenesis in all the daughters.

Here, Srivastav et al. bred fruit flies to create offspring that had different pieces of Har DNA in a genetic background that was normally free from P-elements; they then analyzed the ‘hybrid’ offspring to identify which pieces of the Har genome caused gonadal dysgenesis in the daughter flies. These experiments showed that Har flies possess a very short variant of the P-element (named “Har-P”) that is more mobile than other variants. However, the Har-P variants still depended on an enzyme known as P-transposase encoded by the full-length P-elements to move around the genome. Further experiments showed that other strains of fruit flies that cause severe gonadal dysgenesis also had very short P-element variants that were almost identical to Har-P.

These findings may explain why Har and some other strains of fruit flies drive severe gonadal dysgenesis. In the future, it may be possible to transfer P-transposase and Har-P into mosquitoes, ticks and other biting insects to make them infertile and help reduce the spread of certain diseases in humans.

Structures of defective P transposable elements prevalent in natural Q and Q-derived M strains of Drosophila melanogaster

Several DNA sequences with homology to the complete 2.9-kilobase (kb) P element from a P strain in the United States were isolated and characterized from two Drosophila melanogaster strains collected on Chichi Jima, an island 1000 km south of Tokyo. Except for a missing central region and trivial unsequenced regions of 38 base pairs, the 2.1-kb element isolated from a Q strain had the same DNA sequence as that of the complete P element. Seven other elements cloned from genomic DNAs of the Q strain and a Q-derived M strain all possessed the same restriction sites as those of the 2.9-kb P element except for one deleted region in each element. The finding of sequence conservation in P elements have had a common ancestor relatively recently. Thus, it is suggested that the P element family was a recent invader of the species. By contrast, no complete P element was found in these Japanese strains so far as surveyed, indicating the possibility that P elements in the Chichi Jima population are almost all defective. The implication of this possibility is discussed in relation to the uniqueness of the population on Chichi Jima where Q strains predominate and no P strains have yet been found.

Selection components in background replacement lines of Drosophila

Selection components analysis was performed in lines of Drosophila melanogaster at three times during substitution backcrossing. The initial two lines were inbred isofemale lines from natural populations in California, and one had the spread wing mutation eagle. The selection components analysis revealed aspects of the genetic structure of the determinants of fitness by demonstrating changes in the marginal fitnesses of the eagle locus. Differences among backgrounds essentially disappeared by the 20th generation of backcrossing, suggesting that the previously observed differences were attributable to linkage disequilibrium. The method of bootstrapping was used as a novel means of determining statistical confidence in selection components.

The population dynamics of transposable elements

This paper describes analytical and simulation models of the population dynamics of transposable elements in randomly mating populations. The models assume a finite number of chromosomal sites that are occupable by members of a given family of elements. Element frequencies can change as a result of replicative transposition, loss of elements from occupied sites, selection on copy number per individual, and genetic drift. It is shown that, in an infinite population, an equilibrium can be set up such that not all sites in all individuals are occupied, allowing variation between individuals in both copy number and identity of occupied sites, as has been observed for several element families in Drosophila melanogaster. Such an equilibrium requires either regulation of transposition rate in response to copy number per genome, a sufficiently strongly downwardly curved dependence of individual fitness on copy number, or both. The probability distributions of element frequencies, generated by the effects of finite population size, are derived on the assumption of independence between different loci, and compared with simulation results. Despite some discrepancies due to violation of the independence assumption, the general pattern seen in the simulations agrees quite well with theory.

Data from Drosophila population studies are compared with the theoretical models, and methods of estimating the relevant parameters are discussed.

P-element-induced mutation and quantitative variation inDrosophila melanogaster: lack of enhanced response to selection in lines derived from dysgenic crosses

Dysgenic and non-dysgenic base populations were made by reciprocal crossing of Harwich (P) and Canton-S (M) strains. From each cross, two up and two down selection lines were established, with selection on abdominal bristle number for ten generations. The intensity of selection was 10 out of 50 individuals from each sex. Mean bristle number, phenotypic variation and heritabilities were compared between dysgenic and non-dysgenic populations under selection. Except for an anomalous non-dysgenic downline in which a mutation of large effect occurred, all lines showed similar responses to selection. These results contrast with the results reported by Mackay (1984, 1985) in which substantial increases were obtained for response to selection, phenotypic variation and heritability in the dysgenic compared to non-dysgenic lines. There are some indications that the higher response in our aberrant non-dysgenic downline is the result of transposition. Possible explanations for the occurrence of transposition and dysgenesis in the lines derived from nondysgenic crosses are discussed.

Temporal stability of P–M cytotype polymorphism in a natural population ofDrosophila melanogaster

The P–M system of hybrid dysgenesis inDrosophila melanogasteris a syndrome of genetic abnormalities which appears in the progeny of crosses between strains different with regard to their possession of ‘P’ transposable elements. Cytotype is an extrachromosomal property which regulates the mobility of the P element. We report here data showing that a cytotype polymorphism previously observed in a natural population from North-Africa is stable over a period of 5 years. A potentially high rate of mutation is associated with this cytotype polymorphism. Explanations of the appearance of a cytotype polymorphism are proposed and the consequences for the genetic load induced by transposable elements are discussed.

Seasonal analysis of 23.5 MRF (hobo) and P-M hybrid dysgenesis determinants in a Greek natural population of Drosophila melanogaster

Genetic analysis of 23.5 MRF (hobo) and P-M hybrid dysgenesis determinants in a Greek natural population in six collections over 24 months, showed the existence of hobo activity in the population at rates higher than P activity. Moreover, seasonal differentiation in hobo GD-sterility potential and hobo repressor abilities were observed. The P activity was low in the population but some tendency for seasonal differentiation of the cytotype was detected. The two systems operate independently in nature. Analysis of isofemale lines, established from inseminated wild-caught females, showed rapid differentiation of their hybrid dysgenesis determinants in the laboratory. This shows that results obtained from isofemale lines do not necessarily reflect the original population structure. The seasonal differentiation may be correlated with seasonal environmental factors, and may be attributed to differences in structure and function of the elements that consequently affect their regulation and transposition.

Endogenous factors regulating mosquito host-seeking behaviour

The physiological state of the mosquito can modulate behaviours that are normally activated by external stimuli. Even though host stimuli may be present at certain times, the insect may not always express host-seeking behaviour, depending upon the physiological factors that predominate. Traditional views of the gonotrophic cycle characterize mosquitoes as engaging in blood feeding only once before depositing eggs. However, physiological state, including such factors as age, nutritional state, presence of eggs, mating condition, circadian rhythmicity and the number of gonotrophic cycles completed, can affect the expression of this behaviour.

Prevalence of full-size P and KP elements in North American populations of Drosophila melanogaster

The P transposable element invaded the Drosophila melanogaster genome in the middle of the twentieth century, probably from D. willistoni in the Caribbean or southeastern North America. P elements then spread rapidly and became ubiquitous worldwide in wild populations of D. melanogaster by 1980. To study the dynamics and long-term fate of transposable genetic elements, we examined the molecular profile of genomic P elements and the phenotype in the P-M system of the current North American natural populations collected in 2001-2003. We found that full-size P and KP elements were the two major size classes of P elements present in the genomes of all populations ("FP + KP predominance") and that the P-related phenotypes had largely not changed since the 1980s. Both FP + KP predominance and phenotypic stability were also seen in other populations from other continents. As North American populations did not show many KP elements in earlier samples, we hypothesize that KP elements have spread and multiplied in the last 20 years in North America. We suggest that this may be due to a transpositional advantage of KP elements, rather than to a role in P-element regulation.

Recent advances in transgenic arthropod technology

The ability to insert foreign genes into arthropod genomes has led to a diverse set of potential applications for transgenic arthropods, many of which are designed to advance public health or improve agricultural production. New techniques for expressing foreign genes in arthropods have now been successfully used in at least 18 different genera. However, advances in field biology are lagging far behind those in the laboratory, and considerable work is needed before deployment in nature can be a reality. A mechanism to drive the gene of interest though a natural population must be developed and thoroughly evaluated before any field release, but progress in this area has been limited. Likewise, serious consideration of potential risks associated with deployment in nature has been lacking. This review gives an overview of the most promising techniques for expressing foreign genes in arthropods, considers the potential risks associated with their deployment, and highlights the areas of research that are most urgently needed for the field to advance out of the laboratory and into practice.

A theoretical approach to predicting the success of genetic manipulation of malaria mosquitoes in malaria control

BACKGROUND

Mosquitoes that have been genetically modified to better encapsulate the malaria parasite Plasmodium falciparum are being considered as a possible tool in the control of malaria. Hopes for this have been raised with the identification of genes involved in the encapsulation response and with advances in the tools required to transform mosquitoes. However, we have only very little understanding of the conditions that would allow such genes to spread in natural populations.

METHODS

We present here a theoretical model that combines population genetical and epidemiological processes, thereby allowing one to predict not only these conditions (intensity of transmission, evolutionary cost of resistance, tools used to drive the genes) but also the impact of the spread of refractoriness on the prevalence of the disease.

RESULTS

The main conclusions are 1) that efficient transposons will generally be able to drive genes that confer refractoriness through populations even if there is a substantial (evolutionary) cost of refractoriness, but 2) that this will decrease malaria prevalence in the human population substantially only if refractoriness is close to 100% effective.

CONCLUSIONS

If refractoriness is less than 100% effective (because of, for example, environmentally induced variation in the effectiveness of the mosquito's immune response), control programmes based on genetic manipulation of mosquitoes will have very little impact on the epidemiology of malaria, at least in areas with intense transmission.

Differences in P element population dynamics between the sibling species Drosophila melanogaster and Drosophila simulans

Patterns of P element establishment and evolution were compared in populations of D. melanogaster and D. simulans. For each species, mixed populations were initiated with M strain flies lacking P elements together with P strain flies having similar P element copy numbers and phenotypes. The mixed populations were subsequently maintained under similar environmental conditions. On the basis of gonadal sterility assays, P elements tended to be significantly more active in D. melanogaster than in D. simulans populations. This activity difference between the two species was positively associated with P element copy number, determined by restriction enzyme analysis, and transposition frequency, as determined by a transposition assay. Host factors are the most likely explanation for the observed species variation. Difficulty of establishment may be a factor determining the absence of P elements in natural populations of D. simulans.

The origin of P elements in Drosophila melanogaster

The P family of transposable genetic elements is thought to be a recent addition to the Drosophila melanogaster genome. New evidence suggests that the elements came from another Drosophila species, possibly carried by parasitic mites. The transposition mechanism of P elements involves DNA gap repair which may have facilitated their rapid spread through D. melanogaster worldwide. These results provide new insight into the process of a transposon's invasion into a new species and the potential risk of extinction such an invasion might entail.

Can transposable elements be used to drive disease refractoriness genes into vector populations?

A number of biological procedures are currently being considered as alternatives to insecticide-based methods for the control of insect vectors of disease. Among these are the adaptation of various genetic mechanisms to drive genes of interest, such as refractoriness to malaria in mosquitoes, into natural populations, for vector control purposes. Here, Margaret Kidwell and Jose Ribeiro develop a rationale for the possible use of transposable genetic elements, one of these potential drive mechanisms, and some of the problems being faced in seeking to determine the feasibility of such a strategy are described.

Horizontal transfer of P elements and other short inverted repeat transposons

Evidence for horizontal transfer of the P family of transposable elements in the genus Drosophila is reviewed and evaluated, along with observations consistent with the recent invasion of Drosophila melanogaster by these elements. Some other examples of horizontal transfer involving other groups of transposable elements having short inverted terminal repeats are also briefly described. The sequential mechanistic steps likely to be involved in a horizontal transfer event are explored, including the requirement for suitable interspecific vectors or carriers. Finally, the frequency and significance of horizontal transfer of transposable elements are briefly discussed within an evolutionary framework.

Comparative study of P element activity in two natural populations of Drosophila melanogaster

Population structure concerning P element activity was investigated in two natural Drosophila populations. These populations are very different as far as in the viability spectrum is concerned. In one population, the Raleigh, United States population, genetic loads related to viability have been kept at a fairly high level. In the other population, the Nagasaki, Japan, population, the genetic loads tend to be stable at very low levels. In the Raleigh population it is estimated that on the average 4 copies of intact P elements that possess transposase activity exist in the genome. On the other hand only 0.7 complete copies are estimated to exist in the genome of the Nagasaki population. Heterogeneity in the P element copy number and significant positive linkage disequilibrium among occupied sites were detected in the Raleigh population. Our results, with some evidences which indicate that high mutation rate was caused by the P element, suggests that the large genetic loads in the Raleigh population are caused by the rapid invasion of P element in this population.

Cytotype control of Drosophila P element transposition: the 66 kd protein is a repressor of transposase activity

Drosophila P transposable elements encode two proteins, an 87 kd transposase protein and a 66 kd protein that has been hypothesized to repress transposition. We have made germline transformants carrying modified P element derivatives that encode only the 66 kd protein and shown that these elements repress transposase activity in both the germline and the soma. The position of these elements in the genome quantitatively affected their ability to negatively regulate transposase and to express the 66 kd protein. Single 66 kd element-containing strains did not exhibit the maternal inheritance of P cytotype characteristic of P strains. However, we demonstrated that a true P strain produced higher levels of the 66 kd protein during oogenesis than single 66 kd P elements. Thus, the expression of the 66 kd repressor during oogenesis may be a major determinant of the maternal effect of P cytotype.

Hybrid dysgenesis-induced response to selection in Drosophila melanogaster

In Drosophila melanogaster, the P-M and I-R systems of hybrid dysgenesis are associated with high rates of transposition of P and I elements, respectively, in the germlines of dysgenic hybrids formed by crossing females of strains without active elements to males of strains containing them. Transposition rates are not markedly accelerated in the reciprocal, nondysgenic hybrids. Previous attempts to evaluate the extent to which hybrid dysgenesis-mediated P transposition contributes to mutational variance for quantitative characters by comparing the responses to selection of P-M dysgenic and nondysgenic hybrids have given variable results. This experimental design has been extended to include an additional quantitative trait and the I-R hybrid dysgenesis system. The selection responses of lines founded from both dysgenic and nondysgenic crosses showed features that would be expected from the increase in frequency of initially rare genes with major effects on the selected traits. These results differ from those of previous experiments which showed additional selection response only in lines started from dysgenic crosses, and can be explained by the occasional occurrence of large effect transposable element-induced polygenic mutations in both dysgenic and nondysgenic selection lines. High rates of transposition in populations founded from nondysgenic crosses may account for the apparently contradictory results of the earlier selection experiments, and an explanation is proposed for its occurrence.

Are Drosophila melanogaster populations under a stable geographical differentiation due to the presence of P elements?

In order to simulate the outcome of the P-M status of Eurasian populations of Drosophila melanogaster, the evolution of experimental mixed-strains was monitored for up to 50 generations. The results were compared with the evolution in natura of European populations sampled in 1981-83 and 1986-87 over a similar period of time. Different combinations of P and M' strains, Q and M' strains, M' and M' strains and M and M' strains were set up at 25 degrees C and duplicated at 13 degrees C night-21 degrees C day. The possibility of a change towards a P type only appeared with the introduction of the strong P strain Harwich into Eurasian strains. Strains with strength similar to that of Harwich are not currently found in wild populations. With the introduction of weak P and Q strains of the strength presently observed in western Europe, experimental populations evolved slowly towards a Q state or a weak M' state. M'-M' mixed populations resulted in strong M' strains, as was the case for M-M' populations. In these cases. P sequences were not eliminated. In wild populations both genetic and molecular analyses showed no significant differences, over a five year period, for GD sterility potentials, for total P copy number or for distribution of the full-sized and KP elements. Changes in Eurasian populations are probably taking place at a very low rate and may even have stopped, leading to a quasi-stable differentiation over the continent. During the different steps of progressive invasion of P transposable elements, several deleted elements developed and natural selection may have acted on them. The possibility of the selection of different types of regulatory mechanisms according to the presence of different kinds of derivative elements, leading to a world-wide differentiation between P-Q and M' strains is discussed.

Comparison of the regulation of P elements in M and M' strains of Drosophila melanogaster

M and M' strains of Drosophila melanogaster in the P-M system of hybrid dysgenesis were compared in two series of tests, with the following results. (1) The singed-weak hypermutability regulation test showed that M' strains had lower P excision rates than M strains, suggesting that P-elements repression must occur in M' strains although it is not detectable by gonadal dysgenesis assays. (2) The evolution of mixed P+M and mixed P+M' populations was compared, using a strong P strain. The P+M cultures invariably evolved in a few generations into strong P cultures, while the P+M' cultures evolved into P-type cultures with reduced P-factor potentials. However, after 30 generations of culture, both these types of mixed cultures had similar P copy numbers, suggesting that regulation of copy number had occurred in them.

Evolution of hybrid dysgenesis potential following P element contamination in Drosophila melanogaster

P elements were introduced into M strain genomes by chromosomal contamination (transposition) from P strain chromosomes under conditions of P-M hybrid dysgenesis. A number of independently maintained contaminated lines were subsequently monitored for their ability to induce gonadal (GD) sterility in the progeny of reference crosses, over a period of 60 generations, in two experiments. The efficiency of chromosomal contamination was high; all tested lines acquired P elements following the association of M and P chromosomes in the same genome for a single generation. All the contaminated lines also sustained an initial unstable phase, marked by high frequencies of transposition and sterility within lines, in the absence of P element regulation. Subsequently, each of the lines rapidly evolved to one of three relatively stable strain types whose phenotypic and molecular properties correspond rather closely to those of the P, Q and M' strains that have previously been characterized. The numbers and structures of P elements and the presence or absence of P element regulation during the early generations appeared to be critical factors determining the subsequent course of evolution. On the basis of GD sterility frequencies, both the mean level of P activity, and the average capacity for P element regulation, were reduced in lines raised at 25 degrees, relative to those raised at 20 degrees, during the early generations. This latter result is consistent with the expectation that natural selection will tend to modify the manifestation of dysgenic traits, such as high temperature sterility, which cause a reduction of fitness. However, overall, stochastic factors appeared to predominate in determining the course of evolution of individual lines.

A latitudinal cline in P-M gonadal dysgenesis potential in Australian Drosophila melanogaster populations

Isofemale lines ofDrosophila melanogasterfrom six localities along the east coast of Australia, spanning 2900 km and 26 degrees of latitude, were assayed for their gonadal dysgenesis characteristics in theP–Msystem of hybrid dysgenesis. A strong clinal pattern with latitude was discovered. From north to south, the first two populations were typical strongPpopulations, and the next population was moderateP. The next population to the south was neutral (Q), with some weakPand weakMcharacteristics. The two southernmost populations were typicalMpopulations. Much variance inPactivity inPpopulations and in susceptibility toPactivity inMpopulations was detected among isofemale lines. This clinal pattern with latitude of theP–Msystem is paralleled by similar clinal patterns for frequencies of common cosmopolitan inversions and of certain allozymes in Australia. A model of introductions of flies with different characteristics in the north and south could account for theP–Mclinal pattern, but cannot account for an intermediateQpopulation, nor establish the inversion and isoenzyme dines at the same time. Current models of transposable element population dynamics are limited to single population dynamics, and are therefore inadequate for these clinal data.

The distribution of P-element sequences in Drosophila: the willistoni and saltans species groups

This report describes the distribution of P-element sequences among members of the closely related willistoni and saltans species groups of the subgenus Sophophora. Gel-blotting analyses showed that many, but not all, species from each of these groups possess sequences with homology to the P transposable element of Drosophila melanogaster, a sophophoran species belonging to the melanogaster species group. Furthermore, P-homologous fragments are present in lower numbers in willistoni- and saltans-group species than in D. melanogaster P strains, and, in some species of those two groups, exhibit species-characteristic hybridization patterns. On the basis of these results, it is proposed that P elements have had a long evolutionary history in the willistoni and saltans lineages.

Molecular analysis of P element behavior in Drosophila simulans transformants

In this report we describe the successful transformation of Drosophila simulans with an autonomous P element from Drosophila melanogaster without the use of a selectable marker. This result demonstrates that there is no species barrier for P element transposition. Utilizing gel blotting and in situ hybridization techniques, we have monitored the behavior of newly-introduced P elements in several D. simulans transformed lines over twelve generations. In most instances, an overall increase in the number of P elements was observed. An examination of the frequency of P-element-bearing individuals in one line revealed the rapid spread of P elements through the population. Analysis of well-characterized sublines confirmed that P elements increase in number by transposition to new genomic sites. The formation of degenerate elements occurred in at least one case. These observations suggest that P elements may behave similarly in D. melanogaster and D. simulans.

Quantitative models of hybrid dysgenesis: rapid evolution under transposition, extrachromosomal inheritance, and fertility selection

A model of the P-M system of hybrid dysgenesis is presented which incorporates single-site transposition of P factors in M cytotype, determination of offspring cytotype by both maternal cytotype and maternal or offspring nuclear genotype, and strong fertility selection in dysgenic individuals. The conditions required for the initial invasion of P factors into a pure M population, information concerning stable polymorphisms, and results of numerical iterations depicting the dynamic, nonequilibrium behavior of the system are summarized. While conditions for initial increase are independent of the rate of cytotype switching, the rate of evolution is accelerated by increased production of dysgenic individuals. If the transposition rate is sufficiently high to overcome the fertility barrier opposing P factors introduced into M populations, then convergence to high frequencies of the P factor occurs very rapidly. Under intense fertility depression, the phase of rapid increase may be preceded by an extended period of gradual increase at low frequencies.

Hybrid dysgenesis in Drosophila melanogaster: the evolution of mixed P and M populations maintained at high temperature

The ability of hybrid dysgenesisPfactors to survive and multiply under conditions of strong negative sterility selection was studied in mixedPandMlaboratory cultures. Eight populations were initiated with varying proportions ofPandMstrains. Mixed populations and controls were maintained for seventeen generations at 27 °C, a temperature sufficiently high to induce maximum frequencies of sterility in dysgenic hybrids. The two components of dysgenesis,Pfactor activity and cytotype, were monitored every generation for the first ten generations and intermittently thereafter. With one exception, all the mixed populations evolved to thePtype indicating thatPfactors can survive and multiply, despite low initial frequency and strong negative selection against dysgenic hybrids. However, the average level ofPfactor activity attained at equilibrium was considerably lower than that of thePstrain control population maintained under the same conditions. It was also lower than the equilibrium level ofPfactor activity attained in a similar experiment carried out at a lower temperature, suggesting that selection favouredPfactors with weak rather than strong sterility potential.

Sequences homologous to P elements occur in Drosophila paulistorum

P elements, a class of mobile genetic elements that cause hybrid dysgenesis in Drosophila melanogaster, have thus far been shown to occur only in this species. Using whole genome blot analysis, we present evidence which indicates that sequences homologous to D. melanogaster P elements also occur in Drosophila paulistorum, a distant relative. D. paulistorum is considered a species complex, consisting of six known incipient species or semispecies. All six semispecies possess P element homologous sequences. We further show that there is conservation of restriction enzyme recognition sites between the D. melanogaster and D. paulistorum P element sequences. The presence of P elements in these two species may clarify the roles of recent invasion and rapid loss in the temporal distribution of P elements in Drosophila.

An SD (segregation distribution)-MR (male recombination) chromosome isolated from a natural population of Drosophila melanogaster

A second chromosome line ofDrosophila melanogaster(S-90), isolated from a northern California natural population, is able to induce (1) an increased frequency of X-chromosome visible mutations, (2) male recombination activity subject to reciprocal cross suppression, and (3) strong meiotic drive from heterozygous males. Based upon several lines of evidence (including the response to suppressor chromosomes of both systems) we conclude thatS-90contains bothSD(Segregation Distortion) andMR(PorI) chromosome activity. The two systems appear to behave independently and simultaneously, and a small centromeric region of theS-90chromosome appears to contain the major genetic elements of both systems.

Evolution of hybrid dysgenesis determinants in Drosophila melanogaster

Hybrid dysgenesis is manifested as a group of correlated aberrant genetic traits such as sterility, increased mutation rate, and male recombination. Previous work has shown that it appears when males of strains carrying either of two independent families of transposable elements called I and P factors are hybridized with females of susceptible strains called R and M, respectively. Here the results of an extensive survey for dysgenic potential in Drosophila melanogaster strains are reported. Striking temporal trends in the distribution of strains were observed with respect to the two transposable element systems; in particular, the frequency of R and M strains is positively correlated with laboratory age. In recent tests of strain samples, those collected from nature about 50 years ago were the earliest observed to possess I characteristics. The I type was increasingly frequent in samples from strains more recently originating in the wild. This type is apparently ubiquitous in present day natural populations. the P type was not found in strain samples collected before 1950, and collections made subsequently showed increasing frequencies of P-factor activity with decreasing laboratory age. Marked geographical patterns are documented in the contemporary worldwide distribution of variant strains within the P-M system. M strains are currently fairly common in natural populations from various parts of the world, except on the American continent where they are rare. The degree and distribution of quantitative variation within M and P strain categories is related to their time of origin in the wild. The implications of these results are discussed in relation to the hypothesis that hybrid dysgenesis determinants have evolved recently in natural populations and to an alternative hypothesis of laboratory evolution.