Molecular characteristics of the heterochromatic I elements from a reactive strain ofDrosophila melanogaster

Summary There are two categories of strains inDrosophila melanogaster with respect to the I-R system of hybrid dysgenesis. The inducer strains contain particular transposable elements named I factors. They are not present in the strains of the other category called reactive (R) strains. Defective I elements are present in the pericentromeric regions of both categories of strains. This last subfamily of I sequences has not yet been described in detail and little is known about its origin. In this paper, we report that the defective I elements display an average of 94% of sequence identity with each other and with the transposable I factor. The results suggest that they cannot be the progenitors of the present day I factors, but that each of these two subfamilies started to evolve independently several million years ago. Furthermore, the sequence comparison of these I elements with an active I factor fromDrosophila teissieri provides useful information about when the deleted I elements became immobilized.     

Evidence for rapid limitation of the I element copy number in a genome submitted to several generations of I-R hybrid dysgenesis in Drosophila melanogaster

Summary When Drosophila melanogaster males coming from a class of strains known as inducer are crossed with females from the complementary class (reactive), a quite specific kind of sterility is observed in the F₁ female progeny (denoted SF). The inducer chromosomes differ from the reactive chromosomes by the presence of a transposable element (called the I factor) that is responsible for the induction of this dysgenic symptom. In the germ line of dysgenic females, up to 100% of the reactive chromosomes may be contaminated, i.e. they acquire I factor(s) owing to very frequent replicative transpositions. A contaminated reactive stock was obtained by reconstructing the reactive genotype in the offspring of SF females and its kinetics of invasion by I elements was followed in the successive inbred dysgenic generations. The results show that the mean copy number of I elements increased very quickly up to the level of inducer strains and then stayed in equilibrium even though the dysgenic state was perpetuated by selection for SF sterility at every generation. The possible mechanisms of this copy number limitation are discussed.     

Defective I elements introduced intoDrosophila as transgenes can regulate reactivity and prevent I-R hybrid dysgenesis

The I-R hybrid dysgenesis syndrome is characterized by a high level of sterility and I element transposition, occurring in the female offspring of crosses between males of inducer (I) strains, which contain full-length transposable I elements, and females of reactive (R) strains, devoid of functional I elements. The intensity of the syndrome in the dysgenic cross is essentially dependent on the reactivity level of the R females, which is ultimately controlled by still unresolved polygenic chromosomal determinants. In the work reported here, we have introduced a transposition-defective I element with a 2.6 kb deletion within its second open reading frame into a highly reactive R strain, by P-mediated transgenesis. We demonstrate that this defective I element gradually alters the level of reactivity in the three independent transgenic lines that were obtained, over several generations. After > 15 generations, the transgenicDrosophila show strongly reduced reactivity, and finally become refractory to hybrid dysgenesis, without, however, acquiring the inducer phenotype. Induction of a low reactivity level is reversible reactivity again increases upon transgene removal and is maternally inherited, as observed for the control of reactivity in natural R strains. These results demonstrate that defective I elements introduced as single-copy transgenes can act as regulators of reactivity, and suggest that some of the ancestral defective pericentromeric I elements that can be found in all reactive strains could be the molecular determinants of reactivity.     

Defective I elements introduced intoDrosophila as transgenes can regulate reactivity and prevent I-R hybrid dysgenesis

The I-R hybrid dysgenesis syndrome is characterized by a high level of sterility and I element transposition, occurring in the female offspring of crosses between males of inducer (I) strains, which contain full-length transposable I elements, and females of reactive (R) strains, devoid of functional I elements. The intensity of the syndrome in the dysgenic cross is essentially dependent on the reactivity level of the R females, which is ultimately controlled by still unresolved polygenic chromosomal determinants. In the work reported here, we have introduced a transposition-defective I element with a 2.6 kb deletion within its second open reading frame into a highly reactive R strain, by P-mediated transgenesis. We demonstrate that this defective I element gradually alters the level of reactivity in the three independent transgenic lines that were obtained, over several generations. After > 15 generations, the transgenicDrosophila show strongly reduced reactivity, and finally become refractory to hybrid dysgenesis, without, however, acquiring the inducer phenotype. Induction of a low reactivity level is reversible reactivity again increases upon transgene removal and is maternally inherited, as observed for the control of reactivity in natural R strains. These results demonstrate that defective I elements introduced as single-copy transgenes can act as regulators of reactivity, and suggest that some of the ancestral defective pericentromeric I elements that can be found in all reactive strains could be the molecular determinants of reactivity.     

Control of expression of the I factor, a LINE-like transposable element in Drosophila melanogaster.

I factors are LINE-like transposable elements in the genome of Drosophila melanogaster. They normally transpose infrequently but are activated in the germline of female progeny of crosses between males of a strain that contains complete elements, an I or inducer strain and females of a strain that does not, an R or reactive strain. This causes a phenomenon known as I-R hybrid dysgenesis. We have previously shown that the I factor promoter lies between nucleotides 1 and 30. Here we demonstrate that expression of this promoter is regulated by nucleotides 41-186 of the I factor. This sequence can act as an enhancer as it stimulates expression of the hsp7O promoter in ovaries in the absence of heat-shock. Within this region there is a site that is required for promoter activity and that is recognized by a sequence-specific binding protein. We propose that this protein contributes to the enhancer activity of nucleotides 41-186 and that reduced I factor expression in inducer strains is due to titration of this protein or others that interact with it.     

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.     

Characterization of 5' truncated transposed copies of the I factor in Drosophila melanogaster.

I factors in Drosophila melanogaster are transposable elements structurally related to Mammalian LINEs. Their transposition is activated at high frequencies during I-R hybrid dysgenesis and is associated with the production of mutations of various sorts. Very few of these mutations have been studied at the molecular level; those reported so far result either from chromosomal rearrangements or from insertions of complete I factors. We have analysed three I-R induced yellow mutations and have found that one of them is due to the insertion of an I element very similar to the complete I factor, whereas the other two are due to insertions of I elements that are truncated at their 5' ends; one of them exhibits an unusual 3' end. We discuss possible mechanisms of production of such modified I elements.     

The molecular basis of I-R hybrid dysgenesis in Drosophila melanogaster: identification, cloning, and properties of the I factor

We have analyzed two mutations of the white-eye gene, which arose in flies subject to I-R hybrid dysgenesis. These mutations are associated with insertions of apparently identical 5.4 kb sequences, which we have cloned. We believe that these insertions are copies of the I factor controlling I-R hybrid dysgenesis. The I factor is not a member of the copia-like or fold-back classes of transposable elements and has no sequence homology with the P factor that controls P-M dysgenesis. All strains of D. melanogaster contain I-factor sequences. Those present in reactive strains must represent inactive I elements. I elements have a remarkably similar sequence organization in all reactive strains and are located in peri-centromeric regions. Inducer strains appear to contain both I elements, located in peri-centromeric regions, and 10-15 copies of the complete I factor at sites on the chromosome arms.     

The I-R system of hybrid dysgenesis in Drosophila melanogaster: influence on SF females sterility of their inducer and reactive paternal chromosomes.

A specific kind of sterile F1 female, denoted SF, arises when females from strains known as reactive are crossed with males from the complementary class of strains (inducer). It has been shown that this sterility results from the interaction between the maternal reactive cytoplasm and any one of the paternal inducer chromosomes. This interaction yields other dysgenic traits including non-disjunction and mutations. In this note, the abilities of paternal gametes containing various combinations of inducer and reactive chromosomes to give more or less sterile SF females when fertilising standard reactive oocytes were compared. Although they did not cause SF sterility, reactive chromosomes, when present in sperm containing at least one inducer chromosome, were found to influence the intensity of sterility: variations of SF sterility were observed between SF females which differed only by one paternally inherited reactive chromosome. Reactive chromosomes are known to control the cytoplasmic state of reactive females. The present results suggest that this chromosomal control also takes place in SF females.     

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.