The Uq transposable element is one of two transposable elements consistently found in maize (Zea mays L.) populations. Populations developed from two independent recurrent selection programs initiated in the Iowa Stiff Stalk Synthetic (BSSS) maize population were tested for the frequency of Uq transposable elements to determine how Uq frequency has changed with cycles of recurrent selection. In the first programme, 13 cycles of half-sib and S2 progeny recurrent selection [BSSS(S)C13] have been completed and 10 of the 13 cycles were assayed for active Uq elements. In the second programme, 11 cycles of reciprocal recurrent selection [BSSS(R)C11] have been completed and five of the 11 cycles were assayed for active Uq elements. The frequency of Uq was different for the two recurrent-selection programmes. The percentage of plants containing active Uq elements increased from 19% (BSSS) to 91% [BSSS(S)C13] at a linear rate after 13 cycles of half-sib and S2 progeny recurrent selection, whereas the percentage of plants containing active Uq elements decreased from 19% (BSSS) to 0% [BSSS(R)C11] after 11 cycles of reciprocal recurrent selection, with extinction of the Uq element occurring between the fifth and sixth cycles of selection. Our data suggest that the increase in frequency of Uq with half-sib and S2 progeny recurrent selection was predominantly due to random genetic drift coupled with a selective advantage possibly associated with a region of the genome linked to Uq. Neither replicative transposition or chromosome assortment and segregation can be invoked to explain the change in frequency of Uq in these populations. The extinction of Uq after reciprocal recurrent selection was best explained by random genetic drift.

The stability of conversion frequencies with time was investigated in Ascobolus immersus. There were usually marked reductions in gene conversion frequencies at locus wl as crosses matured, to about one third of the initial values. This applied to all six wl alleles tested, irrespective of their molecular nature, and at all temperatures used. Asci dehiscing early from an apothecium had much higher conversion frequencies than those dehiscing late, but there were no differences between apothecia maturing at different times within a cross. Alleles at four loci unlinked to wl were also tested. All four showed significant changes, though not in all crosses; three loci showed decreases in conversion frequency with time, while one showed an increase. The relative frequencies of different conversion classes often changed with time. These changes appear to result from alterations in locus-specific recombination initiation and in repair of base mispairs in hybrid DNA, not from differential maturation rates of different ascal segregation classes. These effects could cause misinterpretation of quantitative tests of recombination models from gene conversion data.

We show that parasitism by the trematode Prosorhynchus squamatus in parental and introgressed Mytilus edulis/galloprovincialis (Bivalvia) mussels occurs in individuals with a predominantly M. edulis genome. This result suggests that the restricted specificity of P. squamatus is dependent on genetic factor(s) present in M. edulis. Because of its strong pathogenic effects (i.e. total castration and possible death), this parasite may be a source of intense selection against M. edulis genomes when they are present in a site. As a consequence, it may favour the geographic extension of the M. galloprovincialis genome. Previous studies have indicated that, in hybrid zones, recombinant genotypes are more susceptible to parasitic infections than either parental genotype. We demonstrate that this is not the case for the M. edulis/M. galloprovincialis system, and that the parental genotype alone determines susceptibility.

Twelve inbred lines derived from an M′ strain of Drosophila melanogaster were used to study the repression of P-element-mediated hybrid dysgenesis. Initial assessments indicated that the lines differed in the ability to repress gonadal dysgenesis, and that this ability was highly correlated with the ability to repress snw hypermutability. Later assessments indicated that most of the lines with low or intermediate repression potential evolved to a state of higher repression potential; however, Southern analyses failed to reveal significant changes in the array of genomic P elements that could account for this evolution. In addition, none of the lines possessed the incomplete P element known as KP, which has been proposed to explain repression in some D. melanogaster strains. One of the lines maintained intermediate repression potential throughout the period of study (52 generations), indicating that the intermediate condition was not intrinsically unstable. Genetic analyses demonstrated that in some of the lines, repression potential was influenced by factors that were inherited maternally through at least two generations; however, these factors were not as influential as those in a classic P cytotype strain. Additional tests with a dysgenesis-inducing X chromosome called T-5 indicated that repression itself was mediated by a combination of maternal effects and paternally inherited factors that were expressed after fertilization. These tests also suggested that in some circumstances, the P transposase, or its message, might be transmitted through the maternal cytoplasm.

The B mutation is associated with a tandem duplication of 16A1–16A7. It is unstable, mutating to wild type and to a more extreme form at a frequency of one in 1000 to 3000. The reversion to wild type is associated with the loss of one copy of the duplication, whereas the mutation to extreme B is associated with a triplication of the region. The instability of B has been attributed to unequal crossing-over between the two copies of the duplication. Recent molecular data show that there is a transposable element, B104, between the two copies of the duplication and support the hypothesis that this element generated the duplication via a recombination event. These data suggest that unequal crossing-over within the duplication may not be the cause of the instability of B. Instead, the instability may be caused by a recombination event involving the B104 element. This issue was addressed using probes for the DNA on either side of the B104 element at the B breakpoint. All of the data indicate that the B104 element is not involved in the instability of B and support the original unequal crossing-over model.

Several genetic mosaics for larval body marking of the silkworm, Bombyx mori, have been induced by X-ray irradiation. It is hypothesized that the occasional loss of chromosomal fragments carrying the genes for body marking during development may give rise to this type of mosaicism. Using pulsed field gel electrophoresis (PFGE), we find that a DNA molecule of about 2·5 megabases (Mb) is present in one type of mosaic (mottled striped strains pSm788 and pSm872), and not in any other strain. This DNA fragment hybridizes strongly with some chorion genes which are less than 6·9 cM away from the ps locus, and hence it corresponds to a chromosomal fragment containing genes for both striped marking (ps) and the chorion. In the non-mottled ps strain, the phenotype before X-ray irradiation, no band was detected either on a PFGE gel or after hybridization with the chorion probe. These results suggest that the mottled ps strains carry short chromosome fragments which are lost differentially during cell divisions.

Resistance to the cyclodiene insecticide dieldrin maps to a single gene (Rdl) on the left arm of chromosome III in Drosophila melanogaster (Meigen). The gene was further mapped by the use of chromosomal deficiencies to a single letter sub-region, 66F, on the polytene chromosome. The cross-resistance spectrum of a backcrossed strain lacking elevated mixed function oxidase activity, a common resistance mechanism, was examined. Levels of resistance similar to those found in other insects were found to dieldrin, aldrin, endrin, lindane, and picrotoxinin. Strong similarity of this single major gene with that found in other cyclodiene resistant insects is suggested by its cross-resistance spectrum and chromosomal location, via homology with other Diptera. The significance of major genes in insecticide resistance is discussed.

The ability to repress P-element-induced gonadal dysgenesis was studied in 14 wild-type strains of D. melanogaster derived from populations in the central and eastern United States. Females from each of these strains had a high ability to repress gonadal dysgenesis in their daughters. Reciprocal hybrids produced by crossing each of the wild-type strains with an M strain demonstrated that repression ability was determined by a complex mixture of chromosomal and cytoplasmic factors. Cytoplasmic transmission of repression ability was observed in all 14 strains and chromosomal transmission was observed in 12 of them. Genomic Southern blots indicated that four of the strains possessed a particular type of P element, called KP, which has been proposed to account for the chromosomal transmission of repression ability. However, in this study several of the strains that lacked KP elements exhibited as much chromosomal transmission of repression ability as the strains that had KP elements, suggesting that other kinds of P elements may be involved.

We conducted classical genetic analysis of the difference in male genitalia and hybrid sterility between the island-dwelling sibling species Drosophila sechellia and D. mauritiana. At least two loci (one on each autosome) are responsible for the genital difference, with the X chromosome having no significant effect. In contrast, male hybrid sterility is caused by at least four gene loci distributed among all major chromosomes, with those on the X chromosome having the largest effect.

We also show that the large difference in ovariole number between D. sechellia and its mainland relative D. simulans is due to at least two gene substitutions, one on each major autosome. The X and the left arm of the second chromosome, however, have no significant effect on the character. This implies that the evolution of reduced ovariole number involved relatively few gene substitutions.

These results extend previous findings that morphological differences between Drosophila species are caused by genes distributed among all chromosomes, while hybrid sterility and inviability are due primarily to X-linked genes. Because strong X-effects on male sterility have been found in all three pairwise hybridizations among D. simulans, D. sechellia and D. mauritiana, these effects must have evolved at least twice independently.

The experiments reported here are aimed at determining whether mutations deleting the function of the Sex-lethal (Sxl) gene are able to suppress the lethality of X0 clones, induced in females after the time when the state of activity of Sxl is irreversibly fixed by the ratio of the number of X chromosomes to sets of autosomes (X: A). This analysis was carried out by comparing the frequency of induced male clones (X0 constitution) in SxlfLS/ + and Sxl+/Sxl+ females, following irradiation at blastoderm and larval stages. The genotype used in these experiments, however, could also give rise to 2X; 2A cells homozygous for SxlfLS, and such cells would also differentiate male structures. To minimize this possibility, we have constructed a genotype made up of a ring and a rod X chromosome. In such ring-rod females the production of 2X; 2A clones homozygous for SxlfLS is a rather rare event, if possible at all. X0 male clones were produced in both types of females following irradiation at blastoderm stage, while X0 male clones were only observed in SxlfLS/ + females when irradiation took place at larval stage. In this latter case, the only X0 male clones were those that contained the SxlfLS mutation. These results support the idea of Sánchez & Nöthiger (1983) that the X: A signal irreversibly sets the state of activity of Sxl at blastoderm stage, and in addition show that X0 clones generated after that time are viable if they contain a Sxl− mutation. These results are compatible with the idea of Sxl being the only gene that responds to the X:A signal.

Mitochondrial DNA (mtDNA) variation in Drosophila simulans was studied to determine whether the cytoplasmic state of mtDNA heteroplasmy persists in natural populations in Réunion. For this purpose, 172 isofemale lines, newly collected from two local populations, were examined, among which three types of mtDNA (siII, siIII and siIII′) were found, based on the Hpa II restriction pattern. Ten of the lines were heteroplasmic for a combination of siII and siIII, as determined by autoradiography. The same type of heteroplasmy had been noted in one of the two local populations 8 years before (Satta et al. 1988). The present results suggest that the heteroplasmic state occurs recurrently in natural populations of D. simulans in Réunion.

We describe herein, a new unstable mutant of the vestigial locus, isolated from a French natural population. From this mutant vestigialalmost (vgal) wild-type flies (vgal+) and extreme vg phenotypes (vge) arose spontaneously without genomic shock. The occurrence of vgal+ or vge alleles depends mostly on the breeding temperature; vgal+ revertants arose principally at low temperature (21 °C) and vge at 28 °C. These events occur mainly in the male germ line and the phenomenon appears to be premeiotic. Our results with in situ hybridization experiments and Southern blots show that the vgal mutation is due to a 2 kb DNA insertion, which is a deleted hobo element. Genetic and molecular analyses show that two distinct events may underly the wild-type revertants. One is the excision of the resident hobo element, the other a further deletion (about 300 bp in the example characterized herein). The vge mutation is probably due to a deletion of vestigial sequences flanking the hobo insertion.

The white locus is inactivated in a cell-by-cell variegated pattern when juxtaposed with the proximal or distal parts of the nucleolus organizer region (NO) by X chromosome inversion. Recombinants for two such inversions, wm51b and wm4, were obtained and randomized for genetic background. White locus activity was much higher in the wm4 recombinant duplicated for most of the NO and much lower in the wm51b recombinant deficient for it. Although there may be other molecular differences between the heterochromatic regions of the recombinants, the most obvious is the dosage of NO. Suppression of a NO region-evoked variegated phenotype by additional NO doses is discussed in relation to four different classes of models for position-effect variegation (PEV): chromatin structure, nuclear geometry, incomplete transposition of mobile elements, and heterochromatin promoter-driven transcription. A corollary of the structural model is functional subdivision of heterochromatin, which would enable the use of PEV as a tool for its study.

A spontaneous mutation ‘tich’ (gene symbol tch) appeared as a recessive mutation in inbred mice of strain A. TL. Homozygotes are rather dumpy mice of approximately normal weight but with short limbs and tail. Skeletal measurements on backcross siblings show that the mandible bones are almost normal but long bones and some parts of the pelvic and pectoral girdles are short. Although tich resembles brachypodism phenotypically it is not linked to agouti, and does not match the description of any other skeletal mutation. There was some evidence for weak linkage with albinism on chromosome 7. The mutation has reappeared amongst the A. TL mice of a UK commercial breeder and may have been accepted as the norm for A. TL amongst some European users of this mouse.

This paper examines models of the population dynamics of transposable elements when chromosomal sites vary with respect to the effect on fitness of mutations caused by element insertions. Element abundance is assumed to be stabilised solely by the joint results of transposition, excision, and selection against insertional mutations. When there are only two classes of site, selected and neutral, it is hard to find parameter values for which numbers of elements are maintained that match the findings from surveys of Drosophila populations, as elements tend to accumulate at high frequencies at the neutral sites. It is similarly hard to produce realistic equilibria with three classes of site (strongly selected, weakly selected, and neutral), when elements can transpose out of the neutral sites. If transposition from neutral sites is impossible, as might be the case for elements inserted into centric heterochromatin, then realistic equilibria can be generated if there is very weak selection against elements inserted into the majority of non-neutral sites. This model predicts a modest over-representation of elements at the neutral sites. It also predicts that elements should be under-represented on the X chromosome compared with the autosomes, but this is not generally found to be the case. It is concluded that selection against insertional mutations is unlikely to be the major factor involved in the containment of element abundance.

Mutations in the structural genes for triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase activity in the mouse, selected after mutagen treatment, were used to estimate the map distance between the two loci. It is shown that Tpi-1 and Gapd are closely linked on chromosome 6, with a recombination frequency of 0·1±0·1%.

Segregation of sperm abnormality level and the pattern of major urinary proteins (MUPs) were investigated in F2 and B1 hybrid males obtained from crosses involving two contrasting inbred strains of mice: CBA/Kw (Mup-1a1a, 3·3% abnormal sperm) and C57BL/Kw (Mup-1b1b, 21·9% abnormal sperm). In the progeny of both crosses mean levels of abnormal spermatozoa were significantly higher for males typed as Mup-1b1b than for heterozygous Mup-1a1b males. Moreover, all F2 hybrid males showing very high percentages of abnormal sperm were Mup-1b1bhomozygotes. Similarly, among B1 males with a high level of deformed spermatozoa, a statistically significant majority were Mup-1b1b genotypes. Our results suggest that at least two genes which influence sperm abnormality level are segregating in these crosses. Both appear to be recessive for high sperm abnormality level, and one shows weak linkage to Mup-1 on chromosome 4.

The tissue-specific effects of 17 mutations affecting the synthesis of brown eye pigment (xanthommatin) have been investigated by combining them with chocolate and red cells, two mutations causing ectopic pigmentation of the Malpighian tubules and larval fat body (which normally only synthesize pigment precursors). The majority of mutations block the pigmentation of four organs: the normally pigmented eyes and ocelli, and ectopically pigmented tubules and fat body. They represent genes that would appear to be required for the normal operation of the pathway per se and are likely to encode structural proteins. Mutations at 5 loci affect pigmentation of a subset of organs: cd and po affect only the eyes and ocelli; kar affects the eyes, ocelli and fat body; car causes excretion of pigment from tubules; and z affects pigmentation of the eyes alone. Of these loci, only z has been shown to encode a regulatory protein and the role of the remaining four gene products is not clear. Two mutations affecting the red eye pigments (drosopterins), bw and mal, do not substantially perturb brown pigment synthesis in any of the four organs.

We have examined the effects of major histocompatibility (H-2) haplotypes and genetic background (all loci other than the H-2 region) on hormone-induced ovulation rate in congenic strains of mice. In comparison with the H-2a haplotype, the H-2b haplotype increased hormone-induced ovulation rate 92% on the A/J (A) genetic background. However, H-2 haplotype did not affect hormone-induced ovulation rate on the C57BL/10J (C57) genetic background. The H-2b-linked gene(s) increased hormone-induced ovulation rate on the A/J genetic background largely by (1) enhancing the maturation of follicles in response to pregnant mare's serum gonadotrophin (PMSG) and (2) altering the stages of follicular development which can be induced to ovulate in response to human chorionic gonadotrophin (hCG). The observed effects of H-2 on hormone-induced ovulation rate were not explained by differences in the timing of puberty, the number of follicles present in untreated females, or the incidence of follicular atresia. The effect of genetic background on hormone-induced ovulation rate was much greater than was the effect of the H-2 region. We found that hormone-induced ovulation rate was five- to six-fold higher on the C57 genetic background than on the A genetic background. The C57 genetic background increased hormone-induced ovulation rate by (1) enhancing the induction of follicular maturation in response to gonadotropins and (2) by reducing the incidence of follicular atresia.

A mutation resulting in increased triosephosphate isomerase (TPI) activity in blood was recovered in offspring of procarbazine hydrochloride-treated male mice. Breeding experiments indicated a codominant mode of expression. Compared to the wild type, heterozygous and homozygous mutants have mean erythrocyte TPI activities of approximately 140 and 190%, respectively. Besides blood and erythrocytes the increased activity is expressed to a similar degree in spleen, and to a lesser degree in liver, lung, kidney, muscle and brain. Enhanced activity was absent in the heart. Heterozygous and homozygous mutants are viable, fully fertile and exhibit no significant differences in haematological or other physiological traits studied. Biochemical investigations of TPI in both mutant genotypes revealed neither physicochemical nor kinetic differences compared to the wild type. Moreover, immunoinactivation studies showed no difference in the amount of antiplasma required to inactivate a constant amount of TPI activity in all three genotypes, strongly suggesting that the differences in enzyme activity are attributable to differing amounts of enzyme protein expressed per cell. Mapping studies indicated that the mutation is closely linked to the Gapd locus and consequently is located either adjacent to or within the Tpi-1 structural locus. It is hypothesized that the mutation affected a regulatory element contiguous to the Tpi-1 structural locus which acts by increasing the amount of TPI expressed.