Analysis of mutation frequency following continuous and fractionated application of ethyleneimine and ethyl methanesulfonate to male sex cells of Drosophila melanogaster]

The increase in the frequency of recessive lethal sex-linked mutations induced by fractionated effect of ethylene imine (EI) an ethylmethanesulphonate (EMS) on mature sperm of Drosophila melanogaster was observed and compared uith prolonged treatment (8h) and with the additive effect. This effect of dose fractionation was observed in the case of the treatment of sperms in male gonads and in female spermathecas. The increase of the mutation frequency was noted by brood-pattern method after fractionated treatment of spermatocytes and spermatogonia only with EMS. This increase was not observed under the effect of EI on spermatocytes and spermatogonia because of the high sierilization activity of EI. Possible mechanisms of the effects observed are discussed.     

Evidence of an essential difference between point mutations and chromosome breaks induced by triethylene melamine inDrosophila spermatozoa

Summary Storing of triethylene melamine-treated mature spermatozoa in untreated females was found to result in increased frequencies of both sex-linked recessive lethals and translocations involving the Y, II and III chromosomes. Frequencies of these mutations in effectively unstored spermatozoa were determined from progenies produced using sperm 2–4 days after treatment. The increase in translocation frequencies was on the order of 12-fold in progenies from sperm utilized 11–13 days after treatment when the sperm were stored at 25°C, and 3- to 6-fold when comparable sperm were stored at 12.5°C. Consistent but much smaller increases in frequencies of sex-linked lethals were found, with the increase in lethals tending to be correlated with relative increase in translocation frequency in a given experiment. On the assumption that sex-linked lethals related to chromosome breakage would be expected to increase in frequency in the same proportion as do translocations, approximate agreement was obtained when the proportions of breakage-related lethals among unstored lethals were estimated from the data in the four experimental series. The data are thus consistent with the hypothesis that chromosome breaks but not point mutations are realized during storage of treated spermatozoa. Possible interpretations of a differential effect of storage on treated chromosomes are discussed.Studies carried out while the author was a guest investigator at the Institute of Animal Genetics on sabbatical leave from the University of Minnesota.     

Cytological analysis of storage effects on various types of complete and mosaic change induced in drosophila chromosomes by some chemical mutagens

Structural chromosomal changes induced by ethyleneimine (EI) in Drosophila spermatozoa and the effect of storage on the relative frequencies of mosaic and complete changes were studied cytologically, using salivary gland chromosomes. The pattern of EI-induced changes in unstored spermatozoa differes from that induced by X-rays by (1) a higher proportion of mosaics, (2) a higher proportion of repeats and deficiencies and a lower one of translocations and inversions, (3) a lower proportion of interchromosomal changes and a lower proportion of breaks located in heterochromatin. All of these characteristic feautures that distinguish mutagenic effects of EI from those of X-rays are present also after treatment with triethylene melamine (TEM) and formaldehyde food (FF), but the differences between EI and X-rays are on the whole smaller than those between X-rays and TEM or FF.Effect of storage on the frequencies of EI-induced changes is similar to that found when TEM of FF were used as mutagens: the whole pattern of changes became more like that found after irradiation. The only qualitative difference between EI and TEM of FF is the way in which storage affects the frequencies of mosaics.The peculiar pattern of EI-, TEM-, or FF-induced changes and the effect of storage is attributed to the high proportion of pre-breakage lesions with delayed fixation of breaks.Residual-mutagen hypothesis, as the possible cause of storage effects is also discussed.     

The murine Rb(6.16) translocation: evidence for sperm selection and a modulating effect of aging

Summary The segregation products of the Rb(6.16) translocation were studied at first cleavage metaphase. Male mice heterozygous for the translocation mated at 3- and 14-day intervals to superovulated random-bred ICR females. Chromosome preparations of the recovered one-cell embryos were sequentially G- and C-banded and male and female complements analyzed cytogenetically. Of the 309 zygotes analyzed from both mating groups, no unbalanced segregants of the translocation were observed. In the 3-day group there was a highly significant difference (P<0.001) from the expected 1:1 ratio between sperm with normal (70.5%) and balanced segregants (26.2%) of alternate segregation. In the 14-day group the level of significance for the difference was ten times lower (P<0.01) as normal segregants were observed in 56.4% of the sperm and balanced ones in 36.5%. A comparison of the two groups using a 2×2 contingency table showed that segregant type was related to mating frequency (P<0.05). There was a highly significant distortion (P<0.01) of the sex ratio, with 178 Y-bearing and 131 X-bearing sperm in the combined populations. When sex ratio was analyzed according to mating intervals, the distortion was significant (P<0.01) only for the 3-day group. An analysis of the sex ratio according to the segregant type showed no significant deviation from 1:1 for type 1 segregants, which had normal chromosomes, while type 2 segregants, with the translocation, had a deficiency of X-bearing sperm. This deficiency was significant (P<0.05) only for the 3-day population. Analysis of meiotic preparations showed no association between the translocation trivalent and the X-Y bivalent. Our results, obtained under physiological conditions, provide definitive evidence for sperm selection and support previous findings that aging of sperm can modify the effect of selection.     

Studies on storage effects and the negative synergism between trenimon and X-rays in Drosophila

The experiments reported in this paper were designed to answer some questions relating to the augmenting effect of storage of sperm in the inseminated female, on the frequency of translocations in spermatozoa treated with the trifunctional alkylating agent trenimon. To see whether, upon storage, more chromosome breaks become available for interaction, sperm cells that had been treated with trenimon in the male were exposed to X-irradiation before or after 6 days storage in the female. The data of the first experiment indicated that in unstored sperm the translocation yield after treatment with both trenimon and X-rays, was lower than that expected on the basis of additivity of yields of the single treatments. The negative synergism between trenimon and X-irradiation has been confirmed in further experiments with both translocations and dominant lethals. The latter finding does not support an interpretation in terms of selective elimination of translocationsby cell death. Following storage, the translocation frequencies increase and after combination of trenimon and X-rays, yields corresponding to additivity of frequencies with single treatments are observed.To study whether changes in the maternal repair system contribute to the storage effect, trenimon-treated males were mated with aged females and the frequencies of translocations were determined. It was found that these frequencies were similar to those in young (unstored) females; this result suggests that the possibility raised above is unlikely.     

A comparison of the frequency and type of chromosomal abnormalities in human sperm after different sperm capacitation conditions.

Human sperm karyotypes can be prepared after fusion of human sperm with Golden hamster oocytes. Most laboratories use one of two methods of sperm capacitation: incubation of freshly-ejaculated sperm in Biggers, Whitten, and Whittingham (BWW) medium for 5-7 h at 37 degrees C or sperm storage in (N-tris [hydroxymethyl]methyl-2-aminoethanesulfonic acid; 2-([2-hydroxy-1,1-bis(hydroxymethyl)ethyl]amino)ethanesulfonic acid) (TES)-Tris yolk buffer (TYB) for 1-3 days at 4 degrees C. Since there have been conflicting reports as to whether there is a difference in the frequency of structural chromosomal abnormalities between BWW capacitation and storage in TYB for 2 days, we analyzed a larger number of karyotypes (8974) from 136 donors to determine if there was any difference in the frequency or type of chromosomal abnormalities in sperm treated by fresh BWW capacitation, storage in TYB for 1 day (TYB-1), or storage in TYB for 2 days (TYB-2). There was no difference in the frequency of numerical chromosomal abnormalities or sex ratio in any of the three treatment groups. However, there was a significantly increased frequency of structural chromosomal abnormalities after storage in TYB-1 and TYB-2. There was no difference in the frequency or type of structural chromosomal abnormalities after sperm storage in TYB-1 compared to TYB-2.     

Effects of storing x-rayed spermatozoa on the frequency of translocations and sex-linked lethalsin Drosophila melanogaster

Summary x-rayed adult males ofDrosophila melanogaster were left with untreated females from 2 to 3 days after which the males were discarded. Sex-linked recessive lethals and translocations were scored in progeny produced during the first 2 or 3 days following irradiation, and after storage of the spermatozoa in the females for 6 days.The results obtained show that the frequencies of sex-linked lethals and of translocation involving the two large autosomes and the x-chromosome were unchanged by storage. In experiments in which Y, 2, 3 translocations were scored both the 2–3, and the Y translocations showed a slight increase. These experiments show that the strong storage effect on translocations produced by certain alkylating agents is peculiar to chromosomes treated by these chemicals.Guest investigator at the Institute of Animal Genetics, Edinburgh, on leave from Assuit University Egypt.     

Transmission Ratio Distortion,Sterility, and Control of the t-Complex Function in Sperm

Mouse t-complex located on chromosome 17 contains genes affecting only male fertility. Some genes of this complex are recessive lethals; nonetheless, the high frequency of the t-complex carriers in a population is maintained due to a mechanism referred to as transmission ratio distortion (TRD), i.e., after crosses with wild-type females, males heterozygous for the t-complex transmit the t-bearing chromosome to nearly all their offspring, which suggests that the t-complex genes control sperm function. Analysis of this phenomenon shows that the resultant TRD is determined by the ratio between the distorter genes (Tcd) and a responder gene (Tcr) located within the t-complex region. Many authors believe that two to six distorter genes currently known have an additive effect. A genetic model of the non-Mendelian inheritance in the progeny of heterozygous male mice specifically explains sterility of animals carrying the t-complex with complementary lethal genes. The model suggests that some distorter gene products interacting with the responder gene have a selective effect on motility of both mutant and wild-type sperm. Insufficient sperm motility and/or their unsuccessful capacitation result in poor if any fertilization. Information on the t-complex genes is necessary for understanding the biological mechanisms of male sterility and may be used in medical practice.     

Increment kinetics of recessive lethal mutations and dominant lethals in offspring of Drosophila melanogaster on storage of methyl-methanesulfonate-treated sperm in females

The frequencies of sex-linked recessive lethal mutations in F1 males after feeding adult male Drosophila melanogaster with 0.25 and 0.5 mM methyl methanesulfonate (MMS) orally for 24 h increased approximately linearly with storage of the treated spermatozoa in females, whereas the number of hits of dominant lethals in the sperm after feeding 0.3 and 0.5 mM MMS increased approximately with the square of the storage time. Chromosome losses and mosaics in F1 males also increased with the dose of MMS to males, but their yields were too low to be analyzed quantitatively, only indicating a slight increase of chromosome loses and a slight decrease of mosaics with the time of storage of sperm. Maternal non-disjunctions (or chromosome losses), detected in F1 males, decreased with the dose of MMS to spermatozoa and their yield decreased with the time of storage of sperm of both MMS-treated and the control groups. A unitary model is proposed to explain the effect of storage on the dominant lethals and recessive lethal mutations.     

Autophagy and apoptosis have a role in the survival or death of stallion spermatozoa during conservation in refrigeration

Apoptosis has been recognized as a cause of sperm death during cryopreservation and a cause of infertility in humans, however there is no data on its role in sperm death during conservation in refrigeration; autophagy has not been described to date in mature sperm. We investigated the role of apoptosis and autophagy during cooled storage of stallion spermatozoa. Samples from seven stallions were split; half of the ejaculate was processed by single layer centrifugation, while the other half was extended unprocessed, and stored at 5°C for five days. During the time of storage, sperm motility (CASA, daily) and membrane integrity (flow cytometry, daily) were evaluated. Apoptosis was evaluated on days 1, 3 and 5 (active caspase 3, increase in membrane permeability, phosphatidylserine translocation and mitochondrial membrane potential) using flow cytometry. Furthermore, LC3B processing was investigated by western blotting at the beginning and at the end of the period of storage. The decrease in sperm quality over the period of storage was to a large extent due to apoptosis; single layer centrifugation selected non-apoptotic spermatozoa, but there were no differences in sperm motility between selected and unselected sperm. A high percentage of spermatozoa showed active caspase 3 upon ejaculation, and during the period of storage there was an increase of apoptotic spermatozoa but no changes in the percentage of live sperm, revealed by the SYBR-14/PI assay, were observed. LC3B was differentially processed in sperm after single layer centrifugation compared with native sperm. In processed sperm more LC3B-II was present than in non-processed samples; furthermore, in non-processed sperm there was an increase in LC3B-II after five days of cooled storage. These results indicate that apoptosis plays a major role in the sperm death during storage in refrigeration and that autophagy plays a role in the survival of spermatozoa representing a new pro-survival mechanism in spermatozoa not previously described.     

Role of rabbit prostate granules on sperm viability and acrosome reaction evaluated with different methods

In the present study, the role of rabbit seminal granules was observed. Their influence on motility, capacitation and acrosome reaction, as well as the presence of apoptosis and the morphology of rabbit sperm, were compared in different conditions. Ejaculated sperm from five mature New Zealand White rabbit bucks during three series of collections were studied, comparing raw semen, Percoll-selected sperm and Percoll-selected sperm plus prostate granules. We observed sperm motility kinetic traits by computer-assisted sperm analyzer (CASA) analysis in each sample. Acrosome status was evaluated by FITC-labeled Pisum sativum Agglutinin staining and chlortetracycline fluorescence assay, phosphatidylserine translocation was determined by AnnexinV/Propidium iodide assay and sperm morphology was studied using transmission electron microscopy (TEM). All traits were observed after 30 min incubation at 37 °C in 5% CO₂. Data showed that sperm motility and viability markedly improved in the presence of prostate granules, whereas capacitation, acrosome reaction and phosphatidylserine translocation were lowered. TEM confirmed these results. In conclusion, the role of granules was confirmed in synchronizing sperm capacitation and acrosome reaction with egg availability; indeed, rabbit ovulation occurs only 6 to 10 h after mating.     

An evaluation of the mouse sperm morphology test and other sperm tests in nonhuman mammals. A report of the U.S. Environmental Protection Agency Gene-Tox Program

The literature on the mouse sperm morphology test and on other sperm tests in nonhuman mammals was reviewed (a) to evaluate the relationship of these tests to chemically induced spermatogenic dysfunction, germ-cell mutagenicity, and carcinogenicity, and (b) to make an interspecies comparison to chemicals. A total of 71 papers were reviewed. The mouse sperm morphology test was used to assess the effects of 154 of the 182 chemical agents covered. 4 other murine sperm tests were also used: the induction of acrosomal abnormalities (4 agents), reduction in sperm counts, (6 agents), motility (5 agents), and F1 sperm morphology (7 agents)). In addition, sperm tests for the spermatogenic effects of 35 agents were done in 9 nonmurine mammalian species; these included analyses for sperm count, motility, and morphology, using a large variety of study designs. For the mouse sperm morphology test, 41 agents were judged by the reviewing committee to be positive inducers of sperm-head shape abnormalities, 103 were negative, and 10 were inconclusive. To evaluate the relationship between changes in sperm morphology and germ cell mutagenicity, the effects of 41 agents on mouse sperm shape were compared to available data from 3 different mammalian germ-cell mutational tests (specific locus, heritable translocation, and dominant lethal). The mouse sperm morphology test was found to be highly sensitive to germ-cell mutagens; 100% of the known mutagens were correctly identified as positives in the sperm morphology test. Data are insufficient at present to access the rate of false positives. Although it is biologically unclear why one might expect changes in sperm morphology to be related to carcinogenesis, we found that (a) a positive response in the mouse sperm morphology test is highly specific for carcinogenic potential (100% for the agents surveyed), and (b) overall, only 50% of carcinogens were positive in the test (i.e., sensitivity approximately equal to 50%). Since many carcinogens do not produce abnormally shaped sperm even at lethal doses, negative findings with the sperm test cannot be used to classify agents as noncarcinogens. We conclude that the mouse sperm morphology test has potential use for identifying chemicals that induce spermatogenic dysfunction and perhaps heritable mutations. Insufficient numbers of chemicals agents have been studied by the other sperm tests to permit similar comparisons. A comparison of 25 chemicals tested with sperm counts, motility, and morphology in at least 2 species (including man, mouse and 9 other mammals) demonstrated good agreement in response among species. With further study, interspecies comparisons of chemically induced sperm changes may be useful for predicting and evaluating human effects.     

Cytogenetic analysis of sperm from a male heterozygous for a 13;14 Robertsonian translocation

Summary Cytogenetic analysis of 121 sperm from a man heterozygous for a t(13;14) Robertsonian translocation was performed using the technique of in vitro penetration of hamster eggs. The frequency of sperm that were chromosomally unbalanced with respect to the translocation was 27%. The frequency of chromosomally normal (36%) and balanced (38%) complements was approximately equal, as theoretically expected. There was no evidence for an interchromosomal effect since the frequency of numerical chromosomal abnormalities (2.5%) and structural chromosomal abnormalities (10.7%) — both unrelated to the translocation — were within the normal range of control donors. The ratio of X-and Y-chromosome bearing sperm was equal, and there was no evidence for preferential segregation of the X chromosome with the translocation.     

Function of Prolonged Copulation in Nysius huttoni White (Heteroptera: Lygaeidae) Under Male-Biased Sex Ratio and High Population Density

Prolonged copulation as a response to sex ratio and population density has been demonstrated for a number of species but the functions behind it remain controversial. It is difficult to determine the function of prolonged copulations without examining the mechanisms of sperm transfer and storage, and fertilization success–copulation duration relationships. Here, we report our work on a polygamous seed bug, Nysius huttoni White (Heteroptera: Lygaeidae), where we manipulated the population density and sex ratio for a series of mating trials, quantified the relationship between copulation duration and ejaculate amount as well as reproductive outputs, and examined the sperm transfer and storage mechanisms. Our results suggest that males prolong copulations to increase their fertilization success relative to their rivals in response to high sperm competition intensity under male-biased sex ratio and high population density. The mechanism behind the positive correlation between copulation duration and fertilization success may be attributed to the characteristic structures of aedeagus and spermatheca in this insect, which allow direct transfer of sperm to a storage site of fixed size during copulation.     

Comparative study of the clastogenic efficiency of ethyl methanesulfonate and diethyl sulfate in Drosophila melanogaster mature sperm

Chromosome loss and translocation tests were carried out in Drosophila melanogaster sperm, stored in untreated females for up to 24 days, to compare the clastogenicity of ethyl methanesulfonate (EMS) and diethyl sulfate (DES). The sex-linked recessive lethal test was used as a "biological dosimeter" and the following results were obtained: The yield of 2-3 translocations induced by both mutagens increased steadily with storage, being significantly higher after EMS than after DES treatment. The frequencies of partial losses induced by EMS and DES were similar and increased with storage. With up to 11 days' storage, the frequency of complete loss induced by DES was higher than that induced by EMS and remained unchanged when storage was extended to 24 days. Complete loss induced by EMS increased significantly with further storage (12-24 days). With DES, complete (but not partial) loss was detected with a dose at which EMS failed to modify the control values. These data suggest that the lower recovery of II-III translocations after treatment with DES does not result from a low breaking capacity but from a diminished or delayed rejoining of the induced breaks. This could be due to a physiological impairment of the treated cells by the high toxicity of DES or to an actual lower rejoinability of the broken ends. The differential recovery of complete and partial losses after DES treatment further suggests that the mechanisms leading to the fixation of both types of damage are somehow different, and that processes intervening in the recovery of partial losses are less affected, or not at all, by the proposed reduced rejoining of chromosome breaks.     

Sperm chromosome analysis in a man heterozygous for a paracentric inversion of chromosome 7 (q11q22)

Summary Human sperm chromosomes were studied in a man heterozygous for a paracentric inversion of chromosome 7 (q11q22). The pronuclear chromosomes were analysed after in vitro penetration of golden hamster (Mesocricetus auratus) eggs. Ninety-four sperm chromosome spreads were examined, of which 34 contained the normal number 7 chromosome and 59 the inverted 6. This segregation was significantly different from the expected 1:1 ratio. The number of X- to Y-bearing sperm was 48 and 46 respectively. No sperm contained a recombinant chromosome caused by a crossover within the inversion. The frequency of chromosomal abnormalities in other chromosomes was 9.6%, which is not significantly different from the frequency observed in normal donors (8.9%) in our laboratory. These result suggest that the risk of chromosomally unbalanced sperm is not high for this paracentric inversion.     

Mating strategy in Aleochara bilineata: sperm storage and allocation

Abstract By contrast to females that can maximize reproductive success with only one or a few copulations, males generally increase their fitness with frequency of mating. Sperm storage and allocation is therefore crucial for both male and female fitness. Sperm storage in Aleochara bilineata (Coleoptera; Staphylinidae) is investigated by measuring the number of spermatozoa stored in the female spermatheca after single, double or triple successive copulations with different males. The potential advantages of polyandry are studied in terms of the number of sperm stored by females mated twice with the same male (i.e. repeated copulation), compared with females mated twice with two different virgin males (i.e. polyandry). Level of polygyny is also estimated by measuring sperm allocation when ten successive mates are offered to a virgin male. Aleochara bilineata females store the sperm of the same or different males additively, suggesting no advantage for polyandry in terms of the number of sperm stored. A virgin male is able to inseminate ten different females but the number of sperm transferred decreases linearly. Finally, the latencies and durations of copulations are measured in all experiments to estimate changes according to the male or female status (i.e. virgin or mated). The latency before mating is higher when females are virgin than when females have already mated.     

Meiotic segregation, recombination, and gamete aneuploidy assessed in a t(1;10)(p22.1;q22.3) reciprocal translocation carrier by three- and four-probe multicolor FISH in sperm.

Meiotic segregation, recombination, and aneuploidy was assessed for sperm from a t(1;10)(p22.1;q22.3) reciprocal translocation carrier, by use of two multicolor FISH methods. The first method utilized three DNA probes (a telomeric and a centromeric probe on chromosome 1 plus a centromeric probe on chromosome 10) to analyze segregation patterns, in sperm, of the chromosomes involved in the translocation. The aggregate frequency of sperm products from alternate and adjacent I segregation was 90.5%, and the total frequency of normal and chromosomally balanced sperm was 48.1%. The frequencies of sperm products from adjacent II segregation and from 3:1 segregation were 4.9% and 3.9%, respectively. Reciprocal sperm products from adjacent I segregation deviated significantly from the expected 1:1 ratio (P < .0001). Our assay allowed us to evaluate recombination events in the interstitial segments at adjacent II segregation. The frequencies of sperm products resulting from interstitial recombination in chromosome 10 were significantly higher than those resulting from interstitial recombination in chromosome 1 (P < .006). No evidence of an interchromosomal effect on aneuploidy was found by use of a second FISH method that simultaneously utilized four chromosome-specific DNA probes to quantify the frequencies of aneuploid sperm for chromosomes X, Y, 18, and 21. However, a significant higher frequency of diploid sperm was detected in the translocation carrier than was detected in chromosomally normal and healthy controls. This study illustrates the advantages of multicolor FISH for assessment of the reproductive risk associated with translocation carriers and for investigation of the mechanisms of meiotic segregation of chromosomes.     

Different photodynamic action of proflavine and methylene blue on bacteriophage

Summary The irradiation with visible light (Li) of temperate Serratiaphage that is maximally sensitized with either proflavine (PF) or methylene blue (MB) induces—apart from lethal lesions—mutations which are phenotypically expressed as clear (c) or lightly turbid (l) plaques. The mutagenicity of the MB+Li and PF+Li treatment differs in several respects: (i) Up to an inactivation of 6 to 8 lethal hits MB+Li is a much more potent mutagen than PF+Li. (ii) at low levels of survival the dose curve of mutation frequency with MB+Li reaches a peak and then decreases drastically while the mutation frequency after PF+Li continues to increase in proportion to lethal hits induced. (iii) Mapping of 100 MB+Li and 77 PF+Li induced c or l mutants indicates significant difference in the electiveness of the four genomic regions of c or l mutants.     

The murine Rb(6.16) translocation: alterations in the proportion of alternate sperm segregants effecting fertilization in vitro and in vivo

Summary The segregation products of the mouse Rb(6.16)24 Lub male translocation carrier were analyzed at first cleavage metaphase to determine whether the proportion of normal, balanced, and unbalanced sperm segregants differ in fertilizations occurring in vivo and in vitro. From 34 males, the sperm genomes in 268 firstcleavage mouse embryos were analyzed cytogenetically: 137 and 131 following in vivo and in vitro fertilization, respectively. Both systems demonstrated a preponderance of alternate (67.2% and 54.2%) as compared to adjacent segregation (10.2% and 13.7% as estimated). A contingency table showed that the distribution of reciprocal alternate segregants differed significantly between the two fertilization environments (x ²=20.64, P<0.0005). Whereas chromosomally normal sperm were 3.6 times more likely than the balanced reciprocals to fertilize in vivo (78.3% normal:21.7% balanced), 11 ratios were recovered following in vitro fertilization (43.7% normal: 56.3% balanced). The data also showed an excess of Y-bearing sperm with the translocation in both in vivo and in vitro fertilization groups. In the latter these segregants were 3 times more likely than X-bearing ones to effect fertilization. These data suggest a phenotypic disadvantage of translocation-X-bearing sperm, possibly mediated through altered haploid gene expression on chromosome 6 and gene expression on the Y. The results show clear evidence for prezygotic selection in vivo and indicate that the environment in which fertilization occurs significantly affects the transmission frequency of this specific translocation.