Triploidy is one of the most frequent chromosomal errors responsible for reproduction failure. This paper encompasses, in one conceptual frame, four recent findings in reproduction biology: predominant dispermic origin of triploids, paternal centrosome inheritance, eccentric cleavage divisions of dispermic triploid zygotes and certain intricate cases of mosaicism/chimerism. It is argued that dispermic zygotes, in contrast to digynic ones, are characterized by cytogenetic phenomenon described here as postzygotic diploidization of triploids (PDT). PDT embraces three main developmental scenarios: (i) the maintenance of the triploid state accompanied by regular segregation of 2n cells and the 2n/3n mixoploid populations; (ii) immediate diploidization with elimination of an odd haploid set of chromosomes and regular appearance of 1n/2n, 2n/3n and other mixoploids and (iii) tripolar spindle formation leading to gross aneuploidy, cell death with occasional survival of 2n+1 or 2n+1+1 trisomics and uniparental disomics. According to the PDT concept, a trisomy and disomy might occur due to generalized karyotype instability of dispermic triploids. PDT may provide a natural explanation for the regular appearance of 2n homozygous androgenic moles, various 2n/3n, 2n/2n molar/twin complexes without necessitating the concept of the 'empty' oocyte fertilization. Convincing evidence for a reservoir of anuclear oocytes does not exist. Peculiar implications are expected in the case of two rounds of diploidizations or involvement of triploid cell derivatives in the twinning process. Cryptic mosaic/chimeras and unusual twins intermediate between monozygotic (MZ) and dizygotic (DZ) are expected. Thus, PDT could have an explanation for the broad spectrum of odd reproductive cytogenetic events and might provide additional alternatives and definite predictions.
Drosophila melanogaster telomeres have two DNA domains: a terminal array of retrotransposons and a subterminal repetitive telomere-associated sequence (TAS), a source of telomere position effect (TPE). We reported previously that deletion of the 2L TAS array leads to dominant suppression of TPE by stimulating in trans expression of a telomeric transgene. Here, we compared the transcript activities of a w transgene inserted between the retrotransposon and TAS arrays at the 2L telomere in genotypes with different lengths of the 2L TAS. In contrast to individuals bearing a wild-type 2L homologue, flies with a TAS deficiency showed a significant increase in the level of telomeric w transcript during development, especially in pupae. Moreover, we identified a read-through w transcript initiated from a retrotransposon promoter in the terminal array. Read-through transcript levels also significantly increased with the presence of a 2L TAS deficiency in trans, indicating a stimulating force of the TAS deficiency on retrotransposon promoter activity. The read-through transcript contributes to total w transcript, although most w transcript originates at the w promoter. While silencing of transgenes in nonhomologous telomeres is suppressed by 2L TAS deficiencies, suggesting a global effect, the overall level of HeT-A transcripts is not increased under similar conditions.
A series of eleven independent mutants at the X chromosome singed bristle (sn) locus of Drosophila melanogaster is described. All mutants descend from flies caught in the wild and bred in the laboratory. On Table 1 has its counterpart among the conventional sn mutants already described (2).Mutability of the sn Mutants. Subsequent to its discovery each sn mutant was crossed to attached-X females and a stock was established. Examination of the male progeny in each stock revealed the presence of phenotypically wild-type (sn + ) males, suggesting that the sn mutants were reverting at inordinately high frequencies. Therefore, a systematic study of the mutability of a number of the sn mutants was undertaken. A simple crossing procedure was adopted. For each sn mutant 20 to 25 newly enclosed males were crossed individually to harems of six females homozygous for the recessive X chromosome mutants y2 w spl sn3(cf. ref. 2 for a description of the mutants). Exceptions were scored among the F1 females on the basis of their bristle phenotype. Two classes of exceptions were sought: (i) reversions to wild type in which the F1 females exhibited a normal (sn +) bristle phenotype; (ii) mutation to a more extreme an mutant identified as a bristle phenotype of F1 females whose departure from wild type is greater than that of their sisters.The results of testing the mutability of 11 separate an mutants are given in Table 2 and merit a few comments. Among 11 mutants tested, 10 were mutable. On the basis of their phenotype, three classes of mutants were detected. One class, designated sn + in Table 2, represents reversions to wild-type bristles. The reversion chromosome produces a wild-type phenotype in compound with either Sn 3 or an X chromosome carrying a cytologically visible deletion of the sn locus. A second class, designated ansu (singed subliminal) in Table 2, represents those revertants that appeared wild type in compound with sn3 but manifest a slight but distinct wavy bristle phenotype in compound with the sn deletion. The third class, designated snex in Table 2, represents mutation to a phepotypically more extreme allele. These could not always be objectively identified, especially when the sn mutant under study evoked an extreme bristle phenotype.It is obvious from the data of Table 2 that the several sn alleles mutate at inordinately high rates. As recorded, many mutants occur in clusters, suggesting that some, perhaps most, mutants are premeiotic in occurrence. If all the mutants of a cluster are assumed to stem from one premeiotic mutational event, then the mutation frequencies range between O-4 and O-3. It is of relevance to note here that in addition to producing mutations transmitted through the germ line, some alleles were also somatically unstable. The allele 77-27 was especially noteworthy in this respect and genotypically 77-27 males and fe-
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