Drosophila simulans Lethal hybrid rescue mutation (Lhr) rescues inviable hybrids by restoring X chromosomal dosage compensation and causes fluctuating asymmetry of development.

The Drosophila simulans Lhr rescues lethal hybrids from the cross of D. melanogaster and D. simulans. We describe here, the phenotypes of Lhr dependent rescue hybrids and demonstrate the effects of Lhr on functional morphology of the salivary chromosomes in the hybrids. Our results reveal that the phenotypes of the 'Lhr dependent rescued' hybrids were largely dependent on the genetic background and the dominance in species and hybrids, and not on Lhr. Cytological examination reveal that while the salivary chromosome of 'larval lethal' male carrying melanogaster X chromosome was unusually thin and contracted, in 'rescued' hybrid males (C(mel)X(mel)Y(sim); A(mel)A(sim)) the X chromosome showed typical pale staining, enlarged diameter and incorporated higher rate of (3)H-uridine in presence of one dose Lhr in the genome. In hybrid males carrying simulans X chromosome (C(mel)X(sim)Y(mel); A(mel)A(sim)), enlarged width of the polytene X chromosome was noted in most of the nuclei, in Lhr background, and transcribed at higher rate than that of the single X chromosome of male. In hybrid females (both viable, e.g., C(mel)X(mel)X(sim); A(mel)A(sim) and rescued, e.g., C(mel)X(mel)X(mel); A(mel)A(sim)), the functional morphology of the X chromosomes were comparable to that of diploid autosomes in presence of one dose of Lhr. In hybrid metafemales (C(mel)X(mel)X(mel)X(sim); A(mel)A(sim)), two dose of melanogaster X chromosomes and one dose of simulans X chromosome were transcribed almost at 'female' rate in hybrid genetic background in presence of one dose of Lhr. In rescued hybrid males, the melanogaster-derived X chromosome appeared to complete its replication faster than autosomes. These results together have been interpreted to have suggested that Lhr suppresses the lethality of hybrids by regulating functional activities of the X chromosome(s) for dosage compensation.

X-chromosome Inactivation Patterns in Korean Women with Idiopathic Recurrent Spontaneous Abortion

Recurrent spontaneous abortion (RSA) defines as two or more consecutive losses at or =90%) and 4 (8.9%) had mild skewed inactivation (> or =85%). In 54 heterozygous control subjects, 5 (9.3%) women showed extreme skewed X inactivation and 7 (13.0%) had mild one. The frequency of skewed X inactivation between RSA patients and control group was not significantly different (p>0.05). This finding suggests that skewed x romosome be not associated with unexplained RSA patients.

The behavior of sex chromosomes in two human X-autosome translocations: failure of extensive X-inactivation spreading

Two patients, one adult male and one infant girl, bearing different X-autosome translocations, were studied with cytogenetical, ultrastructural and chromosome-painting techniques. The adult male, is a carrier of a reciprocal, balanced translocation involving the X and #2 chromosomes: 46,Y,t(X;2) (q13;p21). This man showed infertility with spermatogenesis arrest at the spermatocyte stage. Synaptonemal complex analysis at pachytene showed the quadrivalent structure and the putative breakage points. Sex-chromatin condensation did not spread towards the autosomal regions of the quadrivalent. The female infant showed diminished body growth and multiple somatic anomalies. She is a 45,Xp-,t(X;21)(p11;p13) carrier, an unbalanced translocation involving chromosomes X and #21, which leads to a monosomy of almost all Xp. The translocated #21 is practically complete, and its centromere is the active one in the rearranged product. The analysis of interphase nuclei with the X-centromere probe shows that the Xq region of the rearranged chromosome is the late -replicating and inactive element. However, X-inactivation does not spread to the attached #21, as shown by the R-banding pattern. Thus, both in the male adult and in the female infant there is a barrier to the spreading effect of X-chromosome inactivation, which is probably due to different mechanisms.

Induction and characterization of premature chromosome condensation in Drosophila synkaryons and implications to dosage compensation.

Premature chromosome condensation (PCC) was induced in Drosophila melanogaster cell hybrids, with Drosophila mitotic cells and interphase cells at different phases (G1,G1-S,S,S-G2 of the cell cycle, and from male and female, using standard cell fusion technique with polyethylene glycol (PEG). A combination of Feulgen and autoradiography was used to enhance the resolution of the PCC plates. It was possible to identify the characteristics of PCC's at G1, S and G2, and the transitory intermediate phases, which are comparable with respect to the characteristics of PCC's previously described for other species. Using the combined Feulgen-autoradiography technique it was possible to critically resolve the different phases including the transitory intermediate phases in greater detail. Analysis and comparison of results obtained from M (female) x S (female/male) and M (female) x G2 (female/male) hybrids have revealed that the X chromosome from the male could be identified as a distinct acrocentric entity which showed clear allocyclic, heteropycnotic characteristics. The results thus lead us to suggest that the X chromosome in such synkaryons is indeed early replicating as is the X chromosome of male larval salivary gland of Drosophila. The X chromosome morphology is also distinctly at an advanced stage of the cell cycle. On the basis of these findings it is concluded that X chromosome is hyperactive in the normally dividing diploid cells.

Replicative activity of X-chromosome and autosomes of Drosophila melanogaster in autosomal hyperploids and the problem of dosage compensation.

Replicative behaviour of two hyperploid autosomal arms (2L and 3L) of D. melanogaster has been analysed by 3H-thymidine autoradiography. Results reveal that hyperploid autosomal arms (2L-trisomy or 3L-trisomy) replicate synchronously with other disomic non-homologous chromosome arms i.e. there is no asynchrony in the initial mid or late phase of replication patterns between the trisomic 2L or trisomic 3L and disomic arms, suggesting that the extra dose of an autosomal arm can not alter the inherent pattern of replication of that arm. Results further reveal that 2L-trisomy or 3L-trisomy does not impart any influence on X-chromosomal replication in either sex. It is suggested from these results that change in the genomic dose of autosome does not play any role in modulating the replicative organization of the autosomes, 2L and 3L. Thus, although a regulatory mechanism of autosomal dosage compensation does exist for Drosophila, the hierarchy of genetic programming of regulation for X-chromosomal and autosomal dosage compensation might be different. Neither hypertranscriptive activity nor faster replication pattern of the male X-chromosome is influenced by 2L- or 3L-trisomy.

Dosage compensation for a chromosome that comprises an X chromosome and an autosome as its two homologs in interspecific hybrids of Drosophila miranda and D. persimilis.

A chromosomal element (C) of an interspecific hybrid of D. miranda (females) and D. persimilis (males) represented a unique situation, where between its two homologous, one derived from an X chromosome (X2 of miranda) and the other from an autosome (3rd chromosome of persimilis). In cytological preparations of polytene nuclei, the X chromosomal homolog in hybrid males exhibited the male-X like inflated structure, known as prerequisite for hypertranscription; whereas the autosomal homolog existed as an haploid autosomal arm. In hybrid females, both the homologs had similar diameter and stainability. This difference in cytomorphology of the X2 homolog between two sexes made the C element potential to transcribe higher in males than in females, raising the inevitable question of compensation. Cellular autoradiography, using 3H-UR, was employed to measure the total transcription of the C element (X2 + 3rd) in hybrid sexes. Results revealed that, although the X2-homolog was hyperactive in males, the total transcription of the C element was equal (relative to autosomal transcription) in both hybrid sexes, and this was achieved in expense of the transcription of the 3rd chromosomal homolog in males. Thus, apart from X-chromosomal and autosomal dosage compensation, the existence of an X-autosomal dosage compensation in Drosophila is evident in the present work.