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The study of the effect of programmed cessation of transcription in a large nuclear domain, on the distribution of elements of the pre-mRNA splicing machinery, is the main aim of this paper. To this end, we took advantage of the nuclear partitioning of mouse spermatocytes early in meiosis into autosomal transcribing and XY nontranscribing compartments. This system also allows to extend this study to stages in sperm differentiation that are accompanied by reduction and eventual cessation of transcription. We show by indirect immunofluorescence in spermatogenetic cells that 1) fluorescent signals of the pre-mRNA splicing factors SF53/4 and SC35, of the Sm antigens, and of RNA polymerase II, are largely absent from the nontranscribing, X-inactivated compartment, but are abundantly present in the transcribing autosomal compartment and 2) the presence, gradual reduction, and absence of transcriptive activity in nuclei undergoing the sperm formation sequence are positively correlated with the fluorescence patterns of the antibodies against SF53/4, SC35, and the Sm antigens. These data suggest that cessation of transcription during spermatogenesis is accompanied by exclusion of the splicing machinery from nontranscribing chromatin to its vicinity.

We show here that histone macroH2A1.2 concentrates at the transcriptionally silent XY body, normally being formed during male meiosis in the mouse. A similar accumulation has earlier been observed on the inactive X chromosomes of somatic adult female mammalian cells by Costanzi and Pehrson (1998). This correspondence in the nature of heterochromatinization of the X chromosomes in males and females adds another property of X chromosome inactivation that is shared by males and females at different phases of their life cycle.

The sex chromosomes of the partly sympatric species of gerbils Gerbillus pyramidum and G. gerbillus (Mammalia: Gerbillinae) were investigated by a variety of light- and electron-microscope methods, including DNA replication banding and synaptonemal complex (SC) techniques. The sex-chromosome mechanism of G. pyramidum is of the JXY : XX type, whereas that of G. gerbillus is of the less common (JXYJY2 : XX system. The results include the demonstration that the X chromosomes of both species are compound. One segment is added to the X chromosome of G. pyramidum, leading to an increase in length from the standard 5% to approximately 7.3%, whereas two different extra segments increase the length of the X chromosome of G. gerbillus to approximately 11 % of the length of the haploid genome. In both cases the extra material is autosomal and is also represented in the respective Y chromosomes. Classifying heterochromatin by the variation in staining quality was helpful in elucidating the possible origin of the different chromosome segments, including the pericentromeric regions. Observations on meiotic chromosome pairing and chiasma formation have confirmed the homologies established by band comparisons. The occurrence of chiasmata between the sex chromosomes supports the autosomal origin of the pairing segments. These and other findings have been interpreted in the framework of a multistep evolutionary model. This sequence starts from a hypothetical pair of sex chromosomes, the X element of which amounts to 5% of the haploid genome, and leads through three translocations involving two pairs of autosomes and one pericentric inversion to the most complex situation of this series, manifested in G. gerbillus. The adaptive value, if any, of autosome incorporation into the sex chromosomes repeatedly occurring here is unknown. It is, however, a remarkable fact that in one species, G. gerbillus, the complex sex-chromosome constitution is conserved over vast geographic distances, and in the other, G. pyramidum, the compound X and Y chromosomes withstand change in the face of extreme autosome restructuring.

The spindle of metazoan cells functions as a dual distributor that guarantees the accurate segregation of both chromosomes and centrioles (basal bodies). This combined mechanism may have evolved from the separate distribution devices for chromosomes and basal bodies found in Protozoa. Typical eupyrene spermatocytes of the silk moth were compared with atypical apyrene spermatocytes. (a) Long microtubules, persisting during centriolar movements, develop in the meiotic prophases in continuity with the centrioles (b) There is no longer a centriole-microtubule continuity at metaphase-anaphase (c) Spindles comprise microtubules and endoplasmic reticulum (ER). The microtubules form a barrel-shaped structure terminating far in front of the centrioles. The two types of spermatocytes differ in the structure of the ER, which is vesicular in the eupyrene and lamellar in the apyrene line. It is suggested that the ER influences the anaphase movement of chromosomes, (d) The chromosomes lack localized centromeres. Microtubules penetrate all along the polar faces of the eupyrene metaphase chromosomes. The apyrene chromosomes form irregular blocks that are penetrated by microtubules all around their periphery, (e) The centriole comprises 3 concentric zones. Typical changes in the centriole are correlated with changes in the cell cycle. Replication, de novo formation and disappearance of centrioles are manifestations of the state of flux of the microtubules.

Association of DNA with the nuclear membrane was studied in the naturally synchronous slime mold Physarum polycephalum . Sucrose gradient analyses showed that DNA is bound to the membrane during all stages of interphase. The association is labile, but can be stabilized by spermine, which presumably acts on the membrane. Experiments with pulse-labeled DNA suggest that replication does not occur at a membranal site.

The typical eupyrene and atypical apyrene cycles of sperm differentiation in Bombyx mori were studied, with special attention to centriole number and behavior. Contrary to other reports, there is always only one centriole in the differentiating and in the mature sperm, thus confirming our previous findings that insect sperm has one centriole at the base of the flagellum, in contrast to two centrioles found in many other groups of animals. This numerical difference is discussed in an evolutionary context.

Observations sur la variation intra-specifique du nombre des chromosomes dans deux especes du genreAmeles. On decrit ici 3 types chromosomiaux apparaissant chezA. heldreichi. Les chromosomes de ces 3 types, de meme que ceux de 4 especes apparentees aA. heldreichi ont d'une facon constante 30 bras. L'evolution chromosomique s'effectue selon le principe deRobertson et est probablement tres active puisque on trouve des etats intermediaires: des individus heterozygotes pour un element metacentrique et deux elements acrocentriques. Les 3 types cytologiques decrits par nous en Israel se retrouvent dans une population d'A. heldreichi provenant de la Turquie.

THE Gerbillinae are highly differentiated rodents exhibiting many instances of parallel specializations in external and anatomical features evolved in independent lines. Consequently, the present state of the taxonomy of this group is unsatisfactory. Three years ago we started an extensive study of the chromosome complements in the species occurring in Israel. The comprehensive cytological studies of Matthey1,2 enabled us to compare the idiograms of our species with those of others from different parts of the family range.

A microspread, early-mid diplotene nucleus of a man with a normal karyotype and presumably normal meiosis is compared with a similar, earlier described nucleus of a man with meiotic arrest, heterozygous for a (14;21) Robertsonian translocation (Rosenmann et al., 1985). The axes of the XY bivalent of normal diplotene have an extremely tangled configuration, whereas those of the meiotically-arrested cell are straight, recalling the shape of the XY which is normally found in early pachytene. The morphological reversal from the complex configuration to a simpler shape may be associated with reactivation of the sex chromosome(s). Such a reactivation may be responsible for the sterility of the carrier of the Robertsonian translocation which thus may be considered as chromosomally-derived. The diplotene cells shown here have autosomal bivalents with continuous axes and various degrees of focal separation as is typical for diplotene in general. The observations on axis continuity, bivalent segmental dilatations, and XY tanglement in diplotene are compared with findings by others in human ultrathin sectioned material.

Intimate association between autosomal translocation trivalents and XY bivalents at pachytene was observed in a majority of cells of two men ascertained through primary sterility and found to be heterozygous for a 14;21 Robertsonian translocation. The association, studied by light and electron microscopy of spread first spermatocytes, was between the unpaired short arms of the normal chromosomes of the translocation trivalent and the differential axes of the XY chromosomes. In a minority of cells, this contact was not established, or not maintained, as alternative combinations between the elements available for non-homologous pairing were realized. Following a suggestion of Lifschytz and Lindsley (1972), sterility in these patients was attributed to spermatogenic arrest caused by physical contact of sex chromosomes with autosomal material and consequent interference with the normal metabolism of the sex chromosomes. Autosomal aberrations and polymorphisms, which lead to the presence of unpaired segments at meiosis, may thus play a critical role in a general mechanism of chromosomally-derived male sterility. It is proposed that such a mechanism may also be instrumental in the initiation of reproductive barriers in nature.

Limitation of current techniques in identifying extra chromosomal segments arising de novo is illustrated by a putative case of a duplication of the long arm of chromosome 7. The propositus, demonstrating multiple congenital anomalies and severe mental retardation, had a large extra segment of chromatin on chromosome 7q that was absent in his parents. The banding pattern of this segment resembled that of the long arm of chromosomes 7, 8, or 9. Various procedures indicated that the additional material did not include the secondary constriction of 9q. The phenotype of the propositus did not fit well with that of trisomy 8.

The sex chromosomes of the partly sympatric species of gerbils Gerbillus pyramidum and G. gerbillus (Mammalia: Gerbillinae) were investigated by a variety of light- and electron-microscope methods, including DNA replication banding and synaptonemal complex (SC) techniques. The sex-chromosome mechanism of G. pyramidum is of the maleXY:femaleXX type, whereas that of G. gerbillus is of the less common maleXY1Y2:femaleXX system. The results include the demonstration that the X chromosomes of both species are compound. One segment is added to the X chromosome of G. pyramidum, leading to an increase in length from the standard 5% to approximately 7.3%, whereas two different extra segments increase the length of the X chromosome of G. gerbillus to approximately 11% of the length of the haploid genome. In both cases the extra material is autosomal and is also represented in the respective Y chromosomes. Classifying heterochromatin by the variation in staining quality was helpful in elucidating the possible origin of the different chromosome segments, including the pericentromeric regions. Observations on meiotic chromosome pairing and chiasma formation have confirmed the homologies established by band comparisons. The occurrence of chiasmata between the sex chromosomes supports the autosomal origin of the pairing segments. These and other findings have been interpreted in the framework of a multistep evolutionary model. This sequence starts from a hypothetical pair of sex chromosomes, the X element of which amounts to 5% of the haploid genome, and leads through three translocations involving two pairs of autosomes and one pericentric inversion to the most complex situation of this series, manifested in G. gerbillus. The adaptive value, if any, of autosome incorporation into the sex chromosomes repeatedly occurring here is unknown. It is, however, a remarkable fact that in one species, G. gerbillus, the complex sex-chromosome constitution is conserved over vast geographic distances, and in the other, G. pyramidum, the compound X and Y chromosomes withstand change in the face of extreme autosome restructuring.

Several different types of rodents are found among the specialized mammals exploiting the subterranean ecological niche. Tunnels dug by them can often be detected through the mounds of earth thrown up during the burrowing. Striking anatomical and behavioral adaptations are associated with their fossorial mode of life (Ellerman, 1956; Pearson, 1959). The mole rats of the genus Spalax are an extreme example of such rodents; they are largely confined to a subterranean existence. Their adaptations include the great reduction in the development of their eyes, which are not visible externally.

Four forms of mole rats with diploid numbers of chromosomes of S2, 54, 58, and 60, respectively, were found in Israel and the vicinity. The differences between the chromosome sets are due to whole-arm (Robertsonian) changes and pericentric inversions. The geographic distribution of the different forms is contiguous. Only a few hybrid individuals have been discovered. These chromosome forms are tentatively considered as sibling species, almost completely isolated by cytogenetic and possibly ethological mechanisms. The weak dispersal powers of mole rats may have-contributed to a rapid fixation of adaptive homozygous chromosomal changes.

Centrioles were studied in living and fixed testes of neuropteran insects by light, phase-contrast, and electron microscopy. Giant centrioles, measuring up to 8.0 µ in length (according to the species), develop in meiotic prophase, whereas only minute centrioles are found in spermatogonia. The giant centrioles have essentially the same ultrastructure as described for smaller centrioles, but their periodic structure is more clearly defined. The growth of giant centrioles consists of two phases. First, a procentriole lengthens to the size of its mother centriole, as in mitosis; then the four centriolar rods of one cell continue their growth simultaneously and at the same rate. At their distal ends centriolar vesicles are attached. These originate from invaginations of the cell membrane and apparently contribute to the growth of the centrioles. Neuropteran spermatids, like those of other insects, have only one centriolar rod, while spermatids of many other animals contain two centrioles. The origin and significance of this difference are unknown.

Testes of a Neuropteran insect were treated in vivo with mercaptoethanol. Spermatids with two and four axes were produced in less than 2 hr. It is concluded that spermatid differentiation can go on while the meiotic divisions are inhibited.

Two similar pericentric inversions of chromosome 9 are transmitted in two unrelated families, each of which includes a consanguineous marriage. In both instances the inverted segment comprises the centromere region and the proximal part of the long arm, including the secondary constriction. The inverted chromosome was found in one homozygote and in three heterozygotes of the six karyotyped individuals of one family; in the second family, three of five karyotyped individuals were heterozygotes. The health problems of the propositi are apparently not associated with their chromosomal constitution.