Multiple independent evolutionary losses of XY pairing at meiosis in the grey voles

In many eutherian mammals, X-Y chromosome pairing and recombination is required for meiotic progression and correct sex chromosome disjunction. Arvicoline rodents present a notable exception to this meiotic rule, with multiple species possessing asynaptic sex chromosomes. Most asynaptic vole species belong to the genus Microtus sensu lato. However, many of the species both inside and outside the genus Microtus display normal X-Y synapsis at meiosis. These observations suggest that the synaptic condition was present in the common ancestor of all voles, but gaps in current taxonomic sampling across the arvicoline phylogeny prevent identification of the lineage(s) along which the asynaptic state arose. In this study, we use electron and immunofluorescent microscopy to assess heterogametic sex chromosome pairing in 12 additional arvicoline species. Our sample includes ten species of the tribe Microtini and two species of the tribe Lagurini. This increased breadth of sampling allowed us to identify asynaptic species in each major Microtine lineage. Evidently, the ability of the sex chromosomes to pair and recombine in male meiosis has been independently lost at least three times during the evolution of Microtine rodents. These results suggest a lack of evolutionary constraint on X-Y synapsis in Microtini, hinting at the presence of alternative molecular mechanisms for sex chromosome segregation in this large mammalian tribe.

Meiotic characterization and sex determination system of neotropical primates: Bolivian squirrel monkey Saimiri boliviensis (primates: Cebidae)

The chromosomal sex determination system differs among platyrrhine monkeys more than any other group of primates. Although a number of studies have investigated mitotic chromosomes across platyrrhine species, the meiotic chromosomes of many genera have not yet been described. The goal of this study was to characterize the sex determination system of Saimiri boliviensis. We described for the first time the meiotic cycle, confirming the sexual system in germ cells from testicular biopsies of four adult male S. boliviensis. All specimens were weighed and testicular volume was measured. We observed 22 bivalents corresponding to 2N = 44, and a "human-like" XY bivalent was found in diakinesis/metaphase I. In addition, mitotic studies from blood samples of both sexes were performed and G- and C-banding patterns agreed with previously reported karylogy of S. boliviensis boliviensis. Further meiotic studies should be performed in New World primates based on the great value of those studies for systematic evolutionary biology and conservation programs.

The genetic signature of sex-biased migration in patrilocal chimpanzees and humans

A large body of theoretical work suggests that analyses of variation at the maternally inherited mitochondrial (mt)DNA and the paternally inherited non-recombining portion of the Y chromosome (NRY) are a potentially powerful way to reveal the differing migratory histories of men and women across human societies. However, the few empirical studies comparing mtDNA and NRY variation and known patterns of sex-biased migration have produced conflicting results. Here we review some methodological reasons for these inconsistencies, and take them into account to provide an unbiased characterization of mtDNA and NRY variation in chimpanzees, one of the few mammalian taxa where males routinely remain in and females typically disperse from their natal groups. We show that patterns of mtDNA and NRY variation are more strongly contrasting in patrilocal chimpanzees compared with patrilocal human societies. The chimpanzee data we present here thus provide a valuable comparative benchmark of the patterns of mtDNA and NRY variation to be expected in a society with extremely female-biased dispersal.

Bidirectional transcription of a novel chimeric gene mapping to mouse chromosome Yq

BACKGROUND

The male-specific region of the mouse Y chromosome long arm (MSYq) contains three known highly multi-copy X-Y homologous gene families, Ssty1/2, Sly and Asty. Deletions on MSYq lead to teratozoospermia and subfertility or infertility, with a sex ratio skew in the offspring of subfertile MSYqdel males

RESULTS

We report the highly unusual genomic structure of a novel MSYq locus, Orly, and a diverse set of spermatid-specific transcripts arising from copies of this locus. Orly is composed of partial copies of Ssty1, Asty and Sly arranged in sequence. The Ssty1- and Sly-derived segments are in antisense orientation relative to each other, leading to bi-directional transcription of Orly. Genome search and phylogenetic tree analysis is used to determine the order of events in mouse Yq evolution. We find that Orly is the most recent gene to arise on Yq, and that subsequently there was massive expansion in copy number of all Yq-linked genes.

CONCLUSION

Orly has an unprecedented chimeric structure, and generates both "forward" (Orly) and "reverse" (Orlyos) transcripts arising from the promoters at each end of the locus. The region of overlap of known Orly and Orlyos transcripts is homologous to Sly intron 2. We propose that Orly may be involved in an intragenomic conflict between mouse X and Y chromosomes, and that this process underlies the massive expansion in copy number of the genes on MSYq and their X homologues.

Synaptonemal complexes and XY behavior in two species of Argentinian armadillos: Chaetophractus villosus and Dasypus hybridus (Xenarthra, Dasypodidae)

Spermatocytes from the two armadillo species, C. villosus and D. hybridus were studied in microspreads for synaptonemal complexes (SCs) and in thin sections for electron microscopy (EM). The complete SC karyotype generally agrees with previous reports on mitotic chromosomes, except for the sex chromosomes. The X chromosome is submetacentric in both species and the Y is the shortest one in C. villosus and the second shortest in D. hybridus, and an extremely acrocentric one. A SC is formed along the total length of the Y chromosome, and this SC persists along all the pachytene substages. A single recombination nodule (RN) is located in the region of the SC nearest to the attachment to the nuclear envelope. The lateral element (LE) of the X axis in the SC shows a wavy aspect in most of the SC length distant from the nuclear envelope. Nucleoli are attached to acrocentric or submetacentric bivalents, are visibly double in some cells, and in thin sections show an elaborate nucleolonema. Some differences in the XY are species-specific, as the higher degree of tangling and stronger heteropycnosis in D. hybridus. The effective, single crossover of the XY pair is highly localized, despite the permanence of a long tract of SC.

Heteromorphic sex chromosome system with an exceptionally large Y chromosome in a catfish Steindachneridion sp. (Pimelodidae)

The chromosomes and banding patterns of Steindachneridion sp., a large catfish (Pimelodidae), endemic to the Iguaçu River, Brazil, were analyzed using conventional (C-, G-banding) and restriction enzyme banding methods. The same diploid number (2n = 56) as in other members of the genus and the family was found but the karyotype displayed an XX/XY sex chromosome system. The X chromosome was the smallest submetacentric, while the Y was the largest chromosome in the karyotype. Meiotic analysis showed 27 autosomal bivalents plus one heteromorphic XY bivalent during spermatogenesis. Sex chromosomes had no particular pattern after C-banding but G- and restriction enzyme bandings showed specific banding characteristics. The present finding represents the first report of a well-differentiated and uncommon sex chromosome system in the catfish family Pimelodidae.

The characteristics of karyotype and telomeric satellite DNA sequences in the cricket, Gryllus bimaculatus (Orthoptera, Gryllidae)

The chromosomes derived from the Japanese population of Gryllus bimaculatus were characterized by C-banding and Ag-NOR staining. The chromosome number, 2n = 28 + XX (female)/XO (male), corresponded with that of other populations of G. bimaculatus, but the chromosome configuration in idiograms varied between the populations. NORs were carried on one pair of autosomes and appeared polymorphous. The positive C-bands located at the centromere of all chromosomes and the distal regions of many chromosome pairs, and the size and the distribution pattern of the distal C-heterochromatin showed differences among the chromosomes. In addition, this paper reports on the characteristics of HindIII satellite DNA isolated from the genome of G. bimaculatus. The HindIII repetitive fragments were about 0.54 kb long, and localized at the distal C-bands of the autosomes and the interstitial C-bands of the X chromosome. Molecular analysis showed two distinct satellite DNA sequences, named the GBH535 and GBH542 families, with high AT contents of about 67 and 66%, respectively. The two repetitive families seem to be derived from a common ancestral sequence, and both families possessed the same 13-bp palindrome sequence. The results of Southern blot hybridization suggest that the sequence of the GBH535 family is conserved in the genomic DNAs of Gryllus species, whereas the GBH542 family is a species-specific sequence.

Histone H3 lysine 4 dimethylation is enriched on the inactive sex chromosomes in male meiosis but absent on the inactive X in female somatic cells

Inactivation of the X chromosome occurs in female somatic cells and in male meiosis. In both cases, the inactive X chromosome undergoes changes in histone modifications including deacetylation of core histone proteins and enrichment with histone H3 lysine 9 (H3-K9) dimethylation. In this study we show that while the inactive X in female somatic cells is largely devoid of H3-K4 dimethylation, the inactive X in male meiosis is enriched with this modification. However, the inactive X chromosome in female somatic cells and the inactive X and Y in male meiosis are devoid of H3-K4 trimethylation. Further, trimethylation of H3-K4 is present at discrete regions along most of the autosomes, while H3-K4 dimethylation shows a more homogenous staining. Also, the Y chromosome is largely devoid of H3-K4 di- and trimethylation in somatic cells of both humans and mice, however, the Y chromosome is enriched with H3-K4 di- but not trimethylation throughout spermatogenesis. Our results provide insights into the differences between female somatic cells and male germ cells in inactivating the X chromosome, and suggest that trimethylation, and not dimethylation, of H3-K4 is a more robust indicator of the active regions of the genome.

Has the kouprey (Bos sauveli Urbain, 1937) been domesticated in Cambodia?

The kouprey (Bos sauveli Urbain, 1937) is a very rare bovid species of Cambodia, which may be extinct in the wild, as no living specimen has been observed for a long time. Here, we describe a complete taxidermy mount, which presents astonishing morphological similarities with the kouprey. The animal was mounted in 1871 at the National Museum of Natural History in Paris, where it was referenced as No. 1871-576. It was deposited at the Natural History Museum of Bourges, France, in 1931, where it is still conserved today. To clarify the taxonomic status of the specimen of Bourges, DNA was extracted from a piece of bone taken on the mandible, and two different fragments of the mitochondrial cytochrome b gene were independently amplified and sequenced. The phylogenetic analyses show that the specimen of Bourges is robustly associated with the holotype of the kouprey, and that both are related to other wild species of Bos found in Indochina, i.e., banteng (B. javanicus) and gaur (B. frontalis). Because of doubts for sexing the animal, we applied a molecular test based on the PCR amplification of a DNA fragment specific to the Y chromosome. The results indicate that the specimen of Bourges is a male. The comparisons with male kouprey previously described in the literature reveal important differences concerning the body size, general coloration and horns. As these differences involve phenotypic traits that are strongly selected in case of domestication, we suggest that the specimen of Bourges was a domestic ox. This implies therefore that the kouprey may have been domesticated in Cambodia, and that several extant local races may be directly related to the kouprey.

Localization of MMR proteins on meiotic chromosomes in mice indicates distinct functions during prophase I

Mammalian MutL homologues function in DNA mismatch repair (MMR) after replication errors and in meiotic recombination. Both functions are initiated by a heterodimer of MutS homologues specific to either MMR (MSH2-MSH3 or MSH2-MSH6) or crossing over (MSH4-MSH5). Mutations of three of the four MutL homologues (Mlh1, Mlh3, and Pms2) result in meiotic defects. We show herein that two distinct complexes involving MLH3 are formed during murine meiosis. The first is a stable association between MLH3 and MLH1 and is involved in promoting crossing over in conjunction with MSH4-MSH5. The second complex involves MLH3 together with MSH2-MSH3 and localizes to repetitive sequences at centromeres and the Y chromosome. This complex is up-regulated in Pms2-/- males, but not females, providing an explanation for the sexual dimorphism seen in Pms2-/- mice. The association of MLH3 with repetitive DNA sequences is coincident with MSH2-MSH3 and is decreased in Msh2-/- and Msh3-/- mice, suggesting a novel role for the MMR family in the maintenance of repeat unit integrity during mammalian meiosis.

Retroelements: tools for sex chromosome evolution

Many eukaryotic taxa inherit a heteromorphic sex chromosome pair. It is a generally accepted hypothesis that the sex chromosome pair is derived from a pair of homologous autosomes that has developed after the occurrence of a sex differentiator in an evolutionary process into two structurally and functionally different partners. In most of the analyzed systems the occurrence of the dominant sex differentiator is paralleled by the suppression of recombination within and close by that region. The recombinational isolation can spread in an evolutionary selection process from neighboring regions finally over the whole chromosome. Suppression of recombination strongly biases the distribution of retrotransposons in the genome. Our results and that from others indicate that the major force driving the evolution of Y chromosomes are retrotransposons, remodeling euchromatic chromosome structures into heterochromatic ones. In our model, intact or already eroded retrotransposons become trapped due to their inherent transposition mechanisms in non-recombining regions. The massive accumulation of retrotransposons interferes strongly with the activity of genes. We hypothesize that Y chromosome degeneration is a stepwise evolutionary process: (1) Massive accumulation of retrotransposons occurs in the non-recombining regions. (2) Heterochromatic nucleation centers are formed as a consequence of genomic defense against invasive parasitic elements; the established nucleation centers become epigenetically inherited. (3) Spreading of heterochromatin from the nucleation centers into flanking regions induces in an adaptive process gene silencing of neighbored genes that could either be still intact or in an already eroded condition, e.g., showing point mutations, deletions, insertions; the retroelements should be subjects to the same forces of deterioration as the genes themselves. (4) Constitutive silenced genes are not committed to the same genetic selection pressure as active genes and therefore more exposed to the decay process. (5) Gene dosage balance is reestablished by the parallel evolution of dosage compensation mechanisms. The evolving secondary sex chromosomes, neo-X and neo-Y, of Drosophila miranda are revealed to be a unique and potent model system to catch the evolutionary Y deterioration process in progress.

X chromosome painting in Microtus: origin and evolution of the giant sex chromosomes

Sex chromosomes in species of the genus Microtus present some characteristic features that make them a very interesting group to study sex chromosome composition and evolution. M. cabrerae and M. agrestis have enlarged sex chromosomes (known as 'giant sex chromosomes') due to the presence of large heterochromatic blocks. By chromosome microdissection, we have generated probes from the X chromosome of both species and hybridized on chromosomes from six Microtus and one Arvicola species. Our results demonstrated that euchromatic regions of X chromosomes in Microtus are highly conserved, as occurs in other mammalian groups. The sex chromosomes heterochromatic blocks are probably originated by fast amplification of different sequences, each with an independent origin and evolution in each species. For this reason, the sex heterochromatin in Microtus species is highly heterogeneous within species (with different composition for the Y and X heterochromatic regions in M. cabrerae) and between species (as the composition of M. agrestis and M. cabrerae sex heterochromatin is different). In addition, the X chromosome painting results on autosomes of several species suggest that, during karyotypic evolution of the genus Microtus, some rearrangements have probably occurred between sex chromosomes and autosomes.

XMR, a dual location protein in the XY pair and in its associated nucleolus in mouse spermatocytes

Xlr and Xmr are sex-specific genes which are expressed during the meiotic prophase I in the mouse. In spermatocytes, XMR concentrates on the asynapsed regions of the XY chromosomes, suggesting that XMR plays a role in sex chromosome condensation and silencing. The present study shows that in the mouse, XMR also concentrates in the nucleolus which is closely associated with the XY chromosome pair. In this species, the formation of a large fibrillo-granular nucleolus signals the activation of the ribosomal genes, but release of pre-ribosomal particles is inhibited. Using laser confocal microscopy we characterized the distribution of XMR in the XY body relative to the XY chromatin and the nucleolus. Immunoelectron microscopy showed that XMR concentrates in the fibrillo-granular component and the granular component (GC) of the nucleolus. In (T[X;16]16H) mouse spermatocytes, the nucleolus displays little or no activity and does not associate with the XY pair. XMR concentrated only on the XY chromosomes in (T[X;16]16H) mouse spermatocytes. These data suggest that XMR could play a role both in the XY pair and the nucleolus associated to the sex chromosomes.

A gradual process of recombination restriction in the evolutionary history of the sex chromosomes in dioecious plants

To help understand the evolution of suppressed recombination between sex chromosomes, and its consequences for evolution of the sequences of Y-linked genes, we have studied four X-Y gene pairs, including one gene not previously characterized, in plants in a group of closely related dioecious species of Silene which have an X-Y sex-determining system (S. latifolia, S. dioica, and S. diclinis). We used the X-linked copies to build a genetic map of the X chromosomes, with a marker in the pseudoautosomal region (PAR) to orient the map. The map covers a large part of the X chromosomes--at least 50 centimorgans. Except for a recent rearrangement in S. dioica, the gene order is the same in the X chromosomes of all three species. Silent site divergence between the DNA sequences of the X and Y copies of the different genes increases with the genes' distances from the PAR, suggesting progressive restriction of recombination between the X and Y chromosomes. This was confirmed by phylogenetic analyses of the four genes, which also revealed that the least-diverged X-Y pair could have ceased recombining independently in the dioecious species after their split. Analysis of amino acid replacements vs. synonymous changes showed that, with one possible exception, the Y-linked copies appear to be functional in all three species, but there are nevertheless some signs of degenerative processes affecting the genes that have been Y-linked for the longest times. Although the X-Y system evolved quite recently in Silene (less than 10 million years ago) compared to mammals (about 320 million years ago), our results suggest that similar processes have been at work in the evolution of sex chromosomes in plants and mammals, and shed some light on the molecular mechanisms suppressing recombination between X and Y chromosomes.

The XY pair of the mink (Mustela vision) during different periods of testicular activity

Synaptonemal complexes of the mink (Mustela vison) were examined during different stages of testicular activity to determine whether the distribution of prophase substages and the configuration of the sex complement are altered during pre-quiescent and regenerative phases compared to those detected during the breeding period. Spermatocytes obtained during pre-quiescence showed no differences from those of breeding season in terms of substage distribution, whereas those from regenerating testes were mainly in zygotene and early pachytene substages, reflecting the high mitotic activity of spermatogonia and their subsequent transit to meiosis. Based on the location of kinetochores on the sex complement, the synapsed segments were identified as the short arm of the X (Xp) and the long arm of the Y (Yq), although pairing of the X and Y beyond the "pseudoautosomal region" was frequently observed. In some spermatocytes, the entire Y chromosome synapsed with the X or split into two strands with only one strand "paired" with the X while the other remained unpaired. It is not clear at present whether the Y chromosome splitting is part of the mechanisms that prevent crossing over in the non-homologous segments of the sex complement that often undergo synapsis or a post-crossover phenomenon unrelated to pairing mechanisms.

Role of bone marrow cell trafficking in replenishing skeletal muscle SP and MP cell populations

The multipotent nature of skeletal muscle-derived side population cells is demonstrated by their myogenic and hematopoietic potential in vivo. However, whether muscle side population cells are derived from the bone marrow is unclear. To study the long-term contribution of the hematopoietic system to muscle side population, whole bone marrow cells from Ly5.1 males or from e-GFP transgenic male mice were transplanted into lethally irradiated Ly5.2 females. Long-term cell trafficking of donor bone marrow cells to muscle side population was monitored 17 times in a 34-week study. Fluorescence-activated cell sorter analyses were used to detect Ly5.1 and GFP+ donor cells, which were confirmed by fluorescence in situ hybridization of the Y-chromosome. Analyses post-transplantation indicated that whereas cells of donor origin could be found in the muscle, donor bone marrow cells had contributed little to the muscle side population. Attempts to increase cell trafficking by induced muscle damage again confirmed that more than 90% of side population cells present in the muscle were derived from the host. These results demonstrate that muscle side population cells are not replenished by the bone marrow and suggest a non-hematopoietic origin for this cell population.

Molecular aspects of XY body formation

More than a century ago, a densely stained area inside the nucleus of male meiotic cells was described. It was later shown to harbor the sex chromosomes which undergo transcriptional inactivation in conjunction with heterochromatinisation and synapsis to form the XY body. Formation of the XY body is conserved throughout the mammalian phylogenetic tree and is thought to be essential for successful spermatogenesis. However, its biological role as well as the molecular mechanisms underlying XY body formation are still far from being understood. A lot of effort has already been undertaken to characterize components of the XY body and to investigate their functional implications in sex chromatin heterochromatinisation and meiotic sex chromosome inactivation (MSCI). This review gives an overview of those components and their possible implications in XY body formation and function.

Dual Y chromosome painting and in situ cell-specific immunofluorescence staining in lung tissue: an improved method of identifying donor marrow cells in lung following bone marrow transplantation

Several recent studies have demonstrated localization of donor bone marrow-derived cells in recipient lungs following transplantation from male to female mice or patients. Donor cells are identified by detection of the Y chromosome by fluorescence in situ hybridization (FISH). However, protein digestion pretreatments usually required for tissue FISH significantly limit the ability to detect cell type-specific markers by immunohistochemistry. We have used an alternative protein digest approach that entails heating the slides in 10 mM sodium citrate rather than utilizing a protease digestion. This approach preserves cell proteins following FISH, and allows lung tissue to remain intact for subsequent detection of cell-specific markers by immunohistochemistry. We have examined this technique in mouse lungs using a Y chromosome paint and three cell-specific markers, a pan-cytokeratin for epithelial cells, PECAM-1 for endothelial cells, and CD45 for leukocytes. Excellent visualization of both the Y chromosome and cell-specific surface protein markers was obtained on a single slide. This approach will significantly enhance the ability to detect and identify donor bone marrow cells in recipient mouse lungs following male to female transplantation.

The pairing of X and Y chromosomes during meiotic prophase in the marsupial species Thylamys elegans is maintained by a dense plate developed from their axial elements

Unlike eutherian males, pairing of the sex chromosomes in marsupial males during the first meiotic prophase is not mediated by a synaptonemal complex. Instead, a specific structure, the dense plate, develops during pachytene between the sex chromosomes. We have investigated the development and structural nature of this asynaptic association in males of the marsupial species Thylamys elegans by means of immunolabelling and electron microscopy techniques. Our results show that the behaviour of male marsupial sex chromosomes during first meiotic prophase is complex, involving modifications of their structure and/or composition. Pairing of the sex chromosomes and formation of the dense plate take place in mid pachytene, paralleling morphological changes in the sex chromosomal axial elements. Components of the central element of the synaptonemal complex were not found in the sex body, in agreement with ultrastructural studies that reported the absence of a canonical tripartite synaptonemal complex between male marsupial sex chromosomes. Interestingly, the dense plate is labelled with antibodies against the SCP3 protein of the lateral elements of the synaptonemal complex. Moreover, as sex chromosome axial elements decrease in mass throughout mid-late pachytene, the dense plate increases, suggesting that material moves from the axial elements to the dense plate. Additionally, both sex chromosome axial elements and the dense plate have proteins that are specifically phosphorylated, as revealed by their labelling with the MPM-2 antibody, indicating that they undergo a chromosome-specific regulation process throughout first meiotic prophase. We propose that the unique modifications of the composition and structure of the axial elements of the sex chromosomes in meiotic prophase may result in the prescription of synaptonemal complex formation between male marsupial sex chromosomes, where the dense plate is an extension of the axial elements of sex chromosomes. This replaces synapsis to maintain X and Y association during first meiotic prophase.

Artificial life and speciation, a case study: heterochromatin and speciation in the Microtus savii Group (Rodentia-Arvicolinae)

Artificial life is a tool which is used for simulation of peculiar cases of evolutionary events. The main characteristic of artificial life is that with this technique it is possible to simulate for a high number of generations the evolution of a population of individuals. Each individual is characterised by a small number of parameters, but each individual has its own evolutive story. So far it is possible to simulate the evolution of a population of some thousands specimens, for a high number of generations. The realistic aspect of the simulation is that each specimen is taken individually. In our opinion this instrument is very useful to simulate the evolution of the hybrids barrier during speciation. For this reason it is applied to a peculiar case of speciation, that of the Savi pine vole (Microtus savii) whose experimental data were recently investigated.

Anatomy and chromosomes of two intersexual dasyurid marsupials

The intersexual phenotypes of marsupials with XXY and XO chromosome constitutions imply that not all sexual dimorphisms are under the control of testicular hormones and, ultimately, the SRY gene on the Y chromosome. It has been hypothesised that there is a gene on the X chromosome that determines whether either a scrotum will form (one copy of the gene) or a pouch with teats (two copies of the gene). Here, we describe the anatomy and chromosomes of two intersexual dasyurid marsupials. One, a Dasyuroides byrnei, had a pouch, but the reproductive tract was essentially male. The other, a Sminthopsis douglasi, had a hemipouch and a hemiscrotum and the reproductive tract was essentially female. The S. douglasi was a mosaic for cells with an apparently normal 2n = 14, XX female karyotype and cells with 2n = 14 plus (usually) two dot-like supernumerary elements 2n = 14, XX + 2B. The D. byrnei cells examined also had a 2n = 14, XX + 2B karyotype. In fibroblasts from the male and female sides of the S. douglasi, it was possible to assign the 2n = 14, XX karyotype to the male side and the 2n = 14, XX + 2B to the female side.

Observation of X-Y chromosomal configuration in spermatocytes of infertile PD strain male rats

PD strain male rats that carry an autosomal recessive gene, preaxial duplication (gene symbol: pd), are sterile in the homozygous condition (pd/pd) due to a spermatogenic breakdown in the process of spermatogenesis at the spermatocyte and/or spermatid stage(s), although heterozygotes (pd/+) are normal. In this study, pd/pd males were examined for the presence of abnormal association of the sex chromosomes that might lead to spermatogenic breakdown. Light and electron microscopic observations of the chromosomes at meiotic prophase and metaphase in primary spermatocytes revealed several types of abnormal X-Y association and configurations in pd/pd males. However, the incidences of the abnormal configuration were comparable to those in pd/+ males. These results suggest that abnormal X-Y chromosome association in the germ cells is not a significant cause of spermatogenic breakdown in pd/pd males.

Photographic polytene chromosome maps for Glossina morsitans submorsitans (Diptera: Glossinidae): cytogenetic analysis of a colony with sex-ratio distortion

Photographic polytene chromosome maps from trichogen cells of pharate adult Glossina morsitans submorsitans were constructed. Using the standard system employed to map polytene chromosomes of Drosophila, the characteristic landmarks were described for the X chromosome and the two autosomes (L1 and L2). Sex-ratio distortion, which is expressed in male G. m. submorsitans, was found to be associated with an X chromosome (X8) that contains three inversions in each arm. Preliminary data indicate no differences in the fecundity of X(A)X(A) and X(A)X(B) females, but there are indications that G. m. submorsitans in colonies originating from Burkina Faso and Nigeria have genes on the autosomes and (or) the Y chromosome that suppress expression of sex-ratio distortion.

Recombination between homologous autosomes in medfly (Ceratitis capitata) males: type-1 recombination and the implications for the stability of genetic sexing strains

The sterile insect technique (SIT) is an environmentally safe technology to control insect pests. To improve this technology, genetic sexing strains (GSS) have been developed for the Mediterranean fruit fly, Ceratitis capitata. Such strains are based on Y-autosome translocations linking a selectable marker to the male sex and their long-term stability, especially under large-scale mass rearing conditions, is threatened by genetic recombination in the heterozygous males. We have measured male recombination in order to be able to construct GSS that are more stable. Our results show that male recombination occurs at very low frequencies, that is, below 1% per generation. Furthermore, recombination in medfly males occurs premeiotically. By selecting strains where the Y-autosome translocation breakpoint and the selectable marker are closely linked, the deleterious effects of recombination on the stability of GSS can be minimized. In such strains recombination is reduced by ca. 80% as compared to previously studied GSS. Although recombinants still occur at very low frequencies they still pose a threat to the integrity of the sexing system if they possess a selective advantage. Under mass rearing condition such recombinants will accumulate according to their relative fitness and additional measures, such as improved mass rearing strategies, are required to preserve the accuracy of the sexing system. As a conclusion it is shown that current GSS are stable enough to allow mass rearing at levels exceeding 1000 million male medflies per week.

Genetic stability of sexing strains based on the locus sw of Ceratitis capitata

This report deals with the process of improving the stability of medfly, Ceratitis capitata, genetic sexing strains (GSS) based on the sw mutation on chromosome 2. This gene affects the rate of development as well as the eye colour and iridescence. The improved sexing strains were produced by mapping sw with deletions and then inducing and screening for new translocations with breakpoints close to the marker. The stability was assessed in large populations over many generations. Twenty-two new Y-2 translocations were identified and polytene chromosome analysis was performed to locate breakpoints. The translocation strains were ranked according to the distance of their breakpoints from sw. The map position of sw is region 20D on 2R. As data on the stability of the 22 strains accumulated, Cast 191 was shown to be the most promising as no recombination between sw and the male sex was found. After rearing the strain for 22 generations under semi-mass rearing conditions, with a population size of 15,000 adults and scoring 1000 flies per generation, only one such event was detected (estimated frequency = 3.1 x 10(-6)). Further tests are being carried out with this strain to assess its suitability as a genetic sexing strain for medfly Sterile insect technique (SIT).

Towards a male-only release system for SIT with the Queensland fruit fly, Bactrocera tryoni, using a genetic sexing strain with a temperature-sensitive lethal mutation

Flies that are homozygous for the recessive autosomal mutation bent wings have a limited ability to fly and are less tolerant of high temperatures than normal flies in both the egg and puparial stages. The differences between the mutant and normal flies were found sufficient to be the basis of a genetic sexing strain. Genetic sexing strains were created using translocations of the autosome bearing the wild-type allele of bent wings (chromosome 2) to the Y chromosome, and crossing male flies carrying the translocation to mutant bent wings females. In the resulting strain, the females were homozygous for the bent wings mutation and the males were phenotypically normal for wing characters. Several translocations were recovered after irradiation, but only one translocation involving chromosome 2 was both stable and expressed in a stock that was vigorous enough for long-term viability. Unfortunately, all stocks containing the translocation showed high levels of temperature-dependent lethality, including, inexplicably, both males and females. Translocation stocks showing this effect included bent wings, another second chromosome mutation, white marks, and an otherwise normal stock. This phenomenon is probably rare, as it has not been reported before. It is likely that bent wings could be suitably used with another translocation.

Spectral karyotyping refined the identification of a der(Y)t(Y;1)(q11.1 or.2;q12) in the blast cells of a patient with atypical chronic myeloid leukemia

We report a case of atypical chronic myeloid leukemia who showed leukocytosis with immature granulocytes and dysplastic features but no monocytosis or basophilia. Cytogenetic analysis by conventional G-banding showed an abnormal clone, which was interpreted as 46,X,-Y,+der(?)t(?;1)(?;q?1), and no Philadelphia chromosome. Reverse transcription-polymerase chain reaction did not show either major or minor BCR-ABL chimeric mRNA. Spectral karyotyping (SKY) and fluorescence in situ hybridization (FISH) refined the karyotype to 46,X,der(Y)t(Y;1)(q11.1 or.2;q12). The der(Y)t(Y;1) abnormality was reported previously in 9 cases and associated with myelodysplastic syndrome or chronic myeloproliferative disorders. SKY in combination with the standard banding method and FISH may be useful for exploring undefined chromosome abnormalities in hematological disorders.

Cytogenetics of collared lemmings (Dicrostonyx groenlandicus). II. Meiotic behavior of B chromosomes suggests a Y-chromosome origin of supernumerary chromosomes

The patterns of synapsis and chiasma formation of the B chromosomes of male collared lemmings (Dicrostonyx groenlandicus) were analyzed by light and electron microscopy and compared to expectations for various hypotheses for the intragenomic origin of supernumerary chromosomes. Pachytene analysis revealed a variety of synaptic configurations including B-chromosome univalents, bivalents and trivalents. In approximately one-half of the pachytene nuclei examined, B chromosomes were in synaptic associations with the normally unpaired portion of the Y chromosome. The B-chromosome configurations at pachynema, including those involving the Y chromosome, were maintained into diakinesis and metaphase I. The meiotic behavior of the B chromosomes was inconsistent with their derivation from centric-fusion products, isochromosome formation, small-autosome polysomy, or the X chromosome. However, the frequent synapsis and apparent recombination between B chromosomes and the Y chromosome implicate this sex chromosome as a possible source of the B chromosomes in collared lemmings.

Cytogenetic study of Callicebus hoffmannsii (Cebidae, Primates) and comparison with C. m. moloch

Callicebus is a neotropical primate genus divided into four or five groups of species. Species of the moloch group are distributed in the tropical forests of the Amazon basin. The karyotype of Callicebus hoffmannsii (moloch group) was studied by means of G- and C-banding, Ag-NOR staining and in situ hybridization of telomeric probes. C. hoffmannsii had 2n = 50 chromosomes, with ten biarmed and fourteen acrocentric autosomal pairs. The X chromosome was submetacentric and the Y chromosome was a minor acrocentric. Constitutive heterochromatin was detected in the centromeric regions of all chromosomes; in pairs 7 and 10, it was found in the distal regions of the short arms, and distally in the long arm of the X chromosome. Size heteromorphism in C-bands was detected in pairs 7 and 10. Ag-NOR staining revealed a maximum of three nucleolar organizers. Telomeric probes hybridized only at the terminal regions of all chromosomes. Additionally, a comparison was carried out between C. hoffmannsii and C. m. moloch (2n = 48), as previously reported. Both species shared gross chromosomal similarities diverging by a single rearrangement of centric fusion/fission. A high similarity between C. hoffmannsii and C. donacophilus indicated a close association between the moloch and donacophilus groups.

Meiosis and the Neo-XY system of Dichroplus vittatus (Melanoplinae, Acrididae): a comparison between sexes

The origin of neo-XY sex systems in Acrididae is usually explained through an X-autosome centric fusion, and the behaviour of the neo-sex chromosomes has been solely studied in males. In this paper we analysed male and female Dichroplus vittatus. The karyotype comprises 2n = 20 chromosomes including 9 pairs of autosomes and a sex chromosome pair that includes a large metacentric neo-X and a small telocentric neo-Y. We compared the meiotic behaviour of the sex bivalent between both sexes. Mean cell autosomal chiasma frequency was low in both sexes and slightly but significantly higher in males than in females. Chiasma frequency of females increased significantly when the sex-bivalent was included. Chiasma distribution was basically distal in both sexes. Behaviour of the neo-XY pair is complex as a priori suggested by its structure, which was analysed in mitosis and meiosis of diploid and polyploid cells. During meiosis, orientation of the neo-XY is highly irregular; only 21% of the metaphase I spermatocytes show standard orientation. In the rest of cells, the alternate or simultaneous activity of an extra kinetochore in the distal end of the short arm (XL) of the neo-X, determined unusual MI orientations and a high frequency of non-disjunction and lagging of the sex-chromosomes. In females, the neo-XX bivalent had a more regular behaviour but showed 17% asynapsis in the XL arm which, in those cases orientated its distal ends towards opposite spindle poles suggesting, again, the activity of a second kinetochore. The dicentric nature and the unstable meiotic behaviour of the sex neo-chromosomes of D. vittatus suggest a recent origin of the sex determination mechanism, with presumable adaptive advantages which could compensate their potential negative heterosis. Our observations suggest that the origin of the neo-sex system was a tandem fusion of two original telocentric X-chromosomes followed by another tandem fusion with the small megameric bivalent and a further pericentric inversion of the neo-X. The remaining autosomal homolog resulted in the neo-Y chromosome.

46,XX sex reversal

In humans, sexual differentiation is directed by SRY, a master regulatory gene located at the Y chromosome. This gene initiates the male pathway or represses the female pathway by regulating the transcription of downstream genes; however, the precise mechanisms by which SRY acts are largely unknown. Moreover, several genes have recently been implicated in the development of the bipotential gonad even before SRY is expressed. In some individuals, the normal process of sexual differentiation is altered and a sex reversal disorder is observed. These subjects present the chromosomes of one sex but the physical attributes of the other. Over the past years, considerable progress has been achieved in the molecular characterization of these disorders by using a combination of strategies including cell biology, animal models, and by studying patients with these pathologic entities.

Molecular insights into the causes of male infertility

Infertility is a reproductive health problem that affects many couples in the human population. About 13-18% of couple suffers from it and approximately one-half of all cases can be traced to either partner. Regardless of whether it is primary or secondary infertility, affected couples suffer from enormous emotional and psychological trauma and it can constitute a major life crisis in the social context. Many cases of idiopathic infertility have a genetic or molecular basis. The knowledge of the molecular genetics of male infertility is developing rapidly, new "spermatogenic genes" are being discovered and molecular diagnostic approaches (DNA chips) established. This will immensely help diagnostic and therapeutic approaches to alleviate human infertility. The present review provides an overview of the causes of human infertility, particularly the molecular basis of male infertility and its implications for clinical practice.

Macrophages are eliminated from the injured peripheral nerve via local apoptosis and circulation to regional lymph nodes and the spleen

The present study investigated the fate of macrophages in peripheral nerves undergoing Wallerian degeneration, especially their disappearance from the injured nerves after phagocytosis of axonal and myelin debris. Wallerian degeneration was induced in adult male C57Bl/6 mice by transecting the right sciatic nerve. Five days after transection, the male sciatic nerves were transplanted into female recipient mice by placing them exactly parallel to the host sciatic nerves. Nerves of the female recipient mice were also transected to induce breakdown of the blood-nerve barrier in the host animal. Apoptosis was assessed by morphological, immunohistochemical (activated caspase-3), and molecular (DNA fragmentation) methods in transplanted, recipient, and in control nerves. A subpopulation of macrophages within the degenerating nerves died locally by apoptosis in each experiment. The fate of the male macrophages within the transplanted nerves and the host organism was investigated by in situ hybridization with a Y-chromosome-specific DNA probe (145SC5). In situ hybridization specifically stained cells within the transplanted male nerve. Y-chromosome-positive cells were detected not only inside the transplanted nerve, but also inside the female host nerve, the perineurial tissue, the local perineurial blood vessels, draining lymph nodes and the spleen of the female host, suggesting hematogenous as well as lymphatic elimination of macrophages from the injured nerve. These data indicate that local apoptosis and systemic elimination via circulation to the local lymph nodes and the spleen are involved in the disappearance of macrophages from the injured peripheral nervous system.

Heterochromatin is not an adequate explanation for close proximity of interphase chromosomes 1--Y, 9--Y, and 16--Y in human spermatozoa

Analysis of human spermatozoa and lymphocytes using C-banding techniques and in situ hybridization has shown a higher order packaging of the human genome. Chromosomes are not distributed entirely at random within the nucleus. In particular, chromosomes 1, 9, and 16, carrying large blocks of pericentromeric heterochromatin, and the Y chromosome, carrying heterochromatin in Yq12, are in close proximity to each other within the nucleus and are involved in somatic pairing with nonhomologous chromosomes. In order to determine whether the close proximity of these chromosomes in any way is attributable to the distribution of heterochromatin, double in situ hybridization was performed on chromosomes 1--Y, 9--Y, and 16--Y as well as on 1--X, 9--X, and 16--X-with chromosome X as the other gonosome carrying less heterochromatin-in human spermatozoa. Each pair was found to have a nonrandom spatial distribution. However, comparison of the arrangement of chromosomes 1--Y versus 1--X and 9--Y versus 9--X revealed that heterochromatin cannot be the only cause for the tendency of chromosome fusion, because only the results of the chromosome pair 1--Y/1--X could support this proposition. In conclusion, the heterochromatin effect cannot be, in itself, an adequate explanation for chromosome association, implicating as well other mechanisms.

Y chromosome microsatellite variation in three populations of Jerba Island (Tunisia)

Six Y-chromosome linked microsatellites were typed in a sample of 135 unrelated males representing three different ethnic groups: Arabs, Berbers and Blacks of Jerba Island (Tunisia). Analysis of variation at the six Y chromosome STRs showed significant differences in allele distributions between the Black group and the two other Islander groups. The Black group revealed the highest level of genetic diversity as compared to Arabs and Berbers, while the latter group was the most homogeneous. Allele frequencies obtained for the three islander groups analysed were compared to data available for some European, Mediterranean and African populations. Principal-coordinate analyses showed genetic differentiation between the three geographically closed groups of Jerba. The absence of the YAP insertion marker and the position of Arabs and Jerban Blacks near the European cluster would suggest their relative 'admixture' with European populations.

Evidence for population sub-structuring in São Tomé e Príncipe as inferred from Y-chromosome STR analysis

Seven Y-chromosome STR loci, DYS19, DYS389I, DYS389II, DYS390, DYS391, DYS392 and DYS393 have been analysed in population samples of Angolares, Forros and Tongas, three ethnic groups from the African archipelago of São Tomé e Príncipe (Gulf of Guinea). Complete typings were obtained for 103 chromosomes, which belonged to 79 different haplotypes. The mean heterozygosity per locus in the overall São Tomean sample was 0.566, with the highest value found among Forros and the lowest among Angolares. Angolares also showed the lowest level of haplotype diversity. On average, the mean pairwise difference between two random haplotypes from Angolares, Forros and Tongas was 4.69, 6.74 and 6.23 repeats, respectively. The genetic distances were found to be statistically significant between Angolares and Forros or Tongas. In accordance, AMOVA revealed that the percentage of variation attributable to differences among groups was only significant when we distinguished between Angolares and non-Angolares. Globally, these results indicate that, with respect to the pool of male lineages of São Tomé e Príncipe, some genetic sub-structuring does exist, basically determined by the Angolares ethnic group.

XXY male mice: an experimental model for Klinefelter syndrome

Klinefelter syndrome (47,XXY) is the most common sex chromosome aneuploidy in men. Thus, it is important to establish an experimental animal model to explore its underlying molecular mechanisms. Mice with a 41,XXY karyotype were produced by mating wild-type male mice with chimeric female mice carrying male embryonic stem cells. The objectives of the present study were to characterize the testicular phenotype of adult XXY mice and to examine the ontogeny of loss of germ cells in juvenile XXY mice. In the first experiment the testicular phenotypes of four adult XXY mice and four littermate controls (40,XY) were studied. XXY mice were identified by either Southern hybridization or karyotyping and were further confirmed by fluorescence in situ hybridization. The results showed that the testis weights of adult XXY mice (0.02 +/- 0.01 g) were dramatically decreased compared with those of the controls (0.11 +/- 0.01 g). Although no significant differences were apparent in plasma testosterone levels, the mean plasma LH and FSH levels were elevated in adult XXY mice compared with controls. The testicular histology of adult XXY mice showed small seminiferous tubules with varying degrees of intraepithelial vacuolization and a complete absence of germ cells. Hypertrophy and hyperplasia of Leydig cells were observed in the interstitium. Electron microscopic examination showed Sertoli cells containing scanty amounts of cytoplasm and irregular nuclei with prominent nucleoli. The junctional region between Sertoli cells appeared normal. In some tubules, nests of apparently degenerating Sertoli cells were found. In the second experiment the ontogeny of germ cell loss in juvenile XXY mice and their littermate controls was studied. Spermatogonia were found and appeared to be morphologically normal in juvenile XXY mice. Progressive loss of germ cells occurred within 10 days after birth. This resulted in the absence of germ cells in the adult XXY mice. We conclude that a progressive loss of germ cells occurring in early postnatal life results in the complete absence of germ cells in adult XXY mice. The XXY mouse provides an experimental model for its human XXY counterpart, Klinefelter syndrome.

Orientation of nonrandomly segregating sex chromosomes in spermatocytes of the flea beetle, Alagoasa bicolor L

In males of the flea beetle, Alagoasa bicolor L., spermatocytes have two achiasmate sex chromosomes, X and Y, each of which is approximately five times larger than the ten pairs of chiasmate autosomes. At metaphase I, these univalent sex chromosomes are located on a spindle domain separated from the autosomal spindle domain by a sheath of mitochondria. A single centriole pair is located at each pole of the spindle. In prometaphase I, each sex chromosome appears to maintain an attachment to both spindle poles via kinetochore microtubules (i.e., amphitelic orientation). Before anaphase I, this orientation changes to the syntelic orientation (both sister kinetochores connected to the same pole), perhaps by the release of microtubule attachments from the more distant pole by each of the chromosomes. The syntelic orientation just prior to anaphase I leaves each sex chromosome attached to the nearest pole via kinetochore microtubules, ensuring nonrandom segregation. As the sex chromosomes reorient, the autosomes follow in a sequential manner, starting with the bivalent closest to the sex spindle domain. We report here data that shed new light on the mechanism of this exceptional meiotic chromosome behavior.

Sex-chromosome pairing through heterochromatin in the African rodent Lemniscomys barbarus (Rodentia, Muridae). A synaptonemal complex study

Giemsa-stained spread preparations and microspread preparations of Lemniscomys barbarus spermatocytes were made to investigate the meiotic behaviour of the peculiar sex chromosomes of this species. A typical sex vesicle is absent, as the X and Y chromosomes appear unfolded at zygotene and pachytene. In most cells, the sex chromosomes are associated at distal segments at metaphase I, probably as a consequence of a distal chiasma. The pairing segment is located in the heterochromatic regions of both sex chromosomes, which include silent ribosomal cistrons interspersed throughout the heterochromatin. This may suggest a possible involvement of ribosomal genes in both pairing and recombination processes. X-Y pairing proceeds beyond the pseudoautosomal region, thus involving heterologous segments of the differential regions, a fact that is clearly evident at the Y centromeric region.

Organization of the X and Y chromosomes in human, chimpanzee and mouse pachytene nuclei using molecular cytogenetics and three-dimensional confocal analyses

We used multicolour fluorescence in-situ hybridization on air-dried pachytene nuclei to analyse the structural and functional domains of the sex vesicle (SV) in human, chimpanzee and mouse. The same technology associated with 3-dimensional analysis was then performed on human and mouse pachytene nuclei from cytospin preparations and tissue cryosections. The human and the chimpanzee SVs were very similar, with a consistently small size and a high degree of condensation. The mouse SV was most often seen to be large and poorly condensed, although it did undergo progressive condensation during pachynema. These results suggest that the condensation of the sex chromosomes is not a prerequisite for the formation of the mouse SV, and that a different specific mechanism could be responsible for its formation. We also found that the X and Y chromosomes are organized into two separate and non-entangled chromatin domains in the SV of the three species. In each species, telomeres of the X and Y chromosomes remain clustered in a small area of the SV, even those without a pseudoautosomal region. The possible mechanisms involved in the organization of the sex chromosomes and in SV formation are discussed.

C-banding visualized by atomic force microscopy

C-banding is a method used for studying chromosome rearrangements near centromeres and for investigating polymorphisms. In human chromosomes, the C-bands are located at the centromere of all the chromosomes and the distal long arm of the Y chromosome. In this study, we aimed to detect the structural changes in chromosomes during the stages of C-banding by atomic force microscopy. We observed crater-like structures in the chromosomes after 2xSSC (saline sodium citrate) treatment and measured the relative difference between the heights of chromatid and centromere of the chromosomes. Results showed that the relative difference was 3 nm in chromosomes 1, 9, 16, and Y, whereas in the other chromosomes this value was 11.6 nm. After Giemsa staining, the relative difference increased by a factor of 16 in chromosomes 1, 9, 16, and Y. The other chromosomes showed no such increase, which is in accordance with our suggestion that nonhiston proteins associated with DNA in constitutive heterochromatin can make the constitutive heterochromatin resistant to C-banding.

Mapping of a human centromere onto the DNA by topoisomerase II cleavage

We have mapped the positions of topoisomerase II binding sites at the centromere of the human Y chromosome using etoposide-mediated DNA cleavage. A single region of cleavage is seen at normal centromeres, spanning approximately 50 kb within the centromeric alphoid array, but this pattern is abolished at two inactive centromeres. It therefore provides a marker for the position of the active centromere. Although the underlying centromeric DNA structure is variable, the position of the centromere measured in this way is fixed relative to the Yp edge of the array, and has retained the same position for >100,000 years.

A selective sweep associated with a recent gene transposition in Drosophila miranda

In Drosophila miranda, a chromosome fusion between the Y chromosome and the autosome corresponding to Muller's element C has created a new sex chromosome system. The chromosome attached to the ancestral Y chromosome is transmitted paternally and hence is not exposed to crossing over. This chromosome, conventionally called the neo-Y, and the homologous neo-X chromosome display many properties of evolving sex chromosomes. We report here the transposition of the exuperantia1 (exu1) locus from a neo-sex chromosome to the ancestral X chromosome of D. miranda. Exu1 is known to have several critical developmental functions, including a male-specific role in spermatogenesis. The ancestral location of exu1 is conserved in the sibling species of D. miranda, as well as in a more distantly related species. The transposition of exu1 can be interpreted as an adaptive fixation, driven by a selective advantage conferred by its effect on dosage compensation. This explanation is supported by the pattern of within-species sequence variation at exu1 and the nearby exu2 locus. The implications of this phenomenon for genome evolution are discussed.

Replication of heterochromatin and structure of polytene chromosomes

Heterochromatin is characteristically the last portion of the genome to be replicated. In polytene cells, heterochromatic sequences are underreplicated because S phase ends before replication of heterochromatin is completed. Truncated heterochromatic DNAs have been identified in polytene cells of Drosophila and may be the discontinuous molecules that form between fully replicated euchromatic and underreplicated heterochromatic regions of the chromosome. In this report, we characterize the temporal pattern of heterochromatic DNA truncation during development of polytene cells. Underreplication occurred during the first polytene S phase, yet DNA truncation, which was found within heterochromatic sequences of all four Drosophila chromosomes, did not occur until the second polytene S phase. DNA truncation was correlated with underreplication, since increasing the replication of satellite sequences with the cycE(1672) mutation caused decreased production of truncated DNAs. Finally, truncation of heterochromatic DNAs was neither quantitatively nor qualitatively affected by modifiers of position effect variegation including the Y chromosome, Su(var)205(2), parental origin, or temperature. We propose that heterochromatic satellite sequences present a barrier to DNA replication and that replication forks that transiently stall at such barriers in late S phase of diploid cells are left unresolved in the shortened S phase of polytene cells. DNA truncation then occurs in the second polytene S phase, when new replication forks extend to the position of forks left unresolved in the first polytene S phase.