[Pairing behaviour of the sex chromosomes during male meiosis of Microtus agrestis]

Structural and Evolutionary Relationships in the Giant Sex Chromosomes of Three Microtus Species

The genus Microtus has high karyotypic diversity. The existence of notable differences in the length of its sex chromosomes contributes to this variation. Variations in size are attributed to the enlargement of their heterochromatin content, which is of such magnitude in some species that they are referred to as "giant sex chromosomes". Here, we perform an intra- and interspecific analysis of the molecular composition of the heterochromatic blocks in three species with giant sex chromosomes (Microtus chrotorrhinus, M. cabrerae and M. agrestis). Our results show that the heterochromatic content is very similar in both the X and Y chromosomes of M. chrotorrhinus, and that their molecular composition is more closely related to the heterochromatic blocks of M. agrestis than to the sex heterochromatin of M. cabrerae; however, species-specific differences do clearly exist. Interestingly, the euchromatic regions of the X chromosome of all three of these species share a homologous region composed of heterochromatic-related sequences. Our results therefore reinforce the idea that certain similarities in the original organization of these X chromosomes could have facilitated their later enlargement.

The behavior during pachynema of a normal and an inverted Y chromosome in Microtus agrestis

The pachytene behavior of the chromosomes of Microtus agrestis (L.) (Rodentia, Arvicolidae) males carrying either the standard, or the pericentrically inverted Lund Y chromosome have been examined by electron microscopy of microspread spermatocytes. There is no synapsis between the X and either the standard or the Lund Y chromosomes during any substage of pachynema. Since synapsis is generally considered a prerequisite for crossing over, there appears to be no opportunity for crossover or chiasma formation between the X and Y in this species. The G-, C- and NOR-banded mitotic karyotypes of animals carrying the standard and Lund Y are also presented.

The origin and distribution of the Lund Y chromosome in Microtus agrestis (Rodentia, Mammalia)

The Lund Y (Lu-Y) chromosome of the field vole (Microtus agrestis) is distinguished from the standard Y (St-Y) by its much longer short arm. G-banding revealed that the Lu-Y originated by a pericentric inversion in the St-Y. Chromosome analysis of 297 male field voles from 92 localities in Fennoscandia. Germany, and England, in addition to data from the literature, made it possible to map the distribution area of the Lu-Y. It is restricted to the south-western parts of Sweden. The question of when and where the Lund Y population originated is discussed. Adding data from a hybrid zone (Jaarola et al. 1997) and from females, totally 491 specimens from 120 localities were analyzed without detecting any variation in chromosome number and autosome morphology. Other cases of intraspecific Y chromosome polymorphism in mammals, and the use of Y chromosome variants as population genetic markers, are discussed.

Mammalian sex chromosomes. I. Cytological changes in the chiasmatic sex chromosomes of the male musk shrew, Suncus murinus

The X and Y chromosomes of the musk shrew are the two largest in the complement and they regularly form a single chiasma during meiosis. This chiasma is located in the short arms of the X and Y, both of which show partial C-banding at meiosis. The in vitro incorporation of 5-bromodeoxyuridine/tritiated thymidine during late S reveals that the non-C-band region of the Y finishes replication later than the C-band positive heterochromatin. During meiosis, the sex bivalent opens out early in pachytene to reveal a single chiasma which persists until late metaphase-I. In surface-spread, silver-stained meiocytes, the sex bivalent morphology changes from a phase of extensive pairing to one which includes a visible chiasma through a brief diffuse stage. Observations on C-banded meiocytes show a shift in the sex pair from a C-band positive to a negative state as compared to their corresponding somatic pattern. Comparable changes are also observed in the sex bivalents of other mammals which undergo a chiasmatic exchange. This suggests that in addition to pairing homology, an alteration in the chromatin configuration may be necessary for crossing over to occur between the sex chromosomes.

Differences between genetic and physical centromere distances in the case of two genes for male sterility in barley

Linkage studies with thirty translocations (one of the two chromosomes involved being number 4) in relation to msg24 (chromosome 4) and thirteen translocations (one of the two chromosomes involved being number 6) in relation to msg6 (chromosome 6) show without exception close linkage for all combinations tested. The results indicate that both genes are located genetically in or close to the centromere regions of their chromosomes.Cytological analysis of two BTT stocks (balanced tertiary trisomics) ascertained the respective chromosome arms (both msg24 and msg6 on the short arms) and revealed marked differences between genetic and physical centromere distances. The reason is obviously the high content of centromeric heterochromatin occupying both the chromosome arms involved.

Ultrastructural characterization of the sex chromosomes during spermatogenesis of spiders having holocentric chromosomes and a long diffuse stage

An ultrastructural study has been made of spermatogenesis in two species of primitive spiders having holocentric chromosomes (Dysdera crocata, male X0 and Sergestria florentia X1X2O). Analysis of the meiotic prophase shows a scarcity or absence of typical leptotene to pachytene stages. Only in D. crocata have synaptonemal complex (SC) remnants been seen, and these occurred in nuclei with an extreme chromatin decondensation. In both species typical early prophase stages have been replaced by nuclei lacking SC and with their chromatin almost completely decondensed, constituting a long and well-defined diffuse stage. Only nucleoli and the condensed sex chromosomes can be identified. - In S. florentina paired non-homologous sex chromosomes lack a junction lamina and thus clearly differ from the sex chromosomes of more evolved spiders with an X1X20 male sex determination mechanism. In the same species, sex chromosomes can be recognized during metaphase I due to their special structural details, while in D. crocata the X chromosome is not distinguishable from the autosomes at this stage. - The diffuse stage and particularly the structural characteristics of the sex chromosomes during meiotic prophase are reviewed and discussed in relation to the meiotic process in other arachnid goups.

Hyperthermia induced dissociation of the X-Y bivalent in mice

The mutagenic potential of hyperthermia was examined employing a spermatocyte test on males heat stressed continuously for 2, 3. or 5 days. Stress conditions were 35 +/- 1 degrees C and 65 +/- 3% relative humidity. Males were sacrificed and meiotic preparations made from the testes at various intervals following removal from the stress environment. In this way, chromosome damage could be monitored in all premeiotic and early prophase I stages of spermatogenesis. Results of this study revealed a significant increase in the incidence of X and Y univalents at metaphase I. The significance of this finding is discussed.

Satellite DNA and heterochromatin variants: the case for unequal mitotic crossing over

Variations of constitutive heterochromatin (heteromorphisms) appear to be a general feature of eucaryotes. A variety of molecular and cytogenetic evidence supports the hypothesis that heteromorphisms result from unequal double-strand exchanges during mitotic DNA replication. Constitutive heterochromatin consists of highly repeated DNA sequences that are not transcribed. Thus, heteromorphisms are tolerated without overt phenotypic effect. Several of the highly repeated DNAs that comprise constitutive heterochromatin have been shown to contain site-specific endonuclease recognition sequences interspersed at regular intervals dependent upon nucleosome structure. These interspersed short repeated sequences could mediate unequal crossovers, resulting in quantitative variability of constitutive heterochromatin and satellite DNA. De novo variations of constitutive heterochromatin may be useful as markers of exposure to mutagens and/or carcinogens.

Chromosomes and DNA of Mus. The behavior of constitutive heterochromatin in spermatogenesis of M. dunni

Using C-banded preparations of Mus dunni it is possible to study the behavior of constitutive heterochromatin in early stages of meiotic prophase. The X and the Y chromosomes, both of which contain a large amount of heterochromatin, lie apart in leptotene but move toward each other during zygotene. They then form the sex vesicle at late zygotene. In autosomes zygotene pairing appears to start from the telomeric ends. The centromere of the Y chromosome associates end-to-end with the terminal end of the long arm of the X chromosome. The autosomal heterochromatic short arms show forked morphology in certain bivalents at pachytene, suggesting probable incomplete synapsis.

Myths and mechanisms of meiosis

A comparative analysis of the meiotic secquence in a wide variety of organisms indicates there is no convincing evidence that: (1) Premeiotic pairing plays any role in the synapsis of homologues. (2) Heterochromatic association facilitates homologous pairing. (3) Chiasmata ever form within segments which are positively heteropycnotic at zygotenepachytene. (4) Localisation of chiasmata depends on prior localisation of pairing or on the occurrence of euchromatin-heterochromatin boundaries. (5) Prior association of centromeres plays any role in determing co-orientation. (6) Any form of supra-chromosomal organisation exists involving permanent association between the members of a haploid complement, and (7) Unequal progeny ratios recovered from structurally modified Drosophila complements arise as a consequence of distributive pairing.--On the other hand there is good evidence that: (1) Interlocking of bivalents can occur regularly in species with a chiasma frequency sufficiently high to regularly produce ring bivalents and in which the chiasmata are localised to the ends of the bivalent. (2) Some forms of terminal association cannot represent terminalised chiasmata. (3) U-type exchanges present at diplotene result from errors in crossing over. (4) Pairing and chiasma formation are not necessary for coorientation, and (5) at least some types of elastic constrictions present at first metaphase represent extended nucleolar organisers.