David W. Hale

Heterosynapsis and suppression of chiasmata within heterozygous pericentric inversions of the Sitka deer mouse

The patterns of chromosomal pairing and chiasma distribution were analyzed in male Sitka deer mice (Peromyscus sitkensis) polymorphic for terminally positioned pericentric inversions of chromosomes 6 and 7. Gand C-banding of somatic metaphases indicated that the inversions involved 30% and 40% of chromosomes 6 and 7, respectively. Analysis of silver-stained synaptonemal complexes in surface-spread zygotene and pachytene nuclei from heterozygous individuals revealed that inversion loops were not formed. The inverted segments proceeded directly to heterosynapsis without an intervening homosynaptic phase, and the heteromorphic bivalents remained straight-paired throughout pachynema. C-banded pachytene nuclei corroborated the occurrence of heterosynapsis, as the heteromorphic bivalents exhibited nonaligned centromeres. Analysis of diplonema and diakinesis indicated that crossing over had not occurred within the heterosynapsed inverted segments. The observation of chiasma suppression within the inversions indicates that pericentric inversion heterozygosity does not lead to the production of unbalanced gametes. Heterosynapsis of the inverted segments during zygonema and pachynema and the resulting chiasma suppression therefore represent a meiotic mechanism for the maintenance of pericentric inversion polymorphisms in this population of P. sitkensis.

The mechanism of autosomal synapsis and the substaging of zygonema and pachynema from deer mouse spermatocytes

Surface-spread and silver-stained spermatocytes of Peromyscus maniculatus and P. sitkensis were analyzed in order to develop criteria for the recognition of meiotic substages from early zygonema through early diplonema. The continuous sequence of changes in the morphology of the autosomal axes (lateral elements) of the synaptonemal complexes (SC), sex chromosome axes, and nucleoli enabled the recognition of three substages of zygonema and five of pachynema. The proposed system of subdivision is compatible with descriptions of comparable data from Chinese hamsters and laboratory mice with differences being primarily associated with the timing of sex chromosome synapsis and desynapsis. Within the substages, cytogenetically important details of the synaptic mechanism in deer mice were noted. Autosomal synaptic initiation in deer mice is apparently uniterminal, involving the distal (noncentromeric) end of the homologs. Subsequent pairing is unidirectional towards the centromeric end. Additionally, during mid and late zygonema the homologous axes of late pairing regions of some autosomes were characterized by substantial length differences. These lateral element length differences were not maintained into pachynema and it is hypothesized that differences in the amount of material in the heterochromatic short arms of these species may be subject to synaptic adjustment.

The effect of heterochromatin on synapsis of the sex chromosomes of Peromyscus (Rodentia, Cricetidae)

The pairing behavior of the sex chromosomes in male and female individuals representing seven species of Peromyscus was analyzed by electron microscopy of silver-stained zygotene and pachytene configurations. Six species possess submetacentric or metacentric X chromosomes with heterochromatic short arms. Sex-chromosome pairing in these species is initiated during early pachynema at an interstitial position on the X and Y axes. Homologous synapsis then progresses in a unidirectional fashion towards the telomeres of the X short arm and the corresponding arm of the heterochromatic Y chromosome. The distinctive pattern of synaptic initiation allowed a late-synapsing bivalent in fetal oocytes to be tentatively identified as that of the X chromosomes. In contrast to the other species, Peromyscus megalops possesses an acrocentric X chromosome and a very small Y chromosome. Sex-chromosome pairing in this species is initiated at the proximal telomeric region during late zygonema, and then proceeds interstitially towards the distal end of the Y chromosome. These observations suggest that the presence of X short-arm heterochromatin and corresponding Y heterochromatin interferes with late-zygotene alignment of the pairing initiation sites, thereby delaying XY synaptic initiation until early pachynema. The pairing initiation sites are conserved in the vicinity of the X and Y centromeres in Peromyscus, and consequently the addition of heterochromatin during sex-chromosome evolution essentially displaces these sites to an interstitial position.

Synaptic adjustment in Peromyscus beatae (Rodentia: Cricetidae) heterozygous for interstitial heterochromatin

Chromosomal pairing and chiasma formation were studied two individuals of Peromyscus beatae heterozygous for the presence of a large block of interstitial heterochromatin. Although the modified chromosome was of medium size, analysis of C-banded diakinetic configurations revealed that it was the homolog of one of the smallest autosomes. Analysis of silver stained synaptonemal complexes indicated that synapsis was either unidirectional from initiation at one set of telomeres or was bidirectional from initiation at both sets of telomeres. Each pattern resulted in characteristic heteromorphic pairing configurations (interstitial asynapsis or terminally positioned unpaired segments) in early pachynema. These configurations underwent synaptic adjustment and, by mid-pachynema, the lateral elements of the polymorphic bivalent either appeared typical of homomorphic bivalents or exhibited regional heteropycnosis in one or both axes. Synaptonemal complex data for Peromyscus and many other mammalian species reflect an apparent need for fully paired, linear bivalents prior to the end of pachynema.

The behavior and morphology of the X and Y chromosomes during prophase I in the Sitka deer mouse (Peromyscus sitkensis).

Surface-spread, silver-stained primary spermatocytes from individuals of the Sitka deer mouse (Peromyscus sitkensis) were analyzed by electron microscopy. Pairing of the X and Y chromosomes is initiated at early pachynema and is complete by mid pachynema. The pattern of sex chromosome pairing is unique in that it is initiated at an interstitial position, with subsequent synapsis proceeding in a unidirectional fashion towards the telomeres of the homologous segments. One-third the length of the X and two-thirds the length of the Y are involved in the synaptonemal complex of the sex bivalent. Various morphological complexities develop in the heteropycnotic (unpaired) segments as pachynema progresses, but desynapsis is not initiated until diplonema. Analysis of C-banded diakinetic nuclei indicated that sex chromosome pairing involves the heterochromatic short arm of the X and the long arm of the heterochromatic Y. An interstitial chiasma between the X and Y was observed in the majority of the diakinetic nuclei. The observation of a substantial pairing region and chiasma formation between the sex chromosomes of these deer mice is interpreted as indicating homology between the short arm of the X and the long arm of the Y.

SYNAPTIC ADAPTATION IN DEER MICE: A CELLULAR MECHANISM FOR KARYOTYPIC ORTHOSELECTION

While the kinds of karyotypic differences between closely related species vary greatly from group to group, chromosomal evolution within particular lineages often involves the establishment of similar structural changes in one karyotype after another. This pattern of chromosomal evolution, termed karyotypic orthoselection (White, 1973, 1975), is known to include a diversity of types of structural changes in a variety of groups of organisms. The underlying cellular bases of karyotypic orthoselection, however, remain unknown. White (1975, 1978) suggested four possible explanations for karyotypic orthoselection: differential origin (of mutation types), external adaptation (differential selection), cytomechanical restraints (imposing limits on the number of chromosomes or chromosomal arms), and architectural regularity of the chromosomes themselves and/or their spatial associations in interphase nuclei. Of these, only regularity of the intranuclear architecture was considered likely to explain karyotypic orthoselection (White, 1975, 1978). Recent studies documenting nonrandom orientations of chromosomes in interphase nuclei (Bennett, 1982; Agard and Sedat, 1983; Mathog et al., 1984) suggest that architectural regularities may play a central role in the ways in which chromosomes evolve. Research in this area, however, has yet to document a widely applicable mechanism such as would be necessary to regulate the various expressions of karyotypic orthoselection. Chromosomal evolution involving euchromatic rearrangements is generally considered to involve the fixation of arrangements which substantially reduce the fertility of heterozygotes. If certain rearrangements can be shown to be less susceptible to the production of genetically unbalanced gametes within particular evolutionary lineages, however, then these rearrangements might be expected to become fixed in a pattern of karyotypic orthoselection. In this report, we present data which suggest that synapsis in deer mice, Peromyscus, is characteristically different for two types of spontaneously occurring chromosomal rearrangements, with the orthoselected type of rearrangement apparently resulting in no loss of heterozygote fitness. We hypothesize that such synaptic adaptation may function as a primary factor in the generation of karyotypic orthoselection. All species of Peromyscus for which karyotypic data are available have a diploid number of 48. Variation in the number of arms in the autosomal complement is extensive, however, ranging from 52 to 92. G-banding studies, including 30 species of Peromyscus (Rogers et al., 1984; Stangi and Baker, 1984), indicate that no fewer than 38 separate pericentric inversions have been incorporated during the chromosomal evolution of this genus. Additionally, several species exhibit intraspecific polymorphisms for pericentric inversions (Greenbaum et al., 1978; Baker et al., 1983; Pengilly et al., 1983; Greenbaum and Reed, 1984). Most of the inversions that occur within and among species of Peromyscus are of lengths which should substantially reduce the reproductive fitness of heterozygotes. Except for differences in the presence or absence of heterochromatic segments, no other types of chromosomal rearrangements have been documented in deer mice as either spontaneous events within individuals or as evolutionary events among populations or species. In a light-microscopic study of synaptonemal complex (SC) from P. maniculatus heterozygous for a terminally positioned pericentric inversion of chromosome 6, Greenbaum and Reed (1984) reported regularly formed autosomal pairs in the majority of cells examined; inversion loops were observed at a frequency of less than 1%. It was hypothesized that heterosynapsis of the inverted segment blocks crossing over, eliminates the production of duplication and deletion chromatids, and thus provides an evolutionary mechanism which facilitates the incorporation of pericentric inversions. During electron microscopic studies of SC in Peromyscus, we have obtained data which corroborate the findings of Greenbaum and Reed (1984) and document homologous pairing for three spontaneous rearrangements interpreted as translocations in the primary spermatocytes. Whole cell complements of surface spread SCs were prepared by the method of Counce and Meyer (1973), as modified for mammalian testicular material by Moses (1977), and stained with silver nitrate using the method of Howell and Black (1980). A total of seven Peromyscus (four P. maniculatus and three P. sitkensis) were examined with SCs analyzed for more than 520 zygotene and pachytene nuclei. A detailed analysis of the mechanisms of meiotic pairing based on six of these specimens (excluding the individual of P. sitkensis reported below) is presented by Greenbaum et al. (1986). The somatic karyotype of each specimen was characterized from standard and Gand C-banded mi-

Cytogenetics of collared lemmings (Dicrostonyx groenlandicus) I. Meiotic behavior and evolution of the neo-XY sex-chromosome system

Electron-microscopic analysis of surface-spread synaptonemal complexes at pachynema and light-microscopic analysis of chromosomal configurations at diakinesis/metaphase I corroborate the hypothesized neo-XY derivation of the sex chromosomes of Dicrostonyx groenlandicus. Although an intact neo-XY pairing configuration was observed in a relatively small percentage of the pachytene cells in each individual, the high incidence of neo-XY bivalents at diakinesis/metaphase I suggests that the other observed pachytene configurations were artifacts of the physical stresses of the surface-spreading procedure. The very low frequency (0.6%) of univalent neo-X and neo-Y chromosomes at diakinesis and metaphase I is attributable to consistent synapsis and recombination between their homologous autosomally derived segments. The resultant stability of the sex bivalent through metaphase I may have increased the efficacy of sex-chromosome segregation, and thereby played a mechanistic role in the evolutionary incorporation of the neo-XY sex-chromosome constitution in D. groenlandicus.

Sex chromosomes, heterochromatin, and retrotransposon accumulation in deer mice

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