Autosomal polymorphism due to pericentric inversions in the deer mouse (Peromyscus maniculatus), and some evidence of somatic segregation

A centromere satellite concomitant with extensive karyotypic diversity across the Peromyscus genus defies predictions of molecular drive

A common feature of eukaryotic centromeres is the presence of large tracts of tandemly arranged repeats, known as satellite DNA. However, these centromeric repeats appear to experience rapid evolution under forces such as molecular drive and centromere drive, seemingly without consequence to the integrity of the centromere. Moreover, blocks of heterochromatin within the karyotype, including the centromere, are hotspots for chromosome rearrangements that may drive speciation events by contributing to reproductive isolation. However, the relationship between the evolution of heterochromatic sequences and the karyotypic dynamics of these regions remains largely unknown. Here, we show that a single conserved satellite DNA sequence in the order Rodentia of the genus Peromyscus localizes to recurrent sites of chromosome rearrangements and heterochromatic amplifications. Peromyscine species display several unique features of chromosome evolution compared to other Rodentia, including stable maintenance of a strict chromosome number of 48 among all known species in the absence of any detectable interchromosomal rearrangements. Rather, the diverse karyotypes of Peromyscine species are due to intrachromosomal variation in blocks of repeated DNA content. Despite wide variation in the copy number and location of repeat blocks among different species, we find that a single satellite monomer maintains a conserved sequence and homogenized tandem repeat structure, defying predictions of molecular drive. The conservation of this satellite monomer results in common, abundant, and large blocks of chromatin that are homologous among chromosomes within one species and among diverged species. Thus, such a conserved repeat may have facilitated the retention of polymorphic chromosome variants within individuals and intrachromosomal rearrangements between species-both factors that have previously been hypothesized to contribute towards the extremely wide range of ecological adaptations that this genus exhibits.

Chromosomal polymorphism in mammals: an evolutionary perspective

Although chromosome rearrangements (CRs) are central to studies of genome evolution, our understanding of the evolutionary consequences of the early stages of karyotypic differentiation (i.e. polymorphism), especially the non-meiotic impacts, is surprisingly limited. We review the available data on chromosomal polymorphisms in mammals so as to identify taxa that hold promise for developing a more comprehensive understanding of chromosomal change. In doing so, we address several key questions: (i) to what extent are mammalian karyotypes polymorphic, and what types of rearrangements are principally involved? (ii) Are some mammalian lineages more prone to chromosomal polymorphism than others? More specifically, do (karyotypically) polymorphic mammalian species belong to lineages that are also characterized by past, extensive karyotype repatterning? (iii) How long can chromosomal polymorphisms persist in mammals? We discuss the evolutionary implications of these questions and propose several research avenues that may shed light on the role of chromosome change in the diversification of mammalian populations and species.

Cytogenetical study in Spalax leucodon Nordm. in Moldavia (Rumania)

The karyotype of Spalax leucodon (2n = 56) from Moldavia (Rumania) shows 27 pairs of autosomes: two metacentrics, three submetacentrics, eight suhtelocentrics and 14 acrocentrics. The X chromosomes are the longest subtelocentrics and the Y chromosome is a submetacentric of medium size. In metaphase I we have found a partial homology between the short arm of the X and the Y chromosome.

The population of S. leucodon from Moldavia shows considerable differences of karyotype as compared with other populations of the same species from the south of Europe. This species presents a high degree of chromosomal polymorphism in different populations.

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. G- and 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.

Genome evolution in pocket gophers (genus Thomomys). I. Heterochromatin variation and speciation potential

A basic dichotomy exists in the amount and chromosomal position of constitutive heterochromatin (C-bands) in species of pocket gophers, genus Thomomys. Members of the "talpoides-group" of species (e.g., T. talpoides and T. monticola) have C-bands restricted to the centromeric regions. These taxa are characterized by Robertsonian patterns of karyotypic evolution. In contrast, species within the "bottae-group" are characterized by extensive amounts of heterochromatin, placed as whole-arm and apparent whole-chromosome (T. bottae) or as large interstitial blocks (T. umbrinus). These species are characterized by extensive non-Robertsonian variation in karyotype, variation which may be expressed from local population polymorphism to between population or species polytypy. Within T. bottae, the number of whole-arm heterochromatic autosomes is inversely proportional to the number of uniarmed chromosomes in the complement, which ranges from 0 to 36 across the species populations. In all-biarmed karyotypic populations, upward to 60 percent of the linear length of the genome is composed of heterochromatin. Populations with extensive heterochromatin variation and those with similar amounts meet and hybridize freely in nature. The implications of these date for current ideas on the function of heterochromatin, particularly as related to speciation models, are discussed.

Chromosomes of the Mexican plateau mouse, Peromyscus melanophrys, and a new sex determining mechanism in mammals

A chromosomal analysis of 86 specimens of Peromyscus melanophrys reveals a unique type of chromosomal difference between males and females Females possess three large pairs of subtelocentric autosomes, two pairs of small submetacentric autosomes, and 18 pairs of acrocentric autosomes. The X chromosomes are also subtelocentric. Males possess a similar karyotype with a subtelocentric X chromosome, a minute Y chromosome, and two unmatched autosomes, a large subtelocentric and a large acrocentric. Both sexes have a diploid number of 48. Studies from meiosis and autoradiography indicate that a portion of the original Y chromosome has been translocated to an autosome resulting in a new multiple sex determining mechanism in mammals, an X1X1X2X2/X1X2Y1Y2 system.

Chromosome variation in Peromyscus: a new mechanism

Differences in total chromosome lengths between two karyotypically divergent groups of Peromyscus maniculatus are taken as evidence for an deletion mechanism of chromosomal variation in the species. The differences may be due in part to variation in the amount of constitutive heterochromatin in the two karyotypes.

Chromosome polymorphism in interbred subspecies of Peromyscus maniculatus (deer mouse)

Karyotype analyses of eight animals from an interbreeding colony of three subspecies of Peromyscus maniculatus (P.m. gambelli, P.m. rubidus, and P.m. sonoriensis) demonstrated a chromosomal polymorphism, probably due to pericentric inversions involving at least seven chromosomes. This polymorphism may require consideration in the cytogenetic taxonomy of these animals, and may be related to the widespread distribution of deer mice in North America and their apparent adaptability to many different environments.

Intraspecies autosomal polymorphism and chromosome replication in Akodon molinae (Rodentia-Cricetidae)

Cell spreads from bone marrow, spleen, testis and liver of four male and four female Akodon molinae (Rodentia:Cricetidae) were used for chromosome analysis and sex chromatin scoring. Chromosome replication at the beginning and end of the S period were analysed in bone marrow cells.In five animals (three males and two females) the diploid chromosome number was 42; the other three (1 male and 2 females) had a modal number of 43. In the former animals pairs 1,2,19,20 and the Y chromosome were easily identified morphologically. Chromosomes 1 were large and metacentric. In specimens with 43 chromosomes, pairs 2-20-XY were similar to those of animals with 42. Instead of having two number 1 homologues, these animals showed three unpaired chromosomes, one chromosome 1, one subterminal chromosome (1a) homologue of the long arm of the chromosome 1 and one submetacentric chromosome (1b) homologue of the short arm of the chromosome 1 Chromosomes 1a and 1b were considered to have arisen by a Robertsonian mechanism of centric fission of chromosome 1 plus a pericentric inversion.Studies of sex chromosome replication showed that the Y chromosome was the last to start and to end DNA synthesis in male complements. In females one X chromosome was the last to start replication. No late replicating X chromosome at the end of the S period was found. Coincidently, no sex chromatin could be detected in females.Analysis of late replication patterns in chromosomes 1, 1a and 1b, indicates that pericentric inversions can shift the replicating moment of the chromosomal regions involved in the rearrangement.

Chromosomal Polymorphism in the White-Throated Sparrow, Zonotrichia albicollis (Gmelin)

In a study of 35 white-throated sparrows five distinct karyotypes were observed. A chromosomal polymorphism is present which involves at least two pairs of macrochromosomes. This species is phenotypically polymorphic with selective breeding occurring between morphs. Phenotype is related to chromosomal constitution, and selective breeding appears to maintain heterozygosity within the population.