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PubMed2003

Patterns of nucleotide variation in homoeologous regulatory genes in the allotetraploid Hawaiian silversword alliance (Asteraceae).

Genome-wide duplication (polyploidization) is prevalent in a large number of eukaryotic organisms and is particularly widespread in flowering plants. Polyploid species appear to vary from their diploid progenitors in a variety of ecologically important traits, suggesting that genome duplications provide a mechanism for ecological diversification. Studies of nucleotide variation at duplicate genes that arise via polyploidization allow us to infer the evolutionary forces that act on these polyploid loci. In an effort to examine the evolutionary dynamics of homoeologous loci, molecular population genetic analyses were undertaken for duplicate regulatory genes in the allopolyploid Hawaiian silversword alliance, a premier example of adaptive radiation. The levels and patterns of nucleotide variation for the floral homeotic genes ASAPETALA1 (ASAP1) and ASAPETALA3/TM6 (ASAP3/TM6) were studied in two species representing different lineages within the Hawaiian silversword alliance: Argyroxiphium sandwicense ssp. macrocephalum and Dubautia ciliolata ssp. glutinosa. Homoeologueous copies of ASAP1 and ASAP3/TM6 show differing levels and patterns of nucleotide polymorphism. Duplicate ASAP1 copies have similar levels of nucleotide diversity and haplotype structure in both species; by contrast, duplicate ASAP3/TM6 genes display different levels and patterns of variation in D. ciliolata ssp. glutinosa. Additionally, D. ciliolata ssp. glutinosa appears to be segregating for a moderate frequency null allele in one ASAP3/TM6 homoeologue. These results suggest that differing evolutionary forces can affect duplicate loci arising from allopolyploidization.

Asteraceae↗
PubMed2003

Genes duplicated by polyploidy show unequal contributions to the transcriptome and organ-specific reciprocal silencing.

Most eukaryotes have genomes that exhibit high levels of gene redundancy, much of which seems to have arisen from one or more cycles of genome doubling. Polyploidy has been particularly prominent during flowering plant evolution, yielding duplicated genes (homoeologs) whose expression may be retained or lost either as an immediate consequence of polyploidization or on an evolutionary timescale. Expression of 40 homoeologous gene pairs was assayed by cDNA-single-stranded conformation polymorphism in natural (1- to 2-million-yr-old) and synthetic tetraploid cotton (Gossypium) to determine whether homoeologous gene pairs are expressed at equal levels after polyploid formation. Silencing or unequal expression of one homoeolog was documented for 10 of 40 genes examined in ovules of Gossypium hirsutum. Assays of homoeolog expression in 10 organs revealed variable expression levels and silencing, depending on the gene and organ examined. Remarkably, silencing and biased expression of some gene pairs are reciprocal and developmentally regulated, with one homoeolog showing silencing in some organs and the other being silenced in other organs, suggesting rapid subfunctionalization. Duplicate gene expression was examined in additional natural polyploids to characterize the pace at which expression alteration evolves. Analysis of a synthetic tetraploid revealed homoeolog expression and silencing patterns that sometimes mirrored those of the natural tetraploid. Both long-term and immediate responses to polyploidization were implicated. Data suggest that some silencing events are epigenetically induced during the allopolyploidization process.

DNA, Plant↗
PubMed2000

Selection-mutation balance in polysomic tetraploids: impact of double reduction and gametophytic selection on the frequency and subchromosomal localization of deleterious mutations.

We modeled the behavior of recessive mutations with deleterious effects to either the sporophyte or the gametophyte, or both, in polysomic tetraploid populations by allowing for varying levels of double reduction, mutation, and self-fertilization. Double reduction causes a decrease of the equilibrium frequencies of deleterious alleles, and it has much more influence on genes subjected to gametophytic selection than on genes solely under sporophytic selection. With gametophytic selection, low frequencies of double reduction are enough to reduce equilibrium frequencies severalfold. Double reduction occurs when sister alleles migrate to the same gamete during meiosis. It depends on the frequency at which a locus recombines with its centromere, and on the frequency of multivalent formation. Therefore, a greater accumulation of deleterious mutations should occur on polysomic chromosomes with a prevalence of bivalent pairing and in chromosomal regions between centromeres and proximal chiasmata. Proximal loci should have a greater impact in reducing the fitness of a polyploid population being inbred. This prediction can explain observations that homozygosities at different subchromosomal regions have distinct effects on inbreeding depression in polyploids. Furthermore, even mildly deleterious alleles can lead to large amounts of inbreeding depression because of their high equilibrium frequencies. Molecular studies correlating level of heterozygosity and degree of heterosis should take into account this nonuniform distribution of deleterious alleles in polyploid genomes. Preservation or enhancement of heterozygosity would be more critical at proximal regions than at other chromosome regions in polysomic polyploid species.

Alleles↗
PubMed1995

Bidirectional interlocus concerted evolution following allopolyploid speciation in cotton (Gossypium).

Polyploidy is a prominent process in plant evolution; yet few data address the question of whether homeologous sequences evolve independently subsequent to polyploidization. We report on ribosomal DNA (rDNA) evolution in five allopolyploid (AD genome) species of cotton (Gossypium) and species representing their diploid progenitors (A genome, D genome). Sequence data from the internal transcribed spacer regions (ITS1 and ITS2) and the 5.8S gene indicate that rDNA arrays are homogeneous, or nearly so, in all diploids and allopolyploids examined. Because these arrays occur at four chromosomal loci in allopolyploid cotton, two in each subgenome, repeats from different arrays must have become homogenized by interlocus concerted evolution. Southern hybridization analysis combined with copy-number estimation demonstrate that this process has gone to completion in the diploids and to completion or near-completion in all allopolyploid species and that it most likely involves the entire rDNA repeat. Phylogenetic analysis demonstrates that interlocus concerted evolution has been bidirectional in allopolyploid species--i.e., rDNA from four polyploid lineages has been homogenized to a D genome repeat type, whereas sequences from Gossypium mustelinum have concerted to an A genome repeat type. Although little is known regarding the functional significance of interlocus concerted evolution of homeologous sequences, this study demonstrates that the process occurs for tandemly repeated sequences in diploid and polyploid plants. That interlocus concerted evolution can occur bidirectionally subsequent to hybidization and polyploidization has significant implications for phylogeny reconstruction, especially when based on rDNA sequences.

Base Sequence↗
PubMed2000

Maize as a model for the evolution of plant nuclear genomes.

The maize genome is replete with chromosomal duplications and repetitive DNA. The duplications resulted from an ancient polyploid event that occurred over 11 million years ago. Based on DNA sequence data, the polyploid event occurred after the divergence between sorghum and maize, and hence the polyploid event explains some of the difference in DNA content between these two species. Genomic rearrangement and diploidization followed the polyploid event. Most of the repetitive DNA in the maize genome is retrotransposable elements, and they comprise 50% of the genome. Retrotransposon multiplication has been relatively recent-within the last 5-6 million years-suggesting that the proliferation of retrotransposons has also contributed to differences in DNA content between sorghum and maize. There are still unanswered questions about repetitive DNA, including the distribution of repetitive DNA throughout the genome, the relative impacts of retrotransposons and chromosomal duplication in plant genome evolution, and the hypothesized correlation of duplication events with transposition. Population genetic processes also affect the evolution of genomes. We discuss how centromeric genes should, in theory, contain less genetic diversity than noncentromeric genes. In addition, studies of diversity in the wild relatives of maize indicate that different genes have different histories and also show that domestication and intensive breeding have had heterogeneous effects on genetic diversity across genes.

Biological Evolution↗
PubMed2004

Triploid bridge and role of parthenogenesis in the evolution of autopolyploidy.

Autopolyploidization is considered to play an important role in plant evolution. In polyploidization, the polyploid evolves from the original diploid cytotype, in which the triploid state is considered to mediate the process (triploid bridge). Nevertheless, the fitness of triploid individuals seems to be too low to facilitate the polyploidization process (triploid block). The evolutionary condition of autopolyploidy was analyzed using a mathematical model focusing on the role of parthenogenesis in triploid and tetraploid individuals. In addition, offspring were assumed to arise by sexual reproduction by conjugations between haploid, diploid, and triploid gametes produced by diploid, tetraploid, and triploid individuals. According to the analysis, even if triploid block suppresses the fitness of sexually produced triploids, the polyploidization process can proceed when parthenogenesis occurs frequently. If only triploids frequently reproduce parthenogenetically, the evolutionary consequences tend to depend on the fitness of the tetraploid individuals. On the basis of a predetermined parameter set, if tetraploid fitness is relatively low, all three ploidies can coexist. Otherwise, tetraploidization occurs. In this case, triploid parthenogenesis promotes not only triploidization but also tetraploidization. However, if both triploids and tetraploids frequently reproduce parthenogenetically, the ploidy levels with the highest fitness are likely to dominate in the population through direct competition among cytotypes.

Biological Evolution↗
PubMed1989

DNA ploidy profiles as prognostic indicators in CIN lesions.

The prognostic significance of DNA ploidy measurements in cervical tissues was examined. Microphotometric measurements of 302 Feulgen-stained tissue sections (91 normal squamous epithelia, 14 condylomata, 29 cervical intraepithelial neoplasia (CIN) I, 78 CIN II, and 90 CIN III) were performed with a personal computer (PC)-based video microphotometry system. Analysis of these data shows that the DNA profile provides significant prognostic information: CINs with a polyploid DNA profile are more likely to return to normal than are those exhibiting an aneuploid pattern. Of 211 abnormal cases, 38% had polyploid DNA profiles and 62% were aneuploid. Eight-six percent of the cases that regressed were polyploid and 14% were aneuploid. Of the 130 aneuploid DNA cases, 95% remained static or progressed and only 5% regressed. Of these nonregressing aneuploid lesions, 90 remained static and 34 progressed, whereas within the nonregressing polyploid group 37 remained static and only 6 progressed. This result holds across diagnostic categories. Several other ploidy-related descriptors also showed prognostic significance (including mean ploidy, the 5N exceeding rate and 2N deviation index, and discriminant functions derived from order statistic analysis of the cumulative DNA histograms), but not to the degree or with the consistency of expression as the DNA profile categorization. These results indicate that important information about the prognosis of CIN lesions may be obtained by the DNA profile on Feulgen-stained tissue specimens. The data were acquired by a cytophotometry system of relatively modest cost consisting of readily available hardware components.

Aneuploidy↗
PubMed1992

Proliferative response of smooth muscle cells in hypertension.

A significant increase (up to 20% from about 10% in normals) in the number of smooth muscle cells (SMC) with tetraploid DNA content was found in the media and intima of human hypertensive aorta. A similar process was detected during normal human vessel aging. It was found that SMC from normal human aorta and normotensive rat aorta, which were able to incorporate 3H-thymidine, had diminished proliferative potency and a tendency to polyploidization in primary culture. We failed to detect a similar phenomenon in SMC obtained from aorta from spontaneously hypertensive rats. It was found that 10 mumol/L of norepinephrine significantly increased (by approximately twofold) the frequency of true polyploid cells in a subculture of rat aortic SMC. The effect of norepinephrine was blocked only by simultaneous action of alpha- and beta-adrenoreceptor antagonists. SMC polyploidization was also stimulated by simultaneous application of two direct activators of the second messenger systems, forskolin and phorbol-12-myristate-13-acetate. Thus, a subpopulation of SMC prone to polyploidization exists in normal vessels, and norepinephrine may be one of the mediators of the "hypertensive" response of vessel wall SMC, which probably occurs due to the synergism of two second messenger systems.

Adult↗
PubMed1995

Perturbation of hepatocyte nuclear populations induced by iron and polychlorinated biphenyls in C57BL/10ScSn mice during carcinogenesis.

The induction of hepatocarcinogenesis by polychlorinated biphenyls (PCBs) in C57BL/10ScSn mice is markedly potentiated by iron. To investigate the effects of iron and PCBs on nuclear populations, C57BL/10ScSn mice received a single dose of iron-dextran (600 mg Fe/kg) and were fed a diet containing 0.01% of the PCBs mixture Aroclor 1254 for up to 6 months. DNA content of isolated nuclei and hepatocytes was estimated by flow cytometry. Cell suspensions and nuclei isolated from Aroclor treated mice after 6 months contained increased diploid (2N) populations compared to controls. In contrast, iron treatment of mice markedly enhanced fractions of octoploid (8N) nuclei by 2 weeks and this effect persisted over the 6 month period. When Aroclor 1254 and iron were administered together there was a synergistic increase in the mononucleated diploid fraction which was significant at 2 weeks and highly significant at 6 months. This became the predominant nuclear effect. At six months, Aroclor 1254 and iron, both alone and in combination, also increased the rate of DNA synthesis in hepatocytes as measured by bromodeoxyuridine (BrdU) incorporation. The chronic polyploidizing effect of iron overload alone was investigated further and shown to be proportional to the dose and was detectable as early as 2 days after 600 mg Fe/kg and 1 week after 150 mg Fe/kg. Polyploidization of nuclei was inhibited by the oral iron chelator CP94. Iron also induced a prolonged reduction in the incidence of binucleated cells. Histologically, nuclear enlargement due to iron was confined to the midzonal region of the liver lobule, whereas iron deposition was greatest in the periportal region. Iron (600 mg/kg) also caused increased nuclear polyploid states in hepatocytes of adult rats and gerbils. Similarly, weanling mice with a dominantly diploid cell population, when treated with iron (300 mg/kg), exhibited a significant shift to a tetraploid (4N) population and a marked increase in proliferation as measured by BrdU incorporation and proliferative cell nuclear antigen (PCNA) detection. These results indicate that Aroclor 1254 and iron induce changes in the mouse hepatocyte population that involve 2N and 8N nuclei respectively. The combination treatment leads to the emergence and proliferation of a mononucleated, diploid population as observed frequently in chemical hepato-carcinogenesis. The reason for the chronic polyploidizing effect of iron is unknown, but may imply both increased DNA synthesis and impairment of nuclear division with implications in human conditions of iron overload.

Animals↗
PubMed1994

Monospermic polyploidy and atypical embryo morphology.

Monospermic cleavage-arrested human embryos were analysed by fluorescence in-situ hybridization using specific DNA probes for chromosomes X, Y and 18 simultaneously. Two groups of monospermic polyploid embryos could be distinguished: (i) embryos (n = 13) with only one large cell surrounded by smaller blastomere-sized extracellular fragments. These embryos were polyploid and frequently polyploid mosaic. (ii) Embryos developing from larger than normal oocytes were triploid or triploid mosaics (n = 4). Atypical morphology was not seen in eight other polyploid monospermic embryos. The atypical morphologies described here are quite rare, but are genetically uniform. Polyspermic embryos are the only other known example of a dual genetic-morphological abnormality.

Blastomeres↗
PubMed2026

DNA hypermethylation of abscisic-acid-related genes helps enhance the cold tolerance of tetraploid rice.

Polyploid plants exhibit enhanced stress resistance and superior adaptability to extreme environments, but the underlying molecular mechanisms remain incompletely understood. Here we confirm that tetraploid rice exhibits stronger cold tolerance than diploid rice. This improved tolerance is mediated by reduced malondialdehyde accumulation, elevated antioxidant enzyme activity, and epigenetic regulation of genes involved in abscisic acid (ABA) biosynthesis and signaling. Under cold stress, tetraploid rice induces stress-responsive genes (especially in the ABA pathway) more rapidly and to higher levels than diploid rice. This enhanced gene expression coincides with increased endogenous ABA accumulation. Furthermore, polyploidization and cold stress synergistically induce high methylation at CG, CHG, and CHH sites in genes and transposons (TEs). Notably, the methylation level of class II TEs in tetraploid rice is significantly higher than in diploid rice under low temperatures. To suppress TE activation in gene promoter regions under cold stress, tetraploid rice enhances the methylation level of ABA pathway-related gene promoters, thereby silencing TEs and maintaining genome stability. Collectively, these results enrich the theoretical understanding of the strong stress tolerance in polyploid plants and provide theoretical support for breeding cold-tolerant polyploid rice varieties.

ABA↗
PubMed2006

Phylogenetic relationships in Elymus (Poaceae: Triticeae) based on the nuclear ribosomal internal transcribed spacer and chloroplast trnL-F sequences.

To estimate the phylogenetic relationship of polyploid Elymus in Triticeae, nuclear ribosomal internal transcribed spacer (ITS) and chloroplast trnL-F sequences of 45 Elymus accessions containing various genomes were analysed with those of five Pseudoroegneria (St), two Hordeum (H), three Agropyron (P) and two Australopyrum (W) accessions. The ITS sequences revealed a close phylogenetic relationship between the polyploid Elymus and species from the other genera. The ITS and trnL-F trees indicated considerable differentiation of the StY genome species. The trnL-F sequences revealed an especially close relationship of Pseudoroegneria to all Elymus species included. Both the ITS and trnL-F trees suggested multiple origins and recurrent hybridization of Elymus species. The results suggested that: the St, H, P, and W genomes in polyploid Elymus were donated by Pseudoroegneria, Hordeum, Agropyron and Australopyrum, respectively, and the St and Y genomes may have originated from the same ancestor; Pseudoroegneria was the maternal donor of the polyploid Elymus; and some Elymus species showed multiple origin and experienced recurrent hybridization.

DNA, Chloroplast↗
PubMed1975

A morphometric study of normal human liver cell nuclei.

Using the projecting method, measurements were made of the size of nuclei of liver cells in liver biopsies taken from ten patients with normal liver histology and normal liver function tests and from two patients with acromegaly. For the statistical analysis a parametric model of the distributions of the nuclear radii, the chi distributions, was used, making an estimation of the frequencies of di-, tetra- and octaploid nuclei possible. The distribution of the lengths of the nuclear radii was consistent with normal distributions corresponding to the different ploidy classes. Diploid nuclei constituted 84 to 99 per cent of the liver cell nuclei, and the frequency of polyploid nuclei increased with increasing age. Samples of nuclei within the same biopsy revealed significant differences in the mean radius of diploid nuclei and in the frequency of polyploid nuclei. This biological variation must be taken into account in the interpretation of karyometric data. One of the patients with acromegaly had a higher frequency of polyploid nuclei than seen in normal patients, and in both patients with acromegaly the size of diploid nuclei was large compared with normals. The frequency of binuclear nuclei was unrelated to age and frequency of polyploid nuclei, but males were found to have more binuclear liver cells than females.

Acromegaly↗
PubMed2025

From bottleneck to boom: Polyploidy, genetic instability and response to artificial selection resolve the peanut paradox.

This study, the second in a three-part series, shows how peanut's polyploid origin enabled rapid diversification and enhanced domestication potential. Building on the knowledge that cultivated peanut (Arachis hypogaea) originated from a narrow hybridization between Arachis duranensis and Arachis ipaënsis less than 10 000 years ago, we are confronted with a paradox: how did such a narrow origin give rise to so much diversity-two subspecies, six botanical varieties, and thousands of landraces differing in growth habit, seed size, and pod morphology? Although several diploid Arachis species were cultivated earlier, only the allotetraploid became fully domesticated and widely adopted. The global success of peanut, despite its narrow genetic origin, suggests that polyploidization itself facilitated domestication. To test this hypothesis, we investigated how the two diploid progenitors and neoallotetraploids derived from a single hybridization and polyploidization event responded under artificial selection. In a pollinator-free greenhouse, we advanced lineages of the neoallotetraploid and its diploid parents over 6 years, selecting for divergent seed weights. The neoallotetraploid showed a much stronger response to artificial selection than its diploid parents, while also spontaneously generating diverse phenotypic variation-including flower color, pod reticulation, and chlorophyll content-traits that distinguish A. hypogaea subspecies and landraces. These traits mirrored directional shifts in parental genome dosage caused by homoeologous exchange, supporting a causal connection with phenotype. These findings offer a compelling rationale for a domestication advantage in polyploid peanut, and provide a living demonstration of how a single ancestral tetraploid, despite an extreme genetic bottleneck, generates a phenotypic boom.

Arachis↗
PubMed1970

Changes in deoxyribonucleic acid synthesis regulation in Chinese hamster cells infected with simian virus 40.

Infection of primary or secondary cultures of Chinese hamster embryo cells with simian virus 40 at a multiplicity of 20 to 50 induced synthesis of the virus-specific intranuclear T antigen in 80 to 90% of the cells within 48 to 72 hr. In the infected cultures, 30 to 50% more cells were recruited into deoxyribonucleic acid (DNA) synthesis than in the controls, whether or not the cultures were confluent. The newly synthesized DNA was mostly cellular, since little virus was produced (as shown by various techniques: immunofluorescence for viral antigen, virus growth curves, and isolation of viral DNA from infected cultures). Transformed cells could be detected a few weeks after infection and produced tumors when inoculated into irradiated animals. Chromosomal changes were observed soon after infection (24 hr). Initially, there was a marked increase in the proportion of polyploid cells (8 to 14%), most of which were chromosomally normal. In a few weeks, a large majority of the infected population was polyploid (30 to 50%). Thus, the polyploid cells have the ability to proliferate. Evidence is presented to suggest that polyploid cells arise by stimulation of cells in the G(1), G(2), or S phases to undergo two or more successive periods of DNA synthesis without an intervening mitosis. With a subsequent loss or redistribution of chromosomal material, this may lead eventually to a biologically transformed cell; thus, it is suggested that the initial event(s) relevant to transformation occurs at the level of control of cellular DNA synthesis.

Animals↗
PubMed2004

The utility of the nontranscribed spacer of 5S rDNA units grouped into unit classes assigned to haplomes - a test on cultivated wheat and wheat progenitors.

Data is presented on the evolutionary dynamics of non-transcribed spacers (NTSs) of 5S rRNA genes in some diploid and polyploid Triticum and Aegilops species. FISH experiments with probes representing different unit classes revealed presence and (or) absence of these sequences in genomes or separate chromosomes of the species. Among the three diploid species only Aegilops speltoides has all of the different unit classes in ribosomal clusters as detected by the probes. Triticum urartu does not have the long D1 signals and Aegilops tauschii does not have the long A1 signals. Both polyploids possess all types of sequences, but because of genome rearrangements after polyploidization there is significant repatterning of single different rDNA unit classes in chromosomal positions when compared with those in diploid progenitors. Additional refined work is needed to ascertain if the sequences in the polyploids are mixed or are located in mini clusters in close proximity to each other. Mantel tests for association between the presence of the FISH signals of the A, B, and D genomes together and separately with the unit class data of the material, i.e., the probes used in FISH, indicated that all signals were associated with their respective probe material, but that there was no association of the unit classes found and the signals to each haplome. All combinations of the partial Mantel tests, e.g., between the A and B haplomes while controlling the effect of the all probes signals, with correlations ranging from 0.48 to 0.79 were all significant. Principal coordinate analysis showed that the signals of most unit class specific probes were more or less equally distant except for the long (S1 and short G1 signals, which were not different, and that the short A1 signals were closely related to the former two, whereas the signals of the long G1 were even less related.

Chromosomes, Plant↗
PubMed1995

Molecular analysis of evolutionary patterns in U genome wild wheats.

The theory of pivotal-differential evolution states that one genome of polyploid wheats remains stable (i.e., pivotal) during evolution, while the other genome or genomes may become modified (i.e., differential). A proposed mechanism for apparent modification of the differential genome is that different polyploid species with only one genome in common may exchange genetic material. In this study, we analyzed a set of sympatric and allopatric accessions of tetraploid wheats with the genomic constitutions UM and UC. The U genome of these species is from Triticum umbellulatum and is considered to be the pivotal genome. The M and C genomes, from T. comosum and T. dichasians, respectively, are considered to be the differential genomes. Low copy DNA was analyzed using "sequence tagged site" primer sets in the polymerase chain reaction, followed by digestion with restriction enzymes. Genetic similarity matrices based on shared restriction fragments showed that sympatric accessions of different U genome tetraploid species did not tend to share more restriction fragments than did allopatric accessions. Thus, no evidence for introgression was found. Analysis of the diploid progenitor species showed that the U genome was less variable than the M and C genomes. Additionally, comparison of diploid and polyploid species using genome-specific primer sets suggests a possible polyphyletic origin for T. triunciale and T. machrochaetum. Thus, our results suggest that the differential nature of the M and C genomes may be the result of variability introduced by the diploid progenitors and not the result of frequent introgression events after formation of the polyploid.

Base Sequence↗
PubMed1975

A quantitative study of the second meiotic metaphase in male mice (Mus musculus).

Over 11,000 second meiotic metaphase spreads stained for the pericentromeric region have been studied quantitatively in male mice of 14 strains. The sex-chromosome constitution of a cell could be judged objectively if X and Y chromosomes and ploidy were all scored. A bias arose if only Y chromosomes and ploidy were scored but could be corrected statistically. There was no sign of other forms of bias. The original contiguity of X and Y second metaphases in vivo was very occasionally evident in the preparations. Most of the subhaploid aneuploid counts were assumed to be artifactual. The incidence of truly aneuploid second metaphases in 13 strains was estimated as 0.38+/-0.12%. The estimated average rate per chromosome was 0.019+/-0.006%, with a comparable order of magnitude for the sex chromosomes alone. Simultaneous aneuploidy of two or more chromosomes of the haploid set was estimated to be very rare. Of the spreads from 13 strains, 9.6% were polyploid (2N, 3N, 4N) and showed most of the possible combinations of sex chromosomes. Nearly all the polyploid spreads were considered to arise by artifactual cell fusion at the time of second metaphase during the preparative technique, especially of the X and Y daughter-cell products of the first meiotic division. Other modes of origin (true polyploidy, accidental superposition of cells during preparation) were unlikely. The data could be accommodated by a statistical model with only four parameters. It allowed for artifactual fusion mainly between daughter cells but also between non-daughter cells, bias in one scoring method, and bias in the numbers of cells with given ploidy successfully mounted. Current techniques of chromosome preparation were thought to be wholly unsuitable for the recognition of true polyploidy. The artifactual origin of polyploid spreads was borne out by an absence of polyploid spermatozoa in 14 strains. There appeared to be a virtually constant transmission rate of paternal X and Y chromosomes from early meiosis to late blastocyst. The estimated rate of 49.05+/-0.67% with a Y chromosome also estimated the primary sex ratio. There was evidence of polymorphism in autosomal pericentromeric staining in 3 strains. No measure of the numbers of autosomes or sex chromosomes varied significantly between duplicate preparations or between duplicate males of a strain.

Aneuploidy↗