Chromosomal drive and the evolution of meiotic nondisjunction and trisomy in humans

Hoisted with his own petard: How sex-ratio meiotic drive in Drosophila affinis creates resistance alleles that limit its spread

Meiotic drivers are selfish genetic elements that tinker with gametogenesis to bias their own transmission into the next generation of offspring. Such tinkering can have significant consequences on gametogenesis and end up hampering the spread of the driver. In Drosophila affinis, sex-ratio meiotic drive is caused by an X-linked complex that, when in males with a susceptible Y chromosome, results in broods that are typically more than 95% female. Interestingly, D. affinis males lacking a Y chromosome (XO) are fertile and males with the meiotic drive X and no Y produce only sons-effectively reversing the sex-ratio effect. Here, we show that meiotic drive dramatically increases the rate of nondisjunction of the Y chromosome (at least 750X), meaning that the driver is creating resistant alleles through the process of driving. We then model how the O might influence the spread, dynamics and equilibrium of the sex-ratio X chromosome. We find that the O can prevent the spread or reduce the equilibrium frequency of the sex-ratio X chromosome, and it can even lead to oscillations in frequency. Finally, with reasonable parameters, the O is unlikely to lead to the loss of the Y chromosome, but we discuss how it might lead to sex-chromosome turnover indirectly.

Selfish centromeres and the wastefulness of human reproduction

Many human embryos die in utero owing to an excess or deficit of chromosomes, a phenomenon known as aneuploidy; this is largely a consequence of nondisjunction during maternal meiosis I. Asymmetries of this division render it vulnerable to selfish centromeres that promote their own transmission, these being thought to somehow underpin aneuploidy. In this essay, I suggest that these vulnerabilities provide only half the solution to the enigma. In mammals, as in utero and postnatal provisioning is continuous, the costs of early death are mitigated. With such reproductive compensation, selection can favour a centromere because it induces lethal aneuploidy: if, when taken towards the polar body, it instead kills the embryo via aneuploidy, it gains. The model is consistent with the observation that reduced dosage of a murine drive suppressor induces aneuploidy and with the fact that high aneuploidy rates in vertebrates are seen exclusively in mammals. I propose further tests of this idea. The wastefulness of human reproduction may be a price we pay for nurturing our offspring.

The origins and evolution of genetic disease risk in modern humans

Patterns and risks of human disease have evolved. In this article, I review evidence regarding the importance of recent adaptive evolution, positive selection, and genomic conflicts in shaping the genetic and phenotypic architectures of polygenic human diseases. Strong recent selection in human populations can create and maintain genetically based disease risk primarily through three processes: increased scope for dysregulation from recent human adaptations, divergent optima generated by intraspecific genomic conflicts, and transient or stable deleterious by-products of positive selection caused by antagonistic pleiotropy, ultimately due to trade-offs at the levels of molecular genetics, development, and physiology. Human disease due to these processes appears to be concentrated in three sets of phenotypes: cognition and emotion, reproductive traits, and life-history traits related to long life-span. Diverse, convergent lines of evidence suggest that a small set of tissues whose pleiotropic patterns of gene function and expression are under especially strong selection-brain, placenta, testis, prostate, breast, and ovary-has mediated a considerable proportion of disease risk in modern humans.

Trisomy and triploidy are sources of embryo mortality in the zebra finch

Hatching failure is a surprisingly common phenomenon given that natural selection constantly works against it. In birds, an average of about 10 per cent of eggs across species fail to hatch, often owing to the death of embryos. While embryo mortality owing to inbreeding is both well-documented and evolutionarily plausible, this is not true for other sources of mortality. In fact, the basis for hatching failure in natural populations remains largely unexplained. Here, we demonstrate that embryo mortality in captive zebra finches (Taeniopygia guttata) follows from chromosomal aneuploidy or polyploidy. As part of microsatellite genotyping of a captive breeding population, we found 12 individuals (3.6%) with three alleles among 331 embryos that had died during development, while there were no such cases observed among 1210 adult birds. Subsequent genotyping of 1920 single nucleotide polymorphism markers distributed across the genome in birds with three alleles at microsatellite loci, and in greater than 1000 normal birds, revealed that the aberrant karyotypes involved cases of both trisomies and triploidy. Cases of both maternally and paternally inherited trisomies resulted from non-disjunction during meiosis. Maternally inherited cases of triploidy were attributable to failure of meiosis leading to diploid eggs, while paternally inherited triploidy could have arisen either from diploid sperm or from dispermy. Our initial microsatellite screening set only had the power to detect less than 10 per cent of trisomies and by extrapolation, our data therefore tentatively suggest that trisomy might be a major cause of embryo mortality in zebra finches.

Adaptive-filtering of trisomy 21: risk of Down syndrome depends on family size and age of previous child

The neonatal incidence rate of Down syndrome (DS) is well-known to accelerate strongly with maternal age. This non-linearity renders mere accumulation of defects at recombination during prolonged first meiotic prophase implausible as an explanation for DS rate increase with maternal age, but might be anticipated from chromosomal drive (CD) for trisomy 21. Alternatively, as there is selection against genetically disadvantaged embryos, the screening system that eliminates embryos with trisomy 21 might decay with maternal age. In this paper, we provide the first evidence for relaxed filtering stringency (RFS) to represent an adaptive maternal response that could explain accelerating DS rates with maternal age. Using historical data, we show that the proportion of aberrant live births decrease with increased family size in older mothers, that inter-birth intervals are longer before affected neonates than before normal ones, and that primiparae exhibit elevated levels of DS incidence at higher age. These findings are predicted by adaptive RFS but cannot be explained by the currently available alternative non-adaptive hypotheses, including CD. The identification of the relaxation control mechanism and therapeutic restoration of a stringent screen may have considerable medical implications.

MTRR and MTHFR polymorphism: link to Down syndrome?

Polymorphisms in genes encoding the folate metabolizing enzymes methylenetetrahydrofolate reductase (MTHFR C677T) and methionine synthase reductase (MTRR A66G) have been linked to the etiology of Down syndrome. We examined the prevalence of these variant genotypes in mothers who had given birth to a child with Down syndrome (n = 48) and in control mothers (n = 192), and investigated the biochemical factors influenced by the presence of MTRR A66G and MTHFR C677T. The frequency of the MTRR variant genotypes (AG, GG) was significantly higher in mothers of children with Down syndrome compared to controls (P = 0.0028). MTHFR C677T genotype frequencies were not significantly altered in mothers of children with Down syndrome (P = 0.74). However, mothers who had a MTHFR CT or TT genotype and a MTRR GG genotype had a 2.98-fold increased risk of having a child with Down syndrome (P = 0.02). The MTRR polymorphism did not increase plasma homocysteine. Higher homocysteine was found with the presence of the MTHFR T allele. In conclusion, MTRR A66G is significantly more common in mothers of children with Down syndrome but does not appear to increase the risk for Down syndrome by changing homocysteine metabolism. Women who have both the MTRR and MTHFR variant genotypes are also at increased risk of producing offspring with Down syndrome.

Nonrandom segregation of the mouse univalent X chromosome: evidence of spindle-mediated meiotic drive

A fundamental principle of Mendelian inheritance is random segregation of alleles to progeny; however, examples of distorted transmission either of specific alleles or of whole chromosomes have been described in a variety of species. In humans and mice, a distortion in chromosome transmission is often associated with a chromosome abnormality. One such example is the fertile XO female mouse. A transmission distortion effect that results in an excess of XX over XO daughters among the progeny of XO females has been recognized for nearly four decades. Utilizing contemporary methodology that combines immunofluorescence, FISH, and three-dimensional confocal microscopy, we have readdressed the meiotic segregation behavior of the single X chromosome in oocytes from XO females produced on two different inbred backgrounds. Our studies demonstrate that segregation of the univalent X chromosome at the first meiotic division is nonrandom, with preferential retention of the X chromosome in the oocyte in approximately 60% of cells. We propose that this deviation from Mendelian expectations is facilitated by a spindle-mediated mechanism. This mechanism, which appears to be a general feature of the female meiotic process, has implications for the frequency of nondisjunction in our species.

Tertiary trisomy due to a reciprocal translocation of chromosomes 5 and 21 in a four-generation family

Tertiary trisomy, or double trisomy, is a rare occurrence. We present two individuals with a previously unreported tertiary trisomy for chromosomes 5p and 21q in an eight-generation pedigree. Their phenotypes are compared with other partial trisomies of either 5p or 21q from the literature. The propositus was diagnosed with trisomy 21 at 2 years of age after a karyotype study for short stature and developmental delay. His phenotype was described as atypical for Down syndrome. He presented at 9 years of age because of pervasive behavioral problems and obesity. He was brachycephalic with a flattened nasal bridge, but he lacked other characteristics of trisomy 21. Because of lack of phenotypic evidence of Down syndrome, a repeat karyotype was obtained and showed 47,XY, +der(21)t(5;21)(p15.1; q22.1), incorporating partial trisomies of both chromosomes 5 and 21. Mother had a balanced translocation, 46, XX,t(5;21)(p15.1; q22.1); 8 other relatives were examined. The translocation originated from the maternal great-grandmother, but only the propositus and his mentally retarded aunt had a similar phenotye and the derivative chromosome. Fluorescence in situ hybridization showed absence of band 21q22.2 in the derivative chromosome of the propositus and his aunt, indicating that neither had trisomy for the Down syndrome critical region. These cases represent a unique double partial trisomy of chromosome arms 5p and 21q that occurred because of 3:1 malsegregation of a reciprocal translocation. These cases further demonstrate that phenotypic discordance with cytogenetic results dictate further investigation using advanced cytogenetic hybridization.

A generalization of Pontryagin's maximum principle for dynamic evolutionary games among relatives

We present two theorems that generalize Pontryagin's maximum principle to the setting of dynamic evolutionary games between genetically related individuals. The two theorems correspond to two types of interactions among individuals: patch-structured populations in which individuals locally "play the field" and pairwise interactions. These generalizations can be used in the same way that Pontryagin's maximum principle is used and they are valid for diploid organisms under a single locus, diallelic genetic model. These generalizations involve an interesting, dynamic version of Hamilton's Rule from inclusive fitness theory. We illustrate how these theoretical results can be applied by modeling the evolution of lifetime resource allocation to growth and reproduction in an annual plant when there is competition for resources among related individuals.

Genetic variation in rates of nondisjunction: association of two naturally occurring polymorphisms in the chromokinesin nod with increased rates of nondisjunction in Drosophila melanogaster

Genetic variation in nondisjunction frequency among X chromosomes from two Drosophila melanogaster natural populations is examined in a sensitized assay. A high level of genetic variation is observed (a range of 0.006-0.241). Two naturally occurring variants at the nod locus, a chromokinesin required for proper achiasmate chromosome segregation, are significantly associated with an increased frequency of nondisjunction. Both of these polymorphisms are found at intermediate frequency in widely distributed natural populations. To account for these observations, we propose a general model incorporating unique opportunities for meiotic drive during female meiosis. The oötid competition model can account for both high mean rates of female-specific nondisjunction in Drosophila and humans as well as the standing genetic variation in this critical fitness character in natural populations.