Natural variation in reproductive timing and X-chromosome nondisjunction in Caenorhabditis elegans

Caenorhabditis elegans hermaphrodites first produce a limited number of sperm cells, before their germline switches to oogenesis. Production of progeny then ensues until sperm is depleted. Male production in the self-progeny of hermaphrodites occurs following X-chromosome nondisjunction during gametogenesis, and in the reference strain increases with age of the hermaphrodite parent. To enhance our understanding of the reproductive timecourse in C. elegans, we measured and compared progeny production and male proportion during the early and late reproductive periods of hermaphrodites for 96 wild C. elegans strains. We found that the two traits exhibited natural phenotypic variation with few outliers and a similar reproductive timing pattern as previous reports. Progeny number and male proportion were not correlated in the wild strains, implying that wild strains with a large brood size did not produce males at a higher rate. We also identified loci and candidate genetic variants significantly associated with male-production rate in the late and total reproductive periods. Our results provide an insight into life-history traits in wild C. elegans strains.

Natural variations in reproductive aging phenotypes reveal the importance of early reproductive period in Caenorhabditis elegans

Although reproductive capacity is a major factor in individual fitness, aging of the reproductive system precedes somatic aging and may reduce the total brood size. Genetic studies have led to the development of a body of evolutionary theory in the nematode Caenorhabditis elegans, but these studies did not take into account current knowledge about the natural history of C. elegans. To enhance our understanding of reproductive aging in C. elegans, we measured and compared two reproductive aging-related traitsthe number of progeny and the X-chromosome nondisjunction rateof 96 wild strains during early, late and total reproductive periods. We found that the two traits exhibited natural phenotypic variation, with few outliers, and that the brood size and the X-chromosome nondisjunction rate were not genetically correlated. Contrary to a previous hypothesis, that reproductive aging contributes to the generation of an optimal total number of offspring, we found that the total brood size did not converge to an optimal value, and early brood size was more constant than total brood size among wild strains. We speculate that reproductive aging is a by-product of a rapid increase in the initial population size, which might be related to the boom-and-bust lifestyle of C. elegans. We also identified loci and candidate genetic variants significantly associated with X-chromosome nondisjunction rate in the late and total reproductive periods. Our results provide an insight into reproductive aging in wild C. elegans strains.

The Interchromosomal Effect: Different Meanings for Different Organisms

The term interchromosomal effect was originally used to describe a change in the distribution of exchange in the presence of an inversion. First characterized in the 1920s by early Drosophila researchers, it has been observed in multiple organisms. Nearly half a century later, the term began to appear in the human genetics literature to describe the hypothesis that parental chromosome differences, such as translocations or inversions, may increase the frequency of meiotic chromosome nondisjunction. Although it remains unclear if chromosome aberrations truly affect the segregation of structurally normal chromosomes in humans, the use of the term interchromosomal effect in this context persists. This article explores the history of the use of the term interchromosomal effect and discusses how chromosomes with structural aberrations are segregated during meiosis.

Prolonged mitosis causes separase deregulation and chromosome nondisjunction

Intercellular karyotype diversity, or aneuploidy, is a widespread feature of cancers propagated through continuous mitotic chromosome segregation errors, the condition called chromosomal instability. The protease separase is an essential enzyme that sever cohesin between sister chromatids, and a probe for separase activity has articulated that separase undergoes abrupt activation shortly before anaphase onset, after being suppressed through metaphase; however the relevance of this robust control remains unclear. In this study, we found that separase activates precociously during prolonged metaphase, consistently in multiple types of cancer cell lines. An artificial extension of metaphase alone in chromosomally stable diploid cells was found to induce precocious activation. These kinetic changes resulted in an incomplete removal of cohesin and emergence of chromosomal bridges in anaphase. Conversely, in transformed cells, shortening back of their prolonged metaphase restored the robust activation of separase and ameliorated anaphase bridge formation. These observations suggest that retarded metaphase progression directly affects the separase activation profile, which provides a previously unanticipated etiology for chromosomal instability in cancers and underscores the significance of the swift mitotic transitions for fail-safe chromosome segregation.

Ассоциация полиморфизма Т657С гена SYCP3 с потерей беременности на ранних сроках у женщин русской национальности

Background. Aneuploidy is a consequence of the chromosome nondisjunction. Majority of aneuploid embryos die in utero between the 6th and 10th week, resulting in early pregnancy loss of approximately 15% of all clinically recognized pregnancies. SYCP3 plays an important role in pairing and recombination of homologous chromosomes in meiosis I, and it has been shown that the lack of this gene leads to sterility in male and subfertility in female mice due to chromosome nondisjunction. So SYCP3 is a candidate gene to evaluate the hypothesis of fetal aneuploidy arisen from meiosis gene mutations as a cause for human early pregnancy loss. Methods. In our study, the SYCP3 T657C polymorphism in 100 Russian women with early pregnancy loss (EPL) that were classified into 2 subgroups: with sporadic pregnancy loss (SPL, n=50) and recurrent pregnancy loss (RPL, n=50), as well as 56 normal fertile women (control), was examined to determine whether there is an association between the polymorphism and early pregnancy loss in Russian population. Genomic DNA was extracted from peripheral blood samples using the standard procedure of a commercially available kit. Genotyping of SYCP3 gene was determined by allele-specific polymerase chain reaction method. Data were analyzed using SPSS statistical software version 22. The chi-square test and Fisher’s exact test were used to compare genotype and allele frequencies between analyzed groups. The odds ratio (OR) and 95% confidence intervals (CI) were used for risk estimation. Results. Frequency of the heterozygous genotype (CT) was significantly higher in the group of women with early pregnancy loss as well as in women with sporadic pregnancy loss (p<0.05). In women with RPL, we found that the frequency of this genotype tended to increase but could not make a significant difference when compared with the control group. Conclusion. Our findings postulate that the T657C polymorphism of the SYCP3 gene is possibly associated with a sporadic early pregnancy loss. Актуальность. Анеуплоидия возникает вследствие нарушения расхождения хромосом, и большинство анеуплоидных эмбрионов погибают до 10 недели гестации, что приводит к потере на ранних сроках приблизительно 15% клинически диагностированных беременностей. Белок SYCP3 играет важную роль в конъюгации и рекомбинации гомологичных хромосом в первом мейотическом делении. Было показано, что отсутствие гена данного белка у мышей приводит к стерильности самцов и снижению фертильности самок вследствие нарушения расхождения хромосом. В этой связи SYCP3 можно рассматривать в качестве кандидатного гена для оценки гипотезы о фетальной анеуплоидии, возникающей в результате мутаций генов, продукты которых участвуют в мейозе, и ведущей к ранним репродуктивным потерям. Методы. В представленной работе был исследован полиморфизм Т657С гена SYCP3 у 100 женщин русской национальности с ранними репродуктивными потерями, подразделенных на две подгруппы: со спорадической потерей беременности (n=50) и привычным невынашиванием (n=50), а также у 56 фертильных женщин (контроль). Целью работы явилось изучение наличия ассоциации между данным полиморфизмом и ранними репродуктивными потерями в русской популяции. Геномная ДНК была выделена из периферической крови, генотипирование выполнялось с использованием аллель-специфической полимеразной цепной реакции. Для сравнения частот генов и генотипов в изучаемых группах применялись критерий Хи-квадрат и точный тест Фишера, для определения которых использовалось программное обеспечение SPSS Statistics, версия 22. Оно также было использовано для расчета отношения шансов (OR) и 95% доверительного интервала. Результаты. Частота гетерозиготного генотипа СТ была достоверно выше в группе женщин с ранними репродуктивными потерями и, в частности, в подгруппе со спорадической потерей беременности (р<0,05). Несмотря на тенденцию к увеличению частоты данного генотипа у женщин с привычным невынашиванием, статистически значимых отличий от контроля обнаружено не было. Заключение. Полученные результаты позволяют предполагать наличие ассоциации между полиморфизмом Т657С гена SYCP3 и спорадической потерей беременности на ранних сроках.

Chromosome errors in human eggs shape natural fertility over reproductive life span

Chromosome errors, or aneuploidy, affect an exceptionally high number of human conceptions, causing pregnancy loss and congenital disorders. Here, we have followed chromosome segregation in human oocytes from females aged 9 to 43 years and report that aneuploidy follows a U-curve. Specific segregation error types show different age dependencies, providing a quantitative explanation for the U-curve. Whole-chromosome nondisjunction events are preferentially associated with increased aneuploidy in young girls, whereas centromeric and more extensive cohesion loss limit fertility as women age. Our findings suggest that chromosomal errors originating in oocytes determine the curve of natural fertility in humans.

Insights about variation in meiosis from 31,228 human sperm genomes

Meiosis, while critical for reproduction, is also highly variable and error prone: crossover rates vary among humans and individual gametes, and chromosome nondisjunction leads to aneuploidy, a leading cause of miscarriage. To study variation in meiotic outcomes within and across individuals, we developed a way to sequence many individual sperm genomes at once. We used this method to sequence the genomes of 31,228 gametes from 20 sperm donors, identifying 813,122 crossovers, 787 aneuploid chromosomes, and unexpected genomic anomalies. Different sperm donors varied four-fold in the frequency of aneuploid sperm, and aneuploid chromosomes gained in meiosis I had 36% fewer crossovers than corresponding non-aneuploid chromosomes. Diverse recombination phenotypes were surprisingly coordinated: donors with high average crossover rates also made a larger fraction of their crossovers in centromere-proximal regions and placed their crossovers closer together. These same relationships were also evident in the variation among individual gametes from the same donor: sperm with more crossovers tended to have made crossovers closer together and in centromere-proximal regions. Variation in the physical compaction of chromosomes could help explain this coordination of meiotic variation across chromosomes, gametes, and individuals.