Recovery of 2R.2Sk Triticale-Aegilops kotschyi Robertsonian Chromosome Translocations

Robertsonian translocations (RobTs) in the progeny of triticale (×Triticosecale Wittmack) plants with monosomic substitution of Aegilops kotschyi chromosome 2Sk (2R) were investigated by fluorescence in-situ hybridization. Chromosome 2Sk of Ae. kotschyi is reported to possess many valuable loci, such as Lr54 + Yr37 leaf and stripe (yellow) rust resistance genes. We used a standard procedure to produce RobTs, which consisted of self-pollination of monosomic triticale plants, carrying 2R and 2Sk chromosomes in monosomic condition. This approach did not result in RobTs. Simultaneously, we succeeded in producing 11 plants carrying 2R.2Sk compensatory RobTs using an alternative approach that utilized ditelosomic lines of triticale carrying 2RS (short arm) and 2RL (long arm) telosomic chromosomes. Identification of molecular markers linked to Lr54 + Yr37 genes in the translocation plants confirmed that these resources can be exploited in current triticale breeding programmes.

Apoptosis in Spermiogenesis of Multiple Robertsonian Heterozygotes of Mus musculus domesticus

Mus musculus domesticus is a species that is characterized by a diploid number of 40 chromosomes, all telocentrics (acrocentrics). In natural populations of Mus with high frequency Robertsonian chromosome translocations (RBs), a fusion at centromere-level between two autosomal telocentric, producing metacentric Rb chromosomes and a variety of natural subspecies with diploid chromosome numbers below 40. Rb chromosomes do not affect the viability of individuals but mainly fertility of Rb heterozygotes. In their meiosis the metacentric Rb and the homologous telocentrics form trivalents that have problems in synapsis and segregation. This paper presents a comparative analysis of spermatogenesis considering the stages of the epithelium and the number of germ cells loss by apoptosis comparing heterozygote males 2n=32, carriers 8 trivalents, and homozygotes 2n=40 and 2n=24. It was found that the number of spermatocytes in first prophase was similar in the different seminiferous epithelium stages in all the chromosome constitutions. In the 2n=32 heterozygotes a significant decreased number of spermatids was reflected in the proportion of spermatocytes and spermatids that was 1:1.7. In the homozygote males rare germ cell in apoptosis (positive for caspase 3) were observed, which were mainly concentrated in the XII stage of the seminiferous epithelium. In heterozygote spermatocytes apoptotic germ cell number was significantly higher than that registered in homozygote males, and generally correspond to spermatocytes in meiotic divisions. This selective removal of cells possibly carrying anomalies, either in chromosome alignment or segregation, is consistent with the smaller number of spermatids and the relative sub-fertility observed in 2n=32 Rb heterozygotes.

Evidence for meiotic spindle checkpoint from analysis of spermatocytes from Robertsonian-chromosome heterozygous mice

Mice heterozygous for Robertsonian centric fusion chromosomal translocations frequently produce aneuploid sperm. In this study RBJ/Dn× C57BL/6J F1 males, heterozygous for four Robertsonian translocations (2N=36), were analyzed to determine effects on germ cells of error during meiosis. Analysis of sperm by three color fluorescence in situ hybridization revealed significantly elevated aneuploidy, thus validating Robertsonian heterozygous mice as a model for production of chromosomally abnormal gametes. Primary spermatocytes from heterozygous males exhibited abnormalities of chromosome pairing in meiotic prophase and metaphase. In spite of prophase abnormalities, the prophase/metaphase transition occurred. However, an increased frequency of cells with misaligned condensed chromosomes was observed. Cytological analysis of both young and adult heterozygous mice revealed increased apoptosis in spermatocytes during meiotic metaphase I. Metaphase spermatocytes with misaligned chromosomes accounted for a significant proportion of the apoptotic spermatocytes, suggesting that a checkpoint process identifies aberrant meioses. Immunofluorescence staining revealed that kinetochores of chromosomes that failed to align on the spindle stained more intensely for kinetochore antigens CENP-E and CENP-F than did aligned chromosomes. Taken together, these observations are consistent with detection of malattached chromosomes by a meiotic spindle checkpoint mechanism that monitors attachment and/or congression of homologous chromosome pairs. However, the relatively high frequency of gametic aneuploidy suggests that the checkpoint mechanism does not efficiently eliminate all germ cells with chromosomal abnormalities.

Nondisjunction rates and abnormal embryonic development in a mouse cross between heterozygotes carrying a (7, 18) robertsonian translocation chromosome.

Mice bearing Robertsonian translocation chromosomes frequently produce aneuploid gametes. They are therefore excellent tools for studying nondisjunction in mammals. Genotypic analysis of embryos from a mouse cross between two different strains of mice carrying a (7,18) Robertsonian chromosome enabled us to measure the rate of nondisjunction for chromosomes 7 and 18. Embryos (429) were harvested from 76 litters of mice and the parental origin of each chromosome 7 and 18 determined. Genotyping these embryos has allowed us to conclude the following: (1) there were 96 embryos in which at least one nondisjunction event had taken place; (2) the rate of maternal nondisjunction was greater than paternal nondisjunction for teh chromosomes sampled in these mice; (3) a bias against chromosome 7 and 18 nullisomic gametes was observed, reflected in a smaller than expected number of uniparental disomic embryos; (4) nondisjunction events did not seem to occur at random throughout the 76 mouse litters, but were clustered into fewer than would be expected cy chance; and (5) a deficiency of paternal chromosome 18 uniparental disomic embryos was observed along with a higher than normal rate of developmental retardation at 8.5 days post coitum, raising the possibility that this chromosome has at least one imprinted gene.