Meiotic recombination is essential for the segregation of chromosomes and the formation of normal haploid gametes, yet we know very little about the meiotic process in humans. We present the first (to our knowledge) recombination maps for every autosome in the human male obtained by new immunofluorescence techniques followed by centromere-specific multicolor fluorescence in situ hybridization in human spermatocytes. The mean frequency of autosomal recombination foci was 49.8+/-4.3, corresponding to a genetic length of 2,490 cM. All autosomal bivalents had at least one recombination focus. In contrast, the XY bivalent had a recombination focus in 73% of nuclei, suggesting that a relatively large proportion of spermatocytes may be at risk for nondisjunction of the XY bivalent or elimination by meiotic arrest. There was a very strong correlation between mean length of the synaptonemal complex (SC) and the number of recombination foci per SC. Each bivalent presented a distinct distribution of recombination foci, but in general, foci were near the distal parts of the chromosome, with repression of foci near the centromere. The position of recombination foci demonstrated positive interference, but, in rare instances, foci were very close to one another.

Recombination allows faithful chromosomal segregation during meiosis and contributes to the production of new heritable allelic variants that are essential for the maintenance of genetic diversity. Therefore, an appreciation of how this variation is created and maintained is of critical importance to our understanding of biodiversity and evolutionary change. Here, we analysed the recombination features from species representing the major eutherian taxonomic groups Afrotheria, Rodentia, Primates and Carnivora to better understand the dynamics of mammalian recombination. Our results suggest a phylogenetic component in recombination rates (RRs), which appears to be directional, strongly punctuated and subject to selection. Species that diversified earlier in the evolutionary tree have lower RRs than those from more derived phylogenetic branches. Furthermore, chromosome-specific recombination maps in distantly related taxa show that crossover interference is especially weak in the species with highest RRs detected thus far, the tiger. This is the first example of a mammalian species exhibiting such low levels of crossover interference, highlighting the uniqueness of this species and its relevance for the study of the mechanisms controlling crossover formation, distribution and resolution.

Reciprocal translocations, the most frequent structural aberration in humans, are mainly transmitted by one of the parents. In order to analyze the chromosomal content of the spermatozoa from carriers of chromosomal reorganizations, two methods have been used, karyotyping of sperm chromosomes by the human-hamster system and fluorescence in situ hybridization (FISH) in decondensed sperm nuclei. In this work, we review 92 sperm chromosome segregation studies from 85 different reciprocal translocation carriers, including a triple translocation carrier. Using the human-hamster method, a total of 5,818 spermatozoa from 44 reciprocal translocation carriers have been analyzed, 43 of them carrying a single reciprocal translocation and one was a carrier of a double reciprocal translocation. A segregation analysis in a carrier of a t(2;22;11) has been also reported. Carrying out FISH in sperm nuclei, a total of 237,042 spermatozoa from 46 reciprocal translocation carriers have been analyzed. Six of these were also analyzed by the human-hamster system. Taking into account both methods, a total of 76 different reciprocal translocations have been studied. In 74 of these 76 translocations, the reorganization occurs between autosomes, and in the other two, the Y chromosome is involved. Although along general lines, there are similarities between the results obtained by the two methods of analysis, variations are observed when the distribution of the different types of segregations that produce imbalances is compared. As a general rule reciprocal translocation carriers produce more unbalanced sperm than normal or balanced sperm. The results reported also corroborate that the proportion of unbalanced forms depends on the characteristics of the reorganization and that it varies widely. Thus the importance of performing a detailed meiotic behavior analysis for each particular translocation in order to obtain enough information to give adequate genetic counseling is stressed. Aspects as to the possible overestimation of 3:1 segregations or the presence of interchromosomal effects still need to be elucidated.

Endothelial cell chimerism has nothing to do with the induction of clinical tolerance in liver transplant patients after withdrawal of immunosuppression.

Despite the existence of formal models to explain how chromosomal rearrangements can be fixed in a population in the presence of gene flow, few empirical data are available regarding the mechanisms by which genome shuffling contributes to speciation, especially in mammals. In order to shed light on this intriguing evolutionary process, here we present a detailed empirical study that shows how Robertsonian (Rb) fusions alter the chromosomal distribution of recombination events during the formation of the germline in a Rb system of the western house mouse (Mus musculus domesticus). Our results indicate that both the total number of meiotic crossovers and the chromosomal distribution of recombination events are reduced in mice with Rb fusions and that this can be related to alterations in epigenetic signatures for heterochromatinization. Furthermore, we detected novel house mouse Prdm9 allelic variants in the Rb system. Remarkably, mean recombination rates were positively correlated with a decrease in the number of ZnF domains in the Prdm9 gene. The suggestion that recombination can be modulated by both chromosomal reorganizations and genetic determinants that control the formation of double-stranded breaks during meiosis opens new avenues for understanding the role of recombination in chromosomal speciation.

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By comparing these two different translocation carriers with different fertility outcomes, we discuss the possible mechanisms by which translocations might cause the spermatogenesis process to fail.

Noncentromeric heterochromatic regions are the last to synapse. The inter-individual variation observed in the incidence of gaps and splits in these regions may be explained by the heteromorphism of these regions in the general population. The presence of synaptic anomalies in other SC regions may indicate nuclei with a severely affected synapsis. Noncentromeric heterochromatic regions might play a role in the association of autosomal SC15 and SC21 with the XY pair.