Recombination map of the common shrew, Sorex araneus (Eulipotyphla, Mammalia)

Transmission ratio distortion results in asymmetric introgression in Louisiana Iris

BACKGROUND

Linkage maps are useful tools for examining both the genetic architecture of quantitative traits and the evolution of reproductive incompatibilities. We describe the generation of two genetic maps using reciprocal interspecific backcross 1 (BC1) mapping populations from crosses between Iris brevicaulis and Iris fulva. These maps were constructed using expressed sequence tag (EST)- derived codominant microsatellite markers. Such a codominant marker system allowed for the ability to link the two reciprocal maps, and compare patterns of transmission ratio distortion observed between the two.

RESULTS

Linkage mapping resulted in markers that coalesced into 21 linkage groups for each of the reciprocal backcross maps, presumably corresponding to the 21 haploid chromosomes of I. brevicaulis and I. fulva. The composite map was 1190.0-cM long, spanned 81% of the I. brevicaulis and I. fulva genomes, and had a mean density of 4.5 cM per locus. Transmission ratio distortion (TRD) was observed in 138 (48.5%) loci distributed in 19 of the 21 LGs in BCIB, BCIF, or both BC1 mapping populations. Of the distorted markers identified, I. fulva alleles were detected at consistently higher-than-expected frequencies in both mapping populations.

CONCLUSIONS

The observation that I. fulva alleles are overrepresented in both mapping populations suggests that I. fulva alleles are favored to introgress into I. brevicaulis genetic backgrounds, while I. brevicaulis alleles would tend to be prevented from introgressing into I. fulva. These data are consistent with the previously observed patterns of introgression in natural hybrid zones, where I. fulva alleles have been consistently shown to introgress across species boundaries.

Two types of meiotic crossovers coexist in maize

We apply modeling approaches to investigate the distribution of late recombination nodules in maize (Zea mays). Such nodules indicate crossover positions along the synaptonemal complex. High-quality nodule data were analyzed using two different interference models: the "statistical" gamma model and the "mechanical" beam film model. For each chromosome, we exclude at a 98% significance level the hypothesis that a single pathway underlies the formation of all crossovers, pointing to the coexistence of two types of crossing-over in maize, as was previously demonstrated in other organisms. We estimate the proportion of crossovers coming from the noninterfering pathway to range from 6 to 23% depending on the chromosome, with a cell average of approximately 15%. The mean number of noninterfering crossovers per chromosome is significantly correlated with the length of the synaptonemal complex. We also quantify the intensity of interference. Finally, we develop inference tools that allow one to tackle, without much loss of power, complex crossover interference models such as the beam film. The lack of a likelihood function in such models had prevented their use for parameter estimation. This advance will allow more realistic mechanisms of crossover formation to be modeled in the future.

Broad-scale recombination patterns underlying proper disjunction in humans

Although recombination is essential to the successful completion of human meiosis, it remains unclear how tightly the process is regulated and over what scale. To assess the nature and stringency of constraints on human recombination, we examined crossover patterns in transmissions to viable, non-trisomic offspring, using dense genotyping data collected in a large set of pedigrees. Our analysis supports a requirement for one chiasma per chromosome rather than per arm to ensure proper disjunction, with additional chiasmata occurring in proportion to physical length. The requirement is not absolute, however, as chromosome 21 seems to be frequently transmitted properly in the absence of a chiasma in females, a finding that raises the possibility of a back-up mechanism aiding in its correct segregation. We also found a set of double crossovers in surprisingly close proximity, as expected from a second pathway that is not subject to crossover interference. These findings point to multiple mechanisms that shape the distribution of crossovers, influencing proper disjunction in humans.

In humans, as in most sexually reproducing organisms, recombination plays a fundamental role in meiosis, helping to align chromosomes and to ensure their proper segregation. Recombination events are tightly regulated both in terms of their minimum number (the rule of “crossover assurance”) and placement (due to “crossover interference”). Accumulating evidence, however, suggests that recombination patterns are highly variable among humans, raising numerous questions about the nature and stringency of crossover assurance and interference. We took a first step towards answering these questions by examining patterns of recombination in gametes inherited by viable, non-trisomic offspring. We found that the minimum number of crossovers is tightly regulated at the level of a chromosome (rather than chromosome arm), but with a notable exception: in females, chromosome 21 appears to frequently segregate properly in the absence of a crossover. We also found a set of double recombination events in surprisingly close proximity, consistent with a pathway not subject to crossover interference. These findings suggest that there are multiple mechanisms of recombination in human meiosis, which may buffer the effects of inter-individual variation in rates.

Is there variation in crossover interference levels among chromosomes from human males?

We demonstrate that recent data from human males are consistent with constant interference levels among chromosomes under the two-pathway model, whereas inappropriately fitting shape parameters of Gamma distributions to immunofluorescent interfoci distances observed on finite chromosomes generates false interpretations of higher levels of interference on shorter chromosomes. We provide appropriate statistical methodology.

General pattern of meiotic recombination in male dogs estimated by MLH1 and RAD51 immunolocalization

The aim of this study was to estimate a general pattern of meiotic recombination in the domestic dog (Canis familiaris) using immunolocalization of MLH1, a mismatch repair protein of mature recombination nodules. We prepared synaptonemal complex (SC) spreads from 124 spermatocytes of three male dogs and mapped 4959 MLH1 foci along 4712 autosomes. The mean number of MLH1 foci for all autosomes was 40.0 foci per cell. Total recombination length of the male dog autosomal genome map was estimated as 2000 cM. A global pattern of MLH1 foci distribution along the autosomal bivalents was rather similar to that found in the mammals studied: a high frequency near the telomeres and a low frequency near the centromeres. An obligate MLH1 focus in the X-Y pairing region was usually located very close to Xp-Yq telomeres. The distances between MLH1 foci at autosomal bivalents were consistent with crossover interference. A comparison of the interference estimates coming from the distribution of MLH1 interfocus distances and RAD51/MLH1 focus ratio indicated a substantial variation between species in the strength of interference.