ISH-FACILITATED ANALYSIS OF MEIOTIC BIVALENT PAIRING

Chiasmata constitute one of the cornerstones of sexual reproduction in must eukaryotes. They mediate the reciprocal genetic exchange between homologues and are essential to the proper orientation of the homolog ous centromeres in meiosis I. As markers of recombination, they offer a cytological means of mapping. Rather than trying to accurately count individual chiasmata, we have examined properties of the mathematical relationship between frequencies of nonadorned disomic configurations in meiosis (ring, rods, and univalents) and the probabilities at whic h arms of the respective chromosomes are chiasmate (one or more chiasm a per arm). Numerical analyses indicated that conventionally analyzed bivalents with nonidentified arms yield statistically biased estimates of chiasma probabilities under a broad range of circumstances. We sub sequently analyzed estimators derived from adorned configurations with ISH-marked arms, which were found to be statistically far superior, a nd with no assumptions concerning interference across the centromere. We applied this methodology in the study of chromosomes 16 and 23 of c otton (Gossypium hirsutum). and estimated their arm lengths in centimo rgans. The results for chromosome 23, the only one of the two chromoso mes with a documented RFLP map, were consistent with the literature. S imilar molecular-meiotic configuration analyses can be used for a wide variety of eukaryotic organisms and purposes: for example, providing far more powerful meiotic comparisons of genomes of chromosomes, and a rapid means of evaluating effects on recombination.