Meiosis in Mesostoma ehrenbergii ehrenbergii. IV. Recombination nodules in spermatocytes and a test of the correspondence of late recombination nodules and chiasmata.

Abstract

Male meiosis in Mesostoma ehrenbergii ehrenbergii (2x = 10) is characterized by extreme restriction of chiasma formation; 3 pairs of chromosomes form bivalents at metaphase I which are associated by single very distally localized chiasma, while two pairs of chromosomes remain as unpaired univalents. Electron microscopical three-dimensional reconstruction analysis of serial sections has been applied to 20 pachytene spermatocyte nuclei. In each nucleus three short stretches of synaptonemal complex (SC) were found, confined to a localized branched lobe of the nucleus, confirming the findings of an earlier study. The majority of reconstructed nuclei show that each of the three SC segments has a single prominent recombination nodule ("late" RN) associated with it. Late RNs in this system therefore show an excellent correspondence with metaphase I chiasmata, in contrast to a previous report. M.e. ehrenbergii is therefore not an exception to the hypothesis that meiotic exchange requires a functional late RN. A few nuclei had two, one or no RNs; these presumably represent nuclei that are not at the stage of maximum RN presence. Although M. e. ehrenbergii shows pronounced chiasma localization at the light microscope level, at the ultrastructural level RNs are widely distributed along the 5-10 microns of SC formed in each bivalent, indicating that genetic exchange are not restricted to particular localized sites but occur at a large number of DNA sequence.

Meiosis: Dances Between Homologs

“…Finally, the SC might have evolved in two stages, first to promote events specifically at sites of crossovers and later to ensure postinterference pairing. The first stage would explain the widespread nucleation of SC formation at crossover sites (see Section 6), the persistence of the SCs at crossover sites throughout the diplotene stage when most SCs have been disassembled (see Section 7), and the fact that some organisms only install SCs locally at crossover/chiasma sites, without extension along the lengths of the chromosomes (e.g., 69,350).…”