Abstract
At metaphase I during spermatogenesis in Mesostoma ehrenbergii ehrenbergii (2 × = 10), 3 bivalents and 4 univalents form. The bivalents each have a single distally localised chiasma. Electron microscope serial section reconstructions have shown that 3 short pieces of synaptonemal complex (SC) are present in pachytene nuclei in the testis. All the SC is contained in a lobe which is separated by an invagination from the main body of the nucleus. The SC pieces vary in length from approximately 2 μm to 5 μm, and the greatest amount found in one nucleus was approximately 13 μm. It is proposed that the pieces of SC correspond to the paired regions of the 3 bivalents present at metaphase I. This system is therefore an example of localised pairing leading to localised chiasmata. Later prophase stage nuclei have also been reconstructed, in which stacks of polycomplex are present but no SC has been found. The polycomplex is thought to be an association of discarded SC sub-units.
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Oakley, H.A., Jones, G.H. Meiosis in Mesostoma ehrenbergii ehrenbergii (Turbellaria, Rhabdocoela). Chromosoma 85, 311–322 (1982). https://doi.org/10.1007/BF00330355
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DOI: https://doi.org/10.1007/BF00330355