Two-Dimensional Spreads of Synaptonemal Complexes from Solanaceous Plants. II. Synapsis in Lycopersicon esculentum (Tomato)

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Synapsis in tomato (Lycopersicon esculentum, 2n = 24) pollen mother cells (PMCs) was examined using a hypotonic bursting technique along with squashing and partial three-dimensional reconstructions from serial thin sections. Tomato has all the meiotic stages expected from classical descriptions of meiosis. Based on observed synchrony of PMCs within anthers and between anthers of a bud, leptotene through pachytene was estimated to last 24 hr. The hypotonic bursting technique disperses most protoplasm including chromatin to reveal components of synaptonemal complexes (SCs) in two dimensions. Leptotene chromosomes have single axial cores that sometimes show presynaptic alignment and associate with polycomplexes. Zygotene chromosomes contain segments of SCs and axial cores that may be associated with polycomplexes. Axial cores often show presynaptic alignment in zygotene. Synapsis tends to be initiated toward the ends of chromosomes, but interstitial initiation occurs as well. Typically synapsis occurs last in pericentromeric heterochromatin. Recombination nodules (RNs) are numerous on zygotene SCs, indicating the process of recombination probably starts at this time. Zygotene RNs can occur on either face of the central element of SCs. In pachytene, SCs are formed throughout the length of bivalents. Telomeres are visible at synapsed ends, and kinetochores develop as prominent structures by late pachytene. In early pachytene there appears to be a precipitous loss of RNs that typically leaves only one or two RNs per SC. These RNs may lie at the sites of reciprocal recombination events. In the diffuse stage, desynapsis begins with separation of lateral elements distally and/or near kinetochores. Polycomplexes often form at these early sites of desynapsis. Desynapsis appears to be progressive but is temporarily inhibited at telomeres, RNs, and kinetochores. By the end of the diffuse stage, lateral elements have separated completely and have begun to fragment. At the beginning of diplotene, components of SCs are no longer visible.