Presynaptic Alignment Research Articles

SC-formation in someAllium species, and a discussion of the significance of SC-associated structures and of the mechanisms for presynaptic alignment

Earlier observations on synaptonemal complex (SC) formation inAllium are supplemented by data from diploidA. sphaerocephalon, pentaploidA. oleraceum and allotetraploidA. senescens. Accumulating information about structures like lateral element thickenings and -doublings allows to draw conclusions about their nature. The occurrence of discrete intercalary and terminal homologous associations prior to synapsis is confirmed for a range ofAllium species and it is argued that they are a general phenomenon. Several hypotheses on homologous recognition and/or attraction are discussed in the light of the observations on homologous alignment inAllium.

Synaptonemal complex spreading in Allium

A surface-spreading technique for synaptonemal complexes was applied to triploid Allium sphaerocephalon L. (Liliaceae). In early pachytene two of the three axial elements of each set of three homologues are synapsed, the third is intimately aligned with and accompanies them throughout their whole length. The unsynapsed axis is attached to the synaptonemal complex of the other 2 at up to 50 association sites per trivalent. The distribution of these sites within the trivalents is not even; they are under-represented in the proximal regions. From nought to eight switches (pairing partner exchanges), where the accompanying axis joins in synapsis in exchange for one of the two other strands, occur per trivalent. Very often the telomeres of the aligned axes are attached to their synapsed counterparts by dense spherules, which makes this type of association different from the interstitial ones. Frequently the unsynapsed axes show a double structure along short distances. In late pachytene the intercalary associations are abolished, allowing the unsynapsed axes to engage in various types of non-homologous pairing. Since the association sites involve homologous chromosomes and are less abundant in the pericentric regions (which are usually the last to synapse), it is conceivable that similar structures are responsible for the pre-synaptic alignment of homologues and provide the initiation sites for synaptonemal complex formation in diploids.

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

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.

TWO‐DIMENSIONAL SPREADS OF SYNAPTONEMAL COMPLEXES FROM SOLANACEOUS PLANTS. II. SYNAPSIS IN LYCOPERSICON ESCULENTUM (TOMATO)

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.

Chromosome pairing in allohexaploid wheat var. Chinese Spring. Transformation of multivalents into bivalents, a mechanism for exclusive bivalent formation

Meiosis in allohexaploid wheat, Triticum aestivum var. Chinese Spring has been investigated by complete three dimensional reconstructions of a late zygotene and an early pachytene nucleus. At zygotene, specific synapsis of homologous chromosomes and chromosome segments gives rise to extensive multivalent formation with shifts of pairing partners between apparently homoeologous chromosomes. These observations were confirmed by the identification of shifts of pairing partners in three serial sectioned, but only partly reconstructed zygotene nuclei. In striking contrast, the pairing at early pachytene was exclusively in bivalents. These findings, which are consistent with the pairing pattern described in tri- and tetraploid Bombyx oocytes, definitely exclude a presynaptic alignment as the primary cause for a regular bivalent formation, in wheat. Instead, multivalents form during the specific zygotene pairing but are later transformed into bivalents through dissolution and reassembly of synaptonemal, complexes. It is proposed that the regular bivalent formation in allohexaploid wheat is due to a temporal delay of crossing over until the pairing correction is completed at early pachytene. This temporal delay may be mediated by the action of the Ph gene(s) on chromosome 5B. Finally, interlockings were revealed in the completely reconstructed late zygotene nucleus but were resolved prior to early pachytene.