Homologous chromosome pairing and recombination during meiosis in wild type and synaptic mutants of tomato = Homologe chomosoomparing en recombinatie tijdens de meiose van wild type en synaptische mutanten van de tomaat

F.W.J. Havekes

Research output: Thesisinternal PhD, WU


<p>This thesis presents the results of a cytogenetic investigation of meiotic prophase I stages in tomato (Lycopersicon esculentum). The aim of this study was to analyse the relationships between chromosome pairing, and formation of recombination nudules and chiasmata. Tomato was chosen as study material for the following reasons: 1) the species features an extensive genetic map, a pachytene chromosome map and a recombination map based on the distribution of recombination nodules along late pachytene synaptonemal complexes. These three maps together allow a direct comparison between genetic data on the one hand and meiotic chromosome organisation and behaviour on the other; 2) both light and electron microscope observations of spread pachytene nuclei depict detailed chromosomal substructures including centromeres/kinetochores and heterochromatic blocks in the centromere regions, which enables identification of individual chromosomes; 3) a collection of four meiotic mutants, which are disturbed at different phases of chromosome pairing and crossing over (formation of RNs and chiasmata), and a haploid are the basis of this study on the relationship of chromosome pairing and crossing over.</p><p>Chapters 2 and 4 deal with various ultrastructural aspects of homologous chromosome pairing in wild type and mutant tomato. In wild type tomato, the first unmistakable ultrastructural manifestation of homologous chromosome associations was the alignment of homologous axial cores (ACs) at a distance of approximately 300 nm. Shortly later, the aligned ACs converged at several sites, often to centrally positioned recombination nodules (RNs). This phase of pairing initiation is known as presynaptic alignment. Subsequently, homologous ACs approached each other to within 100 nm distance were then incorporated into the tripartite structure of the synaptonemal complex (SC), which was then assembled along each pair of homologous chromosomes.</p><p>During the processes of pairing initiation and SC formation (letotene - late zygotene) numerous RNs appeared at and between the lateral elements of the SC. Most of these RNs disappeared during early pachytene, so that only one or a few RNs per SC persisted in late pachytene. Light and electron microscopic preparations of spread pollen mother cells from the meiotic mutants displayed the following characteristics (Chapter 2): mutant as6 was completely asynaptic, virtually without any presynaptic alignment or SC. Mutants asb and as1 were partially asynaptic, with average values of 6.1% and 25 % synapsis, respectively. The desynaptic mutant as5 had normal, wild type levels of synapsis and about the same number of late pachytene RNs as wildtype, but showed strongly reduced chiasma frequencies at metaphase I. This suggests that this mutant is disturbed in crossing-over and/or the formation of functional chiasmata.</p><p>The three asynaptic mutants asb, as6 and as1 showed a good correlation between the number of bivalents connected by at least one tripartite SC segment at pachytene and the number of chiasmate bivalents at metaphase I (Chapter 2). In mutant asb, morphologically normal late RNs were only observed on SCs, and not on presynaptically aligned segments (Chapter 5). This indicates that in tomato functional late RNs and chiasmata are only formed in the context of the tripartite SC.</p><p>In mutant asb, long chromosomes participated more frequently in synapsis, and displayed a higher percentage of synapsis than short chromosomes. Presynaptic alignment, however, occurred equally frequently in all chromosome pairs, and the length of synapsed segments was proportional to chomosome length (Chapter 4). This indicates that synaptic initiation, and probably crossing-over, are under constraints that act differentially on chromosomes of different length.</p><p>Chapter 3 gives a comparative light microscopic analysis of male and female meiosis in wild type tomato and in the meiotic mutants as6, asb and as5. Only mutant asb showed significant differences between female and male meiosis. We found 2.6 fold more chiasmate bonds between chromosome arms in embryosac mother cells (EMCs) than in pollen mother cells (PMCs). In wild type tomato, the percentage of chromosome arms there was bound by a chiasma was only 7% higher in EMCs than in PMCs. We ascribe this sex effect on chiasmate bonds to the comparatively long duration of female prophase I, particularly of the substage when chromosomes synapse, rather than to higher cross-over frequencies in bivalents of EMCs.</p><p>We also analysed how meiosis proceeds in haploid tomato, where chromosomes do not have homologous counterparts (Chapter 6). In the absence of a homologue, synapsis still occurred, though at the low average frequency of 3.3 SC segments per cell. Presynaptic alignment, though, was absent. Furthermore, the SCs in haploid tomato showed various aberrations. The ACs were locally split at several sites, and we found spherical cloud-like structures along the ACs, which we interpreted as morphologically abnormal early RNs. In the haploid, we also found thirteenfold more SC-segments in prophase I than chiasmata in metaphase I. Apparently, SC-formation in the haploid is not necessarily accompanied by the formation of a chiasma.</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Heyting, C., Promotor
  • de Jong, J.H., Promotor, External person
Award date8 Jan 1999
Place of PublicationS.l.
Print ISBNs9789054859550
Publication statusPublished - 1999


  • cytogenetics
  • meiosis
  • solanum lycopersicum
  • tomatoes

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