Electron microscopic observations on the meiotic karyotype of diploid and tetraploid Saccharomyces cerevisiae.

Abstract

Certain strains of Saccharomyces cerevisiae contain visible segments of synaptonemal complex which are apparent components of bivslents in pachytene of meiotic prophase. The synaptonemal complex has the typical width in the frontal plane but is unusually thin in the sagittal plane, thus accounting for its poor visibility. Amorphous densities situated adjacent to the central element occur at intervals suggesting their coincidence with sites of crossing over. Reconstruction of the synaptonemal complex from serial sections has permitted karyotypic analysis. The number of segments of synaptonemal complex and the distribution of their lengths is consistent with the genetic map. Two, possibly three, segments enter the nucleolus as if bearing sequences encoding ribosomal RNA. Reconstruction of tetraploid nuclei reveals an approximate doubling of the diploid chromosome number and confirms the pattern of nucleolar entry. Quadrivalent pairing is evident between the pairs of synaptonemal complex segments in the tetraploid nuclei. Although the budding yeast, Saccharomyces cerevisiae, has proven highly amenable to genetic analysis, cytogenetic analysis has been limited by the absence of a karyotype visible by light microscopy. The greater resolution of electron microscopy has revealed several features of spindle behavior during nuclear division but has failed to reveal the individual chromosomes. Nevertheless, genetic studies of the spores derived from meiosis indicate that yeast chromosomes probably undergo synaptic behavior similar to that of other eukaryotes. Meiotic nuclei have occasionally been found to contain the synaptonemal complex (SC) both in aggregates and as individual segments (1, 2). Recent observations by Zickler and Olson (3) demonstrate the typical phases of meiosis in yeast, confirming that the individual segments of SC represent components of pachytene bivalents as they do in other species. We have now achieved serial section electron microscopy of meiotic yeast adequate for reconstructing the distribution of these segments of the synaptonemal complex. On this basis, we report here the numbers and lengths of meiotic chromosomes in diploid and tetraploid strains of S. cerevisiae. MATERIALS AND METHODSStrains. The original diploid strain used for these observations is strain 212-1, constructed by Simchen (4). It is homozygous for the temperature-sensitive cell division cycle allele cdc 4-3 (5). This strain sporulates well, forming over 75% four-spored asci with viable spores after transfer from acetate growth medium to sporulation medium under permissive conditions, as described by Simchen (4). The tetraploid derivative, 212-1-T, was obtained by crossing clones that had been induced by UV irradiation to become capable of mating (by conversion of the mating type to aa or aa); the latter were generously supplied by Ms. Hannah Klein. The Abbreviation: SC, synaptonemal complex. cells of 212-1-T were significantly larger than 212-1 and sporulated equally well at the permissive temperature (200), which w...

Chromosome pairing in autotetraploid Bombyx females. Mechanism for exclusive bivalent formation

“…These studies have shown that at pachytene, homologous chromosomes are held in register by a continuous synaptonemal complex from telomere to telomere revealing an apparently specific site-to-site matching of homologous regions. Both light and electron microscopical studies agree in that the number of irregularities is surprisingly low (2,4,6,9,13,14,16,22,33). Furthermore, it has been demonstrated that prior to the onset of pairing, homologous chromosomes are either located at random in the nucleus (1 i, 12, 22, 25) or in separate nuclei (4, 0105-1938/79/0044/0101/$ 05.00 32, 33) the only exception being organisms with a permanent somatic pairing of homologous chromosomes.…”

“…These data show that an increase in ploidy level is not accompanied by a comparable increase of the total length of the lateral components as is the case in di-and tetraploid yeast (2). In the diploid Bombyx oocytes, the lateral component length for the individual genome at early pachytene is 196 ~m, that for triploid oocytes 137 ram and or more interlockings) and in tetraploid oocytes (one interlocking in 7 early pachytene nuclei).…”

Chromosome pairing and chiasma formation in spermatocytes and oocytes of Dendrocoelum lactem (Turbellaria, Tricladida); a cytogenetical and ultrastructural study

“…The RNs of D. lacteum are small spherical structures, not exceeding 70 nm in diameter. They therefore occupy the lower end of the observed range of RN sizes and are comparable to the rather small RNs which characterise many fungal species (e.g., Byers and Goetsch, 1975;HoIm et al, 1981).…”

“…lacteun and has been encountered, for example, for the small RNs of yeast (Byers and Goetsch, 1975) ellipsoid nodules of wild-type Drosophila oocytes (Carpenter, 1981), spherical and ellipsoid RNs of some meiotic mutants of Drosophila (Carpenter, 1979b) and zygotene RNs in the fungus Coprinus (Hoim et a!., 1981). In D. lacteum the problem is compounded by the dense central element of the SC which makes it difficult to detect with certainty any but the largest and most prominent RNs.…”

Biochemical analysis of genetic recombination in eukaryotes

“…The tripartite structure of the synaptonemal complex is remarkably uniform in eukaryotes, although the ratio of synaptonemal complex length to DNA length ranges from 1: 100 in yeast (Byers and Goetsch, 1975) to l'5 : 10,000 in maize (Gilles, 1973). The lateral elements are proteinaceous rods with a diameter of 300 A-600 A which may contain RNA (Westergaard and von Wettstein, 1970;Esponda and Stockert, 1971) and histones (Strokov et al, 1973).…”