Meiosis in haploid rye: extensive synapsis and low chiasma frequency

Abstract

Extensive synaptonemal complex formation was found at prophase I in whole mount spread preparations of a spontaneous haploid rye, Secale cereale, with values of up to 87.8 per cent of the chromosome complement synapsed. Pairing-partner switches were frequent, giving rise to multiple associations in which all or most of the chromosomes were involved. However, the distribution of synaptonemal complex stretches suggests that synapsis does not occur at random. The frequency of multivalents and the mean frequency of bonded arms at metaphase I were 0.03 and 0.39, respectively. Associations between chromosome arms without heterochromatin were more frequent than between the remaining arms. The observation of recombinant chromosomes for telomeric C-bands at anaphase I indicates that metaphase I bonds are true chiasmata. The correspondence between the location of pairing initiation sites and chiasmata indicates that early synapsis could be confined to homologous regions.

Increased ploidy and KAR3 and SIR3 disruption alter the dynamics of meiotic chromosomes and telomeres

“…These observations agree with genetic data indicating that bouquets form in haploid yeast (Rockmill and Roeder, 1998). Because a chromosomal bouquet has been observed in haploid rye (Santos et al, 1994), it appears that the signal for telomere clustering in synaptic meiosis does not depend on the presence of homologous chromosomes.…”

“…It is intriguing that foldback pairing is observed in haploid meiosis of plants and yeast (Levan, 1942;Loidl et al, 1991;Santos et al, 1994). Heterosynapsis is thought to occur late in first meiotic prophase, when homology is no longer required (Moses et al, 1984;von Wettstein et al, 1984).…”

“…In addition to its occurrence in synaptic and asynaptic organisms, meiosis can also be induced in auto-and allopolyploids, and even in the absence of homologues, as in haploid plants, in which heterosynapsis (synapsis between nonhomologous chromosomes) is observed (Gillies, 1974;Levan, 1942;Santos et al, 1994). Similarly, haploid yeast strains that express both mating-type loci can be induced to undergo…”

The relationship between synapsis and recombination: two different views

“…In addition, similar mutants to hop2 have been also described in rye (Fedotova et al, 1994). However, the nonavailability of allelic homology in haploid meiosis promotes ectopic reciprocal recombination both in yeast (about 6% of events per meiosis; Loidl & Nairz, 1997) and plants (about 0.38 chiasmata per meiocyte in rye; Santos et al, 1994). The differences between both materials probably being due to the rareness of repeated sequences in yeast compared with higher plants.…”

Nuclear Organization and Chromosome Segregation

“…The axial elements of the SC do not appear to contain the information for distinguishing homology because they will synapse randomly in a haploid organism undergoing meiosis (Loidl et al, 1991;Santos et al, 1994). Thus, the general consensus is that the homology search is DNA based, so that pairing occurs despite the presence of chromosome inversions, translocations, and alien chromosomes.…”