Two well-known mutants of chiasma maintenance function in maize are allelic

By a series of traditional crosses, allelism has been tested for two maize recessive mutants of independent origin, dy1 and dsy1, both called desynaptic. These mutants both display loss of chiasmate association during diakinesis (late prophase I) but at differing frequencies. This chiasma loss happens before nucleolar loss and nuclear membrane system breakdown. That crossovers have occurred to establish the chiasmata in the first place has been documented by diakinesis-stage separation of heterozygous heterochromatic regions in univalents formed by bivalent-association breakdown. In the present work, the two mutants have been found to be allelic by the outcome of traditional crosses that produced variant plants which were heterozygous for the two alleles. These plants express a unique phenotype at diakinesis, but are essentially normal at pachytene, metaphase I, anaphase I, and later stages of meiosis.Key words: chiasma, crossover, complementation.

CYTOLOGICAL DIPLOIDIZATION IN THE CULTIVATED OCTOPLOID STRAWBERRY FRAGARIA × ANANASSA

Chromosome pairing during meiosis was studied in 9 cultivated genotypes and 32 seedlings produced by selfing several different genotypes of the octoploid strawberry Fragaria × ananassa Duch. (2n = 8x = 56). In all genotypes studied the chromosomes paired exclusively as bivalents indicating cytological diploidization. Meiotic studies of one of the S1 plants, which was completely sterile, revealed that the chromosomes were paired at pachytene but desynapsed during diplotene, so that very little bivalent association was observed in late prophase of meiosis. The pairing in the F. × ananassa × F. nubicola pentaploid hybrids showed an average of 11.6 bivalents per PMC and frequent multivalent formation. There is thus an indication of residual homology between the ancestral genomes of the octoploid strawberry.