GENOME RELATIONSHIPS IN TETRAPLOID AVENA

Abstract

Relationships between the A and B genomes of the tetraploid Avena species (A. magna excluded) and between these and the AS genome of the diploid A. hirtula and strigosa were studied in triploid (ASAB) and tetraploid (ASASAB) hybrids. Chromosome affinities were estimated on the basis of the karyotypes and the pairing behaviour of the chromosomes.The main findings were:1. The AS karyotype of the 2x and the A of the 4x species were identical and the chromosomes of the genomes A and B were similar. No difference was noticeable in the karyotypes of the 4x species studied. A nucleolar chromosome appeared from the B set in the 3x hybrids. It was concluded that the B genome is probably a modified form of A.2. All the chromosomes were capable of pairing in the ASAB and ASASAB hybrids. High degree of homology between the AS and A genomes was indicated by the high frequency of ring bivalents and ring-rod trivalents. The synaptic success of the B genome chromosomes (up to six trivalents) was interpreted as homoeology between the A and B genome chromosomes. The long chains (up to XII) suggested heterozygosity for numerous large segments.3. Attempting to resolve the conflict between homoeology and bivalent pairing in the 4x species, a diploidizing gene system, effective only in the homozygous condition, was assumed.4. The effect of multiple differences for large segments in the evolution of alloploids was briefly discussed.