EVOLUTION IN THE GENUS AVENA: INHERITANCE OF DIFFERENT FORMS OF RIBULOSE DIPHOSPHATE CARBOXYLASE

Disc electrophoresis of ribulose diphosphate carboxylases from Avena species in polyacrylamide gels of varying concentrations reveals the presence of two distinct forms of the enzyme. One migrates faster than the other and is found exclusively in species possessing the A genome. The other is confined to the C genome species (A. pilosa, A. ventricosa, A. clauda). The association of another characteristic of this enzyme (presence of stromacentres in the chloroplasts) with the A genome was reported previously. Observations on the recently described species A. prostrata, A. canariensis, A. damascena, and A. murphyi show that they all possess stromacentres, confirming reports that they all contain the A genome. Examination of amphiploid hybrids (from crosses between various diploid species and A. sativa) has shown that the mobility character is inherited maternally, and is located on the chloroplast genome. All these hybrids have the A genome and all have stromacentres. The results are discussed in the light of recent findings on the structure and synthesis of this enzyme. Consideration of the species distribution of the different forms of carboxylase places certain restrictions on the possible evolutionary paths from diploids to tetraploids and hexaploids.

GENOME RELATIONSHIPS IN TETRAPLOID AVENA

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.