NEW EVIDENCE ON THE ORIGIN OF THE HEXAPLOID OATS

Oat evolution revealed in the maternal lineages of 25 Avena species

Cultivated hexaploid oat has three different sets of nuclear genomes (A, C, D), but its evolutionary history remains elusive. A multiplexed shotgun sequencing procedure was explored to acquire maternal phylogenetic signals from chloroplast and mitochondria genomes of 25 Avena species. Phylogenetic analyses of the acquired organelle SNP data revealed a new maternal pathway towards hexaploids of oat genome evolution involving three diploid species (A. ventricosa, A. canariensis and A. longiglumis) and two tetraploid species (A. insularis and A. agadiriana). Cultivated hexaploid A. sativa acquired its maternal genome from an AC genome tetraploid close to A. insularis. Both AC genome A. insularis and AB genome A. agadiriana obtained a maternal genome from an ancient A, not C, genome diploid close to A. longiglumis. Divergence dating showed the major divergences of C genome species 19.9–21.2 million years ago (Mya), of the oldest A genome A. canariensis 13–15 Mya, and of the clade with hexaploids 8.5–9.5 Mya. These findings not only advance our knowledge on oat genome evolution, but also have implications for oat germplasm conservation and utilization in breeding.

Discrimination of the closely related A and D genomes of the hexaploid oat Avena sativa L

A satellite DNA sequence, As120a, specific to the A-genome chromosomes in the hexaploid oat, Avena sativa L., was isolated by subcloning a fragment with internal tandem repeats from a plasmid, pAs120, that had been obtained from an Avena strigosa (As genome) genomic library. Southern and in situ hybridization showed that sequences with homology to sequences within pAs120 were dispersed throughout the genome of diploid (A and C genomes), tetraploid (AC genomes), and hexaploid (ACD genomes) Avena species. In contrast, sequences homologous to As120a were found in two A-genome species (A. strigosa and Avena longiglumis) and in the hexaploid A. sativa whereas this sequence was little amplified in the tetraploid Avena murphyi and was absent in the remaining A- and C-genome diploid species. In situ hybridization of pAs120a to hexaploid oat species revealed the distribution of elements of the As120a repeated family over both arms of 14 of 42 chromosomes of this species. By using double in situ hybridization with pAs120a and a C genome-specific probe, three sets of 14 chromosomes were revealed corresponding to the A, C, and D genomes of the hexaploid species. Simultaneous in situ hybridizations with pAs120a and ribosomal probes were used to assign the SAT chromosomes of hexaploid species to their correct genomes. This work reports a sequence able to distinguish between the closely related A and D genomes of hexaploid oats. This sequence offers new opportunities to analyze the relationships of Avena species and to explore the possible evolution of various polyploid oat species.

Intergenomic translocations and the genomic composition of Avena maroccana Gdgr. revealed by FISH

Fluorescence in situ hybridization (FISH) using total genomic DNA from putative diploid progenitors was used to confirm the presence of the A and C genomes in Avena maroccana. These results confirm cytological data that intergenomic translocations are present in A. maroccana.

Nuclear and cytoplasmic genome relationships in the genus Avena: analysis by isoelectric focusing of ribulose biphosphate carboxylase subunits

Comparisons of the isoelectric points of small and large subunits of ribulose biphosphate carboxylase extracted from a number of diploid, tetraploid, and hexaploid Avena species have been used to obtain information on the nuclear and cytoplasmic genome relationships within the genus. All species tested had small subunits with similar isoelectric points, so their analysis provided no information of taxonomic value. Three types of large subunits could be distinguished by this method, and the distribution of each among the available species provides strong evidence against the involvement of a C genome diploid (such as A. ventricosa) as the maternal parent in the formation of either tetraploid or hexaploid species. One type of large subunit was confined to the perennial tetraploid, A. macrostachya, and its position in the genus and possible origin are discussed. The value of this approach in studying genome relationships within the genus Avena and related genera is assessed.

INTROGRESSION BETWEEN THE CULTIVATED HEXAPLOID OAT A. SATIVA AND THE TETRAPLOID WILD A. MAGNA AND A. MURPHYI

Introgression between the hexaploid (2n = 42) oat A. sativa and the newly discovered tetraploid (2n = 28) species A. magna and A. murphyi was studied by the rate of stabilization of chromosome number, restoration of fertility of pentaploid hybrid derivatives and the ultimate gene transfer between the tetraploid and the hexaploid levels. The complete self-sterility of the pentaploid F1 hybrids was overcome by massive back-pollination to the parental species. Great variation in chromosome number (12-48) was found among the viable F1 female gametes. Meiotically stable and reasonably fertile derivatives were selected only at the F2 of the BC and in a relatively small proportion. Gene transfer between the tetraploid and the hexaploid species was demonstrated by introducing the allele for nonshattering seed from the cultivated oat A. sativa to both A. magna and A. murphyi, and lemma hairiness from the tetraploids to the hexaploid level. The possible exploitation of introgression between the polyploid oats for breeding purposes has been pointed out and the potential of A. magna and A. murphyi as cultivated oats has been briefly discussed.

SEED PROTEIN HOMOLOGIES AND THE EVOLUTION OF POLYPLOIDY IN AVENA

Crude seed protein of the nine oat species was fractionated by disc electrophoresis on polyacrylamide gel. Correlation coefficients calculated from optical density curves obtained from the stained gels showed that individual species possessed characteristic, albeit variable, profiles. Pattern variability among and within species was greater in the case of the diploids than in the case of the polyploids. The very few profile types in A. strigosa 4x and in the hexaploid A. sativa suggested that the variation in these polyploids is due more to independent origin of the types than to differentiation following polyploidization. Virtual identity between individual A. strigosa 2x and 4x profiles suggested a strict autopolyploid origin for some 4x types while complementing pairs of A. strigosa 2x profiles indicated an intervarietal origin for other A. strigosa 4x types. A diagnostic band at 11.0 cm in the profiles of A. magna and A. murphyi suggested that these species rather than A. strigosa 4x had functioned as the tetraploid parent of the hexaploid A. sativa. The profile of A. murphyi complemented by a specific A. strigosa 2x profile simulated a specific A. sativa type. The adaptive success of the genus is assessed in the light of variation and homologies in the seed protein patterns of the various species.

A THIN-LAYER CHROMATOGRAPHIC STUDY OF SOME AMPHIPLOIDS IN AVENA

A thin-layer chromatographic study was carried out using five synthetic Avena amphiploids and their parental genotypes. The Dass-Nybom solvent and an extraction time of 120 hr were found to be optimum for Avena. About 42% of 3 total of 62 spots were common to all, or all but one species, and these were considered characteristic for the genus. The over-all additiveness of the parental spectra was low (32.5%) in the experimental alloploids; therefore, comparisons of total spectra of natural alloploids and their putative progenitors may not be useful in Avena. A number of hybrid spots, due to complementary gene interactions, was detected. Selected spots specific for either the A- or C-genome diploids could be traced in the polyploids. These suggested the presence of the C genome in the new tetraploid species A. magna, and in the hexaploids, and confirmed A. magna as the putative tetraploid ancestor of the hexaploid species. The chromatographic phenotype, due to its large number of well defined traits, may be very useful in studying gene interactions in polyploid Avena and for marking linkage groups in chromosome deficient lines.

CHROMOSOME RELATIONSHIPS BETWEEN TETRAPLOID (2n=28) AVENA MURPHYI AND SOME DIPLOID, TETRAPLOID AND HEXAPLOID SPECIES OF OATS

On combined evidence of cytology and morphology it is apparent that the newly discovered tetraploid Avena murphyi shares a common genome with the tetraploid A. magna. Lack of crossability and limited pairing in triploid hybrids involving A. murphyi indicate that none of the presently known diploid oats participated in the formation of A. murphyi. Meiotic behavior of A. murphyi × A. sativa pentaploid hybrids shows considerable homology between the chromosomes of the parents. The origin of the hexaploid oats is discussed in the light of the cytogenetic status of A. murphyi.

A PROTEIN ELECTROPHORETIC STUDY OF THREE AMPHIPLOIDS AND EIGHT SPECIES IN AVENA

Seed proteins of three synthetic amphiploids, one autotetraploid and eight species that include representatives of all ploidy levels and karyotypes of Avena were studied by acrylamide gel electrophoresis. Complete or nearly complete additiveness in banding pattern of the parental protein was obtained in the amphiploids and mechanical mixtures. The pattern of autotetraploid A. pilosa was more similar to the amphiploid derived from its diploid and A. sativa than to its diploid form. This was interpreted as a dosage effect indicating the presence of the same genes in both parental species. No distinct zonation of protein bands was apparent in the basic gels, but in the acidic gels a gap separated the fastest moving bands from the slower bands. About 50% of the bands were common to all species studied, indicating extensive residual homology. The A and C genome diploids were readily separated by band differences, the A genome protein patterns were essentially identical and the C genomes differed primarily in band number. The tetraploid protein patterns differed but the hexaploids had high homology and intraspecific polymorphism was detected in the hexaploid cultivars. Almost complete homology occurred between the A genome spectrum and the tetraploid and hexaploid patterns, as well as between the C genome spectrum and the tetraploid A. magna and the hexaploid protein patterns. Protein homology in Avena indicated close genomic relationships.