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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.

Cytogenetic observations, phylogenetic studies and genome analysis using high-density genetic markers have suggested a tetraploid Avena species carrying the C and D genomes (formerly C and A) to be the donor of all hexaploid oats (AACCDD). However, controversy surrounds which of the three extant CCDD tetraploid species— A . insularis , A . magna and A . murphyi —is most closely related to hexaploid oats. The present work describes a comparative karyotype analysis of these three CCDD tetraploid species and two hexaploid species, A . sativa and A . byzantina . This involved the use of FISH with six simple sequence repeats (SSRs) with the motifs CT, AAC, AAG, ACG, ATC and ACT, two repeated ribosomal sequences, and C genome-specific repetitive DNA. The hybridization pattern of A . insularis with oligonucleotide (AC) 10 was also determined and compared with those previously published for A . sativa and A . byzantina . Significant differences in the 5S sites and SSR hybridization patterns of A . murphyi compared to the other CCDD species rule out its being directly involved in the origin of the hexaploids. In contrast, the repetitive and SSR hybridization patterns shown by the D genome chromosomes, and by most of the C genome chromosomes of A . magna and A . insularis , can be equated with the corresponding chromosomes of the hexaploids. Several chromosome hybridization signals seen for A . insularis , but not for A . magna , were shared with the hexaploid oats species, especially with A . byzantina . These diagnostic signals add weight to the idea that the extant A . insularis , or a direct ancestor of it, is the most closely related progenitor of hexaploid oats. The similarity of the chromosome hybridization patterns of the hexaploids and CCDD tetraploids was taken as being indicative of homology. A common chromosome nomenclature for CCDD species based on that of the hexaploid species is proposed.

New sources of resistance to crown rust, Puccinia coronata f. sp. avenae (Eriks.), the major fungal disease of cultivated oat, Avena sativa L. (2n = 6x = 42), are constantly needed due to frequent, rapid shifts in the virulence pattern of the pathogen. Crown rust resistance identified in the diploid oat A. strigosa (Schreb.) (2n = 2x = 14) accession CI6954SP was transferred into cultivated oat using two methods: direct cross of the diploid to a hexaploid cultivar facilitated by embryo rescue, and initial cross of the diploid to a wild tetraploid oat to make a synthetic hexaploid for subsequent crossing to a hexaploid cultivar. Two tetraploids, a crown rust resistant A. murphyi (Ladiz.) accession P12 and a susceptible A. insularis (Ladiz.) accession INS-1, were used in the 2x·4x crosses. Resistant backcross-derived lines were recovered by both methods. Although the 2x·4x synthetic method did not require the laborious discovery and rescue of an infrequent initial hybrid embryo of the direct cross, the direct cross method provided more rapid backcross recovery of plants with high fertility, full transmission of resistance, and desired plant and seed phenotypes. A suppressor effect, present initially but segregating in backcrosses, appeared to come from the CI6954SP donor and is the same as, or analogous to, suppression by an oat line with the crown rust resistance gene Pc38. No resistance from A. murphyi P12 was detected in advanced generations when it was introduced either as a component of a synthetic hexaploid or in direct crosses to A. sativa, indicating suppression of its resistance in interploidy combinations. That the dominant resistance gene transferred from CI6954SP and a gene Pc94 introgressed earlier from a different A. strigosa accession may be the same or quite similar to one another is indicated by their in-common specificity to suppression of resistance expression, susceptibility to a newly recovered rust isolate, and close linkage to the molecular marker SCAR94-2. The introgressed resistance genes from the different sources, even if the same, may have different cultivar genomic introgression sites, which would allow tests of dosage effects on resistance expression.[PUBLICATION ABSTRACT]

The Sicilian wild tetraploid oat Avena insularis is a close relative of A. magna and A. murphyi and is more plausible than either of these two species as the immediate tetraploid ancestor of A. sterilis and the other hexaploid oats. This report presents the C-banding karyotype of A. insularis. As in A. magna and A. murphyi, the chromosomal complement can be divided into two groups, one predominantly heterochromatic and the other predominantly euchromatic. The heterochromatic chromosome group of A. insularis consists of three pairs of metacentric and four pairs of submetacentric chromosomes. The euchromatic chromosome group consists of one pair of metacentric, five pairs of submetacentric, and one pair of subtelocentric chromosomes.

Abstract The tetraploid Avena species in the section Pachycarpa Baum, including A. insularis, A. maroccana, and A. murphyi, are thought to be involved in the evolution of hexaploid oats; however, their genome designations are still being debated. Repetitive DNA sequences play an important role in genome structuring and evolution, so understanding the chromosomal organization and distribution of these sequences in Avena species could provide valuable information concerning genome evolution in this genus. In this study, the chromosomal organizations and distributions of six repetitive DNA sequences (including three SSR motifs (TTC, AAC, CAG), one 5S rRNA gene fragment, and two oat A and C genome specific repeats) were investigated using non-denaturing fluorescence in situ hybridization (ND-FISH) in the three tetraploid species mentioned above and in two hexaploid oat species. Preferential distribution of the SSRs in centromeric regions was seen in the A and D genomes, whereas few signals were detected in the C genomes. Some intergenomic translocations were observed in the tetraploids; such translocations were also detected between the C and D genomes in the hexaploids. These results provide robust evidence for the presence of the D genome in all three tetraploids, strongly suggesting that the genomic constitution of these species is DC and not AC, as had been thought previously.

A new species of oat, Avena insularis, is described. It was collected in southern Sicily where four populations were found on uncultivated clay soil. Morphologically, it is similar to the hexaploid wild oat A. sterilis, but can be distinguished by its smaller and more condensed panicle, less V-shaped dispersal unit and oblong disarticulation scar. Hybrids between A. insularis and the hexaploid cultivated oat A. sativa were obtained only when the latter was the seed parent in crosses. Chromosome pairing of the hybrids at meiosis was irregular with univalents and multivalents, but the mean number of chiasmata per cell was close to that of A. insularis. Furthermore, the hybrids were partially self-fertile. Thus, the newly discovered species seems closer to the hexaploid oats than any other tetraploid species, and is probably the tetraploid progenitor of hexaploid oats. Hybrids between A. insularis and A. magna were sterile because of irregular chromosome pairing at meiosis.

The wild tetraploid oat Avena insularis, which was recently discovered in Sicily has now been detected in two regions in Tunisia. It grows there in a similar habitat as in Sicily and is confined to uncultivated patches. Hybrids between Tunisian populations from Temime and Bargou were partly sterile due to a chromosomal rearrangement and probably other genetic factors. Hybrids between representatives of these populations and Sicilian populations from Gela and Mt. Bubonia were also partial sterile. This sterility suggests that the Sicilian populations of A. insularis have not originated directly from the Tunisian populations.

The newly discovered tetraploid oat Avena insularis was crossed with the diploid A. strigosa and the tetraploid A. murphyi. Considerably reduced chromosome association at meiosis and a low average number of chiasmata per cell of the A. strigosa x A. insularis hybrids indicated that the diploid A. strigosa did not participate in the creation of A. insularis. From A. murphyi, A. insularis differed by four chromosomal rearrangements and the hybrids between them were sterile. The tetraploids A. magna, A. murphyi and A. insularis share the two to four floret large diaspore, which is adapted to heavy alluvial soil. They all, however, diverge from one another by four chromosomal rearrangements. At this point it is not possible to determine whether they have diverged from a single tetraploid progenitor, or developed from different diploid species.