Abstract
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.
Highlights
IntroductionThe hexaploids arose from an original hybridization between a tetraploid species with the genomes A and C, and a diploid species with a D genome
Hexaploid Avena species (2n = 6x = 42), including cultivated A. sativa and A. byzantina, have three genomes of seven chromosome pairs each
Cytogenetic evidence supports A. insularis being closely related to hexaploid oats karyotype analyses involving conventional staining [3], C-banding [4,5,6,7,8,9,10,11,12,13] and fluorescent in situ hybridization (FISH) with repetitive DNA probes [14,15,16,17,18,19,20]
Summary
The hexaploids arose from an original hybridization between a tetraploid species with the genomes A and C, and a diploid species with a D genome. The detection of multivalents during meiosis in interspecific hybrids indicates that the extant Avena species differ in their chromosome structure, mainly a consequence of translocations and inversions that occurred during their evolutionary history. FISH with genome-specific repetitive sequences has identified chromosomes involved in translocations [14, 15, 26] in both tetraploid and hexaploid species. As a result of these profuse rearrangements, none of the extant diploid species has been unequivocally identified as the donor of the A, C and D genome
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