A Reassessment of the Course of Evolution of Wheat

Abstract

Chromosome pairing in hybrids involving Triticum aestivum and new accessions of T. speltoides, and in an amphiploid of these species, indicates that T. speltoides can no longer be considered to be the donor of the B genome of the polyploid wheats. This necessitates a reconsideration of the genome relationships and evolutionary processes that gave rise to cultivated wheats.The evolutionary processes and the species involved in the origin of wheat (Triticum aestivum, L.; 2n = 6x = 42) have been the subject of intensive study for several decades. It is now well accepted that a representative of the diploid wheats contributed the A genome that is found in both the tetraploid and hexaploid species (1). The D genome, found only in the hexaploid and not in the tetraploid species, was donated by T. tauschii (Aegilops squarrosa) (2-4).The donation of the B genome to the tetraploid wheats, from which it was contributed to the hexaploid forms, has been variously ascribed to Agropyron triticeum (3), T. bicorne (Ae. bicornis) (5), and T. speltoides (Ae. speltoides) (6-8).In recent years T. speltoides has come to be widely accepted as the source of the B-genome. A conclusive test of this hypothesis through a demonstration of chromosome homology or nonhomology has unfortunately not been possible, because T. speltoides suppresses the regulatory activity of wheat chromosome 5B and thereby permits pairing not only of homologues but also of homoeologues (related chromosomes).The general acceptance of T. speltoides as the B-genome donor was based on four kinds of evidence: Morphological evidence adduced by Sarkar and Stebbins (7) and karyotypic, synaptic, and geographical evidence gathered by Riley, Unrau, and Chapman (8). Sears (9) has indicated that some of the evidence may not be as conclusive as was earlier assumed. For one thing, a synthetic amphiploid of T. speltoides x T. monococcum does not resemble tetraploid wheat very closely (5). Also, the karyotypic evidence supporting T. speltoides (8) is based on the close similarity of two pairs of large-satellited chromosomes in T. speltoides with two in the polyploid wheats. However, the difference between T. speltoides and other related and excluded forms is only the absence of a very small piece of chromatin in the distal region of the satellite on one of the pairs of chromosomes. Waines and Kimber (unpublished) have established that there is variation in the satellite condition found in T. monococcum, and thus, it is possible that similar variation may exist in relatives of T. speltoides.The synaptic evidence has also been questioned (9). Kimber (10) crossed T. speltoides x T. iongissimum (Ae. sharonensis) and backcrossed twice to T. speltoides, selecting against the special ability of T. speltoides to cause homoeologous pairing. He then crossed to T. aestivum and found that in the low-pairing segregates, only 2.9 bivalents per cell were formed. As Sears (9) points out, this extent of pairing is lower than would be expected if T. speltoides were the donor of the B geno...

Nuclear DNA content in diploid wheat and its relatives in relation to the phylogeny of tetraploid wheat.

“…On the other hand, Kimber and Athwal (1972) considered polyphyletic origin of B genome, i.e. the present B genome being a recombinant genome between two putative B genomes.…”

Discovery and Mapping of Wheat Ph1 Suppressors

“…Ae. speltoides is naturally variable for the ability to elicit homeologous chromosome pairing in hybrids with wheat (Dvorak 1972;Kimber and Athwal 1972). A virtual continuum of chromosomepairing levels, ranging from a few to $16 chiasmata/ metaphase I (MI) cell, has been observed in various T. aestivum 3 Ae.…”

Variation in repeated nucleotide sequences sheds light on the phylogeny of the wheat B and G genomes.

“…(10). However, the chromosomes of T. speltoides pair poorly with the chromosomes of the B genome (11)(12)(13), and it was concluded that T. speltoides could not be a source of the B genome (12). Since the chromosomes of the G genome of T. timopheevii pair, albeit poorly, with the chromosomes of the B genome, the B and G genomes must be very closely related (14,15).…”