Methods are developed that allow a quantitative determination of relative affinity in triploid hybrids. This results in new insights into the analysis of meiosis in hybrids, genomic relationships and the assignment of genome symbols. The separate recording of open and closed chromosome figures in meiotic analyses is shown to be essential.

The somatic chromosomes of common wheat, Tri-ticum aestivum L. (2n = 6x = 42), and those of two of its diploid progenitors and T. speltoides, have been individually identified by a Giemsa staining technique. In wheat, telocentric chromosomes were used to aid the recognition of individual chromosomes, and an ideogram has been constructed depicting the C-band positions. There is no similarity in the C-banding of chromosomes within a homoeologous group, with the possible exception of group 5. Comparisons of the C-banding of the diploid species T. monococcum, T. speltoides, and T. tauschii with that of the A, B, and D genomes, respectively, in hexaploid wheat corroborate that T. speltoides could not be the donor of the B genome to wheat and that T. monococcum and T. tauschii are the probable donors of the A and D genomes, respectively.The Giemsa staining technique for the detection of constitutive heterochromatin in cereal chromosomes has been described by Gill and Kimber (1). The C-bands in cereal chromosomes are usually present in the centromeric area and, additionally, may-or may not be present in interstitial or terminal regions or both. There is a characteristic C-banding pattern for the individual chromosomes in somatic metaphases and, thus, also for the species. These features enabled the identification of all of the somatic metaphase chromosomes of rye and also allowed the recognition of rye chromosomes in wheat-rye addition lines (2). Other species for which C-banded somatic karyotypes have been described are Scilla sibirica (3), AUium flavum (4), and some species of Anemone (5). Quinacrine mustard and other compounds can be used to produce differential fluorescence (Q-banding), and Q-banded somatic karyotypes have also been described in a few plant species (3,4,6,7).A unique situation exists in wheat (Triticum aestivum L. The technique for C-banding is that previously described by Gill and Kimber (1, 2). The dehydration of the slides was usually limited to 1 hr, and the roots were not left in the glacial acetic acid for more than 1 day. Solutions were generally less than 2 weeks old.Photomicrographs, all at the same magnification, were made of somatic cells containing the telocentric chromosomes. The telocentrics were cut out of the prints and mounted to produce a karyotype arranged according to the homoeologous classification (Fig. 1). Since the prefixation treatment used leads to different degrees of contraction from root to root and even from cell to cell, the amount of contraction was not uniform.-Thus, the arm ratios seen in Fig. 1 are not necessarily those of the complete chromosomes. However, the relative positions of the bands within a chromosome arm are assumed to be -constant.An ideogram of the wheat karyotype was produced by drawing the bands at their correct positions within an arm on a chart in which the correct arm ratio and relative size is represented to scale (Fig. 3). The relative sizes and arm ratios used are those determined by Sears (19) for telophase II of cultivar Chinese Sp...

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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...

The chromosomes of rye have been individually identified by their distinctive heterochromatin pattern with Giemsa staining and classified on the basis of their homoeology with wheat chromosomes. The constitutive heterochromatin detected by C-banding has been shown tQ be identical with the classical heterochromatin as seen in the pachytene of meiosis in rye.Recently developed staining techniques that result in differential banding of somatic metaphase chromosomes permit the identification of individual chromosomes and have considerably enhanced cytogenetic studies in mammals (1). With these methods all of the chromosomes have been identified in man, mouse, and many other animal genera; further, in mouse almost all the linkage groups have been correlated with specific chromosomes and chromosome arms (2-8). Unfortunately, the application of these techniques to plant chromosomes has not been particularly successful, although a few reports have appeared (9-14). One of the differential staining techniques, Giemsa C-banding (C = constitutive heterochromatin), which was first applied to animal chromosomes (15-17), involves denaturation-reassociation of DNA, with the highly repetitive DNA reassociating faster and appearing as dark bands. Attempts have been made to identify individual rye chromosomes with conventional staining methods, but the interpretation of the results is difficult (18)(19)(20). In this communication, we report a Giemsa staining procedure in rye that can be routinely used and by which the individual chromosomes can be easily identified. MATERIALS AND METHODSActively growing root tips of rye (Secale cereale L. var. Imperial) prefixed in monobromonaphthalene for 1-3 hr were then fixed in glacial acetic acid. The root tips were softened for 1-2 hr in a 5% solution of pectinase (EC 3.2.1.15; polygalacturonase) and cellulase (EC 3.2.1.4) to which 2-3 drops of 1 N HCl had been added for each 5 ml of the enzyme solution. Softening by this enzyme solution results in banding, whereas the customary hot hydrolysis of roots with 1 N HCl does not. After application of a cover slip, the cells were separated from each other by tapping the slip. The cover slip was separated from the slide by CO2 freezing, and the slide was immersed in absolute alcohol for two to three hours and then dried by air blowing.Denaturation-Renaturation. Dry slides were immersed in a freshly prepared, saturated solution of barium hydroxide for 5 min. After they were washed in three changes of distilled water for a total duration of 10 min, the slides were air dried, incubated in 0.30 M NaCl-0.030 M Na citrate (2) at 600 for 1 hr, washed thoroughly in distilled water, and again air dried. 1247The preparations were stained in Giemsa solution at pH 7 (2) for 1-2 min, washed quickly in water, air dried, stored in xylol over night, and mounted in Canada balsam. RESULTS