| Several new hybrids between different Triticum and Agropyron
species were established at Bozeman in 1961 (SCHULZ-SCHAEFFER 1963). None
of these had the perennial habit coupled with success in winter survival
which is typical of the SANDO Agrotricum hybrids presently under
study at Montana State University. Constant selection in the SANDO material
for winter hardiness under Montana conditions for the last 16 years has
resulted in strains superior in this respect. The difficulty of recovering recombinations of the perennial habit of A. intermedium with the large seed size of T. durum indicates close linkage of the small seed size of A. intermedium with its perennial habit. Although there exists a barrier to natural chromosome recombination in these intergeneric hybrids, it is possible by means of irradiation to induce an exchange between Triticum and Agropyron chromosome segments carrying genes for certain desirable agronomic characters. In order to obtain such segmental exchange, seeds of 3 strains of the Agrotricum material (strains AD 5a-2. AD 5a-3, and AD 7-2/3) were irradiated with X-ray dosages of 14, 15 and 16 thousand r units. Selfed seed of the X1 generation served as parental material in the backcross program described below. In 1960, the amphidiploid Agrotricum material was backcrossed with A. intermedium and with A. trichophorum. Table 2 compares the results of the first backcross with similar attempts at Washington State University (MARKARIAN 1958). Out of 40 seeds obtained in the Montana crossing shown in Table 2, 32 vegetatively propagated first substitution backcross lines (SB1)1) were established in 1963. These lines show very distinct morphological differences in the degree of rhizome formation, the width, rigidity and color of leaves, and number of seed stalks. Additionally, differences in establishment and survival of plants were observed among these strains. Plant survival ranged from 30% in lines SB1-16, SB1-23 and SB1-27 to 100% in line SB1-1. The backcross nature of this material has been verified cytologically in 22 SB1 lines. Disregarding chromosome loss in the amphidiploid parent, the expected chromosome number would be 2n=56. Theoretically, 21 Agropyron bivalents and 14 Triticum univalents are expected in meiosis. The average chromosome number in these 22 lines was 2n=49 with a range from 33 to 54. The average bivalent number was 13 (range 5-21) and the average univalent number was 23 (range 9-37). The low total number may be due to chromosome loss in the amphidiploid parent. The high average number of univalents indicates a tendency for partial asynapsis. In some instances asynapsis is considerable in this material. If for instance only 10 chromosomes pair, there would be about 75% asynapsis. One would expect that there should be no fewer than 14 univalents, however, the chromosomes of the A and B genomes of T. durum can pair allosyndetically, accounting for fewer univalents. Cytological observations of the SB1 strains have been reported (SCHULZ-SCHAEFFER and FENBERT 1969) and will be further discussed in a following paper. |
| 1) The term "substitution backcross" (SB) has been adopted from KIHARA (1951). It implies that in a series of backcrosses with the male parent to an original interspecific or intergeneric hybrid, the genomes of this male parent can be imbedded into a foreign cytoplasm, namely that of the female parent |
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