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Meiotic studies of the second substitution backcross to the amphidiploid hybrid, Triticum durum DESF. x Agropyron intermedium (Host) BEAUV.1)

J. SCHULZ-SCHAEFFER, J. H. KIM and S. R. CHAPMAN

Dept. of Plant and Soil Sci., Montana State Univ., Bozeman, Montana U.S.A.

Individual F7 and F8 perennial amphidiploid (AD) plants of Triticum durum DESF. (2n=28) x Agropyron intermedium (HOST) BEAUV. (2n=42) were used as parent material in this investigation. T. durum was the female parent in these crosses. The original crosses were made by WILLIAM J. SANDO during the period from 1923 to 1935 (U.S.D.A., 1958). We initiated substitution backcrossing2) to Ag. intermedium in 1960. Forty substitution backcross seeds were obtained from the first backcross attempt. Cytological studies of 25 SB1 strains have previously been reported (SCHULZ-SCHAEFFER, et al. 1971). The average number, range and variance of mean bivalents and univalents and the total number of chromosomes of 25 SB1 strains are recorded in Table 1.

Second substitution backcrosses (SB2) were made in 1965. A minimum of two plants of each of 79 vegetatively propagated SB2 strains was established in the field at Bozeman, Montana. Twenty-five SB2 strains were investigated cytologically in meiosis. Eighteen of these strains served as parents for the SB3 generation. The average number, range and variance of bivalents and univalents, and the total number of chromosomes of 25 SB2 strains are shown in Table 2. A marked decrease in the number of univalents from the SB1 (21I) to the SB2 generation (8I) was found (Table 3). This suggests a normalization of pairing of Agropyron chromosomes and a decrease in the number of Triticum chromosomes due to substitution backcrossing. Twenty-one Agropyron bivalents and 14 Triticum univalents are expected in the SB1 (SCHULZ-SCHAEFFER 1972, Fig. 1). The average numbers of bivalents (12) and of univalents (21) in the SB1 differ significantly (P=0.05) from the expected values. The standard heterogeniety chi2 method was employed to compare the goodness of fit of the SB1 strains to an expected ratio of 21II and 14I and the goodness of fit of the SB2 strains to an expected ratio of 21II and 7I. The SB1 strains were heterogeneous and the within strain fit to the expected was poor. Heterogeneity decreased in the SB2 and the within strain fit of the observed to the expected was markedly improved. This suggests that a second substitution backcross tends to stabilize meiotic behavior. The Triticum univalents which decreased from SB1 to SB2 apparently have an asynaptic effect on the normal pairing of the Agropyron homologues. This phenomenon is supported by data of PERSON (1956) who reported partial asynapsis causing excessive univalents in monosomic Triticum aestivum L. TSUCHIYA (1959,1960) ascribed the complete asynapsis of 1% of the sporocytes in single and double primary trisomics of Hordeum vulgare L. to the specific effect of the extra N0.2 chromosomes. An increase of bivalents from the SB1 (12II) to the SB2 (19II) indicates normalization of Agropyron bivalent pairing (Table 3).

The stabilizing effect of substitution backcrossing is further supported by changes in averages and variances of numbers of univalents and bivalents (Table 1 and 2). In general, SB1 strains are more variable than SB2 strains. Also, in the SB1, variances of the mean number of univalents are greater than variances of the mean number of bivalents. This pattern of greater variability in number of univalents than bivalents persists in SB2, but variances in SB2 are lower. It should be noted that numbers of observations were less in SB1 than SB2.

In at least one case, SB2-24-2, Triticum chromosomes are entirely eliminated. A typical diakinesis cell of this strain is shown in Fig.1. This strain is monosomic.

We conclude that substitution backcrossing leads rapidly to normalizing meiotic behavior, the elimination of Triticum chromosomes, and that bivalents stabilize more readily than univalents.

Literature cited

KIHARA, H. 1951. Substitution of nucleus and its effect on genome manifestation. Cytologia 16: 177-193.

PERSON, C. 1956. Some aspects of monosomic wheat breeding. Can. Jour. Bot. 34: 60-70.

SCHULZ-SCHAEFFER, J. 1972. An approach toward the development of hybrid intermediate wheatgrass, Agropyron intermedium (HOST) BEAUV. Jour. Plant Breed. 67: 202-220.

SCHULZ-SCHAEFFER, J., R. I. BAEVA and J. H. KIM 1971. Genetic control of chromosome pairing in Triticum x Agropyron derivatives. Jour. Plant Breed. 65: 53-67.

TSUCHIYA, T. 1959. A preliminary note on cytological abnormalities in barley. Seiken Ziho 10: 49-56.

TSUCHIYA, T. 1960. Cytogenetic studies of trisomics in barley. Jap. Jour. Bot. 17: 177-213. U. S. D. A. 1958. Wheat-grass hybrids combine best features of each. P. M. Release, Nov. 9 (mimeographed).

(Received May 6, 1973)

1) Contribution of the Montana Agricultural Experiment Station, Bozeman, Montana, U.S.A. Paper No. 428 Journal Series, Montana Agr. Exp. Sta., published with approval of the Director.

2) The term "substitution backcrossing" (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 the male parent can be imbedded into a foreign cytoplasm, namely that of the female parent.
       

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