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Materials and methods

 Wheat cv. Chinese Spring (CS) (2n=AABBDD) x A. junccum (j) (syn. Thinopyrum bessarabicum, 2n=14, JuJu) F1 hybrids and their BC1 to CS were described earlier (Sharma and Gill 1983b). Both wheat and A. junccum parents had normal chromosome pairing, mean chromosome pairing being 0.28 I+20.86 II, and 7 II, respectively. As expected, the hybrids had 2n = 28. The level of chromosome pairing in the ABDJu F1 hybrids provided no evidence of homologous or homoeologous pairing: I=26.23, rod II=0.83, ring II=0.04, III=0.01. Among the nine BC1 plants studied, six had 49, one had 48 and two had 46 chromosomes. Chromosome pairing in a 49-chromosome BCl plant (plant no. CS x j x CS-7) was 5-8 I (mean=6.83), 1-4 rod II (mean=2.50), 16-20 ring II (mean=18.33) and 0-1 III (mean=O. 17). Self (BClF2) seeds from this BCl plant were followed in the present study. Chromosome counts were made from root-tips of germinated seeds. For chromosome pairing studies, spikes were fixed in 1:3 acetoalcohol and squashed in 1% acetocarmine.


Results and discussion

Of the 92 BC1F2 seeds set on selfing the BC1 plant CS x j x CS-7, 2n = 49, the chromosome number of 14 seeds was studied. One had 45, four had 46, three had 47, two had 48, two had variable (4448), one had 49, and one had 52 chromosomes. Twelve BC1F3 seeds from the 49-chromosome BC1F2 plant were analyzed. Of these, nine had 2n=49 and three had 2n=49+t (t= telocentric, class)fication of broken chromosomes into telo or fragment is arbitrary). Twenty-eight BC1F4 seeds from one of these BC1F3plants (plant # CS x j x CS-7self- 2self-l, 2n=49) analyzed had a chromosome composition: 1 = 46+2t, 4= 47+t, 3 = 47+2t, 1 = 48, 5 = 48+t, 5 = 48+2t, 3 = 49+t, 2 = 49+2t, 1 = 50, 2 = 50+t and 1 = 51. When ten BC1F5 seeds of the bulk harvest from the three BC1F5 plants with 49+t chromosomes were scored, five had 48, two had 48+t and the other three had 49+t chromosomes. Similarly, when three BC1F5 seeds of the bulk harvest from the two BC1F4 plants having 2n = 49+2t were scored, all had 49+2t chromosomes.

From these results, it is evident that the chromosome number in the wheat x A. junccum derivatives was maintained throughout several generations of selfing, and that the Agropyron chromosomes could not be eliminated. The occurrence of plants with 49 chromosomes up to several generations of selfing shows that the 7 chromosomes of A. junccum have a selective advantage in perpetuation through the gametes from generation to generation. Furthermore, there is evidence of chromosome breakage. Genes on gametocidal chromosomes have been found to cause breakage of chromosomes (Feldman and Strauss 1983, Tsujimoto and Noda 1988). Meiotic behavior observed in 4 cells of a 49-chromosome BC1F3 plant had an average of 6.501 (range=5-7), 2.75 rod II (range=2-3) and 18.50 ring II (range= 18-19) which remained about the same as in BC1. It appears, therefore, that the pair construction of A. junccum chromosomes has not taken place in spite of repeated selfing. It is, therefore, likely that these chromosomes are being transmitted through only female gametes. The study provides an example where the whole genome from a diploid alien species is retained due to the preferential transmission like individual gametocidal chromosomes. Retention of all A. junccum chromosomes solely due to gametocidal effect dictates that these chromosomes must have different non-interacting gametocidal genes. In addition, fertility of the female gametes must be extremely low due to the very low frequency of gametes carrying all the Ju genome chromosomes considering random segregation of univalents. The actual data on fertility were not recorded but casual observations indicated that it was not extremely low. Thus, the phenomenon of directed movement of the seven univalents to one of the cells that forms embryo sac could not be ruled out.

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