| The F1 of Chinese Spring and Ae. variabilis has 35 chromosomes
(21 from wheat and 14 from Ae. variabilis). Therefore, the expected
number in the BC1 plants derived using pollen from Chinese Spring
would be 56 (35 from the unreduced female gamete and 21 wheat chromosomes
from the pollen parent). Twenty BC1 seeds were randomly chosen
for chromosome analysis using N-banding, and a range of chromosome number
was observed between 39 and 63 (Table 1). The aim of the analysis was to use N-banding to determine whether there were changes in the chromosome numbers of wheat, or Ae. variabilis, or changes in both chromosome complements in the BC1 plants. Further, MUJEEB-KAZI (1981) has postulated that the meiosis of the F1 plants must be slightly irregular in order to give rise to the unexpected numbers. The authors explored the possibility that partially reduced eggs may function producing BC1 progeny of varied chromosome number. N-banding analysis of the Chinese Spring/Ae. variabilis F1 backcrossed to Chinese Spring demonstrated the following points: i) At least 1 representative of the N-banded wheat chromosomes was always present. ii) Some triplication of N-banded wheat chromosomes and some duplication of Ae. variabilis chromosomes were observed. iii) The loss or duplication of chromosomes appeared to be random. iv) The 56 chromosome plants did not have the expected 18 banded wheat chromosomes and 14 Ae. variabilis chromosomes, and their spike/morphology was different (Fig. 1). The presence of one representative of each of the N-banded wheat chromosomes is expected from the pollen parent, and was observed. Thus, it would appear that partially reduced F1 female gametes are indeed functional. The partially reduced gametes are assumed to result from random movement of univalents to the poles at anaphase 1 which is the normal occurrence when chromosomes do not have a homologue with which to pair. Further, the duplication of Ae. variabilis chromosomes and the triplication of wheat chromosomes are presumed to have arisen by division of the chromosome(s) involved at metaphase 1, followed by movement of both chromatids to the same pole. Fig. 2 shows the N-banded karyotype of a 39-chromosome BC1 plant of Chinese Spring/Ae. variabilis//Chinese Spring. If, as proposed above, the pollen contributes 21 chromosomes (9 banded and 12 unbanded), (the authors infer that) the functional female gamete had 18 chromosomes. Due to N-banding analysis, these 18 chromosomes can be grouped as follows: 4 of the expected 9 N-banded wheat chromosomes, 7 of the 14 Ae. variabilis chromosomes (one of which is duplicated, thus totaling 8 chromosomes) and, by inference, 6 of the 12 unbanded wheat chromosomes. The 6 unbanded wheat chromosomes may, of course, include duplicates. The results indicate that no one genome present in the F1 is being preferentially excluded from the female gamete. To the best of our knowledge, these findings offer an explanation of unexpected numbers by assuming that some partially reduced gametes, formed by random movement of univalents to poles of anaphase 1, are functional, this being enhanced by the polyploid nature of the material. |
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