The present results from the karyomorphological studies are consistent with our previous results on non-banded chromosomes of several landrace genotypes (Belay et al. 1994). This indicates that, despite the overwhelming morphological diversity of the Ethiopian tetraploid wheat landraces, chromosome arm-ratios are highly conserved. Except for chromosome 7B (see below), the other minor discrepancies could be attributed to differential chromosome contraction and measurement error.
Generally, a considerable degree of C-band polymorphism occurs in these landraces. For the uniqueness of chromosome 7B in B-3-11, it may be difficult to conceive anything other than a structural divergence, which might also explain why its hybrids with the other landraces are highly sterile (Belay and Merker 1997). However, we have discounted the deletion of interstitial bands on 7BS, because otherwise, a higher arm ratio would have been expected.
Meiotic studies in F1 hybrids of K-1-1 and B-3-33 did not indicate the presence of structural differences (Belay and Merker 1997), but the parental morphotypes showed marked C-band variation for chromosomes 5A, 6A, 7A, 3B and 5B. Furthermore, the banding patterns of 2AS (K1-1, B-3-33 and DZ-04-118), 7AL (B-3-33), and the variations observed for 5BL are difficult to account for by structural rearrangements within the AB genome alone. For instance, deletion is less likely to explain the occurrence of only one interstitial band on 5BL (CD-7, DZ-04-118 and A-4-34) since it was observed in T. dicoccoides (Shang et al. 1988). Therefore, two conclusions emerge from the above analysis; (1) it is possible that some of the C-band variations resulted from cytologically (meiotically) undetectable rearrangements. (2) It may be that these variant forms have been maintained by chance in these primitive wheats that have been cultivated in isolation for millennia. In the latter case, "multiple lineage" (see Soltis and Soltis 1995) of the Ethiopian tetraploid wheat landraces may be assumed, which is in line with the results from analysis of Restriction Fragment Length Polymorphisms (Mori et al. 1997).
Because tetraploid wheats in Ethiopia have been grown in mixture with hexaploid wheat for centuries, we have also considered the possibility that D-genome introgression might have contributed to the observed C-band variation. In order to test this possibility, we have PCR-tested for the presence of D-genome chromatin in B-3-11, K-1-1 and DZ-04-118 using primers developed from the published sequence of the Dgas44, a repetitive element which is highly represented in, and is widely dispersed throughout the D-genome (McNeil et al. 1994). These genotypes were chosen because their 2AS was rather similar to 2DS (Friebe and Gill 1994). All three samples gave a negative result, making it very unlikely that these lines carry any A/D or B/ D translocation. Given the non-sterility of the pentaploid hybrids and the possibilities of homoeologous pairing involving D genome chromosomes (Perera et al. 1983), the existence of such translocations in the landrace populations cannot be ruled out, and the PCR assay is seen as an effective and efficient mass screening method to identify rare carriers among the Ethiopian tetraploid wheats (Koebner R: pers commun). The nature of the translocation in positive selections could further be characterized by means of in situ hybridization with D genome-specific probes, or by GISH with total DNA of the D-genome donor (Mukai et al. 1993).
In these landraces, estimating translocation frequencies at the population level (e.g. Joppa et al. 1995), and determining the nature of breakpoints (centromeric vs. noncentromeric) would be of great evolutionary and breeding interest. For example, in T. araraticum, most translocations were of centric-break-fusion types (Badaeva, Badaev et al, 1994), but not those from T. dicoccoides (Nishikawa et al. 1986; Taketa and Kawahara 1996). Given that centromeric positions were altered in a few cases (Belay et al. 1994), it is plausible that most interchanges occurred outside of the centromeric regions that are devoid of C-bands. Alternatively, if their origin was through the centric-break-fusion mechanism, the interchanges were between chromosome arms of similar sizes, which is particularly possible for some of the A genome chromosomes. These two hypotheses may be tested by systematically screening a larger number of genotypes. However, C-banding needs to be complemented by additional information from crossing analysis and other advanced molecular, cytogenetic techniques.
Acknowledgments
We gratefully acknowledge that the PCR analysis was carried out by Dr. RWD Koebner, JI Center, Norwich UK. We thank RWDK for his invaluable comments and discussion, and Dr. B Friebe for his useful comments. This work was carried out when GB was a doctoral student at the Department of Plant Biology, Swedish University of Agricultural Sciences, Uppsala, Sweden. The Swedish Agency for Research Co-operation with Developing Countries (SIDA-SAREC) financed the study through the Ethiopian Tetraploid Wheat Project.