II.10. Fragment trisomics derived from BTT 2-7d ms2.
P. T. Gymer. Rothwell Plant Breeders, Rothwell, Lincoln, United Kingdom. (By kind permission of Rothwell Plant Breeders)
At Rothwell we have three cytologically distinct versions of the balanced tertiary trisomic 2-7d ms2. Type 1 contains the complete extra chromosome 2-7d, as found in the world collection of BTT's. Types 2 and 3 have the extra chromosome fragmented in a way which does not affect the balanced behaviour. The additional chromosome in type 2 is a telocentric whereas in type 3 it is submetacentric (Figure 1). In meiosis, types 1 and 3 can form chains of five chromosomes, while type 2 has no configurations larger than trivalents. Assuming that pairing initiates terminally (Kasha and Burnham, 1965), this means that types 1 and 3 both contain the two terminal segments, one each from chromosomes 2 and 7. Type 2 contains only one of these segments, clearly that from chromosome 7, since the satellite is present. The deletions which have caused the fragmentation are therefore terminal in type 2, and interstitial in type 3. Since Ms2 is closely linked to the 2-7d break point, that region must be present in both types. Therefore, the Ms2 locus and the 2-7d break point must be in the satellited arm of the 2-7d chromosome, since type 2 does not possess the other arm. It is evident that the deletion of type 2 is of the whole of the long arm (or chromosome 2 origin); in type 3, both arms are shorter than in type 1, so that there must be an interstitial deletion on both arms, i.e. a chromosome 2 segment and a chromosome 7 segment. The constitution may, therefore, be assumed to be approximately as in Figure 2.
Figure 1. Three types of 2-7d chromosome, with normal chromosome 7 for comparison, showing estimated relative sizes.
Figure 2. Probable constitution of types 1, 2 and 3.
These are not the first balanced fragment trisomics to be found; others, derived also from BTT 2-7d ms2 were described by Lehmann (1972).
When the male-sterile diploids were pollinated by the fragment trisomics, some trisomic progeny were found, indicating male transmission at about 2%. This is similar to the findings of Lehmann (l.c.). Male transmission was also demonstrated by the discovery of tetrasomics amongst types 2 and 3; they are more vigorous in type 2, whose fragment is smaller. None found hitherto has however been fertile.
Kasha, K. J. and C. R. Burnham. 1965. The location of interchange breakpoints in barley. II. Chromosome pairing and the intercross method. Canad. J. Genet. Cytol. 7:620-632.
Lehmann, L. C. 1972. Male transmission of the extra chromosome of balanced tertiary trisomics of barley (Hordeum vulgare L.). Ph.D. thesis, University of Arizona.
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