II.11. Successive fragmentation in balanced tertiary trisomics.
P. T. Gymer, Rothwell Research Station, Rothwell, Lincoln, United Kingdom (by kind permission of Rothwell Plant Breeders Limited).
Two fragment versions of BTT 2-7d msg2 were reported last year (Gymer, 1973, BGN 3, 16-17). Two further fragmentations are reported here, which are derived from the two former fragments. Their karyotype is illustrated in Figure 1.
In type 4 the extra chromosome has a short arm identical to that of type 3, and is therefore believed to be derived from it. The exact pedigree is not known for certain. The long arm is however shorter, making the chromosome metacentric. In meiosis, chains of five are still produced, indicating an interstitial loss. Type 3 was already believed to have two interstitial deletions; this further loss makes the extra chromosome much smaller than chromosome 5, and thus easily recognized. One tetrasomic has been found, indicating male transmission.
Type 5 is known to be descended from type 2, and is cytologically indistinguishable from it. It was first observed in a family heterozygous for msg2 produced by crossing a BTT with a fertile cultivar; segregation for male-sterility occurred in trisomics as well as diploids. In two further generations, segregation in a number of heterozygous families has been observed; Table 1 gives the number in each of the four possible classes, data from all the heterozygous families being summed. A chi-squared test gave agreement with a 3:1 ratio for diploids and for trisomics.
Table 1. Segregation for msg2 in type 5.
Apparently then the Msg2 locus is absent from the extra chromosome
of type 5. Since no cytological reduction in length could be seen, there
must have been only a very small loss from the chromosome 2 end of the
fragment; this is the centromere end, and since the fragment is telocentric,
the Msg2 locus is assumed to be on the centromere, or else on a
tiny invisible remaining section of the long arm. Such a position has been
confirmed by telotrisomic analysis (Tsuchiya and Singh, 1973, BGN 3, 75-78).
This means that Msg2 is on the opposite side of the centromere to
the T2-7d break-point. Crossovers are however very rare in stocks of BTT
2-7d msg2. Fragments of type 5 are impossible to use in hybrid barley
production.
Both of the new fragments reported here represent deletions which have
occurred in chromosomes which were already fragmented. Such successive
fragmentation must be watched by any workers producing hybrid barley by
the BTT method.
