II.16 Additional information on the position of the centromere on barley chromosome 5.
Jens Jensen, Agricultural Research Department, Danish Atomic Energy Commission Research Establishment Risø, DK-4000 Roskilde, Denmark. "R"
The gene for necrotic leaf spots, nec1a, is allelic with the Parkland spot gene sp, ,b (Jensen, 1974), which is on the long arm of chromosome 5 (Fedak et al., 1972; Tsuchiya, 1972).
The data from a cross of Telo 5L with the double mutant nec1a,wst5e (necrotic leaf spots, white streaks) are given in Table 1. One white-streaked (wst5e,wst5e,wst5e) plant was found among the 25 telotrisomics. This plant must be the result of double reduction. The wst locus must, therefore, be on the telocentric chromosome and at some distance from the centromere. Further, the 16% white-streaked plants found among the disomics are significantly fewer than the 25% that is expected if there is no crossing-over between wst5 and the centromere. With an increasing crossing-over frequency, a decreasing frequency of white-streaked disomic plants is expected. This supports the fact that wst5 is at some distance from the centromere on the telocentric chromosome. Necrotic spotted plants were not found among the telotrisomic plants. This shows that nec1 is on the telocentric chromosome, and it indicates that nec1 is closer to the centromere than wst5. Among the disomic plants, 28% were necrotic-spotted. This indicates that nec1 is close to the centromere. Further evidence indicating that nec1 is on the telocentric chromosome comes from an additional 194 F2 plants that were not chromosome counted, among which only 13% necrotic-spotted plants were found. The overall percentage is 16; this is close to the percentage of 14 reported by Fedak et al. (1972).
Table 1. The F2 segregation from the cross Telo 5L x nec1a,wst5e
The distance on the linkage map (Jensen, 1976) from wst5 to nec1 is 48 cM or 37% recombination. The fs2, which is most likely to be on the telocentric chromosome (Tsuchiya, 1972) is 9 cM or 9°% recombination from nec1, but in the direction opossite to wst5. The fs2 locus is thus probably closer to the centromere than nec1. The loci cer-e and ert-b, which are located between nec1 and fs2 on the map, are both situated near the centromere according to studies with translocation (Fester and Søgaard, 1969; Persson, 1969). The information in the present study about positions of wst5, nec1 and the centromere is thus in agreement with information from other sources.
The available data suggest that the centromere is very close to the fs2 locus that is on the long chromosome arm. The locus nearest to fs2 in the direction of the short chromosome arm is Ml-p, which - like the other genes known in that chromosome region - is a dominant disease-resistant gene. A further localization of the centromere will be difficult because dominant genes close to the centromere will show a deviating segregation in the progeny of a telotrisomic, heterozygous stock only if the dominant gene is carried by the telocentric chromosome. Stocks of this constitution are difficult to obtain.
References:
Fedak. G., T. Tsuchiya and S.B. Helgason, 1972. Use of monotelotrisomics for linkage mapping in barley. Can. J. Genet. Cytol. 14: 949-957.
Fester, T. and B. Søgaard, 1969. The localization of eceriferum loci in barley. Hereditas 61: 327-337.
Jensen, J., 1974. Coordinator's report: Chromosome 5. BGN 4: 102-106.
Jensen, J., 1976. Coordinator's report: Chromosome 5. BGN 6: 98-100.
Persson, G., 1969. An attempt to find suitable genetic markers for dense ear loci in barley. I. Hereditas 62: 25-96.
Tsuchiya, T., 1972. Revision of linkage map of chromosome 5 in barley by means of telotrisomic analysis. J. Heredity 63: 373-375.
