BGN 10: Linkage data with genes near the centromere of barley chromosome 6 BARLEY GENETICS NEWSLETTER, VOL. 10, II. RESEARCH NOTES
Falk et al., pp. 13-16

II. 6. Linkage data with genes near the centromere of barley chromosome 6.

D. E. Falk, M. J. Swartz and K. J. Kasha. Crop Science Dept., University of Guelph, Guelph, Ontario, Canada, N1G 2W1. "R"

A number of genes appear to be closely linked around the centromere region of chromosome 6. While such genes may be closely linked, they could be physically quite far apart as has been demonstrated in corn (Kasha, 1979) and other species. Thus, recombination values tend to vary considerably from population to population and with environments. The objective of this study is to attempt to determine the gene order of genes near the centromere with aid of multiple-point linkage tests and flanking markers.

Only three of the genes used here were not described in our early report (Kasha et al., 1978). They are: fll, chlorina seedlings, BGS No. 0260 described in BGN 9:133, 1979; alb,,t, albino seedlings reported by Clark and Ramage (BGN 6:8, 1976) to be associated with chromosome 6; and nec, necrotic leaf spot, described by Fischbeck and Hauser (BGN 6:28-29, 1976) as mutant 1339/62.

In Table 1 we have presented the linkage data (F2 and/or F3) from 12 different crosses among various marker gene stocks. in some cases, separate populations involving the same stocks have been combined. In some instances, not all the F3 progenies were grown and when no plants fell into one or more F2 phenotypes, the F2 recombination values were not calculated.

Table 1 "Best-fit" recombination values from F2 and F3 data and proposed gene order from 12 crosses among marker genes in chromosome 6 (page 1 of 2).

Table 1 "Best-fit" recombination values from F2 and F3 data and proposed gene order from 12 crosses among marker genes in chromosome 6 (page 2 of 2).

The proposed gene order arising from each cross is proposed in the last column of Table 1. Gene orders were established on both multiple point linkage data and on the basis of F2 recombinants as identified by F progeny tests. In other words, taking the order that would be based upon slngle rather than multiple crossovers. The fact that these genes are near the centromere and there may be no crossover interference across the centromere introduces some caution when involking the criteria of multiple crossovers. The gene order of msg6 - o - sex 1 is based on this hypothesis and conflicts with the 3 point data.

Based on the data in Table 1, possible gene orders have been illustrated in Fig. 1. These are the most probable orders and not the only alternatives possible. Starting with the assumption made earlier (Kasha et al., BGN 8) that msg,,bk is located in the short arm of chromosome 6 and that o is very close to the centromere, the order of the genes in Fig. 1A appears quite reliable. The major problem is the orientation of msg6 and sex 1 in relation to o and the other genes. The order that is most probable is given in Fig. 1B but Fig. 1C is also quite feasible. The orientation of sex 1 will also influence the arm on which alb,,q and alb,,t are placed. Whether alb,,q is distal or proximal to msg,,bk in Fig. 1C is also questionable and larger populations are required for linkage estimates with both albino genes. While the F3 progeny for studies with nec will not be planted until 1980, the F2 data was included as the gene order appears quite certain.

Figure 1. Relative location of genes on chromosome 6. A. Gene order quite certain. B and C. Two of the possible alternative orders when additional genes added.

We have decided to wait another year before trying to combine all data to establish distances between the genes.

Reference:

Kash, K. J. 1979. Viable deficiency-duplications of the centromere regions of Zea mays chromosomes. Maydica 24:23-31.

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