II. 22 Linkage data with genes on chromosome 6.
K.G. Kasha, D.E. Falk, and A. Ho-Tsai. Crop Science Department, Univeristy of Guelph, Guelph, Ontario, Canada N1G 2W1.
We are making a number of crosses each year for the purposes of mapping chromosome 6 marker genes and locating some mutants thought to be on chromosome 6. The description, gene symbols and stocks to be reported are listed in Table 1. In Table 2 are presented the linkage data from 17 crosses with combined p values for F2 and F3, except for cross 15 which is F2 data.
Table 1. Gene symbols and sources of marker genes reported in linkage data in Table 2.
We were surprised to find mul2 and ea7 segregated independently, as both were considered to be in the short arm of chromosome 6. Neither character was easy to classify in either the F2 or F3 progenies of cross 1. However, the F2 in cross 15 were easy to classify for mul2 and the data are consistent with the report of 23% by Walker et al. (1963) for F2 linkage between o and mul2 (described then at mlt2). Our data woula place msg,,bk and mul2 on the opposite sides of o. This may help to explain why ea7 showed no linkage with mul2. Ramage and Paluska (BGN5:49-51) concluded that msg,,bk is in the short arm of chromosome 6 so the position of mul2 should be changed from the short to the long arm. Walker et al. (1963) leaves the possibility that rb could be on the long arm of 6 but independent of o, as we observed in cross 2. We were not able to confirm the loose linkage of o to cu as reported by Walker et al. (1963). The independence of f6 with gs4 and o confirms the conclusion (Kasha BGN7:89-90) that f6 should not be on chromosome 6.
A total of seven sets of 3-point linkage data were obtained and all involved msg,,bk. These are illustrated in Fig. 1. Most distances between msg,,bk and o are consistent with previous reports of 4 to 6 crossover units apart. The major problem in the 3-point data is the reversal of positions of the closely linked genes o and gs4. The reciprocal crosses, 9 and 10, gave the reverse gene orders for these genes which are no more than 4 co-units apart (Fig. lc and Fig. ld). The results from crosses 7 and 8 place uc2 close to o and gs4 but likely in the long arm farther than either o or gs4 from msg,,bk. Cross 14 indicates that alb,,q (described as alb,,f in BGN7:90) may be in the short arm of chromosome 6, distal to msq,,bk. However, the population sizes were small.
Fig. 1. 3-point linkage maps for markers on chromosome 6 based upon data in Table 2.
It is apparent that there are a number of closely linked genes near the centromere of chromosome 6, namely o, gs4, uc2, sex1, and msg6. We are making additional multiple marker gene crosses to attempt to sort out the gene orders for chromosome 6. Linkage calculations presented here have been facilitated by the use of a computer program for data analysis.
Reference:
Walker, G.W.R., J. Dietrich, R. Miller and K. Kasha. 1973. Recent barley mutants and their linkages II. Genetic data for further mutants. Can. J. Genet. Cytol. 5:200-219.
