Threonine-overproducing, lysine-insensitive aspartate kinase (Ask2) map location
--Gary J. Muehlbauer, Burle G. Gengenbach and David A. Somers

Ask2 encodes a threonine-overproducing, lysine-insensitive aspartate kinase mutant (Dotson et al., Planta 182:546-552, 1990). The Ask1 threonine-overproducing trait was previously mapped to chromosome 7S (Azevedo et al., Plant Sci, 70:81-90, 1990) and shown to be nonallelic to Ask2 (Diedrick et al., Theor. Appl. Genet. 79:209-215, 1990). To locate the Ask2 threonine-overproducing trait in the maize genome, a cross was made to wildtype B73 and F3 lines were derived. Free threonine concentrations were measured in the bulked samples of F3 kernels to determine the Ask2 genotype of the segregating F2 plants. DNA blot analysis was conducted on bulked samples of genomic DNA from 16 F3 Ask2 lines. These blots were hybridized with a chromosome 2L marker from a cDNA encoding an aspartate kinase-homoserine dehydrogenase bifunctional enzyme (pAKHSDH2) (Muehlbauer et al., MGN, 1994). Linkage analysis demonstrated that pAKHSDH2 was positioned 6.5 +/- 4.5 cM from Ask2 (Table 1). To further position Ask2, chromosome 2L DNA markers umc55 and umc5 were used on the B73 x A188 F3 Ask2 population. umc55 exhibited no recombination with pAKHSDH2 in this population and was also positioned 6.5 +/- 4.5 cM from Ask2 . umc5 was positioned 13.4 +/- 9.1 cM from Ask2. These data demonstrated that the threonine-overproducing trait conferred by Ask2 is located on chromosome 2L. However, the small population size precludes determination of the marker and gene order.

Table 1. Chi-square analysis of the segregation data for the Ask2 locus (lysine-insensitive AK, threonine-overproducing mutant) with chromosome 2L markers, pAKHSDH2, umc55 and umc5.
Loci pair Data type n X2 df P r +/- S.E.
Ask2/pAKHSDH2 F3 16 22.04 4 0.0002
6.5 +/- 4.5
Ask2/umc55 F3 16 22.04 4 0.0002
6.5 +/- 4.5
Ask2/ umc5 F3 16 8.12 2 0.017
13.4 +/-9.1
umc5/pAKHSDH2 F3 16 11.07 2 0.0039
6.4 +/- 6.3
n = Number of F3 lines.
P = Probability of a greater value of X2.
r +/- S.E. = recombination frequency plus or minus the standard error.
The Linkage-1 computer program (Suiter et al., J. Hered. 74:203-204, 1983) was used in this linkage analysis.

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