Wendel et al. (MNL 60:109-110) reported isozyme segregation data that suggested Adk1 (adenylate kinase) is located near the centromere on chromosome 6, and tentatively proposed a location on 6S. Subsequent work has confirmed that Adk1 is on 6S, which is significant in that this is one of the most poorly mapped chromosome arms of maize. New F2 data (Table 1), in conjunction with the previously presented data, indicate the following gene order and map distances for various markers near the centromere of chromosome 6:

Adk1 - 8.1 - rgd1 - 7.6 - Pgd1 - 3.7 - Enp1

These data are in agreement with previous distance estimates for the loci Adk1, Pgd1, and Enp1. Additional evidence that Adk1 is on 6S was gained from a cross of TB-6Sa (as male) onto the inbred line Mt42, which carries the rate variant Adk1-5. Seven hypoploids (those that display only the tester allele) were recovered out of 51 seedlings analyzed. These same seven plants were heterozygous for numerous other isozyme markers, ruling out the possibility that they resulted from contaminating self-pollinations. These data thus confirm that Adk1 is on 6S. Because neither rgdl nor Pgd1 is uncovered by either TB-6Sa or TB-6Lc (Enp1 is uncovered by TB-6Lc, however; Wendel et al., MNL 59:89-90) their placement relative to the centromere is still uncertain.

Table 1. Joint F2 segregation data and maximum likelihood estimates of the recombination fractions (r) for four markers near the centromere of chromosome 6.

Jonathan F Wendel and J.B. Beckett

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