The mapping of Pea-2 supports the assignment of Vc-5 and rb to a specific region of chromosome 3

       The establishment of linkage between Vc-5 and rb (5) is an important step in the process of mapping both genetic loci in the pea genome. Vc-5 is one member of the small multigene family that encodes vicilin, a major seed storage protein. The rugosus gene, rb, affects several traits, including the lipid content of mature seeds. The tentative assignment of the pair to chromosome 3 (1) originated with a loose linkage between rb and st reported by Gritton (4). Marx (6) was unable to confirm this finding, however, using st and chi-6 markers from the "b end" of chromosome 3; but, he did demonstrate another loose linkage between rb and tac, located at the opposite (M) end of the chromosome (7). The absence of linkage between rb and apu, which is located between st and tac, further suggested that rb is located distal to tac, and much nearer the end of chromosome 3.

Ellis et al. (3) have recently assigned a small Vc-5 gene cluster that is linked with rb to one end of their chromosome 3 RFLP linkage group; but, the absence of accompanying standard marker loci disallows direct comparison and integration with the extant pea linkage map (10). Thus, Weeden et al. (10) still tentatively place rb, and now Vc-5, near the b end of chromosome 3.

Data obtained in this study for the F₂ progeny from two independent, reciprocal crosses, A1078-234 x 82-14n and A1078-234 x PI179449, verify that Vc-5 is, in fact, linked to st and isozyme marker Lap-1 (see Table 1 and Fig. 1), both located in chromosome 3. The genetic locus Vc-5, defined by cDNA clone pDUB2 (2), is the same RFLP marker found linked with rb (5), thereby establishing the location of the rugosus gene on chromosome 3 by association.

Further evidence supporting the assignment of the Vc-5–rb gene pair to chromosome 3 involves the RFLP marker Pea-2, a putative light-regulated gene which corresponds to pea cDNA clone pEA2 (9). In the A1078-234 x 82-14n cross, it exhibits linkage with both Vc-5 and st (see Fig. 1). In the A1078-234 x PI179449 cross, in which Vc-5 was not scored, Pea-2 displays linkage with st and Aat-c, the cytosolic isozyme of aspartate aminotransferase, which is unambiguously located at the M end of chromosome 3 (10, 11).

These findings support both the proposal by Gritton (4) that rb is linked with st and the subsequent study by Marx (7) establishing the location of the rb gene near the M end of chromosome 3.

Acknowledgements. The authors thank both the Biomedical Research Grant Program, National Institutes of Health, and the Northern Illinois University Graduate School for financial support.

Table 1. Joint segregation analyses for chromosome 3 genetic loci.

^aGenotypic designations: 11 = 82-14n or PI 179449 homozygotes, 22 = A1078-234 homozygotes, and 12 = heterozygotes (for Vc-5, 1 and 2 represent haplotypes).

^b1:2:1:2:4:2:1:2:1 expected ratio.

^c3:1:6:2:3:1 expected ratio; due to dominance, st heterozygote listed as 11.

^dReciprocal cross.

^eCalculated using the LINKAGE-1 computer program (8).

Fig. 1. Linkage relationships among Vc-5, Pea-2 and associated marker loci for chromosome 3. The linkage maps show intrachromosomal arrangements among a group of genetic loci segregating in F₂ progeny from reciprocal crosses A1078-234 x 82-14n and A1078-234 x PI 179449. Aat-c was not scored in the former cross, while neither Vc-5 nor Lap-1 were scored in the latter cross.

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