A DNA marker correlated with tolerance to Aphanomyces root rot is tightly linked to Er-1.

Common root rot, caused by Aphanomyces euteiches Drechs., is a serious disease on pea in many parts of the world including the United States, northern Europe, Tasmania and  New Zealand (1). Several sources of tolerance to root rot have been identified, and breeders are attempting to combine these various sources in commercial varieties. Unfortunately, the transfer and pyramiding of genes conferring tolerance has proven very difficult because the trait appears to be polygenic and its expression is influenced by many environmental factors.

In the breeding program at Wisconsin, 12 pea lines (five with some degree of tolerance and seven susceptible) were intercrossed and subjected to seven cycles of selection for tolerance to root rot as well as other horticultural characters (2). Several pea lines have been produced from this experiment with increased levels of tolerance. We used the original parents and selected lines to search for genetic markers linked to the genes in the original parents responsible for tolerance to common root rot

DNA was extracted from four of the original lines (MN 494-A11, MN 108, CSC 8221, and B275-191) displaying some tolerance to root rot as well as from all seven of the susceptible lines (Badger, a backcross derived line of 'Dark Skin Perfection', L1073, L1532, WIS 7101, 8615, and 8617) and seven of the derived lines with enhanced tolerance. RAPD phenotypes for 318 primers were determined on each of the extracts. We identified 21 primers (Table 1) that generated fragments present in one or more of the tolerant parents and most or all of the selected lines, but rare or lacking in the susceptible lines.

At present, most of these RAPDs have been shown only to be correlated with the inheritance of tolerance to root rot, and we do not know if any will be particularly useful for selecting tolerant genotypes in other crosses. However, one of the RAPDs, OPA5₆₈₀ also was segregating in the set of recombinant inbred lines derived from the cross JI1794 x Slow, which has been used to help develop the pea linkage map. The RAPD mapped to a position on linkage group VI very close to Er-1, a locus controlling resistance to powdery mildew. Indeed, OPA5₆₈₀ is our closest marker to Er-1, with no recombinants found among the 45 recombinant inbred lines analysed.

The linkage to Er-1 complicates our analysis slightly because powdery mildew resistance was screened for in the early cycles of selection and two of the root rot susceptible parents possessed the marker and also displayed resistance to powdery mildew. Hence, the high incidence of OPA5₆₈₀ in the derived lines could be attributed to selection of powdery mildew resistance rather than enhanced root rot tolerance. However, despite the early screening for powdery mildew resistance, most of the derived lines (5 out of 7) with enhanced tolerance to root rot are susceptible to powdery mildew. Thus, in at least these five derived lines, the OPA5₆₈₀ marker did not come from those parents carrying er-1, but instead must have come from the root rot tolerant parents. At present, we can not determine if the remaining two lines with enhanced resistance possess a marker derived from a powdery mildew resistant parent.

Table 1. Primers generating amplified fragments that occur in at least one of the root rot tolerant parents, few or none of the susceptible parents, and most or all of the derived lines.

We do not know if the other RAPDs we have identified map to the same region because they were not polymorphic in the JI1794 x Slow RILs and have to be mapped in other populations. However, several of the RAPDs gave slightly different patterns of distribution among the original parents and derived lines, and it is probable that several regions of the genome are tagged by these RAPDs.

1. Hagedorn, D.J. 1985. In The Pea Crop, Eds. P.D. Hebblethwaite, M.C. Heath and
   T.C.K. Dawkins, Butterworths, London, pp. 205-213.

2. Lewis, M.E. and Gritton, E.T. 1992. J. Am. Soc. Hort. Sci. 117:638-642.