--J. T. Gerdes and W. F. Tracy
Purdue 39 (P39) was the most widely used sweet corn inbred line in the 1930's and 1940's. This inbred is reported to be highly mutable, and a number of strains of P39 were previously identified. These strains are thought to be single gene mutations. Four such strains are C30 (a small mutant of P39 and the source of the rd gene), P39M94, P39M96, and IP39 (narrow grained mutant). These inbred strains all closely resemble P39 morphologically, even C30 which looks like a small P39 plant. All four of these inbred strains gained use in hybrid sweet corn production.
P39 and these four strains were subjected to RFLP analysis. Seventy-seven markers spread throughout the genome were scored. Thirty-two of the 77 markers showed differences from P39 for at least one strain. At 14 of these markers, two to all four strains differed from P39. Furthermore, three different alleles were seen at 7 markers among the inbreds. The four strains were compared to P39 and each other for similarity. The differences of the strains from P39 ranged from 9 markers (11.7%) for IP39 to 18 (23.4%) for P39M96 (Table 1). The strains differed from each other by as few as 5 markers (6.5%) for IP39-P39M94 to as many as 28 (36.4%) for C30-P39M96.
Table 1. Number of differences observed among P39 and four inbred strains
derived from P39 at 77 RFLP markers.
| P39 | C30 | P39M94 | P39M96 | IP39 | |
| P39 | -- | 17 | 14 | 18 | 9 |
| C30 | -- | 14 | 28 | 11 | |
| P39M94 | -- | 23 | 5 | ||
| P39M96 | -- | 21 |
The exact cause of these differences is not known. Outcrossing is an
unlikely explanation because of the strong morphological resemblance of
these inbreds along with the historical references of instability. Also
residual heterozygosity is not a likely candidate because as many as three
alleles were seen at a number of markers. These results raise an interesting
question on the origin of variability in maize. (The authors wish to thank
the Agrigenetics RFLP Mapping Group for their assistance in RFLP data collection
and analysis. Agrigenetics Corporation, Madison, WI.)
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