The symbol pro refers to a recessive lethal gene mutant strictly auxotrophic for proline. Homozygous pro seeds are easily identified on the basis of the collapsed endosperm morphology. Upon germination the mutant dies before the first leaf emergence in the genetic background of W 22, while in other backgrounds it can reach a more advanced stage before dying. When necrosis ensues after the first leaf emergence, the leaf blade appears white with green stripes along the veins.

The data to be presented have been collected in order to locate the pro marker to chromosome arm following the procedures outlined in MGNL 45:144, using an improved set of B-A translocations. The procedure consisted of planting selfed seeds of a +/pro heterozygote, crossing at least three plants by each of the B-A translocation set and scoring the resulting ears for the appearance of kernels with collapsed endosperm expected in hypoploids with the pro marker uncovered. The set of B-A translocations used covered 16 chromosome arms to some degree. Since the results of these crosses were all negative, pro must lie in a portion of the genome not uncovered by the set of B-A translocations. One hundred kernels from each ear were then planted in the field, using at least three ears for each B-A translocation. Hypoploid plants were then identified on the basis of pollen sterility (50%), ear semisterility and, at least with some translocations, abnormal plant morphology. Hypoploids yield only one kind of viable gametes, i.e., those carrying the A chromosome or a crossover strand with an exchange between the centromere and the point of exchange, as outlined in the scheme below, left:

Figure.

Accordingly only hypoploids for the chromosome marked with pro are expected to show, upon selfing, a segregation for the mutant significantly higher than the expected 1/4, and more so the closer the marker is to the exchange point.

The results of Table 1 indicate that pro is on chromosome 8. The frequency of the mutant in the selfed progeny of plants with TB-8 is in fact 26.4% (799/3024) and 52.6% (1545:2935) in normal and hypoploid sibs respectively.

Table 1.

These data are in accord with a location of pro either in the long arm in a region proximal to the centromere or in the short arm. A more precise location of the mutant will be attempted with the aid of markers in both arms of chromosome 8.

G. Gavazzi, G. Todesco and M. G. Neuffer

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