Several of the zebra crossbands (zb) mutants in maize exhibit a similar phenotype in which the leaves display transverse bands of normal green tissue alternating with yellowish, frequently grainy, bands of tissue deficient in chlorophyll. To begin to explore the genetic interaction among this class of zb mutants, we intercrossed the Coop's zb1, zb2, and zb3 stocks with the goal of producing double zb mutants. Surprisingly, in the F1 progeny of zb1 x zb2, all plants had a zebra phenotype, suggesting that these two mutants are alleles of the same gene. Two sets of F1 progeny were obtained by crossing zb1 and zb2 to two distinct zb3 stocks. The progeny from crosses of zb1 and zb2 to one zb3 parent (Coop Stock 504C) displayed a zebra phenotype, whereas the progeny from crosses of zb1 and zb2 to a second zb3 parent (Coop Stock 519G) displayed a normal phenotype. To help sort out the relationship among these zb mutants, we created separate F2 populations segregating for zb1, zb2, or zb3 (504C). Molecular segregation analysis was performed using an RFLP marker from chromosome 5L, the reported location of zb3. The results showed that zb1, zb2 and zb3 (504C) all map on 5L, suggesting that all are allelic. To help resolve these discrepancies, pedigrees of the zb stocks maintained by the Coop were reviewed. The zb1 (U340B) and the zb3 (519G) mutants, both originally obtained from Punnett's and Burnham's collections, appear to be bona fide. However, pedigree searches reveal that the zb3 stock maintained as 504C really carries zb1, not zb3, and that this zb1 allele is identical to the zb1 allele in U340B. The identity of zb2 is still a mystery. It was originally described as a seedling zebra (Stroman, GN. 1924. Genetics 9:493-512), but the mutant phenotype of the Coop's zb2 stock (U340C) is evident in the mature plant (like zb1) and not in the seedling. A pedigree search of the Coop's zb2 stocks gives no reason to suspect a recent stock mix-up. The zb2 stock was propagated from an intercross between separate zb2 stocks obtained from Eyster and Burnham. The F1 cross of these two sources produced zebra plants, indicating allelism. Also, notes taken when the Burnham stock was first grown by the Coop in 1943 reveal that this zb2 stock was a mature plant zebra, not a seedling zebra. Therefore, if the zb1 and zb2 stocks were mixed up, the mix-up occurred before these mutants came into the Coop's hands, and had to have been present in both the Eyster and Burnham sources. Based on our allelism test and mapping results, we conclude that the zb in the U340C zb2 stock is at least allelic (and maybe identical) to zb1. To reflect the allelism between the Coop's stocks of zb1 and zb2, the zb1 allele in the zb2 stock has been renamed zb1-2

In summary, we conclude that zb1 maps to the long arm of chromosome 5. zb3, as found in stock 519G, represents a separate gene whose map location is unassigned. TB tests are in progress in order to place zb3 to chromosome arm. All three distinct stocks, zb1, zb1-2, and zb3, have been assigned new stock numbers (519H, 519I, and U840E, respectively) to reflect their chromosomal locations.
 

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