Linkage studies provided preliminary evidence for the presence of a major anodal esterase locus on chromosome 3 (Goodman et al., Genetics 96:697, 1980). The E8 gene, originally defined by MacDonald and Brewbaker (J. Hered. 65:37, 1974) using null alleles, encodes a highly active, ubiquitous anodal esterase. Several electrophoretic mobility classes for E8 have now been described, including naturally-occurring (Ferl, pers. comm.; Goodman and Stuber, 35th Ann. Corn and Sorghum Res. Conf.) and EMS-induced variants (Birchler, Genetics 92:1211, 1979). Heterozygotes for two different electrophoretic forms produce a three-banded pattern in sporophytic tissues, indicating that E8 is active as a dimer. Pollen from heterozygous plants exhibits only the two homodimer bands, suggesting that the E8 present in pollen is synthesized post-meiotically.
In order to confirm the localization of E8 to chromosome 3, plants carrying the extremely slow-migrating, EMS-induced E8-S allele (Birchler, 1979) were pollinated by TB-3Sb (kindly provided by J. Beckett). Immature kernels were harvested 20 days post-pollination and embryonic (scutellar) and endosperm tissues were separately tested for E8. Of 5 tested individuals with the slow allele, one was euploid; i.e., the endosperm and embryo each carried both the paternal fast and the maternal slow forms. In the embryo, a 1:2:1 FF:FS:SS band ratio was observed, while in the triploid endosperm, the banding pattern was skewed towards the SS form, due to the fact that the maternally-derived S allele is represented twice. Three kernels had only the E8-S allele present in the endosperm and an approximate 4:4:1, FF:FS:SS band ratio in the corresponding embryos. These kernels show uncovering of the maternal form in the endosperm and the complementary duplicated condition in the embryo. One kernel exhibited the reverse of this phenotype: the maternal slow allele was uncovered in the embryo and a 1:2:1 band ratio was observed in the correspondingly hyperploid (S/S/F/F) endosperm.
When coleoptile tissue from a similar cross was examined, five tested seedlings carried the slow allele. One was hypoploid, exhibiting only the maternal, slow allozyme. Another was hyperploid as evidenced by an approximate 4:4:1, FF:FS:SS banding pattern. The remaining three were apparent euploids, with equal paternal and maternal E8 contributions. The hypoploid seedling could not be explained by self-contamination, since this individual was heterozygous for maternal and paternal alleles of Glu (chr. 10), Phi (chr. 1), Pgm1 (chr. 1), and Mdh2 (chr. 6). In the exact reciprocal cross (using the TB-3Sb plant as a female), no-cases of uncovering of the E8-S allele were found.
Collectively, these data place the E8 locus in the region of 3S included in TB-3Sb and confirm the linkage of this gene to other chromosome 3 genes. Since E8 is easy to work with and the dosage of 3S can be readily discerned by its banding ratios in heterozygotes, it should serve as a useful marker for dosage studies involving the short arm of chromosome 3.
Kathleen Newton, Major Goodman and Charles Stuber
Return to the MNL 56 On-Line Index
Return to the Maize Newsletter Index
Return to the Maize Genome Database Page