Gene modifies electrophoretic properties of malate dehydrogenase in Norway spruce (Picea abies (L.) Karst.)

The mobility of two malate dehydrogenase (MDH) isozymes (encoded by Mdh‐2 and Mdh‐3) was found to be altered by a modifier gene designated Mm(2,3) in an electrophoretic analysis of megagametophytes of Picea abies. This modifier gene exists in two allelic variants. The dominant allele Mmd(2,3)‐N gives rise to the common electrophoretic phenotype whereas the recessive allele Mmd(2,3)‐n produces faster migrating forms of both MDH‐2 and MDH‐3. In diploid tissues, genotypes heterozygous at Mmd(2,3) cannot be differentiated from homozygotes carrying dominant alleles. Certain allozymes encoded by Mdh2 and Mdh3 have very similar relative migration rates under the given electrophoretic conditions. If diploid tissue is analyzed, the enzyme products cannot be traced back to specific alleles. Therefore, haploid megagametophytes have to be used to identify properly MDH genotypes. It is suggested to analyze at least 13 megagametophytes per tree to ensure a misclassification smaller than the 5% level. Linkage of Mmd(2,3) to Aat3, Lap1, ‐2, Mdh3, Pepca, 6Pgd2, ‐3, Pgi2, Pgm1, ‐2, and Skdh1 was tested, revealing independent segregation. One locus (Mdh1) presumably codes for the cytoplasmatic form of MDH.