Mae - 1 , a malic enzyme coding gene on chromosome 5 Chetelat, R . T . 1 , D . F . Adams 2 , and D . O . Adams 3 1 Department of Vegetable Crops, University of California, Davis, CA   95616 2 Campbell Research and Development, 28605 County Rd . 104, Davis, CA   95616 3 Department of Viticulture and Enology, University of California, Davis, CA   95616 Malic enzymes (MAE) are NADP + - dependent malate dehydrogenases that catalyze the metabolism of malic acid to pyruvate .   MAE activity is found in all plant organs, and increases during fruit development in many plants, where it is an important determinant of flavor .   We were interested in studying MAE isozymes in tomato .   Whereas four malate dehydrogenase genes ( Mdh - 1, 2, 3, 4 ) are known, of which several have been placed on the genetic map in interspecific mapping populations, the number and location of Mae genes has not been reported . We first detected MAE polymorphisms in a F 1 L . esculentum cv . VF36 x Solanum lycopersicoides LA2951 hybrid and its backcross derivatives .   This exceptionally wide cross has proven a rich source of isozyme variation, allowing the determination of map locations of several previously unmapped genes, including Mdh - 1 and – 4 , Dia - 1, - 2, - 3, and – 4 , Fdh - 1 , and Tpi - 1 (Chetelat et al . 1997, and unpublished data) .   For the resolution of MAE isozymes, several starch gel electrophoresis buffer systems were tested, including sodium - borate/tris - citrate pH 7 . 8, citrate/histidine pH 7 . 0, and histidine - citrate pH 5 . 7 (Wendel & Weeden 1989) .   Of these, the pH 7 . 8 gel system produced the sharpest banding, yet failed to reveal a polymorphism between the parental species .   In contrast, the pH 7 . 0 gel produced lower resolution electrophoregrams, but nonetheless revealed a putative polymorphism between the parental species .   Of the plant tissues assayed, including stems, leaves, anthers, and roots, only roots produced satisfactory results under these gel conditions . The allele of S . lycopersicoides was slightly retarded relative to that of L . esculentum , but due to their broad zone of activity, the two alleles overlapped in heterozygotes, producing an extended smear .   The fact that active MAE is normally a tetramer (Weeden & Wendel 1989), would tend to make the bands in heterozygotes more difficult to resolve, since as many as 5 different combinations of subunits could be formed .   Despite this inherent complexity,