Genetics of how corn ticks

In MNL 58:38-46 we reported a generally consistent association in nearly all chromosomes among Krn (kernel row number), FIt (flint or dent endosperm type), and Ger (glucoside earworm resistance), including Mer and Zer (Maya and Zapalote Chico earworm resistance). Chromosome # 4 was not covered and in # 10 only the flint factor was detected.

M.M. Goodman, C.W. Stuber, K.J. Newton & H.H. Weissinger (MNL 54:101-102) reported two parallel cases of linkage in isozymes, one involving MDH (malate dehydrogenases), and PGM (phosphoglucomutases), and the other between IDH (isocitrate dehydrogenases) and MDH pairs. They concluded that the duplication of chromosome segments involving isozyme loci may not be as uncommon as the cytological evidence would imply. Going one step further the logical sequences in Table 1 can be posed, using K for Krebs cycle, EMP for Embden-Meyerhoff-Parnass cycle, and PS for Pentose Shunt. Data are from working maps (MNL 59:168-186), from M.M. Goodman et al. (Maize for Biological Research, 1982), articles of J.Y Wendel et al. (MNL 59:87-90), and Genetic Maps Vol. 2 (Cold Spring Harbor Laboratory). In the Table are shown 5 sequences of the respiration cycle in which there are isozyme duplications and/or overlapping involving chromosomes 1L, 3, 4S, 6 and 8.

Once it is seen that the respiration cycle is repeated among all chromosomes, the next step is to see whether other biochemical cycles are present and if they are in order within a chromosome. Rough sequences can be identified more or less in the following order: glucosides, anthocyanin and endosperm factors, the Krebs cycle, Embden-Meyerhoff-Parnass and Pentose Shunt, and the block Krn Flt Ger. They are more developed (not necessarily in this order) in 1S p1 dek1 Car Cat2, in 1L bz2 Cat3 Mdh4 mmm Pgm1 Adh1 Phi1 Dia (Ger5 Flt11 Krn11), in 3L E Hex1 Tpi4 Pgd2 Me a3 Mdh3 a1 sh2, in 5 Pgm2 Amy2 Mdh5 Cat1 a2 ps1 bt1 ae pr1, in 6L Pgd1 Dt2 Pl Bh1 su2 sm Hex2 Idh2 Mdh2 and in 8 Mdh1, Idh1 Tpi3 all with Krn FIt Ger somewhere near.

Besides the sequences shown, using data from J.M. Chandlee and J.G. Scandalios (MNL 58:172-177) we can pose in chromosome 9 the sequence Dt1 Dt5 C1 sh1 bz1 Atc1 Acp1 wx1; Acp1 is acid phosphatase; Sh1 is sucrose synthetase; probably C1 and C2 have control of flavanone synthase (FS); Bz1 is UDP-glucose: flavonoid 3-0 glucosyltransferase (UFGT); Wx1, starch-granule-bound nucleoside diphospho-glucose-starch glucosyl transferase; and somewhere Krn9 Flt9 Ger4. In 4S we have Asr Bx = Mer?, su1 Krn4 Adh2 Dt6 Aco1 (su1 is deficient in a-1,6 glucosidase debranching enzyme), and in 4L Zer1 c2. In 2, B fl1 Dia1 Zer3 Flt2 Krn2, Dia is diaphorase. In 10, Sad1 bf2 Cx Glu1 R1, we have a trend for aromatic amino acid biosynthesis, the shikimic acid pathway with Sad1 (shikimate acid dehydrogenase) and bf2 (blue fluorescent) if similar to bfl yielding anthranilic acid, and also Cx1 (catechol oxidase), which turns phenols to quinones, toxic to pests in wounds, and Glu1 (beta glucosidase) which splits saccharose or maltose yielding glucose, and the cluster distributing anthocyanin pigments around R1. The shikimic acid pathway through phenylalanine leads to the C-15 flavone skeleton.

In corn, the hint is usually for the Krebs cycle isozymes being nearer the glucosides, anthocyanin and endosperm factors, farther away usually on the same side are those for Embden-Meyerhoff-Parnass cycle, Pentose Shunt still farther away not necessarily on the same side, and the Krn FIt Ger block somewhere along the line.

According to the Report of the Tomato Genetics Cooperative, number 30, 1980, in chromosome 4 we find Tpi-2 (triosephosphateisomerase) 6 units from Pgm2 (phosphoglucomutase), both from the EMP cycle. In the anthocyanin series we find in chromosome 1 y colorless fruit epidermis, 2 units from Prx-1, (peroxidase-1): in 2 the sequence is Prx-2 or Prx-3 with aa (anthocyanin absent), are (anthocyanin reduced), and aw (without anthocyanin); in chromosome 3 is pdc (pudica), plant retarded dark pigmented (Prx-7) and r (fruit flesh yellow); in 9 ah Hoffman's completely anthocyaninless with Est-2.

So Px and correlated corn isozymes should also be searched near the above associations with the standard anthocyanin marker series, including also dek1 and vp1 and not forgetting the anthocyanin block in 6L for completeness. Possibly there is still an unknown anthocyanin gene yet to be found in 4S, and perhaps in 8L.

In human genetics, V.A. McKusick and F.H. Ruddle (Science 196:390-405) comment: "Clustering of genes with similar functions suggests a biologic significance of the association. The genes for three enzymes (98) involved in the Embden-Meyerhoff glycolitic pathway, GAPG (glyceroaldehyde-3-phosphate dehydrogenase), TPI (triosephosphate isomerase), and LDH-B (lactate dehydrogenase B) are on chromosome 12. The tight linkage of TK (thymidine kinase) and Galk (galactosidase) is found not only in man and Chimpanzee (171) but also in Chinese hamster and in Mus musculus. The genes for GOT-1 (glutamate oxaloacetic transaminase-1) and GSS (glutamate-y-semialdehyde synthetase) enzymes involved in proline metabolism are on chromosome 10", (sic) etc... We should further point out IDH and MDH in the short arm of chromosome 2, again paired as in corn.

It seems so far, pooling corn and tomato data, that each chromosome is a complete ordered assembly line, comprised of glucoside, anthocyanin or endosperm factors, Krebs cycle, and Embden-Meyerhoff-Parnass cycle and Pentose Shunt usually in that order, and with the Ger Krn FIt block probably near the linking point. The biochemical link between both these groups, respiration and synthesis, is probably the Pasteur effect. Long ago Pasteur noted that the path of yeast metabolism could be affected by oxygen: low oxygen favored fermentation, whereas high oxygen inhibited fermentation and stimulated oxidative respiration, as well as promoting the use of carbons from sugars for synthetic reactions. Px is probably involved in this assembly judging from tomato data, because it oxidizes NADH produced in respiration processes to NAD, which is necessary for the operation of the respiration cycles. There is a distinct possibility that these systems can be at least partially duplicated within a chromosome: in chromosome 1 around genes p dek1, and bz2, and in chromosome 4 (see also MNL 59:23-24) around Asr1 and c2, in both cases about 100 units apart. How can this apparent independence be reconciled with the gene complementary action among blocks marked by anthocyanin genes? Are they some kind of regulators or pacemakers to the same processes?

Compare the distribution of the blocks described with the cytological map in G.F. Sprague, ed., Corn and Corn Improvement, 1977. Taking aside the structures beside the centromeres and nucleolus organizer and satellite in chromosome 6, we verify that there is a rough correspondence with the knobs and prominent chromomeres with the blocks in chromosomes 1, 4, 5, 6, 8, and 10. Chromosomes 2L, 3S, 7S and 9L are discrepant, which could maintain other linked processes. If no more tandem duplications are found. 1 should be homologous to 4 and 2 homologous to 10 by parallelism in the latente super-genes. which are very much elaborated. Finally, note that our conclusions agree and explain much of P.C. Mangelsdorf's results in 4 on the genetic nature of teosinte, pp. 37-52 in Corn, its Origin, Evolution and Improvement, 1974, especially in the linkage relations of teosinte characters with each other and with marker genes of maize. There are several independent blocks ( super-genes) producing the same effect with reinforcing action. By our data there seem to be about 11 or 12 such blocks, instead of the 4 or 5 or 6 currently accepted, affecting floral and fruit characteristics, including kernel row number. In the next article, quantitative estimates of earworm resistance and kernel row number are presented for the groups around P-WW, B and Plp, showing their complementary dominant mode of action.

Table 1. Isozymes of respiration cycles, in biochemical order, which overlap along different chromosomes. It seems so far as if each chromosome has the complete set.

Luiz Torres de Miranda and Luiz Eugenio Coelho de Miranda

Please Note: Notes submitted to the Maize Genetics Cooperation Newsletter may be cited only with consent of the authors.

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