--- E.A. Lee, D.R. Baxter, L.L. Darrah and E.H. Coe, Jr.

Considerable effort is now underway to identify and mark chromosome segments (QTLs) associated with quantitative traits. The approach being used involves mapping QTLs using isozymes and restriction fragment length polymorphisms (RFLPs). An alternative to this approach is to examine the phenotypic changes in a standard hybrid background when specific chromosome arms are either added or lost (dosage effects). The approach is based on the expectation that dosage effects and differential effects between parental inbred lines can be used to identify chromosome segments influencing quantitatively inherited characters. The results presented below are from a pilot experiment examining chromosome arm dosage effects in order to identify chromosome segments (QTLs) associated with quantitatively inherited characters.

Mo17Ht and B73Ht (hereafter shown as Mo17 and B73) versions of TB-5Sc (Katsuta and Coe, MNL 64:48, 1990) were used to generate reciprocal F1 hyperploid and hypoploid hybrids. The aneuploid plants were evaluated in the field during the summer of 1990 at one location with four replications. The seed was planted in three-row plots with hyperploid and hypoploid plants segregating within the same plot. Identification of the aneuploid plants was done initially on the basis of phenotypic characters (plant height, leaf width, overall vigor, etc.). Putative hypoploid and hyperploid plants were further verified by examining percent pollen abortion. Hypoploid plants should have approximately 50% aborted pollen grains while hyperploid plants should have approximately 10 to 15% aborted pollen grains. Competition within the plots was adjusted by 'topping' the non-aneuploid plants in the plot. A normal F1 check plot was included in each replication to serve as a control. Observations were made on 10 competitive plants per plot except for the following characters, which were evaluated on a plot basis among classified types of plants: days to silk, days to anthesis, and pollen-silk interval.

Loss or gain of doses of 5S affected days to silk in all cases (Table 1). Plants having only the Mo17 5S arm also were significantly affected for number of days to anthesis. The interval between anthesis and silk emergence (pollen-silk intv.) was greatly affected by loss of 5S or by the gain of 5S coming from the Mo17 source. Differential effects between B73 and Mo17 were observed in the hypoploids for pollen-silk intv., further suggesting that genes affecting nicking of silk emergence with anthesis are located on the short arm of chromosome 5. Dose of 5S was directly proportional to rind puncture resistance (a measure of stalk strength). Rind puncture resistance may be influenced by internode length or stalk circumference. Yet, stalk circumference exhibits the same dosage effect pattern that rind puncture resistance exhibits, while internode length does not exhibit the same dosage effect pattern. Loss or gain of 5S affected leaf length, and plant and ear height in all cases. Loss of 5S also affected tassel branch number, leaf width, and the number of leaves above the ear. Based on these observations, genes (QTLs) influencing silk emergence, stalk strength, stalk circumference, leaf length, tassel branch number, nicking of silk emergence with anthesis, and plant and ear height may be located on the short arm of chromosome 5.

Genetic materials have been prepared for a full-scale field study involving 18 of the 20 chromosome arms, which will be conducted during the summers of 1991 and 92.

Table 1.  Pedigree means for 12 traits measured in a single environment for genotypes with varying doses of chromosome 5S.

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