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The Genetics of Resistance to the Corn Leaf Aphid Rhopalosiphum Maidis (Fitch) in Corn
|Title:||The Genetics of Resistance to the Corn Leaf Aphid Rhopalosiphum Maidis (Fitch) in Corn|
|Authors:||Chang, Siew Hoong|
|Abstract:||Genetic segregation for resistance to the c o m leaf aphid, Rhopalosiphum maidis (Fitch), in a planting of AA8sh2 inbreeds stimulated this study. Homozygous resistant and susceptible lines were derived from the AA8sh2 population. Appropriate crosses based on a resistant and a susceptible line were critically evaluated in the greenhouse and showed that the genetics of resistance was monogenic and recessive. The aphid resistant gene was designated as aph. The origin of this resistance gene is obscure, and the most probable source is W22.|
Antibiosis was investigated in a number of lines; it was reflected in aphid weight, the number of days to reproduction, aphid longevity, the number of days reproductive and the rate of reproduction. Resistant AA8sh2 line 3360 showed significant antibiosis for all these parameters except aphid longevity and the number of days reproductive. The recessiveness of the antibiotic effects was in agreement with the recessive nature of its inheritance. Resistant line Mol7 showed significant antibiosis in all the parameters measured and unlike the resistance of 3660 was partially dominant or dominant.
Non-preference was investigated using "cafeteria-style" tests with aphids. High variability was obtained, and non-preference was detected only in the early growth stage of the corn plant. The inheritance of non-preference was not clear in the resistant lines derived from the AA8sh2 population. Mol7's non-preference was found to be dominant. A line much preferred by the aphid was the c o m mutant, bxbx, which lacks DIMBOA, a compound generally regarded as a resistance factor against diseases and pests. Antibiosis was also lacking in this mutant.
Tolerance was unexpectedly detected as a result of an exceptionally severe infestation in which susceptible lines were killed while the resistant line 3660 was unaffected. This plant resistance mechanism is difficult to measure and has not been reported in corn before.
Resistance to the aphid was also studied using two diallel cross experiments growing under severe aphid infestations. High genotypic variance was obtained, indicating that the heritability for polygenic resistance is high and corn lines can be effectively improved by simple recurrent selection or even mass selection for greater resistance. Mol7, 0h545 and 3660 were inbreeds showing good general combining ability for aphid resistance. Lines with poor GCA and hence, susceptible, included B37, Ant2D, CM104, CM105 and CM111. GCA mean square was found to be more important than SCA mean square, implying that the major portion of the genetic variance was additive. In general, the results of a greenhouse diallel were more reliable than those of a field diallel because of the greater uniformity of aphid infestation in the former.
A gas-liquid chromatography procedure was successfully adapted for highly accurate determination of individual analogs of the 2(3) — benzoxazolinones, improving upon earlier colorimetric methods. Individual toxicities of the three known analogs to various pests and fungi could now be studied.
MBOA distribution in etiolated corn seedlings was investigated. The concentration in the shoot and roots was high compared to that in the remainder of the embryo. A very small amount was found in the remainder of the endosperm. High MBOA concentration appears to be associated with tissues of high growth activity. Physiological implications of this are not fully understood as yet.
MBOA and dimethoxy-BOA were also found in the teosinte races from Balsas, Chaleo and Jutiapa. MBOA was also present in Tripsacum dactyloides (4N), T. floridanum and T. laxum. All three analogs were detected in a detailed examination of T. dactyloides (2N). This group of unique compounds was not known previously in teosinte and the Tripsacum complexes, close relatives of corn. Studies of the individual analogs in appropriate interspecific crosses could help further our understanding of the origin of corn.
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Ph.D. - Horticulture|
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