Two sources of resistance to the corn leaf aphid, Rhopalosiphum maidis (Fitch) were verified in Hawaii and are the subject of the present genetic study. The aphid has long been investigated as a pest of preferred hosts like maize (Zea mays L.), barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor (L.) Moench). The species is parthenogenetic and viviparous, and it serves as a host of MDMV virus diseases.
Two visual rating methods were adopted in the present study at Waimanalo Experiment Station of the University of Hawaii for the evaluation of resistance to corn leaf aphids. The first rating was based on the degree of infestation in whorl during tassel emergence. The second rating was based on the degree of aphid infestation on ears covered with shootbags about 14 days after pollination. Both rating methods were based on a 1 to 5 scale (1 = no aphids and 5 = severe)
Sub-lines 71 and 72 of commercial inbred Hi38 (bt1 supersweet) have repeatedly shown no aphid development since resistance was discovered in Nov. 1997. Generation mean analysis was conducted on 6 generations [P1 (Hi38-71, resistant); P2 (G24, susceptible); F1; F2; BC1 and BC2] to determine the type of gene action involved in Hi38-71. Means and their SE for parental, F1, F2, and backcross generations are summarized in Table 1 from the two rating methods. Genetics of resistance is shown to be monogenic and recessive in Hi38-71. Chang and Brewbaker also reported a recessive allele, aph, for resistance to corn leaf aphids in AA8sh2 based on generation mean and diallel analyses (MNL 48: 37-38; MNL 50: 31-32). AA8sh2 is among the ancestry of Hi38-71.
High tolerance to the aphids was also observed in inbred Hi34, parents of several sets of RILs at University of Hawaii (MNL 69: 59-60). Molecular markers were used to map the resistance loci to corn leaf aphid in a set of 100 RILs derived from the cross of Hi34 (Resistant) and TZi17 (Susceptible). The two parents differed significantly (P<0.01) for the resistance to corn leaf aphids in both rating methods. The tassel and ear ratings for corn leaf aphids of Hi34 were 2.36 and 2.54, for TZi17 were 3.72 and 3.76, and for the F1 hybrids (Hi34 x TZi17) were 3.67 and 4.27 respectively. The correlation coefficient between the means of tassel rating and ear rating for the RILs was highly significant (P<0.01). The tassel and ear ratings were combined for the identification of QTLs conferring resistance to corn leaf aphids. Composite interval mapping of 100 RILs by 120 marker loci confirmed the presence of a major recessive resistant gene with a LOD peak as 8.2 near the marker bnl12.09 on the short arm of chromosome 2. This gene explained about 14.3% phenotypic variation for resistance to the corn leaf aphid in the RIL, and may trace back to Antigua 2D from which Hi34 was bred. This gene is presently labeled as aph2, and studies are underway to determine its relationship to resistant allele in Hi38-71.
Table 1. The corn leaf aphid ratings for parents Hi38-71 (P1), and G23
(P2), F1, F2, and backcross (B1, B2) generations.
| Generations | Tassels | Ears | Average |
| P1 | 1.37 ± 0.24 | 1.13 ± 0.20 | 1.25 ± 0.15 |
| P2 | 3.58 ± 1.81 | 3.11 ± 1.10 | 3.34 ± 0.97 |
| F1 | 2.70 ± 1.35 | 3.47 ± 1.69 | 3.10 ± 0.79 |
| F2 | 2.57 ± 0.85 | 2.52 ± 1.47 | 2.55 ± 0.65 |
| B1 | 1.60 ± 0.25 | 2.00 ± 1.50 | 1.80 ± 0.50 |
| B2 | 3.00 ± 0.91 | 2.96 ± 1.29 | 2.98 ± 0.59 |
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