African Crop Science Journal, Vol. 9. No. 2, pp. 431-440
RESISTANCE OF MAIZE TO THE MAIZE WEEVIL: I. ANTIBIOSIS
J. Derera, K.V. Pixley1 and P. Denash Giga2
Department of Research & Specialist Services, P. O. Box CY550, Causeway, Harare, Zimbabwe
(Received 9 June, 2000; accepted 14 December, 2000)
Code Number: CS01025
Genetic resistance of maize grain to storage insects is an important component of integrated control for use in rural storage, but progress towards finding and using such resistance has been limited. Farmers in Zimbabwe have been demanding maize with good storability (Jayne, T. personal communication, cited in Blackie, 1994), because most of the available hybrids are susceptible to maize weevil (Sitophilus zeamais Motsch.)(Giga and Mazarura, 1991). Pesticides, especially Malathion (O,O-dimethyl-s-1,2-di(carboethoxy) ethyl phosphorodithioate), have for many years been providing effective control of maize weevil in rural areas of Zimbabwe (Giga and Mazarura, 1990). However, continued use of insecticides may lead to breakdown in their effectiveness and is hampered by potential hazard to users and consumers.
Assessment of the intrinsic levels of weevil resistance in maize grain has shown large variation among maize genotypes from Eastern, Southern and Western Africa and Latin America (Wright et al., 1989; Giga and Mazarura, 1991; Kossou et al., 1993; Tadesse et al., 1995). Inheritance of grain resistance to maize weevil has been reported to involve maternal effects, additive and non-additive gene action (Widstrom et al., 1975).
The basis of grain resistance to weevil has been explained according to Painters (1951) three-fold resistance mechanism as antibiosis, non-preference and tolerance. Antibiosis resistance is the ability of a host to injure the pest, reduce reproduction potential, retard rate of development and or kill the pest (Dent, 1991). Schoonhoven et al. (1975) reported antibiosis resistance of grain to weevil, while Kang et al. (1995), Schoonhoven et al. (1976) and Gomez et al. (1982; 1983) have reported non-preference. Horber (1989) argued that host tolerance was inapplicable or not useful in the case of stored grain, because grain is dormant and damage incurred is terminal. Thus, selection of grain for weevil-resistance should focus on antibiosis and non-preference resistance types. This paper will discuss antibiosis, while a companion paper focuses on non-preference type of resistance to weevil (see Derera et al., 2001, this volume).
Recent studies have shown that hydoxycinnamic acids (phenolics) play an important role in grain resistance to weevil through mechanical and antibiosis resistance types (Serratos et al., 1987; Classen et al., 1990; Arnasson et al., 1992, 1997; Sen et al., 1994; ). Classen et al. (1990), Sen et al. (1994), and Arnasson et al. (1997) reported that kernel hardness was correlated to phenolics, because phenolic acids esterified to carbohydrates form dimmers, feruloyl and p-coummaroyl arabinoxylaus, which result in a mechanical cross-link in the cell wall; hence these dimers make tissues hard and limit biodegradability of the cellwall polysaccharides by the insects. Resistant hybrids have higher levels of phenolic acids, which cause adverse effects to weevil feeding and survival, hence biochemical screening may be used as a first step towards selection of genotypes for resistance. However, we are not aware of any published attempts to develop protocols for biochemical screening of maize for resistance to the maize weevil, despite their apparent advantages compared to timely and costly conventional methods using live insects and long evaluation periods in controlled environment rooms.
Most published studies about weevil resistance have evaluated grain resistance in F1 crosses, yet farmers store F2 grain. We evaluated weevil resistance of both F1 seed and F2 grain to enable us to study inheritance of weevil resistance while also obtaining results relevant to farmers. This research was designed to study the levels of antibiosis effects and the genetic basis of resistance in F1 and F2 hybrids. A companion paper (Derera et al., 2001) presents an analysis of non-preference type of resistance to weevil for the same hybrids. An understanding of the genetic basis and mechanisms of resistance is essential to develop maize with improved levels of resistance.
MATERIALS AND METHODS
Formation of hybrids. The maize hybrids used in this study were derived from 18 inbred lines, 6 of each from southern Africa, Mexico and CIMMYT-Zimbabwe, respectively (Table 1). The lines from Mexico were reported to be resistant to maize weevils at CIMMYT-Mexico (D. Bergvinson, personal communication), while resistance of southern African and CIMMYT-Zimbabwe lines had not been previously evaluated. Two subgroups of 3 lines were formed within each group, and crosses were made according to a North Carolina design II mating scheme with six sets (Comstock and Robinson, 1952). The lines of the same heterotic orientation were assigned to the same subgroup, such that crosses were formed between lines with different heterotic orientation. Sets, consisting of 9 hybrids each, were formed among subgroups of lines (Table 1): resistant x regional (subgroup 1 x 3), resistant x resistant, (2 x 1), regional x regional (3 x 4), CIMMYT x resistant (6 x 2), regional x CIMMYT (4 x 5) and CIMMYT x CIMMYT (5 x 6). The sets were balanced, meaning that each line was used once as male (3 hybrids) and once as female (3 hybrids) in different sets; lines used as females are always listed first, and males second (e.g. Set 1 x 3 used lines 1, 2 and 3 as females for lines 7, 8 and 9 as males). The single crosses from the Design II mating, plus reference entries, including commercial hybrid SR52 were grown at Harare during summer of 1996/97. Plant to plant, full-sib pollination within each plot was conducted to form F2 grain for each hybrid. Pollination for two hybrids in the regional x regional set failed to produce seed; hence 52 of the possible 54 hybrids were screened for maize weevil resistance.
Resistance tests. After harvest, all grain samples were frozen at -20°C for 10 days to kill any insects or eggs that may have been present on grain due to natural, random insect attack in the field. Grain samples were then conditioned in a constant temperature and humidity (CTH) room at 28±2°C and 70±5% relative humidity for 42 days before being subjected to weevil infestation. The local commercial hybrid SR52 was used as a susceptible control, while Mexican composite Oaxaca 179 was included as a resistant check. The local open pollinated variety Kayile, and a local popcorn obtained from the University of Zimbabwe Farm were also used as standard checks.
After removing damaged kernels, each sample was divided into four 50 g sub-samples (replications). Each 50g sub-sample was placed in a glass jar (500 cm3) that was closed with a mesh screen lid. The jars were arranged in the CTH room as a randomised complete block design having four replications. Each jar was infested with 32 unsexed weevils, aged 21 to 28 days, from a laboratory culture. The weevils were removed and the numbers of dead and live insects were recorded after a 10-day oviposition period in the CTH room. The maize sub-samples were then incubated in the CTH room. Progeny emergence counts were made every two days beginning 25 days after the removal of the parent insects and ending when all progeny (F1) had emerged. Emerged progenies were removed from the jars at each count.
The total number of progeny that emerged and their mean development period were derived for each replicate and subjected to an analysis of variance. Median development period was calculated as the number of days from day-5 of oviposition to 50% emergence of progeny. An index of susceptibility (Equation 1) was calculated for each hybrid and parent inbred lines according to Dobie (1977): Equation 1:
Index of susceptibility = 100 x [loge(total number of F1 adults emerged) / (median development period)]
A relative index of susceptibility for each genotype was also calculated as the index of susceptibility expressed as a proportion of the index of susceptibility for the susceptible check, SR52 (Dobie, 1977) and multiplied by 10 for convenience (Giga and Mazarura, 1991).
Statistical analyses. A random-effects model was used for analyses of variance (ANOVA) for adult weevil mortality, number of progeny emerged, mean development period and index of susceptibility data of inbred lines and hybrids. Mean square expectations for the sources of variation for index of susceptibility of hybrids (F1 and F2) were calculated as described by Hallauer and Miranda (1988). The variation due to entries was partitioned into sources due to sets and hybrids-within-sets. Further, hybrids-within-sets was partitioned into variation due to females within sets, males within sets and female x male interaction within sets. Throughout this paper, variation due to females within sets, males within sets and males x females within sets are referred interchangeably as GCAf (general combining ability effects for females within sets), GCAm, and SCA (specific combining ability effects for hybrids within sets), respectively (Hallauer and Miranda, 1988). GCA and SCA effects were fixed, while replications-within-sets were considered random sources of variation.
RESULTS AND DISCUSSION
Grain resistance. Antibiosis resistance, or significant detrimental effects to the biology of weevil was identified among inbred lines and hybrids. Significant differences were recorded for adult weevil mortality, progeny emergence, development period and the index of susceptibility of maize genotypes (Table 2). In general, hybrids exhibited higher levels of antibiosis effects in F1 than F2 generation.
Parent weevil mortality was almost always below 10%, but ranged from 0 to 89% in F1, and from 0 to 42% in F2 hybrids (Table 2). F1 hybrids 5x1, 5x3 and 6x2 had very high adult weevil mortality, suggesting they contained antibiosis factors, which were lethal to weevil. Progeny (F1) emergence ranged from 1 to 169 in F1 and from 18 to 188 in F2 hybrids (Table 2). Similar results, namely, 3 to 143 progeny emerged among 25 maize genotypes, were reported by Tadesse et al. (1995). Progeny development period ranged from 29 to 44 days in F1, but did not differ among F2 hybrids (mean of 36 days), suggesting reduced antibiosis effects in F2 generation (data not presented). Gomez et al. (1982) reported significant differences in number of days to pupation, ranging from 30 to 38 days among maize genotypes.
Relative index of susceptibility ranged from 0.3 to 11.7 in F1, and from 4.4 to 10.6 among F2 hybrids (Table 2). These results are not very different from relative index of susceptibility ranging from 4 to 12 among 217 materials, reported by Dobie (1977), and relative index from 6.5 to 12.1 among 84 materials reported by Giga and Mazarura (1991). The mean relative index of susceptibility for sets of hybrids ranged from 7.4 to 10.2 for F1, and 8.3 to 9.7 for F2 hybrids (Table 3). The resistant x resistant set of hybrids was significantly more resistant to weevil than all other sets both for F1 and F2 hybrids; there were no significant differences for weevil resistance among any of the remaining five sets of hybrids. This result suggests that significant weevil resistance was generally obtained only for hybrids with both parent lines being resistant to weevil.
We classified materials with relative index of susceptibility <4 as resistant, 4.1 to 6 moderately resistant, 6.1 to 8.0 moderately susceptible, 8.1 to 10.0 susceptible, and >10.1 highly susceptible. To provide a basis for comparisons across experiments, we defined the relative index of susceptibility of hybrid SR52 to be 10.0. Hence, genotypes in the susceptible group were similar to SR52. Four percent of the hybrids were classified resistant in F1, but none were resistant in F2 (Table 2). A local popcorn variety was highly susceptible, while the open-pollinated local variety (Kayile) and the Mexican check (Oaxaca179) were susceptible and similar to SR52 (Table 1). The parent inbred lines for the North Carolina Design II ranged from moderately resistant to susceptible. A less conservative classification method might put all materials that were significantly less susceptible than SR52 in the resistant category. Presently, there is no agreed method for classifying genotypes for resistance to weevil. In addition, conclusions are strongly influenced by choice of susceptible check for use in calculating relative index of susceptibility. Dobie (1977) recommended the use of the Mexican variety Cacahuacintle as a reliable susceptible check. We used SR52 as a susceptible check in the current study, because previous researchers (Giga and Mazarura, 1991; Dobie, 1977) classified it susceptible, relative to Cacahuacintle.
Genetics of weevil resistance. Genetic analyses were performed only for the index of susceptibility because it incorporates all resistance parameters (Equation 1). Variance of general combining ability effects of female lines (GCAf) was significant for both F1 and F2, while that of GCAm (GCA effects of male lines) was significant only for F1 hybrids (Table 4). GCA (additive) and SCA (non-additive) effects were of similar importance in explaining differences in antibiosis among hybrids. Greater importance of GCAf relative to GCAm, however, indicated importance of additive maternal effects determining antibiosis type of resistance to weevil in both F1 and F2 grain, and that differences for additive effects were contributed primarily by the female parent involved in each hybrid. These results mean that: 1) SCA effects, which are unpredictable from parent resistance per se, are important for weevil resistance, and 2) in as far as hybrid performance is predictable, resistant hybrids are most likely to be obtained when the more resistant parent is used as the female in producing the hybrid.
Other researchers have also reported significant additive, non-additive and maternal effects determining maize weevil resistance in maize grain (Schoonhoven et al., 1975; Widstrom et al., 1975 and 1983; Tipping et al., 1989). Widstrom et al. (1975) found that dominance effects were important for seed resistance to weevil among sources segregating for maternal and endosperm genotypes.
Negative GCA and SCA effects indicated good (favourable) combining ability for grains antibiosis effects. Estimates of combining ability effects for each line are only relative to the other lines included in the same set with the line. For example, regional line 8 (SC) showed negative GCA as female in F1 hybrids for set "Regional x Regional" and as male in F1 hybrids for set "Resistant x Regional" (Table 5). This result indicates that line 8 was relatively better than lines 7 and 9 when crossed as female for lines 10, 11 and 12, and as male for lines 1, 2 and 3. Estimates of GCA effects were generally not consistent from F1 to F2, suggesting that maternal effects of lines affect resistance in F1 but are dissipated in F2 generation hybrids. The lack of consistency of GCA effects for lines used as female or male may reflect the lesser importance of male contribution to GCA effects, or it might be a consequence of the experimental design, which paired sub-groups of lines differently as female and male (as opposed to a diallel design, where each line would have been crossed to all other lines).
Variance of SCA effects was significant only for F1 hybrids in one set and F2 hybrids in two sets (Table 6). Only the "Resistant x Resistant" set had significant SCA effects in both F1 and F2, and these were not consistent between the two generations. For example, hybrid 5x3 had the best SCA effect in F1, but showed a reversal to unfavourable SCA effect in F2. By contrast, hybrid 6x3 had positive SCA in F1 and negative SCA in F2. These results are difficult to interpret. We can conclude that: 1) SCA effects were not significant for most hybrids, and 2) it is essential to assess weevil resistance for F2 grain because F1 is generally not a reliable predictor of F2 performance. It is resistance shown in F2 which is of practical significance, because farmers store and merchants trade F2 grain.
We can only speculate whether our results would have been different from those reported herein if we had worked with hybrids exhibiting higher levels or greater variation for weevil resistance. Clearly, most of the hybrids in this study were susceptible to weevil and not very different from each other for this trait. Nevertheless, we found that hybrids had significant levels of antibiosis, and that variation for antibiosis among hybrids was larger in F1 than F2 generation. Additive, non-additive, and maternal effects played significant roles in determining antibiosis in F1 seed and F2 grain. Variance of GCA effects for lines used as females was generally more important than that for lines used as males, suggesting that breeders developing weevil resistant hybrids should place greatest emphasis on choice of the female parent. Our results indicate it is essential to evaluate weevil resistance of F2 grain, because we generally found no relationship between performance in F1 and F2, and because it is weevil resistance in F2 that is of practical significance.
We gratefully acknowledge The Rockefeller Foundation for funding this project.