Weevil Resistance Research Articles

Defense mechanisms against herbivory in Picea: sequence evolution and expression regulation of gene family members in the phenylpropanoid pathway

BackgroundIn trees, a substantial amount of carbon is directed towards production of phenolics for development and defense. This metabolic pathway is also a major factor in resistance to insect pathogens in spruce. In such gene families, environmental stimuli may have an important effect on the evolutionary fate of duplicated genes, and different expression patterns may indicate functional diversification.ResultsGene families in spruce (Picea) have expanded to superfamilies, including O-methyltransferases, cytochrome-P450, and dirigents/classIII-peroxidases. Neo-functionalization of superfamily members from different clades is reflected in expression diversification. Genetical genomics can provide new insights into the genetic basis and evolution of insect resistance in plants. Adopting this approach, we merged genotype data (252 SNPs in a segregating pedigree), gene expression levels (for 428 phenylpropanoid-related genes) and measures of susceptibility to Pissodes stobi, using a partial-diallel crossing-design with white spruce (Picea glauca). Thirty-eight expressed phenylpropanoid-related genes co-segregated with weevil susceptibility, indicating either causative or reactive effects of these genes to weevil resistance. We identified eight regulatory genomic regions with extensive overlap of quantitative trait loci from susceptibility and growth phenotypes (pQTLs) and expression QTL (eQTL) hotspots. In particular, SNPs within two different CCoAOMT loci regulate phenotypic variation from a common set of 24 genes and three resistance traits.ConclusionsPest resistance was associated with individual candidate genes as well as with trans-regulatory hotspots along the spruce genome. Our results showed that specific genes within the phenylpropanoid pathway have been duplicated and diversified in the conifer in a process fundamentally different from short-lived angiosperm species. These findings add to the information about the role of the phenylpropanoid pathway in the evolution of plant defense mechanisms against insect pests and provide substantial potential for the functional characterization of several not yet resolved alternative pathways in plant defenses.

Constitutive defenses and damage in Sitka spruce progeny obtained from crosses between white pine weevil resistant and susceptible parents

Attack by the white pine weevil has notably reduced Sitka spruce productivity in British Columbia (BC) (Canada) and western US. By the 1970s, the BC Ministry of Forests established provenance trials of Sitka spruce with the objective of detecting usable genetic resistance to weevil. These early trials reported significant weevil resistance and allowed the production of the first (F1) controlled-cross progeny generation with demonstrable weevil resistance (R) or susceptibility (S). This study reports results of the screening for weevil resistance and the levels of constitutive defenses of this F1 Sitka spruce progeny. Progeny from resistant parents (R × R progeny) sustained significantly fewer weevil attacks than progeny from susceptible parents (S × S progeny) or progeny with one resistant and one susceptible parent (R × S progeny). Individual and family heritability estimates of the weevil resistance were 0.5 and 0.9, respectively. Constitutive defenses, measured by resin canal and sclereid cell density in the cortex, were significantly higher in R × R progeny than in R × S or S × S progeny. We observed a negative correlation between the percentage of trees attacked in each cross and the average density of the resin canals or sclereid cells for each cross.

Comparative genomics of resistance of spruce to the white pine weevil in British Columbia

We present results of two large scale comparative studies of the genomic basis of resistance of to the white pine weevil. Both volume growth, the main objective of the breeding program in British Columbia, and white pine weevil resistance, are examined. Interior spruce is a species complex involving mainly Picea glauca (White spruce) but introgressed with P. engelmannii (Englemann spruce), depending upon locality. In the first study, we compared constitutive expression of 17825 genes between 20 resistant and 20 susceptible trees to the weevil; 54 upregulated and 137 downregulated genes were found in resistant phenotypes, with implications discussed in regard to volume growth. In particular, we will be surveying these genes for SNPs that differ between these two classes of trees in the next year. In the second study, we developed a 1536 Illumina SNP chip based upon candidate genes for weevil resistance. In a novel experimental design, we assayed 945 open-pollinated progeny of the Prince George breeding population (176 parents), and 654 open-pollinated progeny of the Prince Rupert breeding population (134 parents); parents were also genotyped. Within each family of 100 progeny, we identified the highest ranked 3 progeny and the lowest ranked 3 progeny, based upon BC Ministry of Forests scores for volume growth and resistance. These were genotyped and used in a novel test analogous to the transmission disequilibrium test to detect both SNP associations and QTLs linked to SNP markers. Discoveries about associations and QTL are discussed, with the added caution about genotyping error. Both studies illustrate how operational tree breeding populations can provide valuable inferences about tree genomics.

Global transcriptome analysis of constitutive resistance to the white pine weevil in spruce.

Constitutive defense mechanisms are critical to the understanding of defense mechanisms in conifers because they constitute the first barrier to attacks by insect pests. In interior spruce, trees that are putatively resistant and susceptible to attacks by white pine weevil (Pissodes strobi) typically exhibit constitutive differences in traits such as resin duct size and number, bark thickness, and terpene content. To improve our knowledge of their genetic basis, we compared globally the constitutive expression levels of 17,825 genes between 20 putatively resistant and 20 putatively susceptible interior spruce trees from the British Columbia tree improvement program. We identified 54 upregulated and 137 downregulated genes in resistant phenotypes, relative to susceptible phenotypes, with a maximum fold change of 2.24 and 3.91, respectively. We found a puzzling increase of resistance by downregulated genes, as one would think that “procuring armaments” is the best defense. Also, although terpenes and phenolic compounds play an important role in conifer defense, we found few of these genes to be differentially expressed. We found 15 putative small heat-shock proteins (sHSP) and several other stress-related proteins to be downregulated in resistant trees. Downregulated putative sHSP belong to several sHSP classes and represented 58% of all tested putative sHSP. These proteins are well known to be involved in plant response to various kinds of abiotic stress; however, their role in constitutive resistance is not yet understood. The lack of correspondence between transcriptome profile clusters and phenotype classifications suggests that weevil resistance in spruce is a complex trait.

A screening method for banana weevil ( Cosmopolites sordidus Germar) resistance using reference genotypes

The banana weevil (Cosmopolites sordidus Germar) is a serious pest in most banana-growing areas of the world. Host-plant resistance is considered to be the most feasible and sustainable method for its control. However, a quick and effective method for screening banana genotypes for resistance against the banana weevil to facilitate selection and/or development of resistant genotypes is lacking. The objective of the study was to develop an early screening method for weevil resistance by using a set of reference genotypes. Three susceptible genotypes (‘Atwalira’, ‘Namwezi’ and ‘Kibuzi’) and three resistant genotypes (‘Calcutta 4’, ‘Yangambi Km5’ and ‘TMB3x1968-2’) were used in screen-house experiments to assess weevil resistance/susceptibility. Healthy plantlets of the above genotypes were established in buckets in a screen house. Ten adult weevils (5 females and 5 males) were introduced at the base of each plant and the bucket was covered with a weevil-proof mesh. Weevil damage of the corms was estimated as a percentage at 35 and 60 days after the weevil introduction by estimating the peripheral and cross-section corm damage. The resistant genotypes had significantly lower (p < 0.05) peripheral and total cross-section corm damage, and less larvae than the susceptible genotypes. These results indicated that these genotypes can be used as reference genotypes in evaluating resistance or susceptibility against the banana weevil. These experiments were completed in five to seven months, depending on the source of planting material, as compared to field-screening experiments for the banana weevil that can take up to three or more years.

Resistance of Early Generation Maize Inbred Lines and Their Hybrids to Maize Weevil [Sitophilus zeamais (Motschulsky)

Maize weevil [Sitophilus zeamais (Motschulsky)] causes important losses to maize (Zea mays L.) grain stored on‐farm by small‐scale farmers, thereby threatening food security throughout the developing world. We compared weevil resistance of S1, S2, S3, and S4 inbred lines with that of their testcross hybrids, studied gene action for resistance, and developed recommendations for weevil resistance breeding in maize. Weevil resistance was evaluated for lines and hybrids from two maize populations by infesting F2 grain samples with weevils and incubating the samples in a controlled temperature and humidity laboratory. Genotypic variation among lines and hybrids and both general and specific combining ability effects were significant for weevil resistance. Broad sense heritabilities were 62 and 50% for number of weevils emerged and 67 and 55% for grain weight loss among lines and hybrids, respectively. Weevil resistance of inbreds was generally not significantly correlated with, and thus could not be a useful predictor of, resistance of their hybrids. Grain weight loss from infestation and feeding was identified as a useful trait for weevil resistance breeding because of its moderate heritability, moderately high phenotypic correlation with number of weevils emerged (r = 0.55 to 0.77, p &lt; 0.001), and ease of measurement. Useful genetic variation exists, and maize breeding programs can address weevil resistance with strategies that use both additive and nonadditive gene action.

QTL Mapping of Tropical Maize Grain Components Associated with Maize Weevil Resistance

The maize weevil (MW), Sitophilus zeamais (Motsch.), is a globally distributed pest that causes severe losses in stored maize (Zea mays L.) worldwide. A previous study identified quantitative trait loci (QTL) contributing to reduced grain damage and losses by MW in the maize mapping population CML290 × Muneng‐8128C0HC1‐18‐2‐1‐1. This study was conducted to identify QTL associated with the identified biochemical bases of resistance in the maize grain. Eleven traits were analyzed on grain samples from F2 lines across two environments: p‐coumaric acid (p‐CA), cis‐ and trans‐ferulic acid (FA), four isomers of diferulic acids (DiFA), phenolic acid amides (p‐coumaroyl‐feruloyl putrescine [CFP] and diferuloyl putrescine [DFP]), total DiFA and total phenols acids (PhA). Significant variations were observed for most of the traits, while heritabilities ranged from 0.09 to 0.79. Ten QTL were identified for trans‐ and cis‐FA, 8,5′‐DiFA, 8‐O‐4′‐DiFA, 8,5′‐diferulic acid benzofuran form, DFP, and CFP while seven QTL were identified for p‐CA, 5,5′‐DiFA, FA, total DiFAs, and total PhA. These QTL were dispersed across the maize genome and explain between 25 and 47% of the phenotypic variance (σ2p) and 50 to 98% of the genotypic variance (σ2g). Nine regions were common between QTL associated with MW susceptibility and cell wall bound compounds, suggesting a strong genetic association with MW resistance in tropical maize.

Appraisal of Protocol for the Rapid Screening of Maize Genotypes for Maize Weevil Resistance

A simplified, rapid protocol for screening maize genotypes for grain weevil resistance is required, therefore some modifications to the standard method (Dobie 1974) were appraised. The optimum oviposition period and minimum number of randomly selected weevils (Sitophilus zeamais Motschulsky) from a laboratory culture (without the procedure of sexing) to achieve similar progeny emergence to the standard method were determined under laboratory conditions. While results confirmed the effectiveness of the standard method in discriminating genotypes according to resistance, some modifications significantly reducing labour and level of skill required in implementing a rapid screening protocol were identified. Sex ratios in random samples of 10–50 weevils from a large laboratory culture were not significantly different from the 1:1 expected ratio (P >0.05) and therefore sexing of the populations was discarded. Infestation of grain with 32–50 weevils resulted in progeny emergence and kernel damage similar to the ...

Pea weevil, Bruchus pisorum L. (Coleoptera: Bruchidae), resistance in Pisum sativum×Pisum fulvum interspecific crosses

AbstractThe pea weevil, Bruchus pisorum (L.), is one of the most intractable pest problems of cultivated pea, Pisum sativum L., in the world. This study investigated the transfer of pea weevil resistance from two accessions (PI 595946, PI 343955) of wild pea, Pisum fulvum Sibth. &amp; Sm., to interspecific populations derived from crossing these accessions with a weevil‐susceptible pea cultivar (‘Alaska 81’). Partial life tables characterized weevil stage‐specific mortality and survivorship on parents and interspecific progeny in two glasshouse trials. Larval mortality rates on pods (F3 plants) of several F2:3 families were between 36.0% and 52.9%. These means were statistically similar to mean mortality rates on pods of resistant parents (45.4% and 46.2%), but significantly greater than mean rates on the susceptible parent (1.2% and 10.6%). Pod surface characteristics contributed to high neonate larval mortality on pods of resistant parents and interspecific progeny. Seed resistance was not broadly transferred to interspecific progeny [revealed by high weevil survivorship in seeds (means mostly &gt;80%) and high seed damage ratings of 3–5 where ratings of 1–2 denote resistance (production of resistant seed averaged 4.2% to 22.8%)]. Estimates of total weevil mortality on pods and seeds of eight F2:3 families were 50–70%. Thus, weevil resistance in the Pisum secondary gene pool can be transferred to interspecific progeny, thereby providing a potential avenue to develop weevil‐resistant pea cultivars.

Mapping of QTL Associated with Maize Weevil Resistance in Tropical Maize

ABSTRACTThe maize weevil (MW), Sitophilus zeamais (Motsch.), is an important pest of stored maize (Zea mays L.) in tropical areas. Quantitative trait loci (QTL) associated with MW resistance were analyzed. A total of 163 F2 progeny derived from the cross CML290 (susceptible) × Muneng‐8128 C0 HC1‐18‐2‐1‐1 (resistant), were genotyped with 151 molecular markers. Resistance was evaluated using bioassays on replicated F2:3 progenies. Grain damage (GD), grain weight losses (GWL), MW susceptibility index (Dobie index [DI]), and numbers of adult progeny (AP) were evaluated on grain from F2 lines across two environments. Grain hardness and pericarp/grain ratio were evaluated as putative components of resistance. Composite interval mapping was used for QTL analysis. Five QTL dispersed across the genome explained 28% of the phenotypic variance (σp2) and 50% of the genotypic variance (σg2) for GD. Six QTL were detected with effects on GWL and explained 22% of σp2 and 47% of σg2 Seven QTL (27% σp2 and 78% σg2) were identified for DI and three QTL (10% σp2 and 23% σg2) for AP. Both parents contributed resistance alleles. Genetic effects were mainly dominant (GD, GWL, and DI) and additive (AP). Quantitative trait loci × environment interaction was significant for 50% of the QTL. Since the identified QTL accounted for less than a third of σp2, additional mapping populations are required for mapping MW susceptibility.

Screening of Sitka spruce genotypes for resistance to the white pine weevil using artificial infestations

Sitka spruce ( Picea sitchensis (Bong.) Carr), white spruce ( P. glauca (Moench) Voss) and Engelmann spruce ( P. engelmanni (Parry)) plantations in British Columbia (B.C.), Canada, have come under serious attack from the white pine weevil, Pissodes strobi (Peck) (Coleoptera: Curculionidae). This pest attacks and destroys the terminal leader of the tree, causing serious growth losses and stem deformities. Since 1993, we have conducted a series of screening trials to search for spruce with resistance to the white pine weevil. Spruce tree selections from across the range of the species have been planted in several replicated trials. At 12 of these trial sites, in order to accelerate the screening process, local insect populations were augmented by adding reared weevils to the site. Results from our two oldest trials, at Jordan River and Cowichan Lake (planted in 1994), both on Vancouver Island, B.C., indicate that screening for weevil resistance can be effectively accomplished by using the weevil population augmentation method. Consistent selections could be obtained in trials with as few as 10 replicates per family. If weevil attack rates of 50% cumulative attack are obtained, then consistent selections may be obtained in as little as 4 years, which is a fairly quick turn-around time for studying resistance in trees.

First sources of resistance to Sitona weevil (Sitona crinitus Herbst) in wild Lens species

Lentil is one of the important cool-season food legumes grown in many countries in the Mediterranean region. But a substantial yield loss is observed every year due to various biotic stresses. The Sitona weevil (Sitona crinitus Herbst) is a major insect pest limiting lentil productivity mainly in the countries of West Asia and North Africa region. The adult insects feed on the leaflets at seedling stage, and the plant suffers due to reduced photosynthesis. The larvae feed on the root systems and on the nodules, thus decreasing the ability of the plant to fix atmospheric nitrogen. Since sources of resistance to this pest in the cultivated lentil Lens culinaris Medikus subsp. culinaris are lacking, we searched for resistant sources in a collection of wild Lens species available in the ICARDA Gene Bank. We screened 315 accessions of wild lentil covering all known species/sub-species based on nodule damage at ICARDA’s main experimental station (Tel Hadya, Aleppo), a hot-spot for the pest in the region. Large variation was observed in the percent nodule damage among accessions across species. Eight accessions, ILWL 110, ILWL 136, ILWL 166, ILWL 203, ILWL 207, ILWL 245, ILWL 254 and ILWL 258 were identified as resistant, with ≤10% nodule damage, compared to >56% damage recorded on the cultivated lentil. This is the first report of resistance against Sitona weevil in lentil. One resistant accession ILWL 245 belongs to the species L. culinaris Medikus subsp. orientalis (Boiss.) Ponert, progenitor of the cultivated lentil, which is crossable with the cultivated lentil. This line is being used to introgress resistance genes to cultivated lentil and to understand the inheritance of Sitona weevil resistance.

Soluble Peroxidase Activity in Maize Endosperm Associated with Maize Weevil Resistance

Plant peroxidases (PODs) are involved in resistance to pathogens and insects. This study investigated the role of POD in maize (Zea mays L.) resistance to the maize weevil, Sitophilus zeamais (Motsch.). Insect bioassays were performed under controlled conditions to assess maize weevil resistance. Peroxidase activity was measured in the major grain tissues using guaiacol and peroxide. Significant variation (P > 0.001) among genotypes was observed in both the insect bioassay traits and POD activity. Peroxidase was detected in the embryo, endosperm, and pedicel, but it was not detected in the pericarp. Significant correlations were found between endosperm POD activity and maize weevil resistance (r = 0.89, P < 0.001). Histological staining confirmed POD activity in the vascular cylinder of the embryo, while activity in the endosperm was restricted to the aleurone layer. This study shows that POD activity is correlated with maize weevil resistance and may be used as a potential biochemical marker.

Wound-Induced Terpene Synthase Gene Expression in Sitka Spruce That Exhibit Resistance or Susceptibility to Attack by the White Pine Weevil

We analyzed the expression pattern of various terpene synthase (TPS) genes in response to a wounding injury applied to the apical leader of Sitka spruce (Picea sitchensis Bong. Carr.) genotypes known to be resistant (R) or susceptible (S) to white pine weevil (Pissodes strobi Peck.) attack. The purpose was to test if differences in constitutive or wound-induced TPS expression can be associated with established weevil resistance. All wounding treatments were conducted on 9-year-old R and S trees growing under natural field conditions within the range of variation for weevil R and S genotypes. Representative cDNAs of the monoterpene synthase (mono-TPS), sesquiterpene synthase (sesqui-TPS), and diterpene synthase (di-TPS) classes were isolated from Sitka spruce to assess TPS transcript levels. Based on amino acid sequence similarity, the cDNAs resemble Norway spruce (Picea abies) (-)-linalool synthase (mono-TPS; PsTPS-Linl) and levopimaradiene/abietadiene synthase (di-TPS; PsTPS-LASl), and grand fir (Abies grandis) delta-selinene synthase (sesqui-TPS; PsTPS-Sell). One other mono-TPS was functionally identified as (-)-limonene synthase (PsTPS-Lim). No significant difference in constitutive expression levels for these TPSs was detected between R and S trees. However, over a postwounding period of 16 d, only R trees exhibited significant transcript accumulation for the mono- and sesqui-TPS tested. Both R and S trees exhibited a significant accumulation of PsTPS-LASl transcripts. An assessment of traumatic resin duct formation in wounded leaders showed that both R and S trees responded by forming traumatic resin ducts; however, the magnitude of this response was significantly greater in R trees. Collectively, our data imply that the induced resinosis response is an important aspect of defense in weevil R Sitka spruce trees growing under natural conditions.

Interspecific hybrids between Medicago sativa L. and annual Medicago containing Alfafa weevil resistance

Non-embryogenic protoplasts of Medicago rugosa and M. scutellata were electro-fused with iodoacetic acid-treated protoplasts of M. sativa (alfalfa). Putative somatic hybrid callus were obtained and some plants regenerated from both combinations. Hybridity of regenerants was confirmed by morphology, molecular means and cytological observations. Parental specific bands were recognized in somatic hybrids by Southern analysis. The somatic hybrids were perennial and their morphology was similar to M. sativa. Cytological observations were carried out on the somatic hybrids, their vegetative clones and self-pollinated offspring. Original somatic hybrids were aneuploids (2n=31–59), but during vegetative proliferation, their chromosome numbers reduced to 32. Those clones of hybrids formed seeds from M. sativa (+) M. rugosa by self-crossing. Chromosomal rearrangements within the parental genomes were observed in vegetative clones of hybrids and their S1 offspring by Genomic in situ Hybridization (GISH). Some of S1 offspring from M. sativa (+) M. rugosa showed better spring growth than parental M. sativa and tend to be tolerant to Alfalfa weevil. It was considered that these traits were introduced from the genome transferring M.␣rugosa chromosome to M. sativa. The cell fusion may still have a potential in transferring alien chromosomes in order to increase the genetic variation for crop breeding.

Combining Ability for Resistance to Maize Weevil among 14 Southern African Maize Inbred Lines

Maize weevil, Sitophilus zeamais Motschulsky, is an important pest of stored maize (Zea mays L.) in the tropics. We used 14 southern African maize inbred lines to assess (i) combining ability for resistance to maize weevil in F2 and F2–Syn 1 grain, (ii) the importance of maternal effects for resistance of F2 and F2–Syn 1 grain to maize weevil, and (iii) combining ability for grain yield of F1 hybrids. Maize weevil resistance of F2 grain was evaluated in 2000 for a 14‐parent diallel, and in 2002 for F2 and F2–Syn 1 grain of a 10‐parent subset diallel. Fifty‐gram grain samples of each hybrid were infested with 32 weevils for a 10‐d oviposition period, after which the samples were incubated in a laboratory. Grain yield was evaluated for the 14‐parent diallel during the summer of 1999–2000 at four locations in Zimbabwe. General combining ability (GCA), specific combining ability (SCA), and reciprocal effects were highly significant (P &lt; 0.01) for number of F1 weevils emerged from F2 grain samples for the combined 2000 and 2002 analysis for the 10‐parent subset diallel. For weevil resistance of F2–Syn 1 grain, GCA and SCA effects were significant (P &lt; 0.01), while reciprocal effects were not important. The GCA was more important than SCA for grain yield, indicating that yield was controlled mainly by additive gene action among these lines. There was no significant relationship between grain yield and weevil resistance. Although inheritance of weevil resistance was complex, our results suggest using inbred lines with good GCA for weevil resistance as female parents for hybrids or as components of synthetic (open‐pollinated) cultivars for regions where farmers often store maize grain without chemical protection against weevils.

Weevil resistance of progeny derived from putatively resistant and susceptible interior spruce parents

Controlled-cross progeny of interior spruce ( Picea glauca (Moench) Voss × Picea engelmanni Parry ex Engelm.) parents ranked as resistant or susceptible to the white pine weevil, Pissodes strobi (Peck) were screened for resistance to the same insect by augmentation of the trial site with weevils. Progeny from two resistant parents (R × R progeny) sustained significantly fewer weevil attacks (13% of the trees were attacked) in the year following the augmentation, than progeny from susceptible parents (S × S progeny) (68% of the trees were attacked). Progeny obtained by crossing one resistant and one susceptible parent (R × S progeny) sustained intermediate attack levels (47% were attacked). Characteristics of the bark resin canals of the crosses were explored using microscopy techniques. Bark resin canal density was highest in R × R progeny, lowest in S × S progeny and intermediate in R × S progeny. There was a negative correlation between the percentage of trees attacked in each cross and the average density of the outer resin canals for each cross. A discriminant function was developed that distinguished between resistant and susceptible progeny using bark characteristics. The function was characterized by positive coefficients for outer resin canal density and inner resin canal size, and a negative coefficient for bark thickness. Thus, trees with thin bark, large inner resin canals and dense outer resin canals are more likely to be resistant to P. strobi.

Sustainable production of root and tuber crops (potato, sweet potato, indigenous potato, cassava) in southern Africa

Africa, including South Africa, is faced with a problem of increasing rural poverty that leads to increasing urbanisation, joblessness, crime, food insecurity and malnutrition. Root and tuber crops such as sweet potato and potato, as well as cassava and indigenous potato are important crops for food security. The latter are also important due to their tolerance to marginal conditions. Potato and sweet potato are of great economic value in South Africa, with well-organised marketing chains and, for potato, a large processing industry. There is one cassava starch extraction factory in operation in South Africa. A number of diseases are of importance in potato in South Africa: early blight, late blight, bacterial wilt, scab and virus. Insect pests such as tuber moth and leaf miner are also constraints. In sweet potato the occurrence of viruses and weevils, as well as the availability of healthy planting material are the most important limiting factors in production. African Cassava Mosaic Disease (CMD) caused by a virus, is a problem in growing cassava. Plant biotechnology applications offer a number of sustainable solutions. Basic applications such as in vitro genebanking where large numbers of accessions can be maintained in a small space, meristem cultures to produce virus-free plants and mass propagation of popular cultivars in order to make planting material available for sustainable production. More advanced biotechnology applications that may be of value are molecular marker technology and genetic engineering. The latter can play a role in overcoming virus and potato tuber moth in potato, in resistance to CMD in cassava and possibly in sweet potato to incorporate virus and weevil resistance.

English

&nbsp; Banana weevil is a serious pest of bananas and plantains in Africa. The development of resistant cultivars is seen as the long term and more sustainable control strategy. The difficulty in conventional breeding of bananas and plantains has prompted efforts towards the use of genetic transformation for banana and plantain improvement. In this review, the current status of banana weevil resistance, sources of resistance and resistance mechanisms is assessed. Further, current efforts and future prospects for identifying resistance genes outside the genus&nbsp;Musa&nbsp;with potential to control banana weevil in a transgenic approach are outlined and discussed. &nbsp; Key words:&nbsp;Banana weevil, host plant resistance, pest resistance genes, transgenic plants

Divergent Selection for Resistance to Maize Weevil in Six Maize Populations

Maize weevil (Sitophilus zeamais Motschulsky) is an important pest of maize (Zea mays L.) in the tropics, causing serious losses for many resource‐poor farmers who store grain on‐farm for use as food and seed. This study evaluated whether weevil resistance of six maize populations could be divergently changed by S1 and S2 selection, and assessed the importance of replicating grain samples when screening for resistance. Weevil resistance was evaluated for unreplicated S1 and for replicated and unreplicated S2 lines by infesting 50‐g grain samples with 32 young adult weevils, and then incubating the samples in a controlled temperature and relative humidity (CTH) laboratory. Divergent synthetics were formed by recombining the most resistant 10% and the most susceptible 10% of at least 100 lines screened for weevil resistance for each maize population. Replicated S2 selection was successful for both populations where it was applied, resulting in an average of 16% (P &lt; 0.01), 49% (P &lt; 0.05), and 20% (P &lt; 0.01) difference between divergent synthetics for weevil progeny emerged, grain weight loss, and the Dobie index of susceptibility, respectively. S1 unreplicated selection was successful for two of the six populations, while S2 unreplicated selection was not successful. Reciprocal effects were significant (P &lt; 0.01) for weevil resistance of F1 varietal crosses among the divergently selected synthetics, indicating the influence of maternal effects. Nevertheless, the most resistant crosses were those among the most resistant synthetics, confirming that additive gene action for weevil resistance was important. Our results provide practical insights regarding methodologies and demonstrate that it is possible to improve weevil resistance of maize populations.