Efficacy of a cry1Ab Gene for Control of Maruca vitrata (Lepidoptera: Crambidae) in Cowpea (Fabales: Fabaceae)

Cowpea [Vigna unguiculata (L) Walp.] is an important staple legume in the diet of many households in sub-Saharan Africa. Its production, however, is negatively impacted by many insect pests including bean pod borer, Maruca vitrata F., which can cause 20-80% yield loss. Several genetically engineered cowpea events that contain a cry1Ab gene from Bacillus thuringiensis (Bt) for resistance against M. vitrata were evaluated in Nigeria, Burkina Faso, and Ghana (West Africa), where cowpea is commonly grown. As part of the regulatory safety package, these efficacy data were developed and evaluated by in-country scientists. The Bt-cowpea lines were planted in confined field trials under Insect-proof netting and artificially infested with up to 500 M. vitrata larvae per plant during bud formation and flowering periods. Bt-cowpea lines provided nearly complete pod and seed protection and in most cases resulted in significantly increased seed yield over non-Bt control lines. An integrated pest management strategy that includes use of Bt-cowpea augmented with minimal insecticide treatment for protection against other insects is recommended to control pod borer to enhance cowpea production. The insect resistance management plan is based on the high-dose refuge strategy where non-Bt-cowpea and natural refuges are expected to provide M. vitrata susceptible to Cry1Ab protein. In addition, there will be a limited release of this product until a two-toxin cowpea pyramid is released. Other than South African genetically engineered crops, Bt-cowpea is the first genetically engineered food crop developed by the public sector and approved for release in sub-Saharan Africa.

Identification of novel resistance gene sources to cowpea aphid (Aphis craccivora Koch) in cowpea (Vigna unguiculata L.)

The development of cowpea aphid larvae was monitored on seven cowpea genotypes (IAR-48, TVu-15866, IT84S-2246-4, SAKA BABBA SATA, IT90K-76, KANANNADO and TVX 3236). The aim of the study was to determine the developmental response of the larvae as an indication of antibiotic resistance of the genotypes. Highly significant differences (p < 0.01) were observed with respect to fertility, larval development, adult longevity, life span, multiplication rate and intrinsic rate of increase. KANANNADO and TVX 3236 show minimum antibiotic effects while a landrace SAKA BABBA SATA shows relatively high antibiotic effects. This result further reveals the potential of SAKA BABBA SATA as a resistance source to aphid. The reaction of IT84S-2246-4, a hitherto aphid resistant genotype, which supported higher levels of survival of the larvae relative to other known susceptible genotype IAR-48, may be an indication of the presence of a new biotype of Aphis craccivora endemic to Zaria environs, or that of the ability of insects to overcome hindrances to their survival including various forms of resistance.

The spatial genetic differentiation of the legume pod borer, Maruca vitrata F. (Lepidoptera: Crambidae) populations in West Africa

The legume pod borer, Maruca vitrata, is an endemic insect pest that causes significant yield loss to the cowpea crop in West Africa. The application of population genetic tools is important in the management of insect pests but such data on M. vitrata is lacking. We applied a set of six microsatellite markers to assess the population structure of M. vitrata collected at five sites from Burkina Faso, Niger and Nigeria. Observed polymorphisms ranged from one (marker 3393) to eight (marker 32008) alleles per locus. Observed and expected heterozygosities ranged from 0.0 to 0.8 and 0.0 to 0.6, respectively. Three of the loci in samples from Nigeria and Burkina Faso deviated significantly from Hardy-Weinberg Equilibrium (HWE), whereas no loci deviated significantly in samples from Niger. Analysis of molecular variance (AMOVA) indicated that 67.3% level of the genetic variation was within individuals compared to 17.3% among populations. A global estimate of F ST=0.1 (ENA corrected F ST=0.1) was significant (P⩽0.05) and corroborated by pairwise F ST values that were significant among all possible comparisons. A significant correlation was predicted between genetic divergence and geographic distance between subpopulations (R2=0.6, P=0.04), and cluster analysis by the program STRUCTURE predicted that co-ancestry of genotypes were indicative of three distinct populations. The spatial genetic variance among M. vitrata in West Africa may be due to limited gene flow, south-north seasonal movement pattern or other reproductive barriers. This information is important for the cultural, chemical and biological control strategies for managing M. vitrata.

Transcriptome sequencing, and rapid development and application of SNP markers for the legume pod borer Maruca vitrata (Lepidoptera: Crambidae)

The legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), is an insect pest species of crops grown by subsistence farmers in tropical regions of Africa. We present the de novo assembly of 3729 contigs from 454- and Sanger-derived sequencing reads for midgut, salivary, and whole adult tissues of this non-model species. Functional annotation predicted that 1320 M. vitrata protein coding genes are present, of which 631 have orthologs within the Bombyx mori gene model. A homology-based analysis assigned M. vitrata genes into a group of paralogs, but these were subsequently partitioned into putative orthologs following phylogenetic analyses. Following sequence quality filtering, a total of 1542 putative single nucleotide polymorphisms (SNPs) were predicted within M. vitrata contig assemblies. Seventy one of 1078 designed molecular genetic markers were used to screen M. vitrata samples from five collection sites in West Africa. Population substructure may be present with significant implications in the insect resistance management recommendations pertaining to the release of biological control agents or transgenic cowpea that express Bacillus thuringiensis crystal toxins. Mutation data derived from transcriptome sequencing is an expeditious and economical source for genetic markers that allow evaluation of ecological differentiation.

Regulatory considerations surrounding the deployment of Bt-expressing cowpea in Africa: report of the deliberations of an expert panel

Cowpea (Vigna unguiculata spp unguiculata) is adapted to the drier agro-ecological zones of West Africa where it is a major source of dietary protein and widely used as a fodder crop. Improving the productivity of cowpea can enhance food availability and security in West Africa. Insect predation--predominately from the legume pod borer (Maruca vitrata), flower thrips (Megalurothrips sjostedti) and a complex of pod-sucking bugs (e.g., Clavigralla spp)--is a major yield-limiting factor in West African cowpea production. Dramatic increases in yield are shown when M. vitrata is controlled with insecticides. However, availability, costs, and safety considerations limit pesticides as a viable option for boosting cowpea production. Development of Bt-cowpea through genetic modification (GM) to control the legume pod borer is a promising approach to cowpea improvement. Cowpea expressing the lepidopteran-active Cry1Ab protein from Bacillus thuringiensis is being developed as a first generation Bt-cowpea crop for West Africa. Appropriate stewardship of Bt-cowpea to assure its sustainability under West African conditions is critical to its successful development. A first step in this process is an environmental risk assessment to determine the likelihood and magnitude of adverse effects of the Cry1Ab protein on key environmental protection goals in West Africa. Here we describe the results of an expert panel convened in 2009 to develop the problem formulation phase for Bt-cowpea and to address specific issues around gene flow, non-target arthropods, and insect resistance management.

Mitochondrial genome sequence and expression profiling for the legume pod borer Maruca vitrata (Lepidoptera: Crambidae)

We report the assembly of the 14,054 bp near complete sequencing of the mitochondrial genome of the legume pod borer (LPB), Maruca vitrata (Lepidoptera: Crambidae), which we subsequently used to estimate divergence and relationships within the lepidopteran lineage. The arrangement and orientation of the 13 protein-coding, 2 rRNA, and 19 tRNA genes sequenced was typical of insect mitochondrial DNA sequences described to date. The sequence contained a high A+T content of 80.1% and a bias for the use of codons with A or T nucleotides in the 3rd position. Transcript mapping with midgut and salivary gland ESTs for mitochondrial genome annotation showed that translation from protein-coding genes initiates and terminates at standard mitochondrial codons, except for the coxI gene, which may start from an arginine CGA codon. The genomic copy of coxII terminates at a T nucleotide, and a proposed polyadenylation mechanism for completion of the TAA stop codon was confirmed by comparisons to EST data. EST contig data further showed that mature M. vitrata mitochondrial transcripts are monocistronic, except for bicistronic transcripts for overlapping genes nd4/nd4L and nd6/cytb, and a tricistronic transcript for atp8/atp6/coxIII. This processing of polycistronic mitochondrial transcripts adheres to the tRNA punctuated cleavage mechanism, whereby mature transcripts are cleaved only at intervening tRNA gene sequences. In contrast, the tricistronic atp8/atp6/coxIII in Drosophila is present as separate atp8/atp6 and coxIII transcripts despite the lack of an intervening tRNA. Our results indicate that mitochondrial processing mechanisms vary between arthropod species, and that it is crucial to use transcriptional information to obtain full annotation of mitochondrial genomes.

Genetics of rough seed coat texture in cowpea

Seed coat texture is an important trait in determining the acceptability of cowpea varieties in different regions. A rough seed coat is preferred in western and central Africa, since it permits easy removal of the seed coat which is essential for indigenous food preparations. On the other hand, a smooth seed coat is preferred in eastern and southern Africa as well as in parts of South America where cowpea is consumed as boiled beans without removing the seed coats. This study was undertaken to elucidate the inheritance of seed coat texture so that cowpea breeders may adopt appropriate breeding strategy to develop cowpea varieties with preferred seed types for different regions. The F1 plants between smooth- and rough-seeded parents as well as between rough- and rough-seeded parents produced smooth seeds, indicating a complementary gene action and dominance for smooth seed coat. The F2 plants from the smooth x rough cross segregated into a 3 smooth:1 rough seed coat ratio, but the F2 plants from rough x rough crosses segregated into a 9 smooth:7 rough seed coat ratio. The F1 plants from backcross to the smooth parent were all smooth, while the F1 plants from backcross to rough parent segregated in a 1 smooth:1 rough seed coat ratio. However, both the backcross populations in rough x rough crosses segregated into 1 smooth:1 rough seed coat ratio. These results indicate that two pairs of independent recessive genes confer rough seed coat texture in cowpea and the presence of at least one dominant gene at each of the two loci results into smooth seed coat. The gene symbols rt1rt1 and rt2rt2 are being assigned for rough seed coat texture in cowpea.