Population genomics and phylogeography of the boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), in the United States, northern Mexico, and Argentina.

The boll weevil, Anthonomus grandis Boheman (Coleoptera: Curculionidae), is an important pest of commercial cotton across the Americas. In the United States, eradication of this species is complicated by re-infestations of areas where eradication has been previously successful and by the existence of morphologically similar variants that can confound identification efforts. To date, no study has applied a high-throughput sequencing approach to better understand the population genetic structure of the boll weevil. Furthermore, only a single study has investigated genetic relationships between populations in North and South America. We used double digest restriction site-associated DNA sequencing (ddRADseq) to resolve the population genomic structure of the boll weevil in the southern United States, northern Mexico, and Argentina. Additionally, we assembled the first complete mitochondrial genome for this species and generated a preliminary whole genome assembly, both of which were used to improve the identification of informative loci. Downstream analyses revealed two main lineages-one consisting of populations found geographically west of the Sierra Madre Occidental mountain range and the second consisting of populations found to the east-were revealed, and both were sub-structured. Population geographic structure was consistent with the isolation by distance model, indicating that geogrpahic distance is likely a primary mechanism driving divergence in this species. Boll weevil populations from Argentina were found to be more closely related to the eastern lineage, suggesting a recent colonization of South America by the eastern lineage, but additional sampling across Mexico, Central America and South America is needed to further clarify their origin. Finally, we uncovered an instance of population turnover or replacement, highlighting the temporal instability of population structure.

Molecular diagnosis of populational variants of Anthonomus grandis (Coleoptera: Curculionidae) in North America.

The utility of the cytochrome oxidase I (COI) DNA sequence used for DNA barcoding and a Sequence Characterized Amplified Region for diagnosing boll weevil, Anthonomus grandis Boheman, variants was evaluated. Maximum likelihood analysis of COI DNA sequences from 154 weevils collected from the United States and Mexico supports previous evidence for limited gene flow between weevil populations on wild cotton and commercial cotton in northern Mexico and southern United States. The wild cotton populations represent a variant of the species called the thurberia weevil, which is not regarded as a significant pest. The 31 boll weevil COI haplotypes observed in the study form two distinct haplogroups (A and B) that are supported by five fixed nucleotide differences and a phylogenetic analysis. Although wild and commercial cotton populations are closely associated with specific haplogroups, there is not a fixed difference between the thurberia weevil variant and other populations. The Sequence Characterized Amplified Region marker generated a larger number of inconclusive results than the COI gene but also supported evidence of shared genotypes between wild and commercial cotton weevil populations. These methods provide additional markers that can assist in the identification of pest weevil populations but not definitively diagnose samples.

Use of Optical Sorting to Detect Wheat Kernels Infected with Tilletia indica.

Wheat infected with Tilletia indica is subject to international regulation by 78 countries, and U.S. economic losses could exceed $1 billion if T. indica was found throughout major wheat-producing regions and caused wheat exports to be halted. Samples are currently manually inspected for the presence of kernels with Karnal bunt as part of routine survey methods. This visual inspection of all seed in a sample can result in harvest delays due to long inspection times and missed kernels due to inspector fatigue. A high-speed sorter was tested to determine if infected kernels could be rapidly removed from 1,800-g wheat samples. When the sorter removed about 8% or more of the sample, the reject portion contained 100% of the bunted kernels. Concentrating the bunted kernels in a smaller sample size will reduce sample inspection time and should reduce inspection errors. One high-speed sorter can process up to 8,800 kg/h; thus, bunted kernels can be rapidly removed from samples or large lots. Each sample was sorted in less than 1 min. This technology provides the wheat industry with a tool to rapidly inspect samples to aid in regulating Karnal bunt, and to remove bunted grains from seed wheat and wheat destined for food or feed use.