Molecular tools for characterization of the legume pod borer Maruca vitrata fabricius (lepidoptera: pyraloidea: crambidae); mode of action of hermetic storage of cowpea grain

Venu Madhav Margam, Purdue University

Abstract

Cowpea [Vigna unguiculata subsp. unguiculata (L.) Walp] is an important food legume cultivated widely in the arid and semi-arid tropics of the world. Insects are the major challenge to cowpea production and storage in West Africa, where nearly two thirds of the world's cowpea is produced and consumed. In the introductory chapter, I discuss the importance of cowpea, its major field and storage problems, and recent advances in biotechnology that offer hope for solving these problems. In the first chapter, I present results from a field survey looking for wild host plants of Maruca vitrata to assess if these can serve as natural refuges when Bt-cowpea is deployed. In the second, third and fourth chapters I describe (i) applications of the molecular barcoding approach for studying global populations of M. vitrata; (ii) results from expressed sequence tag library sequencing of M. vitrata and molecular marker discovery; and (iii) the mitochondrial genome of M. vitrata. In the fifth chapter, I discuss the mode of action of hermetic storage of cowpea grain. In the first major section of the work, two aspects of insect resistance management - needed for M. vitrata resistant Bt-cowpea deployment — have been investigated. The first is the status of wild alternate host plants of M. vitrata in the cowpea growing regions of northern Nigeria (Kano and Zaria) and southern Niger (Maradi). Results from field surveys done in these regions suggest that wild hosts are not abundantly present and thus aren't available to serve as refugia for M. vitrata. Lack of molecular markers for M. vitrata was one of the constraints to characterizing its populations. I sought to fill this gap by generating genomic DNA- as well as EST-based molecular markers. I also demonstrated that molecular barcoding using cytochrome c oxidase I can be applied to distinguish global populations of M. vitrata. Data from the barcoding study strongly suggests that the genus Maruca may be a species complex or has undergone a recent genetic bottle-neck. In the second major section of the dissertation — dealing with the mode of action of hermetic storage of cowpea grain — I was able to estimate that each cowpea bruchid requires approximately 8 -10 ml of pure O2 during its development to adult hood. Feeding behavior studies, done to investigate the cause for feeding cessation in storage, show that it is the level of O2, not CO2, that influences the bruchid feeding behavior. More importantly the larval susceptibility, visible evidence in the form of desiccated larvae, and the respiratory quotients during development suggest that inability of bruchids to make water in the absence of O2 may be a major contributing factor to the success of the hermetic method of storing cowpea grain.

Degree

Ph.D.

Advisors

Pittendrigh, Purdue University.

Subject Area

Molecular biology|Genetics|Zoology

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