Transcriptomic profiling of North American ash trees (genus Fraxinus)

Darla G French, Purdue University

Abstract

At present, North American ash species (genus Fraxinus) are under attack by the Emerald Ash Borer (Agrilus planipennis Fairmaire; Coleoptera: Buprestidae; EAB), an aggressive, invasive insect native to southeastern Asia. In contrast, native Asian ash species are comparatively resistant to this phloem-feeding insect. Currently, most researchers working on EAB are focused on improving detection, monitoring, and biological control methods, in an attempt to protect North American ashes from the advancing infestation front. To date, however, little work has been done to determine why North American ash species are susceptible to EAB. The focus of this dissertation research has been to determine if the difference in resistance level between the North American and Asian ash species is due to transcripts produced by one species but not another. This is vital information if we intend to understand how EAB affects our North American ashes. The objectives of my study were: (1) To compare transcriptomic profiles of EAB-infested and uninfested green ash in order to better understand what the defense response of a susceptible tree is to EAB infestation; (2) To use qRT-PCR analyses to validate and verify the results found in the transcriptome sequencing project; (3) To determine whether methyl jasmonate is a suitable surrogate for EAB insect feeding; and (4) To optimize growing conditions for ash seed. For objectives 1-3, tissue samples of green ash (Fraxinus pennsylvanica) were harvested and then subjected to high-throughput 454 sequencing and quantitative real-time polymerase chain reaction (qRT-PCR) analyses. Results from the transcriptomic sequencing studies of infested and uninfested green ash trees were used to identify candidate genes that may be involved in producing resistance factors. For objective 4, seeds were subjected to a variety of treatments, and germination rates were compared to identify optimal methods for ash seed germination. The research reported in this dissertation provides a new perspective on the response of Fraxinus to EAB infestation. This research may ultimately lead to many long-term benefits, which may aid in slowing the spread of this invasive insect.

Degree

Ph.D.

Advisors

Meilan, Purdue University.

Subject Area

Molecular biology|Plant biology|Forestry

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