Interaction Between the Seed-Chalcid Wasp, Megastigmus spermotrophus and its Host, Douglas-fir (Pseudotsuga menziesii)




Donaleshen, Kathleen Louise

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Megastigmus spermotrophus is a parasitic chalcid wasp that spends most of its life in the seed of its host, Douglas-fir (Pseudotsuga menziesii). The adult female wasp lays its eggs into the megagametophyte deep within the ovule; the larva prevents an unpollinated ovule from aborting, redirecting resources to feed itself. Host-site selection pressures that influence female oviposition depend on a number of factors. Morphological characteristics of Douglas-fir cones including seed size, seed location, and scale thickness were measured for every ovuliferous scale. Seeds infested by M. spermotrophus as well as seeds fused to galls intiated by a competing conophyte, Contarinia oregonensis were noted. Using a generalized linear mixed effects model, I found that seed position, and the presence of C. oregonensis, were strong predictors of Megastigmus infestation. The percent of M. spermotrophus infested seed was higher in the apical and basal regions of the cone where seeds were smaller, scales were thinner and C. oregonensis were less frequently found. M. spermotrophus was also found to exploit seeds in regions of the cone, where seeds rarely complete development. These data suggest that competitors may not be the only factor influencing infestation; factors of cone morphology are also important. Douglas-fir seed does not show any anatomically detectable defense response to Megastigmus attack. To study mechanisms of host manipulation and defense response of the seed I took a genomics approach. Four types of ovules/seeds were studied: 1. pollinated & uninfested, 2. pollinated & infested, 3. unpollinated & uninfested, and 4. unpollinated and infested. A de novo reference transcriptome was assembled. Transcripts were annotated based on sequence similarity to genes of Pinus taeda, Arabidopsis thaliana, Nasonia vitripennis, and the UniProt database. Expression values were estimated based on the alignment of the original reads back onto the reference transcriptome. Differentially expressed transcripts were identified. Oviposition of M. spermotrophus caused changes in expression of Douglas-fir transcripts. Functional classification of differentially expressed transcripts between infested and uninfested seed revealed genes with possible roles in wounding, but none specific to herbivory. Infested treatments had more transcripts similarly expressed to pollinated than unpollinated seeds suggesting that M. spermotrophus is capable of manipulating gene expression. These transcripts had functional roles related to seed storage, cell division and growth, solute transport, hormone signalling, and programmed cell death among others. Overall, this study reveals a select set of genes that may be involved in stress response to wounding and also genes important for seed development and maturation.



parasitic, infestation, wasps, morphology