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Title: Outcrossing and crossbreeding recovers deteriorated traits in laboratory cultured Steinernema carpocapsae nematodes

item CHASTON, JOHN - Brigham Young University
item DILLMAN, ADLER - Brigham Young University
item Shapiro Ilan, David
item BILGRAMI, ANWAR L. - Rutgers University
item GAUGLER, RANDY - Rutgers University
item Hopper, Keith
item ADAMS, BYRON - Brigham Young University

Submitted to: International Journal for Parasitology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/18/2011
Publication Date: 5/1/2011
Citation: Chaston, J.M., Dillman, A.R., Shapiro Ilan, D.I., Bilgrami, A., Gaugler, R., Hopper, K.R., Adams, B.J. 2011. Outcrossing and crossbreeding recovers deteriorated traits in laboratory cultured Steinernema carpocapsae nematodes. International Journal for Parasitology. 41:801-809.

Interpretive Summary: Entomopathogenic nematodes, also called beneficial nematodes, are small round worms that are used as natural biological agents to control a wide variety of insect pests. When these nematodes are cultured, certain traits that are important to their efficacy in biological pest control may deteriorate, e.g., traits such as virulence (killing-power), reproductive capacity, and heat tolerance. Trait deterioration may be caused be genetic factors such as inbreeding or selection, or by non-genetic factors such as nutritional limitations. In this study our objective was to determine the basis of trait deterioration in the entomopathogenic nematode Steinernema carpocapsae. To address our objective, we conducted controlled crosses between deteriorated nematode populations and non-deteriorated (control) populations. Our results indicated that the trait deterioration observed was due (at least in part) to genetic causes, and the changes were likely due to inbreeding (rather than selection). Understanding the basis for trait deterioration will aid in the development of approaches to overcome the problem and lead to improved natural pest control methods.

Technical Abstract: The nematode Steinernema carpocapsae is used as a natural biocontrol agent for suppression of many economically important insect pests. Culturing of the nematodes prior to distribution for biocontrol commonly results in deterioration of traits that are essential for nematode efficacy. To better understand the mechanisms underlying trait deterioration in entomopathogenic nematodes we explored the evolution of reproductive capacity, heat tolerance, insect virulence, and nictation among inbred lines derived from natural, random mating populations of Steinernema carpocapsae. Nematode populations with trait values below wild-type levels regained wild-type levels of traits when outcrossed to a parental, non-deteriorated strain for reproductive and heat tolerance traits. Virulence and nictation did not deteriorate in our experiments. Crossing two trait-deteriorated populations with each other also resulted in restoration of trait means to wild-type levels in progeny nematodes. Our results implicate inbreeding depression as the primary cause of trait deterioration in the populations tested but are not consistent with the currently favored model of inbreeding depression, the dominance hypothesis. Our results suggest a possible method for improvement of nematode breeding programs and highlight how entomopathogenic nematodes may represent an emerging model for the empirical validation of existing models of inbreeding depression.