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

Abstract

The nematode Steinernema carpocapsae infects and kills many pest insects in agroecosystems and is commonly used in biocontrol of these pests. Growth of the nematodes prior to distribution for biocontrol commonly results in deterioration of traits that are essential for nematode persistence in field applications. To better understand the mechanisms underlying trait deterioration of the efficacy of natural parasitism in entomopathogenic nematodes, we explored the maintenance of fitness related traits including reproductive capacity, heat tolerance, virulence to insects and `tail standing' (formerly called nictation) among laboratory-cultured lines derived from natural, randomly mating populations of S. carpocapsae. Laboratory cultured nematode lines with fitness-related trait values below wild-type levels regained wild-type levels of reproductive and heat tolerance traits when outcrossed with a non-deteriorated line, while virulence and `tail standing' did not deteriorate in our experiments. Crossbreeding two trait-deteriorated lines with each other also resulted in restoration of trait means to wild-type levels in most crossbred lines. Our results implicate inbreeding depression as the primary cause of trait deterioration in the laboratory cultured S. carpocapsae. We further suggest the possibility of creating inbred lines purged of deleterious alleles as founders in commercial nematode growth.

Entomopathogenic Nematode Application Technology

“…Attenuation of beneficial traits, which may result from repeated sub-culturing, can jeopardize biocontrol efforts. Trait deterioration can be genetically based (e.g., inbreeding, drift, inadvertent selection), or stem from non-genetic factors (e.g., disease or nutrition) (Chaston et al, 2011;Hopper, Roush, & Powell, 1993;Tanada & Kaya, 1993). Deterioration in EPNs has been observed under laboratory conditions for various traits such as virulence, environmental tolerance, reproductive capacity, and host-finding (Bai, Shapiro-Ilan, Gaugler, & Hopper, 2005;Bilgrami, Gaugler, Shapiro-Ilan, & Adams, 2006;Shapiro, Glazer, & Segal, 1996;Wang & Grewal, 2002).…”

“…Both the nematodes and their bacterial symbionts may experience trait loss (Bilgrami et al, 2006;Wang et al, 2007). The cause of trait deterioration in EPNs (H. bacteriophora) was reported to be genetically based with inbreeding depression being the prominent issue (Adhikari et al, 2009;Bai et al, 2005;Chaston et al, 2011).…”

Genomics of Entomopathogenic Nematodes and Implications for Pest Control

“…If the new populations prove to be superior they can be further propagated and used. Multiple selected strains with improved traits (as represented by DNA markers) but with variable genetic backgrounds should be maintained and inter-crossed to prevent trait deterioration [17, 18]. …”

“…Moreover, selection of some traits can lead to inadvertent reduction of others or of overall fitness [12, 15, 16]. Inbreeding depression or other means of fitness loss during EPN-mass production or as a result of continuous laboratory culture are also concerns [17, 18]. The second major strategy to improve EPN field efficacy is to use modern genetic and molecular tools.…”

Nematode Pathogenesis of Insects and Other Pests

“…Genome size of these bacteria varies between 4.5 Mb (X. nematophila) and 4.8 Mb (X. szentirmaii). The GC content varies between 44.2 % (X. nematophila) and 45 % (X. bovienii) (Chaston et al, 2011;Gualtieri, Ogier, Pagès, Givaudan, & Gaudriault, 2014;Lanois et al, 2013). Analyses of these genomes has revealed that there is a large number of encoded adhesins, toxins, hemolysins, proteases and lipases, as well as a wide array of antibiotic synthesizing genes (Bode, 2009).…”