Environmental drivers of trait changes in Photorhabdus luminescens

Abstract Biological control agents have become increasingly important in integrated pest management programs. However, certain traits of these agents that are needed for efficient biocontrol often decrease or are lost during in vitro rearing. Entomopathogenic nematodes (EPNs) often exhibit trait deterioration when reared under laboratory conditions. EPN trait deterioration has been attributed (at least in part) to genetic causes; however, the underlying causes of trait deterioration in its bacterial endosymbiont have not been explored. In this study the EPN symbiont Photorhabdus luminescens was monitored for the deterioration of three traits; inclusion body production, reproductive potential, and virulence, in three different nutritional environments; lipid liquid medium (LLM), nutrient broth (NB), and tryptic soy broth + yeast extract (TSY). Significant trait deterioration did not occur for any of the traits in any environment. There was an increase in inclusion body production in TSY. Additionally, there was variation in growth within NB and TSY sub-cultured population lines and one TSY sub-population line was less virulent than the other two. However, returning bacteria to LLM restored all traits to wild-type levels. We infer the observed trait deterioration in Photorhabdus was minimal and appeared to be driven by environmental conditions as opposed to stable genetic changes. Our data suggest that variation among traits of in vitro cultures of Photorhabdus is more likely due to environmental variation than inadvertent laboratory selection or other genetic processes.