The entomopathogenic nematode Steinernema hermaphroditum is a self-fertilizing hermaphrodite and a genetically tractable system for the study of parasitic and mutualistic symbiosis

• Published in: Genetics (Austin) Vol. 220; no. 1
• Main Authors: Cao, Mengyi, Schwartz, Hillel T, Tan, Chieh-Hsiang, Sternberg, Paul W
• Format: Journal Article
• Language: English
• Published: United States Oxford University Press 04.01.2022; Genetics Society of America
• Subjects: Bacteria; Cryopreservation; Entomopathogenic nematodes; Fertilization; Gene mapping; Genetic analysis; Genetics; Hermaphrodites; Hermaphroditism; Insects; Intestine; Investigation; Mutagenesis; Mutants; Nematodes; Parasitism; Parthenogenesis; Phenotypes; Self-fertilization; Steinernema; Symbiosis; hermaphroditism; entomopathogenic; nematode; genetic screen; symbiosis; parasitism; Steinernema; Xenorhabdus; model system; mutualism
• ISSN: 1943-2631; 0016-6731; 1943-2631
• DOI: 10.1093/genetics/iyab170

Abstract Entomopathogenic nematodes (EPNs), including Heterorhabditis and Steinernema, are parasitic to insects and contain mutualistically symbiotic bacteria in their intestines (Photorhabdus and Xenorhabdus, respectively) and therefore offer opportunities to study both mutualistic and parasitic symbiosis. The establishment of genetic tools in EPNs has been impeded by limited genetic tractability, inconsistent growth in vitro, variable cryopreservation, and low mating efficiency. We obtained the recently described Steinernema hermaphroditum strain CS34 and optimized its in vitro growth, with a rapid generation time on a lawn of its native symbiotic bacteria Xenorhabdus griffiniae. We developed a simple and efficient cryopreservation method. Previously, S. hermaphroditum isolated from insect hosts was described as producing hermaphrodites in the first generation. We discovered that CS34, when grown in vitro, produced consecutive generations of autonomously reproducing hermaphrodites accompanied by rare males. We performed mutagenesis screens in S. hermaphroditum that produced mutant lines with visible and heritable phenotypes. Genetic analysis of the mutants demonstrated that this species reproduces by self-fertilization rather than parthenogenesis and that its sex is determined chromosomally. Genetic mapping has thus far identified markers on the X chromosome and three of four autosomes. We report that S. hermaphroditum CS34 is the first consistently hermaphroditic EPN and is suitable for genetic model development to study naturally occurring mutualistic symbiosis and insect parasitism. Entomopathogenic nematodes (EPNs) seek and invade insect hosts and release the mutually symbiotic pathogenic bacteria they carry. Together they kill the insect, and the bacteria act as a food source for the nematode. Cao, Schwartz, Tan, and Sternberg report development of the EPN Steinernema hermaphroditum, which is consistently hermaphroditic through self-fertilization, as an experimental genetic system for the study of symbiosis and parasitism. They describe the sex determination of this species and present efficient methods for in vitro culture, cryopreservation, and mutagenesis.