Novel Integrative Mobilizable Elements (IMEs) in Vibrio Chromosomes Linked to Translucent Post-Larvae Disease (TPD)

Abstract

Translucent post-larvae disease (TPD), caused by specific Vibrio parahaemolyticus strains, poses a global threat to shrimp aquaculture. A key virulence factor in these strains is the vhv gene cluster, which encodes toxins central to TPD pathogenesis. While plasmid-borne vhv genes have been extensively studied, the chromosome-borne vhv genes in TPD-associated strains remain poorly characterized. In this study, we systematically analyzed integrative mobilizable elements (IMEs) on the small secondary chromosomes of TPD-associated V. parahaemolyticus. Seven nearly identical novel IMEs harboring full-length vhv clusters were identified from seven independent strains. These IMEs encode a tyrosine recombinase for site-specific chromosomal integration, yet lack core conjugative transfer machineries including origin-of-transfer (oriT), relaxase, type IV coupling protein (T4CP), and IV secretion system (T4SS), consistent with the definition of non-autonomous IMEs. Each IME carries diverse functional cargo genes, covering five virulence factors (including previously unreported colonization and chemotaxis genes specific to TPD mobile elements), one pollutant-degrading enzyme, and two distinct anti-phage defense systems (PsyrTA and Lamassu-Fam), thereby potentially boosting the host’s fitness and pathogenicity in aquaculture environments. This study provides evidence for the critical role of IMEs in mediating the horizontal transfer of chromosomal vhv, offering new insights into TPD epidemiology and a foundation for developing targeted strategies to mitigate the disease’s impact on shrimp aquaculture.