Identification of the mapkk gene family in large yellow croaker (Larimichthys crocea): Involved in the regulation of immune responses to Aeromonas hydrophila and Pseudomonas plecoglossicida infections.

Mitogen-activated protein kinase kinases (Mapkks) are pivotal components of the highly conserved MAPK signaling pathway, which regulates diverse cellular processes and innate immune responses. These kinases are essential for growth, development, and adaptation to environmental stress. Although mapkk genes have been extensively characterized in diverse vertebrate and invertebrate species, a systematic investigation of these genes in the large yellow croaker (Larimichthys crocea) is lacking. Here, we identified eight Lcmapkks and investigated their molecular characteristics and expression profiles. The evolutionary phylogenetic analysis, combined with exon/intron architecture and conserved motif characterization, demonstrated significant evolutionary conservation within the mapkk gene family in the large yellow croaker. Eight Lcmapkks were mapped across six chromosomes. Lcmapkk2b, Lcmapkk4a, and Lcmapkk6 were colocalized on chromosome 12, whereas Lcmapkk4a and Lcmapkk4b were identified as segmental duplication paralogs. Transcriptomic analysis revealed ubiquitous expression of Lcmapkks across all examined tissues, with Lcmapkk4a exhibiting particularly pronounced expression levels. Aeromonas hydrophila infection significantly upregulated Lcmapkk expression in head kidney tissue samples, with hypoxia-tolerant individuals exhibiting markedly elevated transcript levels compared to control individuals. Following Pseudomonas plecoglossicida infection, the splenic expression of Lcmapkk4b and Lcmapkk7 was significantly upregulated at 24 h post-infection. These results highlight the crucial role of Lcmapkks in mediating innate immune responses in the large yellow croaker. This study reveals the immunoregulatory mechanism of members of the mapkk family in the large yellow croaker and provides a new research foundation for further analysis of the innate immune signaling pathway in fish.