MHC II α polymorphism of Nile tilapia, Oreochromis niloticus, and its association with the susceptibility to Gyrodactylus cichlidarum (Monogenea) infection

Highlights

• Oreochromis niloticus has high level of polymorphism in Orni-DAA alleles.

• The diversity of MHC II α in O. niloticus might be generated by positive selection, which was mediated by parasites.

• Oreochromis niloticus with medium number of Orni-DAA alleles had higher resistance to Gyrodactylus cichlidarum infection.

• The allele Orni-DAA*3601 was highly associated with resistance to G. cichlidarum infection, while Orni-DAA*0201 and Orni-DAA*0701 were significantly linked with susceptibility.

Abstract

Nile tilapia, Oreochromis niloticus, is among the most preeminent aquaculture species in the world. In aquaculture systems, this species is vulnerable to various pathogens including Gyrodactylus cichlidarum (Monogenea). The major histocompatibility complex (MHC) genes, a highly polymorphic gene superfamily, have vital roles in adaptive immunity and are closely associated with disease resistance. In the present study, the relationship between MHC II α polymorphism of Oreochromis niloticus and the susceptibility to G. cichlidarum infection were analyzed. A total of 50 alleles of MHC II α were identified, which demonstrated a high level of polymorphism representing a variability of 89.6% amino acid sites. In the peptide-binding region (PBR) and entire region, the values of d_N to d_S were 1.155 and 1.180, respectively. Infection experiments revealed that hosts with moderate polymorphism of MHC II α (2 to 3 alleles) had relatively higher resistance to G. cichlidarum infection. In addition, the alleles Orni-DAA*0201 and Orni-DAA*0701 were found to be significantly associated with susceptibility (P < 0.05), while Orni-DAA*3601 was highly associated with resistance (P < 0.05). The results suggested that the diversity of MHC II α allele was associated with the susceptibility of tilapia to this monogenean, which would contribute to the study on MHC diversity maintenance and molecular-assisted selection of O. niloticus to enhance disease resistance. Of course, small scale of productive tests is required to further confirm these results.

Introduction

Nile tilapia, Oreochromis niloticus, is one of the most important aquaculture species. This fish is native to Africa, but has been widely introduced and cultured throughout the world. From the Nile River in Sudan, it was first introduced to China by Yangtze River Fisheries Research Institute in 1978, and has become the main freshwater aquaculture species in south China because of its rapid growth and good taste (Li and Li, 2001). China has become the largest tilapia farming and exporting country with an annual production of 1.625 million tons (Chinese Fishery Statistics Yearbook 2019). Although the O. niloticus is a resistant species, outbreaks of disease have become a restriction to the development of Nile tilapia culture. Monogeneans are pathogenic in intensively cultured tilapia due to their direct mechanical injury, and often followed by secondary bacterial infection (Johnsen and Jensen, 1996; Buchmann and Uldal, 1997; Cable et al., 2000; Bakke et al., 2007; Shinn et al., 2015). Infection by the monogenean Gyrodactylus cichlidarum, an ectoparasite of Nile tilapia generally residing on the skin and fins of host, can cause severe disease of juvenile tilapia (García-Vásquez et al., 2007; Grano-Maldonado et al., 2018). There are, however, no documented preventative measures to its infection on O. niloticus. One potential approach is to breed new strains of O. niloticus with enhanced resistance to monogenean infection using marker-assisted selection (MAS). The precondition of MAS is to determine the key marker genes associated with resistance to infection (QTL and genetic characteristics markers).

The Major Histocompatibility Complex (MHC) was first identified for its relevance to histocompatibility and exclusion in organ/tissue transplantation, but subsequently found to be closely associated with immune responses (Klein and Figueroa, 1991; Chen et al., 2010). MHC is a group of closely linked and highly polymorphic genes that code the MHC antigens (Mona et al., 2008). Based on the different structures and functions of these molecules, MHC genes were categorized into class I and class II. Class I genes produce molecules on the surface of all nucleated cells except sperm cells and some neurons, which present endogenously derived peptides to CD8⁺ cytotoxic T-cells, so they are primarily associated with defense against intracellular pathogens such as viruses (Piertney and Oliver, 2006). MHC II molecules in fish are mainly distributed on the membranes of immune cells such as macrophages, B cells, monocytes, and dendritic cells, and can help to present exogenous cellular pathogens (e.g. bacteria, parasites) to CD4⁺ T lymphocytes (Huang and Germain, 1992; Reche and Reinherz, 2003; Rodgers and Cook, 2005). MHC II genes are closely linked and encode proteins for the recognition of extracellular antigens and their presentation to T lymphocytes (Eizaguirre et al., 2012; Stutz and Bolnick, 2017). The α and β genes in MHC class II severally encode the α and β chains that form a heterodimer (Klein et al., 2007). The α1 and β1 domains of MHC II molecule form the peptide-binding region (PBR) in T lymphocyte-mediated immune recognition of pathogens (Zhou et al., 2013).

As a result of balancing selection, class II genes have evolved extensive diversification, generally with the highest in the peptide-binding region (PBR) (Hughes and Nei, 1989; Milinski et al., 2005). Specific MHC diversity of class I and class II genes has been found to be closely related to disease resistance (Yang et al., 2016a; Li et al., 2017; Zhu et al., 2018; Cao et al., 2019). In fish, it has also been reported that the polymorphisms of MHC genes were associated with resistance/susceptibility to parasite infection, and parasite-induced selection could facilitate the maintenance of MHC polymorphism (Wegner et al., 2003a, Wegner et al., 2003b; Rakus et al., 2009a, Rakus et al., 2009b; Spurgin and Richardson, 2010; Eizaguirre et al., 2012; Hablutzel et al., 2016; Phillips et al., 2018). The association between MHC II α and its susceptibility to monogenean pathogens, which are common in aquaculture, have, however, rarely been documented.

By analyzing the MHC II α polymorphism of O. niloticus challenged with G. cichlidarum, the objective of present study is to estimate the association between MHC II α alleles diversity and resistance/susceptibility to monogenean infection. These results will contribute to the study on MHC diversity maintenance and molecular-assisted selection of O. niloticus to enhance disease resistance.