Stress caused by sudden changes in temperature or chronic heat stimuli above normal optimum can disturb cellular homeostasis and lead to severe retardation in growth and development, and even death. Ongoing global climate change is predicted to affect organisms during all life stages, thereby affecting populations of a species, communities and the functioning of ecosystems (Pörtner and Peck, 2010). In the freshwater systems, the general effects of climate change on environmental variables will likely be increased water temperature, decreased dissolved oxygen and the increased toxicity of pollutants. Therefore, as it worsens over time, global climate change will become a more significant stressor for fish living in natural or artificial systems (Ficke et al., 2007). Water temperature is a major seasonal environmental factor that can also undergo daily fluctuations and short erratic lows and highs. Each species of the aquatic ectotherms has evolved physiologically to live within a specific range of environmental variation, and existence outside of that range can be stressful or fatal (Roessig et al., 2004). The acclimation temperature (both constant and cyclic), magnitude and direction of the temperature shift, frequency of temperature change and rate of temperature change can have important effects on the their life history (Todd et al., 2008). A variety of physiological functions such as growth, metabolism, reproduction success, food consumption, and the capacity to maintain internal homeostasis capacity of aquatic species will be affected in response to temperature fluctuation (Pörtner and Peck, 2010).

Catfish species have a wide range of natural habitats, and harbor great plasticity when they encounter temperature variations. It must undergo and adapt seasonal temperature changes from near freezing during winter in the North to over 36^oC in the summer in the South (Ju et al., 2002). Water temperatures in aquaculture ponds currently approach upper thermal tolerance (∼37^oC) levels for channel catfish, particularly in June-August, which routinely see daily maximum values of 29^oC and higher (Arnold et al., 2013). Production rate in catfish ponds could be decreased due to a decreased dissolved oxygen levels and an increased virulence of pathogens caused by high temperature. A drop in dissolved oxygen levels can lower the management capacity of water from uneaten feed, fecal matter, and fish metabolism, which could lower the reproductive capacity of catfish ponds (Ficke, 2005).

Heat stress also adversely affect growth, reproduction, and survival of channel catfish (McCauley and Beitinger, 1992; Arnold et al., 2013; Stewart et al., 2015). The effects of three cycling upper-range temperature regimes (23-27^oC, 27-31^oC, and 31-35^oC) characteristic of aquaculture environments on juvenile channel catfish growth, and feeding performance have been studied (Arnold et al., 2013). The survival rate of catfish was significantly decreased for individuals under the highest temperature regime. The growth rate of channel catfish was reduced as well. Therefore, increased water temperature may present major challenges to the culture and management of catfish (Arnold et al., 2013).

Heat tolerance between channel catfish, blue catfish and hybrid catfish

In United States, the majority of catfish production are in the southeast (92%), where some of the warmest temperatures are found (Stewart et al., 2015). Channel catfish has a natural geographical distribution from southern Canada to northern Mexico, which encompasses a temperature range from 5 to 35^oC (Bennett et al., 1998; Tavares‐Dias and Moraes, 2007; Stewart et al., 2015). Blue Catfish are distributed further south, ranging from the Mississippi River basin and Gulf Coast through Mexico and into Guatemala and Belize (Graham, 1999). In general, blue catfish has a higher level of heat tolerance than channel catfish. The hybrid catfish also have a higher level of   heat tolerance than channel catfish (Stewart et al., 2015). It was reported that the optimum water temperature for channel catfish best growth performance ranging from 27-32°C (Stewart et al., 2015). However, the Southeastern U.S. ponds reach daily maximum as high as of 34-36°C with daily fluctuations averaging 4^oC in May-August (Arnold et al., 2013), indicating that aquaculture catfish is under constant heat stress.

Knowledge about the heat tolerance between different channel catfish strains, blue catfish strains and their hybrid catfish is very limited. It was first found that little to no geographic variation in incipient upper lethal temperature (IULT) of Channel Catfish from Florida and Ohio (Hart, 1952). However, the study was limited by small sample sizes. Thereafter, critical thermal maximum (CTmax) was used to examine the thermal sensitivity of catfish to acute temperature fluctuations, which can provide guidelines for best culture management practices (Bennett et al., 1998). It was observed that the CTmax ranged from 38.6^oC to 40.3^oC for two geographically distinct strains of channel catfish (Stewart and Allen, 2014). Catfish with a southern distribution (Delta Select strain, from the Mississippi Delta, Mississippi) had a greater CTmax than did catfish with a northern distribution (Red River strain, from the Red River, North Dakota). These geographic differences in thermal tolerance were also observed in the hybrid catfish, suggesting a genetic component for thermal tolerance in catfish.

The molecular mechanisms underlying heat stress in catfish

Knowledges from several model species reveal that a series of evolutionarily conserved stress-responsive genes display distinct expression for heat stress, including genes involved protein folding and repair, protein degradation and biosynthesis, energy metabolism, cell cycle and signaling, cytoskeletal reorganization and apoptosis (Kültz, 2005; Logan and Somero, 2011; Liu et al., 2013). Increasing the levels and magnitudes of stress sequentially can lead to different components of the stress response (Feder and Hofmann, 1999; Kültz, 2005; Logan and Somero, 2011; Logan and Buckley, 2015). For instance, under mild heat stress, chaperone proteins could be induced to refold proteins that have unfolded caused by heat perturbation of tertiary structure, so that to maintain protein homeostasis (Feder and Hofmann, 1999). At moderate levels of heat stress, the abnormal folded protein which cannot be rescued through activities of chaperones will be degraded by proteolysis through the ubiquitin-proteasome pathway. In addition, above a certain level of stress, basic cell activities such as cell proliferation may cease and cytoskeletal reorganization may be induced because of the cellular damage, which has attendant effects on cellular structure and function. Especially to DNA, sufficient energy needs to be redirected from housekeeping functions toward the stress response. Furthermore, when suffering severe acute stress, significant enough damage to cell will to trigger induction of apoptotic pathways (Buckley et al., 2006; Logan and Somero, 2011).

Heat shock proteins (HSPs) are a class of proteins that are produced by cells in response to exposure to stressful conditions (Zhu et al., 2016). They were first described in relation to heat shock. Several heat shock proteinase play function as intra-cellular chaperons involved in the folding and unfolding of other proteins in response to heat, oxidative and other cellular stress (Buckley and Hofmann, 2004). In catfish, a number of heat shock genes were first characterized in response to heat. The heat-shock protein (stress-70 family) was isolated from channel catfish liver in 1994 (Abukhalaf et al., 1994). Stress-70s from tissues of a number of fish species share common antigenic determinants of the protein. It has been suggested that levels of synthesis or accumulation of this protein may be useful in determining whether a particular environmental treatment is perceived by the organism as stressful. Increased expression of stress-70s were detected in stressed fish (Welch, 1990). Straight after, the cDNA sequence of a member of the channel catfish heat shock protein 70 (CF Hsp70) family was identified, as well as expression in three leukocyte cell lines was determined in 1996 (Luft et al., 1996). In this study, high levels of CF Hsp70 mRNA were constitutively expressed at optimal culture temperature (27°C), whereas heat shock (37°C) elicited only a modest induction of CF Hsp70 expression.

Transcriptome is one of the most rapid and versatile responses of organism experiencing environmental stress. In order to obtain a broad understanding of heat stress induced gene expression in catfish, RNA-Seq analysis was conducted by using the hybrid catfish, generated from crossing channel catfish female and blue catfish male, which is now widely used in aquaculture production (Liu et al., 2013). In this study, RNA-Seq was carried out on gill and liver samples from intolerant and tolerant catfish groups as well as from the control catfish group. A total of 2,260 unique genes were differentially expressed between control fish and intolerant and/or tolerant fish in gill and/or liver after heat stress. After gene ontology, enrichment and pathway analysis, the differentially expressed genes were classified into six functional categories: 1) protein folding, 2) protein degradation, 3) protein biosynthesis, 4) energy metabolism, 5) molecule and ion transport, and 6) cytoskeleton reorganization. Specifically, genes involved in oxygen transport, protein folding and degradation, and metabolic process were highly induced, while general protein synthesis was dramatically repressed in response to the lethal temperature stress. The most strongly inducible genes in this RNA-Seq study were those of molecular chaperones. Proteotoxic stressors such as heat can cause denatured proteins. The chaperone proteins are critical in maintaining protein homeostasis during cellular response to heat stress through interacting with denatured proteins, preventing their aggregation and degradation (Parsell and Lindquist, 1993). For example, three members of the HSP40 family: DNAJA1, DNAJA4, and DNAJB1B, three HSP70 proteins: HSPA5, HSPA1A, and HSC73L, four members of the HSP90 family, HSP90AA1, HSP90AA2, HSP90AB1, and HSP90B1, and two cofactors, CDC37 and AHAS1, were significantly upregulated after heat challenge. In spite of the protein folding and rescue process participated by chaperone proteins and related factors, some damaged proteins that cannot enter the chaperone pathway are degraded by either autophagy-lysosomal pathway or ubiquitin proteasome pathway (UPP). A large number of proteases such as cathepsins and Legumain were significant induced in this study. However, several genes involved in UPP proteolysis were downregulated in catfish after heat stress, indicating that the ATP-dependent proteolysis way was repressed. This can be explained by the ATP-saving for survival of catfish during exposure to long-time lethal high temperatures (Liu et al., 2013). Genes encoding proteins involved in transporting various molecules and ions throughout the cell were identified after heat stress in catfish, such as glucose, lipids, proteins, oxygen, iron or calcium throughout the cell, as well as others mediate transport though the Golgi. Notably, the gene products of transporting oxygen were most significantly upregulated in both gill and liver, such as several hemoglobin subunits. Although heat stress could reduce the capacity of oxygen delivery system (ventilation and circulation), the requirement for oxygen in transporting an elevated metabolic rate could responsible for the dramatically induced expression of hemoglobin subunit genes (Liu et al., 2013). As expected, the expression of genes involved in regulating metabolism and repair system showed up regulation in response to heat, because these processes are energy-costing. Therefore, heat stress resulted in significant induction of several ATP generating enzymes in catfish. In contrast, several genes encoding enzymes involved in respiratory chain were repressed, including genes coding for mt-ND1, mt-ND2, mt-ND6, and COX2. It’s well recognized that heat stress preferentially leads to upregulation of specific stress-related genes while downregulation of general genes involved in protein synthesis (Buckley and Hofmann, 2002; Buckley et al., 2006). In our study, the ribosomal protein genes were significantly repressed in gills of catfish exposed to high temperature. Ribosomal proteins associate with and stabilize various sub-regions of the ribosome, their repression during heat stress could be an effort to protect ribosome structure and function by replacement or substitution of these key structure components (Buckley et al., 2006; Liu et al., 2013). Besides the effects on internal cellular processes, heat stress can induce the expression of several cytoskeleton-associated proteins, including Ras GTPase activating protein-binding protein 2 (G3BP2), contractile protein tropomyosin (TPM4), matrix metalloproteinase genes (MMP9, MMP13, and MMP18) and collagen genes (COLLA1A and COLLA1B).

QTL associated with heat stress in catfish

Genome-wide association studies (GWAS) allow the detection of linked QTL in families as well as historically accumulated recombination events. Using a reference genome, candidate genes physically close to QTLs can be detected, which is useful for understanding the underlying biology of a trait by identification of genes in proximity to QTL (Dikmen et al., 2013; Geng et al., 2015). Results generated from GWAS can facilitate the genetic selection of breeds and species with valuable performance traits. In catfish, GWAS have been conducted to identify QTLs associated with several important traits, including disease resistance for columnaris (Geng et al., 2015) and ESC (Zhou et al., 2017b; Shi et al., 2018; Tan et al., 2018), growth rate (Li et al., 2017), head size (Geng et al., 2016), body conformation (Geng et al., 2017), and low oxygen tolerance (Wang et al., 2017b; Zhong et al., 2017). Considering the ongoing global climate change, developing heat-tolerant catfish lines becomes an important goal for genetic breeding programs. A genome-wide scan for QTLs conferring resistance to heat stress was conducted by using interspecific backcross progenies and the 250 K catfish SNP array, with the objective of initial understanding of the genomic regions important for heat stress in catfish (Jin et al., 2017a).

In the GWAS analysis, three significant SNP markers were identified in response to heat stress at the genome-wide significance level [-log₁₀(P-value) ≥ 5.209]. One SNP was located on linkage group 14 and the other two SNPs were located on linkage group 16. Their minor allele frequencies ranged from 0.15 to 0.32, and the ratio of phenotypic variation (R²) explained by the SNPs ranged from 0.11 to 0.12. In additional to mapping the heat-related SNPs, heat stress associated genes surrounding each identified SNP were determined. A total of 14 genes with heat stress related functions were detected within the significant associated regions. Among them, five genes-TRAF2, FBXW5, ANAPC2, UBR1 and KLHL29, have known functions in the protein degradation process through the ubiquitination pathway. This may suggest that heat stress causes abnormal folding and irreversible damage to proteins, which are then unable to enter the molecular chaperone pathway. In order to avoid forming cytotoxic aggregates, such damaged proteins need to be removed via proteolytic degradation by covalently tagging with multiple units of ubiquitin when conjugated to a damaged polypeptide (Buckley et al., 2006; Logan and Somero, 2011). Therefore, there is an increasing necessity of degradation for cells that are suffering sufficient levels of protein damage under such lethal heat treatment. However, observations from RNA-Seq analysis reveals that complex molecular mechanisms are involved in heat stress other than simply the induction of a certain category of genes. On the other side, the levels and magnitudes of stress can lead to different components of the stress response (Logan and Buckley, 2015). In this GWAS study, genes involved in protein biosynthesis (PRPF4 and SYNCRIP), protein folding (DNAJC25), molecule and iron transport (SLC25A46 and CLIC5), cytoskeletal reorganization (COL12A1) and energy metabolism (COX7A2, PLCB1 and PLCB4) processes were also identified in the genome-wide significantly associated regions. The results further supported the notion that except for the need of enhanced protein degradation, cellular response to heat stress involves a range of biological mechanisms to stabilize cellular hemostasis.

Considering the population specificity of QTL and the minor allele effect in association analyses, future studies using larger populations or more catfish families and various catfish strains are necessary for fine mapping of the heat tolerance loci. Meanwhile, genetic evaluation of heat resistance in channel and hybrid catfish strains should be further explored in the face of managing genetic enhancement programs for heat tolerance in catfish industry (Stewart et al., 2015).

Comparison of molecular responses of catfish to diseases and heat stress

The heat-shock response (HSR) is one of the highly conserved molecular responses to disruptions of protein homeostasis. HSPs are a class of highly conserved proteins whose expressions are mostly induced to protect the proteome against elevated temperature. As mentioned above, HSPs are mainly induced under acute and mild heat stress by playing roles of molecular chaperone to refold proteins that have unfolded caused by heat perturbation of tertiary structure, so that to maintain protein homeostasis. Likewise, in response to acute stress from pathogen infection and hypoxia, HSPs are also induced to protect core biosynthetic processes. Because of the constitutive expression in non-stressed cells, HSPs are always been used as housekeeping proteins (Song et al., 2014). However, recent studies have revealed that many HSPs play important functions in innate and adaptive immunity. For example, HSPs are considered to mediate humoral and cellular innate immune responses; HSPs in extracellular environment serve as a danger signal to activate innate immune cells such as dendritic cells and macrophages (Srivastava, 2002). The interaction of HSPs with the APCs leads to several peptide-independent activities, including secretion of inflammatory cytokines tumour-necrosis-α(TNF-α), interleukin-1β (IL-1β), IL-12 and granulocyte–macrophage colony-stimulating factor (GM-CSF) by macrophages (Basu et al., 2000). In catfish, families of HSP40, HSP60, HSP70, HSP90 and sHSP were characterized, and their expression profiles after E. ictaluri and F. columnare bacterial infections were determined (Song et al., 2014; Xie et al., 2015; Song et al., 2016). Pathogen-specific, tissue-specific and time-specific patterns were found of these genes after infection with the two diseases. The majority of HSP genes were up-regulated at earliest stages of disease infections, whereas with the progression of the diseases, more and more HSP genes became down-regulated. The significantly regulated expression of catfish HSP genes after bacterial infections suggested their involvement in immune response in catfish.

Likewise, the up-regulation of claudin and cathepsin genes were detected after bacterial infection and heat stimuli by RNA-Seq studies. Claudins are one of the major groups of transmembrane proteins that play crucial roles in tight junctions (Sun et al., 2015). Claudin-1, a, e, i, were significantly induced after heat stress in catfish. These proteins play roles in adjusting cell volume through manipulation of transporters and cytoskeletal reorganization, respectively (Liu et al., 2013). The expression profiles of catfish claudin genes in response to ESC disease were also determined by analyzing existing RNA-Seq datasets. Significant down-regulation of claudin genes were observed in the intestine at 3 h post-infection, indicating that pathogens may disrupt the mucosal barrier by suppressing the expression of claudin genes. Cathepsins are a large group of proteases that serve as enzymes degrading damaged proteins to avoid forming cytotoxic aggregates. After heat stress, the expression of cathepsin Z, B, D, L were dramatically induced with the function of proteolysis through the autophagy-lysosomal pathway (Liu et al., 2013). After infection of E. ictaluri and F. columnare, expression of catfish cathepsin D, H, L, S genes were significantly induced, but the expression patterns varied depending on both pathogens and tissue types, suggesting that these genes may exert disparate functions or exhibit distinct tissue-specific roles in innate and adaptive immune responses (Yeh and Klesius, 2009; Feng et al., 2011; Wang et al., 2015; Dong et al., 2016). However, further studies are needed to expand functional characterization and examine regulation mechanisms of these gene in catfish after pathogen infection and environmental stresses. Same or similar gene pathways apparently regulate both biotic and abiotic stresses, as mimicked by bacterial infections and heat stresses.