**4. Cellular-process-related genes**

#### **4.1. Ubiquitin-conjugating enzyme**

The ubiquitin proteolytic system is involved in various cellular processes such as cell cycle regulation, cellular response to stress, and immune response [33-34]. In shrimps, the ubiquitinconjugating enzyme E2r (UBE2r) gene isolated from *Marsupenaeus japonicus* showed that the gene expression level changed significantly in the developing testis and ovary with higher level in the testis than in the ovary, which indicates the functional role of UBE2r in oogenesis and spermatogenesis of shrimp [35]. The gene expression pattern of ubiquitin in abdominal muscle of 3-month-old shrimps *L. vannamei* at different molt stages on examination did not increase significantly in premolt stages and was found to be relatively stable at all stages of the molt cycle [36]. Antiviral function has been demonstrated with ubiquitin-conjugating enzyme, E2, isolated from *F. chinensis*, which could inhibit WSSV replication and ubiquitinate WSSV RING domain-containing proteins [37]. In *P. monodon* ubiquitin-conjugating enzyme E2, gene was up-regulated in muscle and gill tissues under low salinity stress conditions [20]. In American lobster, *Homarus americanus* the polyubiquitin gene expression revealed signifi‐ cant changes in abdominal muscle and hepatopancreas during osmotic stress, indicating that proteins may be more susceptible to ionic fluctuations [38].

### **4.2. Cathepsins**

**3. Immune genes**

106 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

in shrimps.

shrimp.

**4. Cellular-process-related genes**

**4.1. Ubiquitin-conjugating enzyme**

Crustins, belonging to family of antimicrobial peptides (AMPs) are shown to be differentially expressed in response to various immunostimulants and microbes [22]. Different isoforms of crustins have been isolated from a variety of penaeid shrimps [23-25]. The up-regulation of crustin-like AMP in shrimps suggests functional role of AMPs primarily in the shrimp immune response [26]. Under hyperosmotic stress, crustinPm5, crustinPm1, and crustin-like Pm gene levels were found to be up-regulated [27]. In our study, we observed crustin gene was upregulated in all the three shrimp tissues of gills, gut, and muscle analyzed at low salinity stress [20]. Hence, crustins having antimicrobial activity also functionally respond to salinity stress

Another class of AMPs, the penaeidin gene was found to be down-regulated in gill, gut, and up-regulated in antennal gland tissues of shrimp exposed to high salinity stress (55 ppt) conditions [21]. These changes in the gene expression levels of penaeidin may be regulated through variations in hemocyte numbers in salinity-stressed shrimp. Antibacterial proteins such as lysozymes are involved in nonspecific innate immunity in shrimps [28]. Significant up-regulation of lysozyme gene expression occurred in gill (16.25-fold) tissues of shrimp exposed to low salinity stress [20]. In these shrimps, the anti-lipopolysaccharide factor, which is considered to play an important role in shrimp immune response [29], was significantly down-regulated in gill and gut tissues as compared to up-regulation in the muscle tissues of

In general, as a result of environmental stress, the expression of immune-related shrimp genes transcripts gets affected [30]. The shrimp exhibits reduction in immune parameters when exposed to salinity stress conditions, leading to decreased resistance against pathogens [31-32]. White shrimp *L. vannamei*, when transferred to low salinity conditions showed significant decrease in hemocyte count, phenoloxidase activity, respiratory burst, and superoxide dismutase activity, which further reduced on challenge with *Vibrio alginolyticus.* Hence, innate immunity in shrimps reduces with combined effect of low salinity and bacterial challenge [32].

The ubiquitin proteolytic system is involved in various cellular processes such as cell cycle regulation, cellular response to stress, and immune response [33-34]. In shrimps, the ubiquitinconjugating enzyme E2r (UBE2r) gene isolated from *Marsupenaeus japonicus* showed that the gene expression level changed significantly in the developing testis and ovary with higher level in the testis than in the ovary, which indicates the functional role of UBE2r in oogenesis and spermatogenesis of shrimp [35]. The gene expression pattern of ubiquitin in abdominal muscle of 3-month-old shrimps *L. vannamei* at different molt stages on examination did not increase significantly in premolt stages and was found to be relatively stable at all stages of The proteases are distributed into four major classes: aspartyl proteases, metalloproteases, serine proteases, and cysteine proteases [39]. The cathepsins belonging to cysteine proteases in addition to cellular protein degradation and turnover are also involved in numerous other physiological processes. In fishes, the cathepsin La isoform has been implicated in yolkprocessing mechanism during oogenesis and embryogenesis [40]. In shrimps (*Metapenaeus ensis*), cathepsin L, with predominant expression in hepatopancreas, is suggested to have a role in food digestion. The immunolocalization of cathepsin L in the nucleus of oocyte suggests its specific physiological role in shrimp oocytes [41]. The cathepsin L-like proteinases isolated from *L. vannamei* showed that specific activity and mRNA expression of cysteine protease were associated during the molt cycle [42]. The expression of the cathepsin C gene expression in hemocytes of *L. vannamei* could be induced after *V. alginolyticus* challenge, which reached a maximum level at 4 h post challenge period, indicating that the cathepsins in shrimp may be involved in immune response [43]. The aspartic protease cathepsin D obtained from *M. japonicus* and *P. monodon* were characterized for physical and chemical properties and were shown to have identical subunits and similar optimal pH [44]. The analysis of cathepsin B gene expression in *L. vannamei* tissues revealed high expression levels in midgut gland and gut, which are involved in food digestion process. The changes in the gene expression levels and enzymatic activity were induced by starvation in the midgut gland of the white shrimp [45]. Significant increase in the cathepsin B gene expression levels in gills, gut, and muscle tissues of shrimp exposed to low (3 ppt) salinity conditions suggest that *P. monodon* cathepsin gene responds to the shrimp adaptive mechanism to salinity stress [20].
