4.3.2 High oxygen transport ability improves the hypoxia
tolerance of C. japonica
Although
hemocyanin has a low affinity for oxygen, it is still an essential
oxidizing medium for crustacean tissues in the absence of oxygen (Figure
11) (Charlotte, 1980; Fujieda et al., 2010). Desiccation can inhibit
oxygen supply and induce respiratory acidosis in crustaceans (Defur,
1988). Respiratory acidosis further reduces the ability of oxygen and
hemoglobin to bind, which exacerbates oxygen deficiency and tissue
hypoxia (Allen et al., 2012). However, desiccation-tolerant crustaceans
can regulate ion (including Ca2+ and
Mg2+) concentration to improve the oxygen-binding
ability of hemocyanin and promote oxygen transfer between tissues
(Charlotte, 1980). The expansion or positive selection of the IP3and PRPS1 genes means that C. japonica can maintain
Ca2+ and Mg2+ homeostasis to ensure
the oxygen-binding ability of hemocyanin to resist hypoxia in drought
environments. Hypoxia-inducible factors (HIFs) also play an important
role in hypoxia response by binding with hypoxia response elements near
the downstream gene promoter for transcriptional activation (Hirota,
2002; Martens et al., 2007). The STK gene in C. japonicacan enhance HIF phosphorylation and increase the molecular weight of
HIFs (Richard et al., 1999). In addition, the GSR gene, which
prevents the oxidative decomposition of hemocyanin, was also positively
selected or expanded. Glutathione reductase encoded by the GSRgene can effectively maintain reduced glutathione in cells, which plays
an important role in preventing the oxidative decomposition of
hemocyanin (Kanzok et al., 2001). In conclusion, the maintenance of
hemocyanin level and oxygen-carrying capacity will help C.
japonica cope with hypoxia in drought environment.