hemolytic phagocytosis through interacting with the light chain of
myosin to form a protein complex during pathogen elimination
[27]. The transcription level of Ran gene in WSSV-infected shrimp
was obviously up-regulated in comparison with that in WSSVresistant
shrimp [27]. Further study demonstrated that the depletion
of Ran by RNAi in M. japonicus caused a significant increase of
virus copies, and the overexpression resulted in a significant
decrease of virus copies [50]. In accordance, the expression level of
Ran gene increased by 1.9-folds at 6 hpi, gradually decreased, and
then increased again by 5-folds at 48 h post WSSV injection in this
study. In bacteria V. anguillarum single-infection group, the transcriptional
levels increased significantly within 24 hpi than that in
virus single-infection group. Our results were in agreement with
the up-regulation of Ran gene in large yellow croaker (Pseudosciaena
crocea) post induction with formalin-inactive Vibrio parahaemolyticus
[24]. However, in the multiple-infection group, the
expression levels were suppressed significantly until 48 hpi. The
lower transcription of Ran gene will reduce the phagocytosis of
hemocytes and elimination of pathogens, and result in higher
infection or death.
In summary, this study demonstrated that 1) the shrimp in
multiple infection groups suffered higher mortality and death
occurred more quickly than groups with single infection by WSSV
or V. anguillarum only; 2) the transcription levels of immunerelated
genes, including proPOs, PPAEs, Ran and Mas, were
depressed in multiple infection groups. The depression of immune
genes resulted in low PO activity and phagocytosis and then led to
high mortality by secondary infection. This study provided information
for understanding the effect of multiple infections in
shrimp production. Further study is needed to develop prevention
and management strategies to reduce losses caused by multiple
pathogens in aquaculture.