Spätzle protein is an extracellular ligand of Toll receptor in Toll signaling 
pathway involved in the embryonic dorsoventral patterning and in the innate 
immunity. In this study, a spätzle gene of freshwater prawn, Macrobrachium 
rosenbergii (MrSpz) was isolated and characterized. The open reading frame of 
MrSpz consisted of 747 nucleotides encoding 248 amino acid residues containing a 
signal peptide and C-terminal spätzle activated domain. MrSpz shared high 
similarity to spätzle of Fenneropenaeus chinensis (FcSpz) at 92% identity and 
Marsupenaeus japonicus (MjSpz) at 83% identity. Phylogenetic analysis was 
performed and the results revealed that MrSpz was a member of the clade 
containing LvSpz3 of Litopenaeus vannamei, FcSpz and Penaeus monodon spätzle 
protein. The expression distribution at transcriptional level in various tissues 
of normal prawn revealed that the MrSpz was detected in gills, heart and 
hepatopancreas while no expression was observed in hemocyte, muscle and stomach. 
In the Aeromonas caviae challenged prawn, the expression level of MrSpz in 
hemocyte was increased gradually at 6, 12 and 24 h post-injection. Furthermore, 
in MrSpz knocked down prawn injected with Aeromonas caviae, the mortality rate 
were higher than that of non-related dsRNA group and control group. These 
results suggest that MrSpz protein may play a key role in the innate immunity of 
M. rosenbergii, especially in response to Gram-negative bacteria A. caviae 
invasion.