Ontogenetic and expression of different genes involved in the Toll pathway of black tiger shrimp (Penaeus monodon) following immersion challenge with Vibrio harveyi and white spot syndrome virus (WSSV)

In the present study, ontogenetic expression of different innate immune genes in the Toll pathway of tiger shrimp, Penaeus monodon, such as TLR, myeloid differentiation factor 88 (MyD88) and tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6), was investigated in different developmental stages. Ontogenetic expression by real-time PCR revealed constitutive expression of these genes in all the developmental stages selected. TLR expression was found to be the highest in PL4, whereas MyD88 and TRAF6 showed the highest expression in eggs. The ubiquitous expression of TLR, MyD88 and TRAF6 in different developmental stages of P. monodon indicates the role of these genes in protecting the animals during early development. Immersion challenge of PL 18 with V. harveyi resulted in significant upregulation of TRAF6 at all time-points and significant upregulation of TLR at most of the time-points selected; however, MyD88 showed differential modulation pattern. In contrast to the bacterial challenge, WSSV infection in PL18 did not show any significant change in the expression of TRAF6, except for a downregulation observed at 12 to 48 hpi. However, TLR and MyD88 showed moderate increase in their expression, especially at late time-points. The responses of these genes to V. harveyi and WSSV immersion challenges in the juveniles (average body weight 3 g) of P. monodon were investigated in selected tissues including gill, hepatopancreas, and different parts of gastrointestinal tract such as foregut (stomach), midgut and hindgut. Temporal expression analysis revealed complete downregulation of PmMyD88 at most of the time-points in the gill following V. harveyi challenge and significant induction of PmTRAF6 at all time-points following WSSV infection. Immersion challenge with V. harveyi resulted in enhanced expression of TRAF6 in stomach and MyD88 in hepatopancreas showed similar pattern of expression post-WSSV challenge. Other tissues showed varying levels of induction of these genes at different time-points following pathogen challenge. The results of the present study suggest that both bacterial and viral challenges through immersion modulates the genes involved in the Toll pathway, and this might play an important role in the immune defense of post-larvae and juveniles of P. monodon.