1a. Objectives (from AD-416):
The objective of this cooperative agreement is to conduct collaborative research between the Harry K. Dupree-Stuttgart National Aquaculture Research Center of the Agricultural Research Service (ARS) and Auburn University to characterize changes in gene expression in superior and inferior performing hybrid striped bass with a principal focus on growth and resistance to pathogens.
1b. Approach (from AD-416):
Our approaches are two-fold: 1) Families of hybrid striped bass will be reared in communal ponds for a set duration to determine the performance of different families. After which, various tissues from the fastest and slowest growing fish will be collected and samples will be sent to Cooperator for gene expression analysis. 2) Families of hybrid striped bass will be challenged with Flavobacterium columnare. Tissues from naïve fish, moribund fish, and surviving fish will be isolated and sent to Cooperator for gene expression analysis.
3. Progress Report:
Scientists in this project are actively collaborating in planning research, data collection, and data analysis to better understand the molecular events contributing to disease resistance in farmed fish. During the past year, four studies were completed under this agreement, which led to the submission of four peer-reviewed manuscripts. In the first study, our research team examined the consequences of short-term feed deprivation on immunity. Short-term feed deprivation (or fasting) is a common occurrence in aquacultured fish species and can be brought about by seasonal changes, production strategies, or as a means to combat certain disease outbreaks. In channel catfish, periods of fasting have been suggested to increase the susceptibility of fish to columnaris disease (caused by the bacterium Flavobacterium columnare). Because columnaris primarily affects the gill and skin, we conducted a study to examine changes at the genetic level in these tissues in fish that were subjected to a 7-day period of fasting. In comparison to fish fed daily, fasted fish showed changes in the levels of over 1,500 genes. Many of these changes involved a decrease in the levels of important genes of the immune system and suggest that fasted fish may have impaired immunity which leads to an increased susceptibility to disease. In a separate study conducted with colleagues from the USDA-ARS Catfish Genetics Research Unit in Stoneville, Mississippi, we examined the global gene expression differences in the gill of two families of channel catfish that differed greatly in their susceptibility to columnaris disease. We compared gene expression in the gill before infection and at three timepoints post-infection (1, 2, and 8 h). Our comparisons revealed that over 1,500 unique genes were differentially expressed greater than 1.5-fold at one or more timepoints. A number of critical components of innate immunity, including iNOS2b, lysozyme C, IL-8, and TNF-alpha, were constitutively higher in resistant catfish gill, while susceptible fish showed high expression levels of secreted mucin forms, a rhamnose-binding lectin previously linked to susceptibility, and mucosal immune factors such as CD103 and IL-17. The immune and mucin profiles obtained suggest that susceptible fish may be predisposed to F. columnare infection due to lower levels of innate immune components and higher levels of mucosal immune factors. Finally, in two additional related studies, we conducted experiments to better characterize the immune responses within the skin of fish that were challenged with a highly virulent strain of Aeromonas hydrophila, a bacterial pathogen that has been linked to massive die-offs of catfish in the Southeast. We used a molecular tool called a microarray that allowed for the detection of changes in the levels of immune-related genes in healthy fish versus those exposed to Aeromonas hydrophila. In infected fish, we found robust changes in over 1000 different genes. From these data, we believe that many of the gene expression patterns observed are not protective responses, but instead, actually serve to enhance the ability of Aeromonas hydrophila to infect fish. This research represents a novel approach to addressing one of the major disease issues affecting cultured catfish. Collectively, these findings enhance our understanding of the immune system of warmwater cultured fish, and producers and consumers stand to benefit from the economic returns associated with reducing costly production losses due to bacterial disease outbreaks.