Blooms of surf-zone diatoms along the coast of the Olympic Peninsula,Washington. IV. Nitrate reductase activity in natural populations and laboratory cultures of Chaetoceros armatum and Asterionella socialis

Nitrate reductase (NR) activity in the surf-zone diatom Chaetoceros armatum T. West was studied in its natural habitat at Copalis Beach, Olympic Peninsula, Washington (USA) over a period of 18 months. Variations of enzyme activity were found, depending on the season and the time of day. General seasonal trends in NR activity were correlated with nitrate concentrations in the surf-zone. The results also indicated that several factors may limit the activity of the enzyme. This is one of the reasons why NR activity could not be correlated with environmental nitrate concentration on the basis of individual measurements at a given point in time. Enzyme inhibition due to the presence of ammonium occurred on several occasions throughout the study period, and could be detected by the NR assay. This phenomenon for the most part masked any pattern of diel periodicity in NR activity that might have been expected to exist in cells in the natural environment. Cultures of C. armatum and Asterionella socialis Lewin and Norris were also studied in the laboratory. A diel fluctuation in NR activity corresponding to the pattern shown in natural phytoplankton by other authors was exhibited in C. armatum under a light-dark regime of 8 h light – 16 h dark and a light intensity of 400 lux.     

Geographical variation in parasitism shapes larval immune function in a phytophagous insect

Two of the central goals of immunoecology are to understand natural variation in the immune system among populations and to identify those selection pressures that shape immune traits. Maintenance of the immune system can be costly, and both food quality and parasitism selection pressure are factors potentially driving immunocompetence. In tritrophic interactions involving phytophagous insects, host plants, and natural enemies, the immunocompetence of phytophagous insects is constrained by selective forces from both the host plants and the natural enemies. Here, we assessed the roles of host plants and natural enemies as selective pressures on immune variation among natural populations of Lobesia botrana. Our results showed marked geographical variation in immune defenses and parasitism among different natural populations. Larval immune functions were dependent of the host plant quality and were positively correlated to parasitism, suggesting that parasitoids select for greater investment into immunity in moth. Furthermore, investment in immune defense was negatively correlated with body size, suggesting that it is metabolically expensive. The findings emphasize the roles of host plants and parasitoids as selective forces shaping host immune functions in natural conditions. We argue that kinds of study are central to understanding natural variations in immune functions, and the selective forces beyond.