Many selective forces shape evolution in rocky shore organisms. In the intertidal snail Littorina saxatilis desiccation at emersion is thought to be an important selective force. One approach to study responses to desiccation is through measuring gene expression of genes putatively involved in coping with the exposure. In the exposed ecotype of L. saxatilis a candidate gene for studying response to desiccation is aspartate aminotransferase (Aat). Aat is an enzyme initially active in anaerobic energy production that may be used at emersion. There are two allozyme alleles Aat120 and Aat100 and the allele frequencies vary with shore height. The activity of the enzyme varies depending on the genotype.
This study investigated if there was an up-regulation of the enzyme at transcript level during emersion. Snails were sampled at an exposed atidal shore in the Aat hybrid zone and acclimated to laboratory conditions. An emersion event was simulated and gene expression quantified at different time points with quantitative PCR. The study found no significant response in gene expression for cytosolic Aat or mitochondrial Aat, but found a significant correlation between the two. The hypothesis of up-regulation in response to desiccation however cannot be rejected. Humidity throughout the experimental emersion event was low, but higher than dry air conditions. Snails might therefore not utilise Aat at the humidity and timeframe of the experimental setup.
Based on results, the directions for further studies will be testing extreme stress, difference between genotypes and investigating expression difference at tissue level. The measuring of gene expression and enzyme activity together also allows testing an alternative hypothesis - that regulation is not at transcript level. Simulated emersion and other experimental approaches also should be compared to natural emersion events.