Background/Question/Methods The mechanisms controlling genetic adaptation to stress are not clearly understood. These mechanisms, however, are important to understanding the nature of local adaptation of populations. A wide variety of genetic interactions such as epistasis and maternal effects putatively influence local adaptations of populations. The coastal marshes of Louisiana are subject to a combination of accelerated coastal subsidence and sea-level rise creating a dynamic and stressful environment that favors the local adaptation of fish populations. In a previous study we documented local adaptation of fish populations to salinity stress. However, this raised questions about the various genetic factors that result in local adaptation. Our objective was to examine the nature of that genetic adaptation and understand how it may differ across spatial scales.  To better understand the genetic basis for salinity tolerance we created crosses of populations that were locally adapted to two geographic areas of coastal Louisiana. Within each geographic area we obtained fish adapted to fresh and brackish conditions to initiate experimental lineages. After two generations, offspring from resulting interpopulation, as well as within population crosses were all exposed to a salinity of 25 ‰ and differences in salinity tolerance were examined.

Results/Conclusions   For populations that were historically exposed to freshwater, we found that interpopulation crosses between different geographic areas had lower survival times under stress then fish from control populations. This suggests that that for freshwater populations, outbreeding depression has significant effects on salinity tolerance. For populations from brackish environments, there was evidence of increased salinity tolerance from outbreeding. This effect, however, depended on the site of origin for the males and females used in the cross suggesting the possibility of a nuclear/mitochondrial genome interaction affecting survival under salinity stress.