Adaptation to salinity is an important driving force in the evolution in teleost fishes. Some speciose groups such as minnows and characids are found predominantly in fresh water, while other groups such as tunas and wrasses are found predominantly in marine habitats. Euryhaline groups, such as killifish, contain freshwater species, marine species, and species that can occur in fresh, brackish, and marine conditions. These groups are powerful systems for studying adaptation to salinity as they allow for the comparison of close relatives who differ in salinity tolerance. In chapter 1, I review the biology of Lucania killifish. Lucania contains three species, one of which is a freshwater species (L. goodei) and another of which is euryhaline (L. parva). A third species (L. interioris) is endemic to a small region in Mexico and is not considered in this thesis. Previous studies on L. goodei and L. parva suggest that salinity has dramatic effects on life-history, ecology, physiology, and genetic differentiation at the between-species level. Upon salinity transfer, the two species differ in gene expression in critical osmoregulatory genes. An examination of FST outliers suggests that the two species differ in many genes related to osmoregulation, but that they also possess high levels of differentiation between genes involved in reproduction and spermatogenesis. Within L. parva, preliminary work indicated that freshwater-saltwater population pairs also possessed elevated levels of differentiation in loci related to osmoregulation.
In chapter 2, I used RAD-Seq data from 10 populations across Florida to examine the levels of population structure between freshwater and saltwater populations and the effects of distance on population-wide FST. Here, I found good evidence that differences in salinity increase FST beyond what would be expected from the effects of distance alone.
In chapter 3, I describe a laboratory experiment that tests for both local adaptation and for maternal effects as a function of salinity. Early development is a critical life stage. From their earliest moments in life, embryos cannot regulate their ion and water levels because the physiological traits needed for active osmoregulation have not yet developed. Instead, embryos rely on the properties of the egg and maternal provisioning to maintain proper ion and water levels. Hence, this stage of development is ripe for maternal effects (either genetic or environmental) that influence offspring survival as a function of salinity. In chapter 3, I describe the results of an experiment where I performed within population crosses for a freshwater and a saltwater population from the Wakulla River drainage. In the experiment, I considered the effects of population of origin (fresh versus salt) and the effects of spawning salinity (the salinity in which spawning pairs were housed) and rearing salinity (the salinity in which eggs and fry were kept). Hence, the experiment allows me to examine the effects of population of origin and parental salinity environment on subsequent survival as a function of salinity. Here, I found little evidence for local adaptation as a function of salinity. Maternal effects were present, but the nature of the pattern did not suggest that they were adaptive. I suggest that other life-history stages such as over-winter survival, perhaps in the presence of intraspecific competition, should be assessed.
My thesis indicates that there is evidence for heightened genetic divergence between freshwater and saltwater populations, yet we do not know precisely how these effects emerge. Salinity may affect multiple life-history stages (i.e., growth, survival to adulthood, over-winter survival) beyond the ones examined in this thesis. Salinity may also affect multiple aspects of ecology, including community composition (i.e., potential competitors, predators, and prey items), which may create parallel selection due to ecological demands
