Killifish

Abstract: Populations of common killifish, Fundulus heteroclitus, are distributed along the Atlantic coast of North America through a steep latitudinal thermal gradient. We examined intraspecific variation in whole-animal thermal tolerance and its relationship to the heat shock response in killifish from the northern and southern extremes of the species range. Critical thermal maxima were significantly higher in southern than in northern fish by approximately 1.5 degrees C at a wide range of acclimation temperatures (from 2-34 degrees C), and critical thermal minima differed by approximately 1.5 degrees C at acclimation temperatures above 22 degrees C, converging on the freezing point of brackish water at lower acclimation temperatures. To determine whether these differences in whole-organism thermal tolerance were reflected in differences in either the sequence or regulation of the heat shock protein genes (hsps) we obtained complete cDNA sequences for hsc70, hsp70-1 and hsp70-2, and partial sequences of hsp90alpha and hsp90beta. There were no fixed differences in amino acid sequence between populations in either hsp70-1 or hsp70-2, and only a single conservative substitution between populations in hsc70. By contrast, there were significant differences between populations in the expression of many, but not all, of these genes. Both northern and southern killifish significantly increased hsp70-2 levels above control values (T(on)) at a heat shock temperature of 33 degrees C, but the magnitude of this induction was greater in northern fish, suggesting that northern fish may be more susceptible to thermal damage than are southern fish. In contrast, hsp70-1 mRNA levels increased gradually and to the same extent in response to heat shock in both populations. Hsc70 mRNA levels were significantly elevated by heat shock in southern fish, but not in northern fish. Similarly, the more thermotolerant southern killifish had a T(on) for hsp90alpha of 30 degrees C, 2 degrees C lower than that of northern fish. This observation combined with the ability of southern killifish to upregulate hsc70 in response to heat shock suggests a possible role for these hsps in whole-organism differences in thermal tolerance. These data highlight the importance of considering the complexity of the heat shock response across multiple isoforms when attempting to make linkages to whole-organism traits such as thermal tolerance.

Abstract: Atlantic killifish populations have rapidly adapted to normally lethal levels of pollution in four urban estuaries. Through analysis of 384 whole killifish genome sequences and comparative transcriptomics in four pairs of sensitive and tolerant populations, we identify the aryl hydrocarbon receptor–based signaling pathway as a shared target of selection. This suggests evolutionary constraint on adaptive solutions to complex toxicant mixtures at each site. However, distinct molecular variants apparently contribute to adaptive pathway modification among tolerant populations. Selection also targets other toxicity-mediating genes and genes of connected signaling pathways; this indicates complex tolerance phenotypes and potentially compensatory adaptations. Molecular changes are consistent with selection on standing genetic variation. In killifish, high nucleotide diversity has likely been a crucial substrate for selective sweeps to propel rapid adaptation.

Abstract: The Deepwater Horizon oil rig disaster resulted in crude oil contamination along the Gulf coast in sensitive estuaries. Toxicity from exposure to crude oil can affect populations of fish that live or breed in oiled habitats as seen following the Exxon Valdez oil spill. In an ongoing study of the effects of Deepwater Horizon crude oil on fish, Gulf killifish (Fundulus grandis) were collected from an oiled site (Grande Terre, LA) and two reference locations (coastal MS and AL) and monitored for measures of exposure to crude oil. Killifish collected from Grande Terre had divergent gene expression in the liver and gill tissue coincident with the arrival of contaminating oil and up-regulation of cytochrome P4501A (CYP1A) protein in gill, liver, intestine, and head kidney for over one year following peak landfall of oil (August 2011) compared to fish collected from reference sites. Furthermore, laboratory exposures of Gulf killifish embryos to field-collected sediments from Grande Terre and Barataria Bay, LA, a...

Abstract: Maintenance of ion balance requires that ionoregulatory epithelia modulate ion flux in response to internal or environmental osmotic challenges. We have explored the basis of this functional plasti...