Influence of local adaptation and interstock hybridization on the cardiovascular performance of largemouth bassMicropterus salmoides

Abstract

SUMMARY In this study, the cardiovascular response to exhaustive exercise among differentiated stocks of largemouth bass Micropterus salmoides was compared at 10°C and 20°C to assess the level of their local adaptation. In addition, the impact that interstock hybridization had on adaptive differences was assessed using F1 hybrids. To accomplish these assessments, four genetically distinct stocks of fish were produced using adults from two regions in the midwestern United States identified as distinct conservation management units (central Illinois, IL and southeastern Wisconsin, WI): both P1 stocks and both reciprocal F1interstock hybrids. Cardiac variables (both resting and maximal) were consistently lowest for the IL×IL stock relative to the WI×WI stock and both F1 interstock hybrids. Interestingly, however, all groups of fish were able to maintain similar levels of cardiac scope. All fish responded to exercise by increasing heart rate and decreasing stroke volume,consistent with the notion that largemouth bass modulate cardiac output via frequency. After exercise, cardiac variables returned to resting levels 25-35% more rapidly for IL×IL fish relative to all other groups at 20°C. At 10°C, recovery rates for both P1 stocks were similar but more rapid than the interstock hybrids. Collectively, these results indicate that the locally adapted stock (IL×IL) exhibited cardiovascular adaptations that enabled rapid cardiovascular recovery and maintenance of low resting cardiac output and heart rate. Conversely, the translocated stock (WI×WI) and the interstock hybrids required longer for cardiovascular variables to recover after exercise and exhibited higher resting levels of cardiac output and heart rate. This study provides some of the first direct evidence of a physiological mechanism by which mixing stocks could potentially decrease fitness and illustrates the magnitude of the intraspecific diversity of cardiovascular performance.

Physiological Disturbances and Overwinter Mortality of Largemouth Bass from Different Latitudes

“…We examined lethal and sublethal responses of largemouth bass originating from only two populations due to logistical limitations, which somewhat limited our inference space. Our results, however, are consistent with previous studies that have documented intraspecific variation in the physiology of largemouth bass (e.g., Cooke et al 2001;Cooke and Philipp 2005), killifish (DeKoning et al 2004), and eastern fence lizards (Angilletta 2001) collected from only two latitudinally separated populations. In summary, largemouth bass from northern and southern origins displayed differences in both pre-and postcold-shock physiological parameters, indicating that latitudinal origin affected baseline physiological parameters and the magnitude and nature of the physiological response to acute cold stressors.…”

“…In mammals, constitutive levels of cortisol are responsible for the maintenance of baseline glucose levels and adequate blood pressure (Bamberger et al 1996) and may play a similar role in teleosts (Mommsen et al 1999). Interestingly, cardiac output has been shown to be lower in southern largemouth bass relative to those from northern populations (Cooke and Philipp 2005), highlighting potential relationships among cortisol, cardiac variables, and latitudinal origin. Cortisol production is also linked to hydromineral balance of fish, and elevated cortisol can result in sodium loss via the gills (Gonzalez and McDonald 1992;Wendelaar Bonga 1997).…”

“…Previous studies have shown that resting (prestress) physiological parameters of fish-such as metabolic rate (Galarowicz and Wahl 2003), cardiac function (Cooke et al 2001;Cooke and Philipp 2005), and activity rates of hepatic enzymes (DeKoning et al 2004)-can vary across populations from different latitudes, illustrating that physiological process for fishes can be influenced by geographical origin. In our study, physiological differences between populations of latitudinally separated largemouth bass were observed in the control groups but were apparent only for a few of the physiological parameters after acute cold shock.…”

“…Winter severity can vary considerably across latitudes, and intraspecific variability in thermal adaptations along latitudinal gradients has been observed in many species. For example, intraspecific variability in physiological parameters (e.g., metabolic rate, cardiac variables, conversion efficiency, growth) along latitudinal gradients has been suggested to be due to adaptation to local thermal conditions (e.g., Cooke et al 2001;Galarowicz and Wahl 2003;Garvey and Marschall 2003;Cooke and Philipp 2005). In addition to the physiological parameters listed above, variation in the physiological response of fish to acute cold stressors may be due to evolutionary adaptation to local thermal conditions.…”

“…Growth, survival, and food consumption have been shown to differ among latitudinally separated fish populations (Clapp and Wahl 1996;Fullerton et al 2000;Galarowicz and Wahl 2003). Physiological processes, such as metabolic rates and cardiac variables, have also shown similar division among populations (Clapp and Wahl 1996;Cooke et al 2001;Galarowicz and Wahl 2003;Cooke and Philipp 2005). For example, at high (25ЊC) temperature, the metabolic rate of walleye (Sander vitreus) from southern populations is approximately 50% greater than that of walleye from northern populations (Galarowicz and Wahl 2003).…”