Absence of Cold Acclimation Response in Gray Treefrogs, Hyla chrysoscelis and Hyla versicolor

Abstract

The gray treefrog complex consists of two morphologically similar species, Hyla chrysoscelis and Hyla versicolor. These sibling species can be differentiated by genome size in the laboratory and by mating call in the field. Hyla chrysoscelis is diploid (2n = 24) with a mating call pulse rate of 36 pulses per second or higher, while H. versicolor is tetraploid (2n = 48) with a mating call pulse rate at or below 25 pulses per second depending upon temperature (Zweifel, 1970; Ralin and Selander, 1979). The overall distribution of the two species extends from northern Florida to southern Maine in the east and from central Texas to southern Manitoba in the west (Conant and Collins, 1991). Although the distribution of each species has not been accurately delineated, H. versicolor (the tetraploid species) occurs more frequently in the northern region of this range while H. chrysoscelis occurs primarily in the southern region (Ralin and Selander, 1979). Compensation of metabolic rate in response to lower temperatures (cold acclimation) allows organisms to survive and function under adverse thermal conditions (Rome et al., 1992). For example, Rieck et al. (1960) demonstrated that Rana pipiens held at 5 C had metabolic rates two times greater upon exposure to 10 C than specimens held at 25 C. Such a response allows for greater physiological efficiency at lower temperatures and improved cold tolerance. Previous research provided evidence that H. chrysoscelis (the diploid species) does not have the ability to acclimate to cold temperature (Blem et al., 1986). However, Bailey and Thye (1975), showed that polyploidy enabled gene duplication as well as gene divergence in function. In their study, tetraploid salmonid fish showed a modification in isoenzyme regulation for lactate dehydrogenase (LDH). As with amphibians (Enig et al., 1976; Tsugawa, 1976), a relationship appears to exist between acclimation ability and LDH concentrations in the killifish Fundulus heteroclitus (Crawford and Powers, 1989). The purpose of the current study was to determine if polyploidy imparts a similar physiological advantage to H. versicolor with respect to its thermal acclimation ability. Calling male H. versicolor and H. chrysoscelis were collected between 15 and 31 May 1994 from Hunterdon and Cape May Counties, New Jersey, respectively. The body masses of the frogs ranged from 5.6-10.2 g for H. versicolor (mean = 8.0 g, SE = 0.45, N = 10) and 6.0-7.8 g for H. chrysoscelis (mean = 6.9 g, SE = 0.21, N = 12). Species identification was verified by analyzing recordings of the mating calls (Zweifel, 1970). The two species were held in separate aquaria at a constant temperature (21 C) and fed live crickets 2 to 3 times a week until experimentation in November and December, 1994. Individual frogs were identified by dorsal blotches and/or thigh spots, body mass and snout to urostyle length. Metabolic rates were measured as the amount of