Local adaptation to altitude underlies divergent thermal physiology in tropical killifishes of the genus Aphyosemion

Abstract

In watersheds of equatorial West Africa, monophyletic groups of killifish species (genus Aphyosemion) occur in discrete altitudinal ranges, low altitude species (LA, sea level to ∼350 m) or high altitude species (HA, 350 to 900 m). We investigated the hypothesis that local adaptation to altitude by the LA and HA species would be revealed as divergent effects of temperature on their physiological energetics. Two species from each group (mass ∼350 mg) were acclimated to 19, 25 and 28°C, with 19 and 28°C estimated to be outside the thermal envelope for LA or HA, respectively, in the wild. Wild-caught animals (F0 generation) were compared with animals raised in captivity at 25°C (F1 generation) to investigate the contribution of adaptation versus plasticity. Temperature significantly increased routine metabolic rate in all groups and generations. However, LA and HA species differed in the effects of temperature on their ability to process a meal. At 25°C, the specific dynamic action (SDA) response was completed within 8 h in all groups, but acclimation to temperatures beyond the thermal envelope caused profound declines in SDA performance. At 19°C, the LA required ∼14 h to complete the SDA, whereas the HA required only ∼7 h. The opposite effect was observed at 28°C. This effect was evident in both F0 and F1. Reaction norms for effects of temperature on SDA therefore revealed a trade-off, with superior performance at warmer temperatures by LA being associated with inferior performance at cooler temperatures, and vice-versa in HA. The data indicate that divergent physiological responses to temperature in the LA and HA species reflect local adaptation to the thermal regime in their habitat, and that local adaptation to one thermal environment trades off against performance in another.

Figure 1. Map of Cameroon showing the distribution of the low altitude (LA) and high altitude (HA) Aphyosemion species.

Altitudes below 350 m are in white, above 350 m in yellow. ,The LA species are almost exclusively present below 350 m (black dots), the HA species above 350 m (in blue). The arrows show where the animals were collected for the current study. Photos by Olivier Buisson (A. ahli, A. cameronense and A. exiguum) and Alf Persson (A. splendopleure).

Figure 2

Mean (± SE) rates of oxygen uptake in wild-caught individuals of two species of killifish, A. ahli from low altitude (left column) or A. exiguum from high altitude (right column), at three different acclimation temperatures. In each panel, data are for fish placed into respirometers at 1630 either fasted (white symbols) or when fed 5% of their body mass as live Artemia (black symbols). Rates of oxygen uptake are corrected to a body mass of 350 mg, n = 6 in all cases. See text for further details.

Figure 3

Specific dynamic action (SDA), expressed as rates of oxygen uptake, in two species of killifish, A. ahli from low altitude (LA, upper panel) or A. exiguum from high altitude (HA, lower panel), at three different acclimation temperatures, 19°C (white), 25°C (grey) and 28°C (black). The SDA was calculated as the net difference in oxygen uptake between animals that were either fasted, or fed 5% of their body mass as Artemia (see Figure 2). Values are means of n = 6 individuals from each species, rates are corrected to a body mass of 350 mg. Standard error bars are not shown to ease reading of the figure, but were approximately 10% of the mean value in all cases.

Figure 4. Reaction norms for mean (± SE) peak oxygen uptake rate during the specific dynamic action (Mo _2-peak) as a function of acclimation temperature in four groups of killifish.

Each group comprised a species pair from either low altitude (A. splendopleure and A. ahli), with both an F0 generation (white squares) and an F1 generation (grey squares), or from high altitude (A. cameronense and A. exiguum), with both an F0 generation (white circles) and an F1 generation (grey circles). F0 generation were captured in the wild, F1 generation were bred in captivity. Rates of oxygen uptake are corrected to a body mass of 350 mg, n = between 5 and 7. Two-way ANOVA found a significant effect of temperature (F = 6.064, d.f. = 2, P = 0.003) but not of group (F = 0.926, d.f. = 3, P = 0.430) or their interaction (F = 0.241, d.f. = 6, P = 0.962). This was due to higher overall peak at 25°C than at 19°C, although individual groups did not show significant effects (see Text S8 for details).

Figure 5. Reaction norms for mean (± SE) time required to reach peak oxygen uptake during the specific dynamic action (T_peak) as a function of acclimation temperature in four groups of killifish.

Each group comprised a species pair from either low altitude (A. splendopleure and A. ahli), with both an F0 generation (white squares) and an F1 generation (grey squares), or from high altitude (A. cameronense and A. exiguum), with both an F0 generation (white circles) and an F1 generation (grey circles). F0 generation were captured in the wild, F1 generation were bred in captivity, n = between 5 and 7. A 2-way ANOVA on log 10 transformed values found no effect of group (df = 3, F = 0.849, P = 0.470), but a significant effect of temperature (df = 2, F = 6.652, P = 0.002) and their interaction (df = 6, F = 6.015, P<0.001). Overall T_peak was lower at 25°C than at 19 or 28°C. At 19 and 28°C, a similar letter superscript indicates no significant difference among group means for that temperature. Within a group, † indicates a difference between 19 and 25°C, § indicates a difference between 19 and 28°C, ‡ indicates a significant difference between 25 and 28°C (see Text S9 for details).

Figure 6. Reaction norms for mean (± SE) time required to complete the specific dynamic action response (T_SDA) as a function of acclimation temperature in four groups of killifish.

Each group comprised a species pair from either low altitude (A. splendopleure and A. ahli), with both an F0 generation (white squares) and an F1 generation (grey squares), or from high altitude (A. cameronense and A. exiguum), with both an F0 generation (white circles) and an F1 generation (grey circles). F0 generation were captured in the wild, F1 generation were bred in captivity, n = between 5 and 7. A 2-way ANOVA found no effect of group (df = 3, F = 0.676, P = 0.568) or temperature (df = 2, F = 2.064, P = 0.131) but a significant interaction (df = 6, F = 12.695, P<0.001). At 19 and 28°C, a similar letter superscript indicates no significant difference among group means. Within a group, † indicates a difference between 19 and 25°C, § indicates a difference between 19 and 28°C, ‡ indicates a significant difference between 25 and 28°C (see Text S10 for details).