Genetic variation and the evolution of cold tolerance

Even within species there are often differences in temperature response between populations from different locations, and these differences have frequently been found to be the result of genetic differences rather than being attributable solely to acclimatization. Powerful evidence that cold tolerance varies between geographic races of a species comes from a study of the cactus, Opuntia fragilis. Cacti are generally species of hot dry habitats, but O. fragilis extends as far north as 56°N and at one site the lowest extreme minimum temperature recorded was —49.4°C. Twenty populations were sampled from diverse localities in northern USA and Canada, and were tested for freezing tolerance and ability to acclimate to cold. Individuals from the most freeze-tolerant population (from Manitoba) tolerated —49°C in laboratory tests and acclimated by 19.9°C, whereas plants from a population in the more equable climate of Hornby Island, British Columbia, tolerated only —19°C and acclimated by only 12.1°C (Loik & Nobel, 1993).

There are also striking cases where the geographic range of a crop species has been extended into colder regions by plant breeders. Programs of deliberate selection applied to corn (Zea mays) have expanded the area of the USA over which the crop can be profitably grown. From the 1920s to the 1940s, the production of corn in Iowa and Illinois increased by around 24%, whereas in the colder state of Wisconsin it increased by 54%.

If deliberate selection can change the tolerance and distribution of a domesticated plant we should expect natural selection to have done the same thing in nature. To test this, the plant Umbilicus rupestris, which lives in mild maritime areas of Great Britain, was deliberately grown outside its normal range (Woodward, 1990). A population of plants and seeds was taken from a donor population in the mild-wintered habitat of Cardiff in the west and introduced in a cooler environment at an altitude of freeze-avoidance and freeze-tolerance acclimation and acclimatization

Figure 2.8 (a) Changes in the glycerol concentration per gram wet mass of the freeze-avoiding larvae of the goldenrod gall moth, Epiblema scudderiana. (b) The daily temperature maxima and minima (above) and whole larvae supercooling points (below) over the same period. (c) Changes in glycogen concentration over the same period. (After Rickards et al., 1987.)

Figure 2.8 (a) Changes in the glycerol concentration per gram wet mass of the freeze-avoiding larvae of the goldenrod gall moth, Epiblema scudderiana. (b) The daily temperature maxima and minima (above) and whole larvae supercooling points (below) over the same period. (c) Changes in glycogen concentration over the same period. (After Rickards et al., 1987.)

Figure 2.9 Acclimation to low temperatures. Samples of the Antarctic springtail Cryptopygus antarcticus were taken from field sites in the summer (c. 5°C) on a number of days and their supercooling point (at which they froze) was determined either immediately (•) or after a period of acclimation (•) at the temperatures shown. The supercooling points of the controls themselves varied because of temperature variations from day to day, but acclimation at temperatures in the range +2 to —2°C (indicative of winter) led to a drop in the supercooling point, whereas no such drop was observed at higher temperatures (indicative of summer) or lower temperatures (too low for a physiological acclimation response). Bars are standard errors. (After Worland & Convey, 2001.)

157 m in Sussex in the south. After 8 years, the temperature response of seeds from the donor and the introduced populations had diverged quite strikingly (Figure 2.10a), and subfreezing temperatures that kill in Cardiff (—12 °C) were then tolerated by 50% of the Sussex population (Figure 2.10b). This suggests that past climatic changes, for example ice ages, will have changed the temperature tolerance of species as well as forcing their migration.

Figure 2.10 Changes in the behavior of populations of the plant Umbilicus rupestris, established for a period of 8 years in a cool environment in Sussex from a donor population in a mild-wintered area in South Wales (Cardiff, UK). (a) Temperature responses of seed germination: (1) responses of samples from the donor population (Cardiff) in 1978, and (2) responses from the Sussex population in 1987. (b) The low-temperature survival of the donor population at Cardiff, 1978 (1) and of the established population in Sussex, 1987 (2). (After Woodward, 1990.)

Figure 2.10 Changes in the behavior of populations of the plant Umbilicus rupestris, established for a period of 8 years in a cool environment in Sussex from a donor population in a mild-wintered area in South Wales (Cardiff, UK). (a) Temperature responses of seed germination: (1) responses of samples from the donor population (Cardiff) in 1978, and (2) responses from the Sussex population in 1987. (b) The low-temperature survival of the donor population at Cardiff, 1978 (1) and of the established population in Sussex, 1987 (2). (After Woodward, 1990.)

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