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Cold tolerance and acclimatisation have long been recognised as being controlled by many genes, and the advent of technology that allows us to identify these and recognise their function has led to significant advances in the understanding of cold hardiness. In plants, for example, perception of cold temperatures seems to occur at the plasma membrane and is associated with an increase in calcium concentration that sets in train the activation of a wide variety of genes responsible for the biochemical changes already described (Wisniewski et al., 2014). And in insects, Zhang et al. (2011) identified cold‐responsive genes in the fruit‐fly Drosophila melanogaster associated with muscle structure and function, immune and stress responses and carbohydrate metabolism.

In a laboratory ‘selection’ experiment involving plants of alfalfa (Medicago sativa, an important animal forage species), Castonguay et al. (2011) investigated whether the plant could be selected for improved freezing tolerance. Five weeks after sowing 1500 genotypes of a particular alfalfa cultivar used in eastern Canada, the plants were moved to low‐temperature chambers for two week’s acclimation at 2°C before being transferred to –2°C for an additional fortnight to simulate ‘hardening’ conditions in frozen soil. Subsequently, temperature was progressively dropped to the expected lethal temperature for 50% of the plants (their LT₅₀ – lethal temperature for 50% of the plants in January), using a stepwise decline of temperature. After five weeks of regrowth at 20°C, genotypes that survived the original freezing cycle were intercrossed and subject to another cycle, and so on for six cycles of recurrent selection. The experiment was repeated with a second cultivar for four cycles of selection. Figure 2.14a shows that for both cultivars, several cycles of selection for freezing tolerance led to a significant decline in LT₅₀ between the first cycle and later cycles of selection: in other words, individuals in the populations subject to selection were able to tolerate lower winter temperatures. Associated biochemical (Figure 2.14b, c) and genetic patterns (Figure 2.14d) provide good evidence that recurrent selection for superior freezing tolerance in alfalfa induces marked changes in influential traits. And if deliberate selection can change the tolerance of a domesticated plant we can certainly expect that natural selection has done the same thing for plants, animals and microorganisms in nature.

Figure 2.14 Alfalfa can be selected for improved freezing tolerance. (a) Tolerance to freezing (LT₅₀, 5% confidence levels shown) of populations of two cultivars of alfalfa used for animal forage in eastern Canada, before selection (TF0) or after several cycles of recurrent selection for freezing tolerance (three, four, five or six cycles). (b, c) Starch and sucrose concentrations in crowns of alfalfa plants during autumn and winter (cultivar 1) before (0) and after five or six cycles of selection. (d) Relative expression of the cold‐induced gene cas15 before (0) and after five or six cycles of selection.

Source: From Castonguay et al. (2011).