Selection for canalisation in mice.

HE term canalisation will be used here to describe a developmental buffering system which acts to reduce potential variation and produce a less variable phenotype. Unless such a system acts at the gene level it would act on variation due to either genotype or environment and canalising selection can aim at reducing the response to either genetic or environmental variation. Selection for canalisation has been successful in Drosophila. I t was possible, by selecting for variance of scutellar bristle number in scute flies, to reduce variance and response to temperature (RENDEL and SHELDON 1960). Other attempts to select for canalisation in Drosophila have been made by WADDINGTON (1960) and KINDRED (1965). In each of these experiments a change in the response to temperature was produced. Although differences in the degree of canalisation of mouse vibrissae have been shown (KINDRED 1963), canalising selection has not been attempted in mice. There is a need to demonstrate that canalising selection is possible in animals other than Drosophila. Furthermore, well canalised and poorly canalised lines would provide useful information about the canalisation system. Mice do not produce large enough progenies to allow selection on within-litter variance, and it is extremely difficult to bring about a precise change in the environment of a developing mammal. Another indication of canalisation is symmetry, since presumably asymmetry indicates a response to some localised variable (VAN VALEN 1962). It should thus be possible to select for well canalised and poorly canalised lines by selecting for symmetry and asymmetry. A symmetrical animal may be one which is well canalised and doesn’t respond to environmental changes, or it may not have been exposed to such changes. This dependence on chance differences may slow down response to selection, but it should not prevent a response. Unsuccessful attempts to select for symmetry in Drosophila have been made, but as WADDINGTON (1962) has pointed out, symmetry in Drosophila is complicated by the fact that each side of the adult develops from a different set of rudiments. Much work has been done in this laboratory on the canalisation of secondary vibrissae in mice, and the character secondary vibrissa number was thus a logical choice for canalising selection. Because normal mice very rarely have asymmetrical vibrissae, the Tabby ( T a ) mutant was used. Tabby has been employed to demonstrate that the number of secondary vibrissae is a canalised character (DUN and FRASER 1959).