Overcoming the congenitally disadvantageous mutation through adaptation to environmental UV exposure in land snails

Congenital fitness-disadvantageous mutations are not maintained in the
population; they are purged from the population through processes such as
purifying selection. However, these mutations could persist in the
population as polymorphisms when it is advantageous for the individuals
carrying them to adapt to a specific external environment. We tested this
hypothesis using the dimorphic land snail Euhadra peliomphala simodae in
Japan; these snails have dark or bright-coloured shells. The survival rate
of dark snails at hatching was lower than that of the bright ones, as
observed in the F1 progenies produced through crossing. Dark snails have a
congenital fitness-disadvantageous mutation; however, they also have
protection against ultraviolet radiation. They have a higher survival rate
than the bright snails in a UV environment, as observed using the UV
exposure experiments and UV transmittance measurements. This is a good
example of a congenitally disadvantageous mutation that is advantageous
for adapting to the external environment. These results explain the
maintenance of polymorphism and highlight the genotypic and phenotypic
diversity in the wild population.