Genetic variance in temperature dependent adult size deriving from physiological genetic variation at temperature boundaries

An increase in genetic variation in body size has often been observed under stress; an increase in dominance variance and interaction variance as well as in additive genetic variance has been reported. The increase in genetic variation must be caused by physiological mechanisms that are specific to adverse environments. A model is proposed to explain the occurrence of an i ncrease in genetic variation in body size in Drosophila at extreme temperat ures. The model has parameters specific to the low- and high-temperature re gions of the viable range. Additive genetic variation in the boundary tempe ratures leads to a marked increase in additive genetic variation in develop ment rate and body size at extreme temperatures. Additive genetic variation in the temperature sensitivity in the low- and high-temperature regions ad ds non-additive genetic variation. Development rate shows patterns in addit ive genetic variation that differ from the patterns of genetic variation in body size; therefore, the genetic correlation between development rate and body size changes sign repeatedly as a function of temperature. The existe nce of dominance in the genetic variation in the boundary temperatures or i n the low- and high-temperature sensitivities leads to a higher total genet ic variance due to higher dominance and interaction variance, for both deve lopment rate and body size.