Maintenance and Loss of Migration and Migration-Associated Phenotypes in Monarch Butterflies

North American monarch butterflies maintain alternative migratory phenotypes that develop when individuals experience environmental cues that indicate summer is transitioning into autumn. While the summer generations live relatively short lives in which they reproduce quickly, the migratory generation delays reproduction to migrate thousands of miles to overwinter in Mexico or Southern California. As monarchs are freeze susceptible insects, migration is an adaptation to survive the cold of winter. However, not all monarchs need to migrate. In fact, several populations of monarchs live in habitats that never experience extreme cold. They are freed from the selection imposed by the necessity and difficulty of surviving the migration and are no longer exposed to the environmental triggers that lead to the development of the alternative migration phenotypes. The lack of exposure to the relevant environmental cues could result in the dormancy/non-expression of the migration traits like in summer generation North American monarchs. Alternatively, the change in selection pressure could result in loss of migration due to relaxed selection and drift or adaptation to a novel environment. Using common garden experiments, I investigated under what genetic and environmental conditions migration associated traits are lost and maintained in monarchs. I measured migration associated phenotypes including, directional orientation behavior, body mass, forewing size, and forewing shape in several populations reared under different environmental conditions. I found that maintenance of directional orientation and forewing size is very sensitive to the relaxation of selection; without the consistent seasonal pressure of winter, monarchs can lose the genetic mechanisms responsible for plasticity. In addition, the development of the alternative migratory phenotypes in the known migratory population is very sensitive to environmental conditions. When compared with North American monarchs reared completely outdoors, North American monarchs reared in a variety of autumn-simulating indoor conditions did not develop the alternative migration phenotype.