David Haig

Department of Organismic and Evolutionary Biology, Harvard University

Families are sources of evolutionary cooperation and conflict: cooperation because of the genes that are shared by related genotypic individuals and conflict because of genes that are not shared. Microchimerism brings the complexities of family life into the bodies of phenotypic individuals. Conflict arises if microchimeric cells are able to benefit the fitness of the genotypic individual from whom the cells come at the expense of the fitness of the genotypic individual within whom the cells reside. In the specific case of offspring cells resident in maternal bodies, these conflicts will be most pronounced in the interbirth interval subsequent to the birth of the offspring from whom the cells come. Offspring cells are predicted to favor increased delivery of resources by the mother to the offspring from whom the cells come (for example, during lactation) and favor longer delays until the birth of the next offspring, either by suppressing maternal fertility or by eliminating embryos in early gestation. Many years after a pregnancy, there seems little microchimeric cells could do to favor their own offspring over its siblings. Microchimeric cells are predicted to promote maternal health because offspring benefit from a healthy mother.