Evolution
Thursday, 28.05.2026, Day 2
Time: 16:15-17:45
Consequences of being microchimeric
Amy M. Boddy
Department of Anthropology, University of California Santa Barbara
Internal gestation creates a biological paradox: placental mammals must maintain immunological self-identity while hosting genetically distinct offspring. This challenge extends beyond pregnancy, cellular exchange during gestation (microchimerism) persists for decades, with fetal cells detectable in mothers and maternal cells in offspring throughout life. Mammalian evolution has navigated this paradox through adaptive compromises that facilitate internal gestation while potentially increasing disease vulnerability.
Microchimerism transferred during pregnancy may represent an evolved mechanism for acquired immunological tolerance. Gradual cellular exchange could facilitate tolerance to non-self antigens (e.g., paternal alleles in mothers, non-inherited maternal antigens in offspring) where large scale exposure could trigger rejection. However, this tolerance comes at a cost: the immunological tolerance required for gestation may constrain HLA diversity in mammals. Recent work suggests mammals are more vulnerable to cancer than other species, suggesting a fundamental trade-off between reproductive tolerance and immunological surveillance.
But do the costs and consequences of microchimerism fall equally on mothers and offspring? Building on Medawar’s pioneering work on acquired tolerance, fetal and maternal microchimerism have fundamentally different evolutionary dynamics. Fetal cells colonize a mature maternal immune system, potentially triggering chronic low-grade immunological conflict, while maternal cells are introduced during fetal immune development when self-versus-non-self boundaries are still being established. This temporal asymmetry predicts greater immunological conflict from fetal microchimerism than maternal microchimerism and suggests greater costs on maternal health.
Microchimerism and kinship: evolutionary considerations.
David Haig
Department of Organismic and Evolutionary Biology, Harvard University, USA
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.
