In a beautiful paper that has been an important reference for me in this project, Micro(chimerism), Immunity and Temporality: Rethinking the Ecology of Life and Death (2019) feminist philosopher Margrit Shildrick critiques a range of biological orthodoxies, particularly with regard to the existence of a singular genetic profile for what counts as human.
Genetic identity is assumed to be a definitive method for establishing and defining individual selfhood. Microchimerism is a phenomenon where an individual has cells in their body with two (or more) distinctly different genomes, and therefore distinctly different genetic identities. It is thought to be surprisingly common and calls into question genetic identity (as we currently think it) as definitive of individual identity. It also shows that DNA can wander, and that wandering DNA can become established in the body of another. DNA as a far more fluid and promiscuous entity than we’ve realised.
Genome sequencing has become commonplace for many people outside of scientific and medical practice, who seek a deeper understanding of their ancestry. The information gained from genome sequencing is open to question on multiple levels, some of which I’ll discuss in another post. In this paper Shildrick explores the biopolitical and bioethical implications of microchimerism and the more familiar terrain of the microbiome on foundational ideas in biology and medicine.
Microchimerism
Shildrick writes:
Classically, chimerism is understood as a combination of forms, either intraspecies or transspecies, that nevertheless retain genetic and usually morphological distinctions within a single body. Chimerism at the level of whole bodies represents the conventional use of the term, but it is nowhere near as ubiquitous as the unseen microchimerism that occurs at the cellular level and has limited impact on morphology. Strictly speaking microchimerism indicates that no more than 1 in 1000 cells is genetically distinct from the majority, but in some cases such cells may come to predominate in a particular organ as well as circulating in low numbers throughout the body. As Dupré remarks:
Chimeras do not necessarily experience any unusual symptoms, so the prevalence of full chimerism, chimerism derived from multiple zygotes, is not really known, and may be much higher than suspected. (Dupré, 2010, 22. In Shildrick, 2019, 11)
As Shildrick explains, a chimera is different to a hybrid such as the mule, created when a horse and a donkey mate. In the case of mules, the DNA of both horse and donkey is inside every cell of the mule, with chromosomal mismatches between horse and donkey DNA rendering the hybrid mule unable to have offspring. Chimeras differ in that individual cells in the chimera contain DNA from only one of the originating organisms. These 2 different genetic profiles exist in a cellular patchwork or mosaic fashion throughout the body of the chimera: some areas of the body comprise cells with DNA from one of the originating organisms, other areas comprise cells containing DNA from the other organism. Shildrick cites the example of geeps, who have a goat and a sheep as parents, and who can have offspring because their gametes (eggs or sperm) will be either sheep or goat, not a combination of both as in a hybrid.
How microchimerism occurs
Shildrink writes:
In conventional evolutionary terms, microchimerism is an ancient phenomenon widely detected in plants and invertebrates as well as many vertebrates and mammals including monkeys, cattle, dogs and humans. In human beings, a range of both iatrogenic and natural chimeric states exist, with biomedical interventions such as organ or stem cell transplantations constituting the former, while the latter includes the fusion of dizygotic twins in utero into one body or the more common incidence of foetal cell engraftment into the maternal body, and vice versa. Whatever the provenance, such transformations challenge ‘the traditional evolutionary dogma for the dominancy of genetically homogenous entities in nature’ (Rinkevich, 2011, 1). Bioscientific explanations for the existence of chimerism and microchimerism are disputed. But this has transformative implications for our conventional model of distinct biological objects, including ourselves as human beings, where each organism is coincident with a single genome. What is at stake, at very least, is the principle that DNA is sufficiently stable across individuals and over intergenerational time to provide a reliable guide to the genetic basis of human health, disease, and difference. But as Lappé and Landecker note: ‘(a)s genomic instability becomes an area of increasing focus for life scientists, it opens up a new landscape of genomic multiplicity and temporality in health and disease.’ (Lappé and Landecker. 2015, 161. In Shildrick, 2019, 12)
For me the most fascinating of these (all fascinating) forms of microchimerism is the one referred to by Shildrick above as ‘foetal cell engraftment into the maternal body, and vice versa.’ Rather than this being a strange aberration of biology, an instance of biology going rogue or the result of a biomedical intervention, it appears to be commonplace. In this case foetal cells that pass across the placenta can become established in the bodies of their mothers, establishing enduring, long-term cell lineages. These cells of foetal origin contain the child’s DNA, not the mother’s. The phenomenon also happens in the reverse, mother to child. Studies of autopsied brain and breast tissues of women who have given birth to boys, have revealed cells with XY chromosomes that presumably originated from their son’s cells, that engrafted in their mother’s body during pregnancy. The same process would of course happen with daughters, but the ‘aberrant’ XY chromosomes are easier to spot as they differ from the mother’s XX chromosomes.
The significance of microchimerism
Shildrick writes
… the whole-sale visceral transformations that chimerism implies indicate not simply intercorporeality, but the irreducibility of embodiment into singular and static forms. If once the standard expectation would have been that such incursions could not be tolerated long-term without pathological consequences, there are now at least some indicators of beneficial effect. We might wonder if we could think of such cellular translocations as offering a different model that extends far beyond the privileging of modernist forms of human being. The significance is that if self and other are no longer distinct, if the very rigidity of the terms suggests a certain incoherence, then it is not just the space of the body that is contested but the time of the body too. What does it mean for a body (whose body? which body?) to die? (Shildrick, 2019, 16-17)
And further:
On a theoretical level, the move that is emerging is away from a philosophy of biology that tries to pin down the essence of life, to a biophilosophy ‘concerned with articulating those things that ceaselessly transform life’ (Thacker 2015, 126). In other words, the project is to elaborate a hitherto unregarded network of relations that dispenses with the boundaries of singular location and time and reimagines the concept of living outside oneself. In an embodied hauntology, the other is always within, but equally the self (if we can still call it that) externalises its becoming … At very least the rethinking of microchimerism and immunity entails a different kind of ethics where we should actively seek to ‘enter into modes of relation with multiple others.’ (Braidotti 2015, 34. In Shildrick, 2019, 20, bold text mine)
Braidotti, Rosi. 2015. “Posthuman Affirmative Politics.” In Resisting Biopolitics: Philosophical, Political and Performative Strategies, edited by S. Wilmer, and A. Zukauskaite, 30–56. London: Routledge.
Dupré, John. 2010. “The Polygenomic Organism.” The Sociological Review 58 (1): 19–31.
Lappé, Martine, and Hannah Landecker. 2015. “Sociology in an Age of Genomic Instability: Copy Number Variation, Somatic Mosaicism, and the Fallen Genome.” In Genetics, Health and Society, edited by Brea L. Perry, 157–186. United Kingdom: Emerald Group.
Rinkevich, Baruch. 2011. “Quo Vadis Chimerism?” Chimerism 2 (1): 1–5.
Shildrick, Margrit. 2019. ‘Micro(chimerism), Immunity and Temporality: Rethinking the Ecology of Life and Death.” Australian Feminist Studies 34 (99): 10-24. https://doi.org/10.1080/08164649.2019.1611527