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Is chimerism associated with cancer across the
tree of life?
Stefania E Kapsetaki ( skapseta@asu.edu )
Arizona State University https://orcid.org/0000-0002-9999-8573
Angelo Fortunato
Arizona State University
Zachary Compton
Arizona State University https://orcid.org/0000-0002-0558-2316
Shawn M. Rupp
Arizona State University
Zaid Nour
Arizona State University
Skyelyn Riggs-Davis
Arizona State University
Dylan Stephenson
Arizona State University
Elizabeth G. Duke
North Carolina State University https://orcid.org/0000-0002-5534-0649
Amy M. Boddy
University of California Santa Barbara https://orcid.org/0000-0002-7990-2415
Tara M. Harrison
North Carolina State University https://orcid.org/0000-0002-2072-5936
Carlo C. Maley
Arizona State University https://orcid.org/0000-0002-0745-7076
Athena Aktipis
Arizona State University https://orcid.org/0000-0002-7128-670X
Research Article
Keywords:
DOI: https://doi.org/10.21203/rs.3.rs-2370277/v1
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Title
Is chimerism associated with cancer across the tree of life?
Authors
Stefania E. Kapsetaki1,2,+, Angelo Fortunato1,2, Zachary Compton1,2,3, Shawn M. Rupp1,2, Zaid
Nour1,2, Skyelyn Riggs-Davis1,2, Dylan Stephenson4, Elizabeth G. Duke1,5,7, Amy M. Boddy1,6,
Tara M. Harrison1,5,7, Carlo C. Maley*1,2,3, Athena Aktipis*1,4
+ corresponding author
* co-senior authors
Author affiliations
¹ Arizona Cancer Evolution Center, Arizona State University, Tempe, AZ, USA
² Center for Biocomputing, Security and Society, Biodesign Institute, Arizona State
University, Tempe, AZ, USA
3School of Life Sciences, Arizona State University, Tempe, AZ, USA
4Department of Psychology, Arizona State University, Tempe, AZ, USA
5Department of Clinical Sciences, North Carolina State University, Raleigh, NC, 27607 USA
6Department of Anthropology, University of California Santa Barbara, CA, USA
7Exotic Species Cancer Research Alliance, North Carolina State University, Raleigh, NC,
27607 USA
1
Abstract
Chimerism is a widespread phenomenon across the tree of life. It is defined as a
multicellular organism composed of cells from other genetically distinct entities. This ability
to ‘tolerate’ non-self cells may be linked to susceptibility to diseases like cancer. Here we test
whether chimerism is associated with cancers across obligately multicellular organisms in the
tree of life. We classified 12 obligately multicellular taxa from lowest to highest chimerism
levels based on the existing literature on the presence of chimerism in these species. We then
tested for associations of chimerism with tumour invasiveness, neoplasia (benign or
malignant) prevalence and malignancy prevalence in 11 terrestrial mammalian species. We
found that taxa with higher levels of chimerism have higher tumour invasiveness, though
there was no association between malignancy or neoplasia and chimerism among mammals.
This suggests that there may be an important biological relationship between chimerism and
susceptibility to tissue invasion by cancerous cells. Studying chimerism might help us
identify mechanisms underlying invasive cancers and also could provide insights into the
detection and management of emerging transmissible cancers.
Introduction
Chimerism is widespread across life
Although the concept of a chimera derives from a Greek mythological monster with a
lion’s head, a goat’s body, and a snake’s tail1,2, chimeras are biologically real. A chimera is an
obligately multicellular organism that is composed of non-clonal cells (relatedness < 1),
which do not originate from mutations within the body3–5. Chimeric cells are cells from a
2
different host. This can range from large scale cellular exchange to smaller amounts, called
microchimerism. Chimeras exist in several taxa, from marine sponges to trees and terrestrial
mammals (Table 1 & 2).
The evolutionary road from single cells to obligate multicellularity, shows that
irreversible (obligate) multicellularity only arose when cells divided clonally across the tree
of life6,7. Such clonality allowed somatic cells to cooperate at the most extreme level,
altruistically sacrificing their reproduction, due to the alignment of their genomic fitness
interests with the germ cells and other somatic cells. Chimeric cells, due to their
non-clonality, are expected to have incompletely aligned fitness interests with other cells in
the obligately multicellular body6–8. These unaligned fitness interests and other interacting
ecological factors can lead to conflict among cells6,7,9–12, which can include overproliferation,
avoiding apoptosis and other forms of cheating among cells that can lead to fatal cancers8,13,14.
When non-self cells appear, self/non-self-recognising systems, often in the form of immune
cells, destroy the foreign/non-relatives15,16. This is why, in organ transplantations, it is
necessary to give immunosuppressive drugs to the host, otherwise, the immune system almost
always rejects the transplant17–19.
The existence of chimeras challenges the traditional view that clonality is critical to
obligate multicellularity6–8. Chimeras also appear to break the traditional laws of
inheritance20. Mendel saw that traits can be inherited strictly through the passage of parental
traits in the germline creating the zygote21. Since then, people have discovered that somatic
cells are routinely transmitted from one generation to the next via the placenta, generating
microchimerism22–24. Transplanted cells can also become the germline, as was seen in a
patient who received a bone marrow transplant where after four years, 100% of his semen
DNA derived from the donor25. Similarly, in the colonial tunicate Botryllus schlosseri, when
3
different colonies fuse forming a chimera, the germ cells of one colony can replace the germ
cells of another colony, a phenomenon known as germ cell parasitism26–28.
Obligate chimerism and cancer have common hallmarks
Cancers are composed of genetically and/or epigenetically mutated cells that invade
other tissues and/or organs in the body29–31. Cancer cells can even invade other hosts, such as
humans32–35, Tasmanian devils, Syrian hamsters, dogs, clams, and molluscs as intraspecies or
interspecies transmissible cancers36–42. Across different types of cancers we see similar
phenotypes called hallmarks of cancer43. Cancers and chimerism share some of these
hallmarks. Cancer cells can evade immune destruction44–47; chimeric cells can too23,48. For
instance, transmissible cancers are linked with loss of diversity in the major
histocompatibility complex (MHC), low expression of MHC antigens, in other words
immunological invisibility, and appear to be boundless in terms of the intraspecific hosts they
can invade (e.g., 4,000-8,500 year old distribution of Canine Transmissible Venereal Tumours
across 43 countries)49. The membrane protein CD200 inhibits natural killer cell responses to
cancer, facilitates graft tolerance in humans and mice, and is highly expressed on
transmissible devil facial tumour cells50.Also, both cancer cells and chimeric endothelial
progenitor fetal cells can induce angiogenesis and activate invasion23,43,51–54. Chimeric fetal
cells have been found in several tumours in mothers55–58. Furthermore, chimeric fetal cells are
more often found in cancerous than healthy tissue59–61. Genetic differences among cells can
lead to conflict over limited resources, and overproliferation of one cell type over the other8.
This may be happening when allografts or xenografts lead to hyperplasias in sea cucumbers,
sea stars, and sponges62–64, and interspecific crosses can lead to cancer in plants43,65.
4
Investigating associations between chimerism levels and cancer
across species
It is clear that chimerism exists, but it has not been systematically studied in relation
to susceptibility to cancer. In this study we use literature resources and zoological data across
the tree of life to: (1) organise taxa according to their highest level of chimerism observed;
and (2) test whether there is a positive association between the highest level of chimerism
observed in an obligately multicellular taxon and the highest level of tumour invasiveness
observed in those taxa. Within mammals, we also investigated whether chimerism was
associated with neoplasia (benign or malignant) prevalence and malignancy prevalence. Due
to the limited experimental data on tumour invasiveness in non-vertebrates in the literature,
we classified the data relative to lineages representing broad taxa (from subphyla to
subkingdoms) including: Vertebrata (vertebrates), Tunicata (tunicates), Protostomia
(protostomes), Placozoa (placozoans), Ctenophora (comb jellies), Porifera (sponges),
Echinodermata (echinoderms), Cnidaria (cnidarians), Porifera (sponges), Ascomycota (sac
fungi), Embryophyta (land plants), Rhodophyta (red algae). We chose these taxa based on an
existing phylogeny of tumour invasiveness across taxa43. We hypothesised that chimerism is
positively associated with cancers across the tree of life.
Results
Tumour invasiveness is positively correlated with chimerism across
the tree of life
We classified tumour invasiveness on a scale from no cancer or no cancer-like
growth,cancer-like growth,cancer, to transmissible cancers. We found that across 12
5
obligately multicellular taxa, the highest level of tumour invasiveness observed in a taxon is
positively correlated with the highest level of chimerism observed in a taxon. In other words,
in obligately multicellular taxa that accept foreign cells from different species, higher tumour
invasiveness will be observed (Table 1; Fig. 1 & 3; PGLS analysis: F-statistic = 6.02 on 1 and
10 DF, ML lambda = 1, R² = 0.37, slope = 0.67, P-value = 0.03).
Types and duration of chimeras
Acceptance of foreign cells has been seen either as foreign cells existing inside
species naturally, or species not rejecting graft tissue, or both (Table 1; Supplementary Table
1). We have classified chimeras in these 12 obligately multicellular taxa as natural,
experimentally-induced, or both (Table 1; Supplementary Table 1; Fig. 3). Seven taxa have
been found to tolerate experimentally-induced chimeras. Only Basidiomycota exhibit
evidence of just natural chimeras, and four taxa show evidence of both
experimentally-induced and natural chimeras (Table 1; Supplementary Table 1; Fig. 3).
The length of time graft cells can survive in hosts varies from a few days to a lifetime,
depending on the taxon. Graft cells have been found in many sites including the gonads,
intestinal tissue, and meristem in plants (Table 1; Supplementary Table 1). Chimerism has
been reported early in development in two taxa, later in development in two taxa, both early
and late in development in five taxa, and no information about developmental timing of
chimerism is reported in three taxa (Table 1).
6
Table 1. Examples of natural (A) and experimental (B) chimeras across 12 obligately multicellular taxa. In the majority of cases in the
literature, species reject foreign cells. The list of references in this table is not exhaustive since we do not mention here all the examples of graft
rejection reported in the literature. The examples of chimerism in this table are the examples of graft/foreign cell acceptance from the existing
literature. We provide an extended version of this table, which also includes the highest level of chimerism observed in each taxon, in
Supplementary Table 1.
Table 1A.
Taxon
(common
name)
Examples of
chimerism
Chimerism
early in
development
Chimerism
later in
development
Manipulation of the
graft/recipient
For how long did the
graft cells survive in
the recipient?
How many of the graft
cells survived in the
recipient?
Vertebrata
(vertebrates)
see Table 2A
✓
(see Table
2A)
✓
(see Table
2A)
see Table 2A
see Table 2A
see Table 2A
Tunicata
(tunicates)
chimeric colonies of
2–3 different
genotypes66
N/A
N/A
NA
N/A
N/A
Cnidaria
(cnidarians)
chimeras with
distinct genotypes,
unrelated genotypes
fused67
✓
juveniles 67–69
✓
67–69
“branches were
sampled as far away
from each other as
possible” 67,69
up to 450 days68
“partners shared layers of
endoderm, mesoglea and
ectoderm, and the
gastrovascular cavity”70
Porifera
(sponges)
bispecific
conglomerates71
✓
larvae72
✓
72
N/A
N/A
10% of the whole71
7
Basidiomycota
(filamentous
fungi)
single basidiome with
nine different
genotypes 73
N/A
N/A
N/A
N/A
N/A
Table 1B.
Taxon
(common
name)
Examples of
chimerism
Chimerism
early in
development
Chimerism
later in
development
Manipulation of the
graft/recipient
For how long did
the graft cells
survive in the
recipient?
How many of the graft cells
survived in the recipient?
Vertebrata
(vertebrates)
see Table 2B
✓
(see Table
2B)
✓
(see Table
2B)
see Table 2B
see Table 2B
see Table 2B
Tunicata
(tunicates)
fusing colonies
share at least one
allele74,75;
xenograft
chimera76
✓
77
✓
adult77
sexually
mature75
irradiated subclones
received the grafts66,76
30 days66; 1 week78;
months76; 2
months74; 8–10
months75
both genotypes in a bud74; mass
of gonads and soma came from
the
resorbed genotype75
Protostomia
(protostomes)
allografts79;
allografts and
xenografts80;
xenografts81
N/A
✓
adults81;
parents82
collagen-based
skin-like scaffold
allowed wound
healing79
6 months79; 6
months80; 14 days82
implanted hearts continued to
beat throughout the study79
Placozoa
(placozoans)
cell reaggregation,
no evidence that
the new organism
is functional83
N/A
N/A
dissociation medium,
cells were passed
through a gauze83
weeks 83
N/A
8
Ctenophora
(comb jellies)
“animal consisting
of the mid-pieces
of four animals”84
N/A
N/A
grafts were held in
place with cotton to
permit healing85
3 days84; 2 days, 10
days85
“each piece maintained its
identity”, “and by regenerating
auricles and lobes” 84
Echinodermata
(echinoderms)
viable allografts62
N/A
✓
adults62
anesthetized before
grafting, use of
tetracycline and gelatin
powder 62
>100 days, 129–185
days62; >300 days,
110 days86; average
of 341.8 days87
allografts “gradually resorbed
by ingrowth of recipient
tissue”86
Cnidaria
(cnidarians)
bispecific
chimeras88
N/A
N/A
N/A
>4 months 88
“chimeras had H. attenuata
epithelial cells and P. oligactis
interstitial cell lineage”88
Porifera
(sponges)
bispecific
conglomerates89
✓
larvae90,91
N/A
“forced to settle in
contact”91
48 hours, 72 hours92;
50 days91
N/A
Ascomycota
(sac fungi)
hyphae fuse
between colonies
of the same
species93
✓
germinating
conidia94
✓
mature
hyphae94
“conidia were used to
initiate heterokaryotic
mycelia.”94
N/A
N/A
Embryophyta
(land plants)
interspecific
chimeras95–97
✓
98;
seedlings99;
young
plants97
N/A
“Graft unions were
secured with budding
rubbers”99
46, 53, 70 and 73
days after
decapitation96
“Eventually the N. glauca
tissue in the original menstem
was replaced by N. tabacum”96
Rhodophyta
(red algae)
“interindividual
fusions in red and
brown algal
species”100
✓
100
N/A
laboratory-built bicolor
chimeras100
30–45 days100
“mixed tissue reached
10%–15% the axes length”100
9
Chimerism does not explain the variance in malignancy and neoplasia
prevalence across terrestrial mammals
We searched within mammalian species to see whether malignancy prevalence and
neoplasia prevalence are correlated with chimerism levels. We found that the highest
chimerism level observed does not explain a significant amount of variance in malignancy
prevalence or neoplasia prevalence (Fig. 2; PGLS analysis; malignancy prevalence:
F-statistic = 0.34 on 2 and 8 DF, ML lambda = 0.00006, R² = 0.2, P-value = 0.71; neoplasia
prevalence: F-statistic = 0.48 on 2 and 8 DF, ML lambda = 0.99, R² = 0.19, P-value = 0.63).
The majority of these mammalian chimeras are developmentally young, immunosuppressed
and/or inbred. Across the literature, eleven species are experimentally-induced chimeras, four
species are natural chimeras, and three species including humans are both
experimentally-induced and natural chimeras (Table 2; Supplementary Table 2).
The period of time that the foreign cells survive in the host varies from 179 days in
the Hamadryas baboon to a lifetime in humans. Graft cells have been found in many locations
in the host, including the brain, spleen, liver, heart, pancreas, blood, and the thymus,
depending on the species (Table 2; Supplementary Table 2).
Based on the current literature, chimerism has been reported early in development in
nine mammalian species, later in development in three species, both early and late in
development in five species, and no data on the developmental timing of chimerism were
available in the rabbit Oryctolagus cuniculus (Table 2).
10
Table 2. Examples of natural (A) and experimental (B) chimerism in 18 mammalian species. In the majority of cases in the literature,
species reject foreign cells. The list of references in the table is not exhaustive since we do not mention here all the examples of graft rejection
reported in the literature. The examples of chimerism in this table are examples of graft/foreign cell acceptance from the existing literature. An
extended version with the highest level of chimerism observed in each species is reported in Supplementary Table 2. cGY: centigray; DLA: dog
leucocyte antigen.
Table 2A.
Mammalian species
(common name)
Examples of
chimerism
Chimerism
early in
development
Chimerism
later in
development
For how long did the
graft cells survive in the
recipient?
How many of the graft cells survived
in the recipient?
Saguinus oedipus
(cotton-top tamarin)
microchimerism
101
✓
101
N/A
N/A
chimeric lymphocyte and
monocyte/macrophage populations101
Bos taurus (cattle)
microchimerism
102
✓
102
✓
102
twins were adults when
tested102
N/A
Canis lupus familiaris
(domestic dog)
fetal
microchimerism
103
✓
103
N/A
N/A
N/A
Homo sapiens (human)
blood group
chimerism104
✓
22,23,104,105
✓
22,23,105
years, probably a
lifetime105; until adulthood
22
mothers with scleroderma disease had
“an average of seven male cells per 10
milliliters of blood”, one fetal cell in 1
million cells in maternal circulation 22
11
Callithrix jacchus
(common marmoset)
blood chimerism
between twins
and triplets106
✓
106
N/A
age at birth or age when
euthanized106
N/A
Equus ferus (wild horse)
microchimerism
107
✓
107
N/A
6th month of gestation107
N/A
Macaca mulatta (rhesus
macaque)
maternal
microchimerism
108
✓
108
✓
108
1–1.5 years of age108
0.001–1.9% chimeric cells108
Table 2B.
Mammalian
species
(common
name)
Examples of
chimerism
Chimerism
early in
development
Chimerism
later in
development
Manipulation of the
graft/recipient
For how long did the graft
cells survive in the
recipient?
How many of the graft
cells survived in the
recipient?
Rattus
norvegicus
(common rat)
allogeneic109;
mouse retinae
transplanted into
rats110
✓
110,111
N/A
conditioned with 1100
cGy and T cell
depleted109
300 days after
transplantation, ≥14 months
109
“well-formed grafts
containing numerous
cells”110
Mus musculus
(house mouse)
human
embryonic cells
implanted in
mice112
✓
112–114
N/A
coculturing with
mouse embryonic
fibroblasts in defined
medium112
two months, 18 months112; 2
weeks, 3–5 weeks114
“0.1% of the brain cells
are of human origin.”112
Sarcophilus
harrisii
(Tasmanian
devil)
“All successful
allografts were
acutely
rejected”115
N/A
✓
115
surgical glue on the
borders of grafts, pain
relief medication115
“14 days after surgery”,
“severe rejection on Day
21”115
“necrosis associated with
polymorphonuclear cell
infiltration”115
12
Ovis aries
(sheep)
embryonically
derived human
hematopoietic
stem cells in
sheep
116,117
✓
116,117
N/A
“transplanted early in
gestation when the
recipient is still
largely
immunologically
naive.”117
7 years later
116,117; “at least 9
months.”117
“liver, heart and pancreas,
that are 15% human.”
116,117
Bos taurus
(cattle)
transgene-specifi
c sequence in
cows
with transgenic
fetuses118
✓
118
✓
118
“embryo produced by
in vitro fertilization of
transvaginally
recovered
oocytes”118,119
up to 4 months118
“six circulating male cells
or their corresponding
DNA contents (if
cell-free) per mL of
maternal blood”118
Sus scrofa (wild
boar)
human cells in
pigs
✓
120
N/A
fetal pigs injected
with human T
cell-depleted bone
marrow cells120
>1 year120
N/A
Acinonyx
jubatus
(cheetah)
allograft, “skin
grafts between
unrelated
cheetahs”121
N/A
✓
121
antibiotics were
administered, surgical
area was bandaged121
“2 weeks after surgery”, day
23121
N/A
Canis lupus
familiaris
(domestic dog)
allografts in dogs
bearing identical
DLA
haplotypes122;
skin allograft123
N/A
✓
122,123
200 cGy,
immunosuppression123
; radiation chimeras122
4 weeks, 76 weeks, >5
years123; 10-25 days 122
“T and B cells contained
donor-type cells”123
Papio
hamadryas
(Hamadryas
baboon)
xenotransplantati
on124;
pig-to-baboon
xenografts125
✓
125
N/A
immunosuppressives,
irradiation, aspirin124;
fibrinogen125
up to 6 months, 78 and 179
days124; up to 3 ½ days125
N/A
13
Homo sapiens
(human)
allogeneic
transplantation126
; xenograft127
N/A
✓
126,127
immunosuppression,
“20 units of blood
were given during the
11 h operation.”127
70 days127
N/A
Didelphis
virginiana
(Virginia
opossum)
maternal
allograft, “none
of the 24 young
less than 12 days
of age rejected
the maternal
allografts”128
✓
128
✓
128
“the mother (was)
anesthetised with
sodium
pentobarbital”128
“at least 80 days in most
cases”128
N/A
Oryctolagus
cuniculus
(European
rabbit)
xenografted rat
tissue129
N/A
N/A
N/A
“rat hippocampal grafts
developing for 8 weeks in
the rabbit septum”129
“grafts significantly
increased in their volume
(600 to 800% of the initial
value)”129
Dasypus
novemcinctus
(armadillo)
skin grafts
accepted
between
monozygotic
littermates
130
✓
130
N/A
“grafts were redressed
at the early
inspections to protect
them from trauma.”130
20 days, 50 days, 85th
postoperative day130
N/A
Mesocricetus
auratus (Syrian
hamster)
homografts
between
completely
unrelated
stocks131
N/A
✓
131
“graft fitted into an
appropriately sized
bed on the lateral
thoracic wall.”131
200 days, 24 to 140 days131
N/A
14
Discussion
Chimerism has been observed across a wide variety of species and appears in a
variety of forms (Table 1 & 2). We found that higher levels of chimerism are positively
correlated with tumour invasiveness across 12 obligately multicellular taxa on the tree of life
(Fig. 1 & 3; Table 1; Supplementary Table 1). We did not find a significant association
between chimerism and malignancy prevalence or neoplasia prevalence among terrestrial
mammals (Fig. 2).
This association of chimerism with tumour invasiveness might be a result of similar
underlying mechanisms that allow chimeric and cancerous cells to flourish inside hosts. This
is consistent with several observations in the literature including common clinical signs
between chimeric and cancer cells23,43, hybrid crosses between different species of plants
leading to cancerous growth65, and even cell ploidy levels positively correlating with tumour
invasiveness132,133.
Ecology may influence chimerism
Chimerism may also depend on the environment. Recent research shows that lineages
forming non-clonal multicellular groups are more common in terrestrial environments,
whereas lineages forming clonal groups are more common in aquatic environments9,134,135.
This suggests we might expect to find more cases of chimeric cells that disrupt the
cohesiveness of the multicellular community of cooperating cells in terrestrial environments.
However, we did not find that obligately multicellular taxa that evolved on land had higher
levels of chimerism than obligately multicellular taxa that evolved in water (Supp. Fig. 1).
This could be because we restricted the definition of chimerism to obligately multicellular
15
organisms, whereas Fisher et al.9restricted the definition of chimerism, i.e. non-clonal group
formation, to facultatively multicellular organisms.
Limitations & future directions
There are several limitations in this study. A primary limitation is the lack of data. We
were only able to analyse 12 obligately multicellular taxa across the tree of life and 11
terrestrial mammalian species (Fig. 1 & 2). These large taxa likely include species with
different degrees of chimerism. Additionally, there is likely a sampling bias in the degree of
chimerism that has been observed. We are more likely to have observed chimerism, and
higher levels of chimerism in species that have been studied more and for a longer period per
experiment, compared to species for which there have been few studies. Thus, our analysis is
just a first, imprecise view of the relationship between chimerism and cancer. Future studies
would benefit from testing for both spontaneous and experimentally induced chimerism with
consistent methodologies, across many more species. Such studies would also benefit from
collecting more necropsies in those species (ideally in wild animals) for more accurate
estimates of cancer prevalence.
Not all obligate chimeras are strictly chimeric throughout their lifetime and this
creates additional confusion in the definition of multicellular species. Some chimeras keep
chimeric cells only for a few days or months, while others for a lifetime (Table 1 & 2). Some
of this observed variation may be due to methodological differences among experiments
(Table 1 & 2), but other aspects of this variation are likely the result of biological differences
among species, including differences in immunological barriers136,137. The specific
immunological barriers that determine whether a graft will be accepted and/or passed to the
next generation are largely unknown across species. For example, only recently have
scientists found that chimeric cells can invade the germline of humans25. This reminds us that
16
an individual may not be as defined as we think7,8,138–140. The tips of a phylogenetic tree may
be a single species or even a chimera of two or more species88,141. In other words, according
to Ford Doolittle, “the history of life cannot properly be represented as a tree.”142.
For some species and higher taxonomic groups in our dataset, we know that there are
chimeras early and/or late in development (Table 1 & 2). We have too few species and higher
taxonomic groups, however, to perform a powerful statistical analysis comparing chimerism
levels between these different stages of development. Early embryonic development,
pregnancy, and old age are times when the immune system is relatively weak. Working in a
period when humanity was in desperate need for transplants for the injured, during and after
World War II, Nobel Prize winner Peter Medawar et al. showed that immunological
individuality is “a property that comes into being during the course of development”143. “The
chick, before the eighteenth day of incubation, is almost indiscriminately hospitable” to
extrinsic agents, 29 among 188 chickens that received a graft from a different individual “on
the day of hatching or within a few days thereafter” could keep it almost indefinitely, but
there is “progressive decay, with increasing age, of the power of an antigenic stimulus to
confer tolerance.”144–147. Therefore, we would expect species to have a higher susceptibility in
receiving and accepting chimeric cells during those vulnerable times of early development,
pregnancy, and old age. Future studies should determine which developmental stage
individuals can accept related/foreign cells.
Finally, we lack information on the molecular mechanisms behind the association
between chimerism and tumour invasiveness for any given species. A molecular in vitro and
in vivo approach would be useful to detect and track chimeric cells over several
generations148, identify whether and when they become cancerous, determine if the same
pathways are used to reject chimeric and cancerous cells, and test whether their degree of
invasiveness depends on the level of chimerism.
17
Conclusions
The results of this study are a promising first step in understanding the origins of
chimerism and have implications for the discovery and study of transmissible cancers. We
hypothesise that species that are the most accepting of chimeric cells are also the species
most likely to harbour transmissible cancers. Since chimerism and cancer have common
hallmarks, such as evading immune destruction44–48, inducing angiogenesis, and activating
invasion23,43,51–54, by finding the mechanisms that chimeric cells use to invade other organisms
we may find the mechanisms that transmissible cancer cells use to invade other organisms
and thus design drugs to target those pathways.
Methods
Chimerism
To find chimeras across the tree of life, we searched the Web of Science, StarPlus,
Google Scholar, JStor, and Mendeley using the keywords transplantation, immune tolerance,
immune development, microchimerism, genetic mosaicism, grafting, chimerism, chimera,
chimaera,chimeric hybrids, obligate chimerism, homografts, allografts, xenografts, fusion,
coalescence. We specifically looked for chimerism in 12 obligately multicellular taxa for
which we knew their highest levels of tumour invasiveness (vertebrata, tunicata, protostomia,
placozoa, ctenophora, echinodermata, cnidaria, porifera, basidiomycota, ascomycota,
embryophyta, and rhodophyta)39,43,149 (Table 1), and in 18 terrestrial mammalian species
(Table 2) for which we had data on malignancy prevalence and neoplasia prevalence.
18
We define chimera as an obligately multicellular organism composed of non-clonal
cells not originating from mutations within the organism. Therefore by this definition, we
exclude the following examples: non-genetically-examined chimeric fossil records,
epigenetic phenotypic chimeras, facultative chimeras (e.g., an organism with its microbiome,
including the fungome and virome), genetically modified organisms (GMOs), chimeras
originating from a chimeric zygote by the fusion of two sperms with an egg (e.g.150,151),
hybrids, epigenetic polymorphisms, conjoined twins, interspecific embryo transfers without
having tested for microchimerism, multinucleated cells in fungi and red algae, plant chimeras
originating from mutations within the organism during development, chimeric cells that are
transmissible cancers, chimeric antibodies, no chimerism studies performed in a particular
species.
Levels of chimerism
We categorised organisms into different levels of chimerism from lowest to highest,
as no chimerism [0], accepts cells from a close relative (e.g., twin or mother) [1], accepts
cells from the same species other than a close relative [2], and accepts cells from different
species [3]. We restricted these data only to cases of obligately multicellular organisms
accepting cells and maintaining them beyond reproductive age, if reported. If there was
variation in the level of chimerism among literature in a taxon, we classified that taxon
according to the highest level of chimerism reported for that taxon.
Coding of chimerism levels
We recruited three biology/psychology undergraduates from Arizona State University
to code the examples of highest levels of chimerism the lead author found in the existing
literature into the categories described above. We gave each individual a spreadsheet with: (1)
the definition of chimerism (“An obligately multicellular organism composed of non-clonal
19
cells not originating from mutations within the organism.”); (2) a list of examples that are not
chimeras (“facultative multicellular organisms, genetic hybrids (e.g., mule), epigenetic
polymorphisms, conjoined twins, missing fossil evidence”); (3) the genus
names/clade/division of the organisms in our database; (4) an empty column for the
individuals to write the highest level of chimerism observed in each taxon; (5) a list of
literature and advice to search for more examples in the literature; (6) empty columns for the
individuals to complete whether experiments were performed early or later in development,
and whether chimerism was natural or experimentally-induced; and (7) an empty column for
the individuals to write notes or comments. The individuals completed the task within three
weeks. The final highest chimerism level for each taxon that we report in this article is based
on the “highest level of chimerism observed” score given by the majority of coders. For
example, if among us the “highest level of chimerism observed” we gave for a taxon was 1,
1, 1, 2 (one score from each of the three undergraduate coders and the lead author), then the
final “highest level of chimerism observed” for that taxon is 1. If one level did not dominate
(e.g., 1, 1, 2, 2), then the lead author read additional literature, and decided what was the most
correct “highest level of chimerism observed” for that taxon. Finally, while revising the
manuscript, A.F. proof-read these scores based on the literature.
Tumour invasiveness
We collected tumour invasiveness data for 12 obligately multicellular taxa: vertebrata,
tunicata, protostomia, placozoa, ctenophora, echinodermata, cnidaria, porifera,
basidiomycota, ascomycota, embryophyta, and rhodophyta39,43,149. In order to create a scale of
tumour invasiveness, we classified tumour invasiveness from lowest to highest, as no
cancer/cancer-like growth detected [0] (in studies that sought to find cancer/cancer-like
growth), cancer-like growth [1], cancer [2], and transmissible cancer [3]. If there was
20
variation in the level of tumour invasiveness among species in a taxon, we classified that
taxon according to the highest level of tumour invasiveness found in any species of that
taxon.
Malignancy and neoplasia prevalence data collection
Within mammals, there was not enough variation in tumour invasiveness levels across
the 18 mammalian species in our dataset in order to conduct a powerful analysis within this
class, so we examined the malignancy prevalence and neoplasia prevalence of each
mammalian species instead. We obtained malignancy prevalence and neoplasia prevalence
data across dozens of terrestrial mammalian species. These data are from animals in zoos,
aquariums, and/or private veterinary practices. Neoplasia includes benign or malignant
tumours. Malignancy prevalence or neoplasia prevalence refers to the total malignant records
of a species including non-neoplasia records or neoplasia records excluding non-neoplasia
records, divided by the total records with denominators, respectively. Total records with
denominators refer to data where we know the population size of a species from databases
where non-neoplasia records are also available. We only used species for which we had ≥20
necropsies (supplementary data). We excluded cancer records from wild animals. We also
excluded infancy records from this database as there is usually high infant mortality across
species that is not due to cancer. We added malignancy prevalence data for adult humans
(https://ourworldindata.org) in our analyses, but not neoplasia prevalence since we do not
have an estimate of benign prevalence in adult humans.
Phylogenetic tree construction
To create a phylogenetic tree of the 12 obligately multicellular taxa across the tree of
life and 11 mammalian species with more than ≥20 necropsies in our database, we used the
21
Time Tree of Life (http://timetree.org/). At the end of the tips we placed chimeric taxa based
on the majority of cells of that chimera. For example, if there was a sheep-human chimera of
which the majority of cells were sheep cells and the minority human cells, we would place
that species on a tip as a sheep, in order to make the phylogenetic tree.
Statistical analyses
We performed all analyses in R version 4.0.5152. To compare the association between
highest tumour invasiveness, malignancy prevalence or neoplasia prevalence and highest
chimerism levels, we used the R packages CAPER153, phytools154, geiger155, tidyverse156, and
powerAnalysis (https://github.com/cran/powerAnalysis), and performed a phylogenetic
generalized least squares (PGLS) model which takes into account the phylogenetic
non-independence between taxa. In the analyses where the dependent variable was
malignancy prevalence or neoplasia prevalence, we used a PGLS model weighted by
1/(square root of the number of necropsies per species) (from Revell154).
We first made two trees (phyl file); one including the above mentioned 12 obligately
multicellular taxa across the tree of life and one with the 11 mammalian species, using the
NCBI Tree creator (https://www.ncbi.nlm.nih.gov/Taxonomy/CommonTree/wwwcmt.cgi).
In the PGLS analyses of 12 obligately multicellular taxa across the tree of life, we set
the variables highest tumour invasiveness levels [0, 1, 2, 3], and highest level of chimerism
observed [0, 1, 2, 3], as numerical variables. When comparing malignancy prevalence,
neoplasia prevalence and the highest level of chimerism observed across terrestrial
mammalian species, we set malignancy prevalence and neoplasia prevalence as a dependent
numerical variable and the highest level of chimerism observed as an independent categorical
variable. The linear PGLS model was originally designed for continuous variables, however
Graber157 compared the statistical performance of different ordinal response models, and
22
together with Matthews et al.158 recommend using the PGLS model even when treating
ordinal scaled variables as continuous.
Acknowledgements
We would like to thank the pathologists, veterinarians, and staff at the zoos and
private veterinary practices for contributing to the data collection by diagnosing malignancy
prevalence and neoplasia prevalence. We would like to acknowledge the following
institutions: Akron Zoo, Atlanta Zoo, Audubon Nature Institute, Bergen County Zoo,
Birmingham Zoo, Buffalo Zoo, Capron Park Zoo, Central Florida Zoo, Dallas Zoo, El Paso
Zoo, Elmwood Park Zoo, Fort Worth Zoo, Gladys Porter Zoo, Greensboro Science Center,
Henry Doorly Zoo, Utah’s Hogle Zoo, Jacksonville Zoo, John Ball Zoo, Los Angeles Zoo,
Louisville Zoo, Mesker Park Zoo, Miami Zoo, Oakland Zoo, Oklahoma City Zoo,
Philadelphia Zoo, Phoenix Zoo, Pueblo Zoo, San Antonio Zoo, Santa Ana Zoo, Santa
Barbara Zoo, Sedgwick County Zoo, Seneca Park Zoo, The Brevard Zoo, The Detroit Zoo,
The Oregon Zoo, and Toledo Zoo. Thank you to Michael Garner for his help in collecting and
diagnosing some of the malignancy and neoplasia data. Thank you also to William Cross,
David Quammen, Michael Edwards, Valerie Harris, the Arizona Cancer & Evolution team,
and the Murchison group, for helpful comments. This work was supported in part by NIH
grants U54 CA217376, U2C CA233254, P01 CA91955, and R01 CA140657 as well as
CDMRP Breast Cancer Research Program Award BC132057 and the Arizona Biomedical
Research Commission grant ADHS18-198847. The findings, opinions and recommendations
expressed here are those of the authors and not necessarily those of the universities where the
research was performed or the National Institutes of Health.
23
Author Contributions
S.E.K., A.A., A.F., and C.C.M., conceived and designed the study. E.G.D. and T.M.H.
helped in the collection and coordination of malignancy and neoplasia data across
institutions. Z.C., S.M.R, and A.M.B. provided data on malignancy prevalence and neoplasia
prevalence. S.E.K. collected data on chimerism, habitat, and tumour invasiveness, analysed
the data, and wrote the first draft. Z.N., S.R.D., and D.S. peer-reviewed the chimerism data.
All authors discussed the manuscript and contributed to the final versions of the manuscript.
Competing interests
We declare we do not have conflicts of interest.
Figure legends
Figure 1. Phylogenetic tree of the highest level of chimerism observed and the highest
level of tumour invasiveness observed in each of 12 obligately multicellular taxa across
the tree of life. Red bars show the highest level of tumour invasiveness observed in each
taxon, from low to high tumour invasiveness (no cancer observed, cancer/cancer-like
phenomena, cancer, transmissible cancer). Black bars show the highest chimerism level
observed in each taxon, from low to high chimerism (no chimerism observed, accepts cells
from a close relative e.g., mother or twin, accepts cells from the same species other than a
close relative, accepts cells from different species). The phylogenetic tree was created using
the Time Tree of Life (http://timetree.org/). Images show example species in each taxon
(images from http://phylopic.org/). The time bar shows millions of years (MY).
24
Figure 2. Phylogenetic tree of malignancy, neoplasia prevalence, and the highest levels
of chimerism observed in 12 terrestrial mammalian species. Red shows the malignancy
prevalence of each species. Pink shows the benign neoplasm prevalence of each species. Red
together with pink show total neoplasia prevalence. In the case of adult humans, we only
show their malignancy prevalence, as we do not have data on their benign neoplasm
prevalence. Black bars show the highest chimerism level observed in each species, from low
to high chimerism (no chimerism observed, accepts cells from a close relative e.g., mother or
twin, accepts cells from the same species other than close a relative, accepts cells from
different species). The phylogenetic tree was created using the Time Tree of Life
(http://timetree.org/). We obtained images of species from http://phylopic.org/. Time bar
shows millions of years (MY).
Figure 3. The highest level of tumour invasiveness observed is positively correlated with
the highest level of chimerism observed across 12 obligately multicellular taxa on the
tree of life (PGLS analysis: * P-value < 0.05). We show each taxon with a flask, a globe, or
both, according to whether it consists of experimental chimeras (flask), natural chimeras
(globe), or both. We use minimal jitter to better visualise individual taxa.
Supplementary material
Supplementary Methods
We obtained information about the environmental origins of 12 obligately
multicellular taxa, aquatic [0] or terrestrial [1], from Fisher et al9, https://animaldiversity.org/,
and https://www.iucnredlist.org/. If species in a taxon lived in both aquatic and terrestrial
25
environments, we classified that taxon according to the driest environment of any species in
that taxon, i.e. terrestrial environment. In the PGLS analyses of 12 obligately multicellular
taxa across the tree of life, we set the variables and environment [0, 1] as numerical variables.
Supplementary Results
Obligately multicellular taxa that originated on land do not have higher observed
chimerism levels than obligately multicellular taxa that originated in aquatic environments
(Supp. Fig. 1; PGLS analysis: F-statistic = 0.67 on 1 and 10 DF, ML lambda = 1, R² = 0.06,
P-value = 0.42).
Supplementary Figure 1. Taxa that originated on land do not have higher chimerism
levels than taxa that originated in water (PGLS analysis, P-value > 0.05). An analysis
across 12 obligately multicellular taxa on the tree of life. If a taxon includes both aquatic and
terrestrial species, we have labelled that taxon according to its driest environment, i.e.
terrestrial. We show each taxon with a flask, a globe, or both, according to whether it includes
experimental chimeras, natural chimeras, or both, respectively. We use minimal jitter to
improve visibility of individual taxa.
Supplementary Data
The data across taxa used in this manuscript.
26
Supplementary Table 1. Examples of chimerism across 12 obligately multicellular taxa. In the majority of cases in the literature, species
reject foreign cells. The list of references in this table is not exhaustive since we do not mention here all the examples of graft rejection reported
in the literature. The examples of chimerism in this table are rare examples of graft/foreign cell acceptance, if reported, in the literature.
Taxon
(comm
on
name)
Examples of
chimerism [highest
level of chimerism
observed]
Chimer
ism
early in
develop
ment
Chimeri
sm later
in
develop
ment
Natural
chimerism
Exper
iment
al
chime
rism
Manipulation of
the graft/recipient
For how long did
the graft cells
survive in the
recipient?
How many of the graft
cells survived in the
recipient?
Vertebr
ata
(vertebr
ates)
see Table 2 [3]
✓
(see
Table 2)
✓
(see
Table 2)
✓
(see Table 2)
✓
(see
Table
2)
see Table 2
see Table 2
see Table 2
Tunicat
a
(tunicat
es)
chimeric colonies of
2–3 different
genotypes66;
microchimera78;
fusion of colonies
that share at least one
common allele74,75;
xenograft chimera76
[3]
✓
“chimer
as of an
adult
and
young
partner”
77
✓
“chimera
s of an
adult
and
young
partner”7
7;
sexually
mature
colonies7
5
✓
oyster grown
in aquaculture
facilities at the
Fangar Bay66
✓
66,74–76,7
8
experimental design
“to trigger a fast
fusion/non-fusion”66
; cutting and pairing
of colony
fragments66;
irradiated subclones
received the
grafts66,76
30 days66; 1 week78;
months76; 2 months
after the fusion of
colonies74;
stable chimeras
8–10 months after
the fusions75
presence of both
genotypes in a bud74;
whole mass of gonads,
as well as the soma,
came from the
resorbed genotype75; cell
lineage parasitism159
27
Protost
omia
(protost
omes)
heart allografts inside
the snail B.
glabrata79; allografts
and xenografts inside
B. glabrata80;
xenografts81 [3]
N/A
✓
adult
oysters81
;
“parents
were
used as
donors”8
2
N/A
✓
79–82
mesh made oysters
less likely to
reject implanted
nuclei81,160;
air-drying or
incision with
collagen-based
skin-like scaffold to
allow wound
healing after
transplantation79
6 months79; 6
months80; 14 days
after grafting82
DNA from the donor
oyster detected in the
pearl sac161; whether “the
DNA detected is actively
transcribed, is yet to be
determined.”81;
implanted hearts
continued to beat
throughout the study79
Placozo
a
(placoz
oans)
reaggregation of cells
in Trichoplax
adhaerens, no
evidence that this
new organism is
functional83 [0]
N/A
N/A
N/A
✓
83
they used
dissociation
medium, washed it
out, suspended the
cell complex and
passed it through a
gauze, and then they
observed the free
cells and small
complexes
reaggregate83
some reaggregates
lived for weeks and
“led to apparently
normal Trichoplax
capable of further
growth”83
N/A
Ctenop
hora
(comb
jellies)
grafting
experiments84,85:
“coordination of the
plates as well as the
feeding reactions at
the one mouth were
similar to these
processes of a normal
animal.”, “animal
consisting of the
N/A
N/A
N/A
✓
84,85
“grafts were held in
place by strands of
cotton for about two
hours to permit
sufficient healing.”85
“Fusion took place
within three days
and the graft
continued to live as
a single animal.”84;
within two days,
“Within ten days
after the transplant
had healed in the
host, it had been
“each piece maintained
its identity by forming a
mouth, by coordinating
its own plate movement,
and by regenerating
auricles and lobes”84
28
mid-pieces of four
animals”84 [1]
completely
absorbed.”85
Echino
dermata
(echino
derms)
viable allografts62;
rejection of
allografts, but
autografts remained
fully viable86 [2]
N/A
✓
adult sea
cucumbe
rs and
sea
stars62
N/A
✓
62,86,87
animals were
anesthetized before
grafting, “initial
fusion of the graft
and subsequent
healing appeared to
be promoted by a
light dusting of the
contact zone with
tetracycline wound
powder and
absorbable gelatin
powder”62
“allografts survived
for more than 100
days but all showed
slow rejection”,
grafts survived
from 129 to 185
days62; more than
300 days, 110
days86; an average
of 341.8 days87
allografts “were
gradually resorbed by
ingrowth of recipient
tissue”86
Cnidari
a
(cnidari
ans)
allogeneic
chimeras68,70; three
colonies had “two
genotypes that each
differed by two or
more alleles”69;
chimeras with
distinct genotypes,
unrelated genotypes
fused forming mature
colonies67 [3]
✓
juvenile
soft
coral
species6
8;
“relativ
ely
young
coral
colonie6
9;
juvenile
s67
✓
juveniles
and
adults68;
adult
corals69;
adult
and
juvenile
colonies6
7
✓
from
Canada70;
natural
colonies from
the northern
Gulf of Eilat68;
colonies
from
Magnetic
Island69;
natural
populations67
✓
88
“to increase the
likelihood of
detecting genetic
variability at the
colony level,
branches were
sampled as far away
from each other as
possible across the
colony.”67,69
up to 450 days68;
“more than 4
months after
chimeras were
made.”88
“partners shared layers
of endoderm, mesoglea
and ectoderm, and the
gastrovascular cavity”70;
“movement of cells from
one partner to the
other”68; “chimeras had
H. attenuata epithelial
cells and P. oligactis
interstitial cell lineage”88
29
Porifera
(sponge
s)
not all individuals
fused162 72; “fuse in
twos or threes or in
larger number up to
and over one
hundred.”90 ;
bispecific
conglomerates71,89 [3]
✓
larvae72;
fusion
between
larvae90;
sibling
larvae91
✓
“Both,
adult
and
larval
DNA
was
extracted
”72
✓
naturally
occurring
bispecific
chimeras71;
natural
populations of
marine
sponges72
✓
89–92
fusion at a critical
time when “the
ciliated epithelium
is being replaced by
the permanent flat
epithelium”, “with
pipette and needle
coaxed together into
a clump.”90; “pairs
of sibling larvae
were forced to settle
in contact, they
fused in all cases.”91
48 hours after the
beginning of the
culture, 72 hours92;
50 days91
“the Oxymycale portion
comprises the bulk of the
specimen, the
Sigmadocia part being
about l0 per cent of the
whole”71; “small
aggregates were formed
and sometimes patches
of cells of the two
species loosely
adhered.”89
Basidio
mycota
(filame
ntous
fungi)
“Ten cells isolated
from a single
basidiome produced
nine different
genotypes when
analyzed for variation
at six nuclear loci.”73
[2]
N/A
N/A
✓
samples
collected from
a lawn in
Massachusett7
3
N/A
N/A
N/A
N/A
Ascom
ycota
(sac
fungi)
“branching and
fusion within the N.
crassa hyphal
network mix
genetically diverse
nuclei and create
well-mixed conidial
spores”94; hyphae
“fuse within a colony,
but also between
colonies of the same
species.”93 [2]
✓
fusion
of
germina
ting
conidia
with
nearby
mature
hyphae9
4
✓
fusion of
germinat
ing
conidia
with
nearby
mature
hyphae94
N/A
✓
93,94
“conidia were used
to initiate
heterokaryotic
mycelia.”94
N/A
N/A
30
Embryo
phyta
(land
plants)
“four of the 1321
plants regenerated
from chimeral callus
were chimeras”99;
intraspecific
chimeras99,163;
interspecific
chimeras95–97 [3]
✓
98;
Ten-day
-old
stenle
seedling
s99;
young
plants
of two
species9
7
N/A
N/A
✓
97,99
they used the
semidominant aurea
mutant of tobacco,
interspecific
cultures in vitro
were placed in
media
favouring only N.
tabacum shoot
formation, “Graft
unions were secured
with budding
rubbers”99
46, 53,
70 and 73 days after
decapitation96
“Eventually the N.
glauca tissue in the
original menstem was
replaced by N
tabacum”96; “The two
lower leaves are partly
composed of one layer of
luteum
over tomato and partly of
pure luteum”97
Rhodop
hyta
(red
algae)
“interindividual
fusions in red and
brown algal
species”100 [3]
✓
100
N/A
N/A
✓
laborat
ory-bu
ilt
bicolor
chimer
as100
N/A
30–45 days100
“the cell mix (bicolor
with green and red)
appeared to extend from
the base up to 25% of the
basalmost portion of the
shorter axes”, “the mixed
tissue reached 10%–15%
the axes length”, “a
combination of red and
green alleles (chimeric
tissues) was found at the
apical portions of the
chimeric axis”100
31
Supplementary Table 2. Examples of chimerism in 18 mammalian species. In the majority of cases in the literature, species reject foreign
cells. The list of references in the table is not exhaustive since we do not mention here all the examples of graft rejection reported in the
literature. The examples of chimerism in this table are rare examples of graft/foreign cell acceptance, if reported, in the literature. EGFP:
enhanced green fluorescent protein; cGY: centigray; Tg cells: a subset of T cells with a receptor for immunoglobulin G; HSC: hematopoietic
stem cells; DLA: dog leucocyte antigen.
Mamma
lian
species
(commo
n name)
Examples of
chimerism
[highest level of
chimerism
observed]
Chimerism
early in
developmen
t
Chime
rism
later in
develo
pment
Natur
al
chime
rism
Exp
eri
men
tal
chi
mer
ism
Manipulation of
the
graft/recipient
For how long did
the graft cells
survive in the
recipient?
How many of the graft cells
survived in the recipient?
Saguinus
oedipus
(cotton-t
op
tamarin)
microchimerism101
[1]
✓
101
N/A
✓
“natur
ally
occurr
ing
chimer
ic
bone
marro
w”101
N/A
N/A
N/A
“both the lymphocyte and
monocyte/macrophage
populations of these animals are
chimeric”101
32
Rattus
norvegic
us
(commo
n rat)
allogeneic
chimeras109;
retinae from mice
transplanted to the
midbrain of rats111;
retinae
transplanted into
the brains of rats110
[3]
✓
embryonic
mice as
donors and
neonatal rats
as hosts111;
fetal retinae
from rats110
N/A
N/A
✓
109–11
1
bone marrow cells
conditioned with
1100 cGy, the
bone marrow was
T cell depleted109;
“recipients were
unilaterally
enucleated at this
time in order to
enhance
innervation of the
host by the
transplant”111
300 days after
transplantation, ≥14
months109
grafts in “areas of the brain stem
normally innervated by the
eye”111; “well-formed grafts
containing numerous rosettes
and ganglion cells”, “The
transplanted retinae survived,
differentiated, and grew to
2 mm or more in diameter.”110
Mus
musculus
(house
mouse)
“Tetraparental
mice are formed
by the in vitro
fusion of two
eight-
cell stage mouse
embryos”113;
human cells
implanted in the
brain of
mice,
“immunotolerance
of the embryonic
brain.”112 [3]
✓
112–114
N/A
N/A
✓
112–11
4
“(human
embryonic stem
cells) were
cocultured with
mouse embryonic
fibroblasts in a
defined medium
and were
immunoreactive
for (specific)
undifferentiated
markers”112
“Two months after
transplantation, 18
months112; 2 weeks
following
engraftment, “by
3–5 weeks had
appropriately
differentiated into
oligodendrocytes
and astrocytes”114
“Transplanted cells were
identified in brain slices by
EGFP fluorescence” , “an
average of six EGFP cells were
found in each 40-m brain
microtome section.”, “0.1% of
the brain cells are of human
origin.”112
33
Sarcophi
lus
harrisii
(Tasmani
an devil)
“All successful
allografts were
acutely rejected”115
[0]
N/A
✓
three
year
old
females
, three
year
old
male115
N/A
✓
115
“Surgical glue
was placed on the
borders of the
grafts to secure
the
skin.”, they
administered pain
relief medication
“after skin graft
surgery”115
“14 days after
surgery”,
“progressed to very
severe rejection on
Day 21.”115
“necrosis associated with
polymorphonuclear cell
infiltration, surface
parakeratosis and fibrin
deposition.”115
Ovis
aries
(sheep)
embryonically
derived human
hematopoietic
stem cells in
sheep116,117 [3]
✓
“Using the
fetal
transplant
method, he
injects
embryonical
ly derived
human
hematopoiet
ic stem cells
into sheep.”
116,117
N/A
N/A
✓
116,11
7
“the
HSC are
transplanted early
in gestation when
the recipient is
still largely
immunologically
naive.”117
seven years later
116,117; “the sheep
maintained their
chimeric status for at
least 9 months.”117
“an animal with organs,
including the liver, heart and
pancreas, that are 15%
human.”116,117
Bos
taurus
(cattle)
microchimerism102
; Y chro-
mosome-specific
DNA in naturally
mated heifers
“carrying
conventional bull
calves” and a
transgene-specific
sequence in “cows
✓
vascular
anastomose1
02 ; fetuses118
✓
102
✓
102;
“natur
ally
mated
heifers
”118
✓
118
“conventional
recipient cows
pregnant after
non-surgical
transfer of a single
Tg embryo
produced by
in vitro
fertilization of
transvaginally
“many of the twins
in this study were
adults when they
were tested”102;
“fetal DNA found in
the maternal
circulation up to 4
months
postpartum”118
“the magnitude of the
microchimerism is in the order
of six circulating male cells or
their corresponding DNA
contents (if cell-free) per mL of
maternal blood and about 36 Tg
cells, respectively.”118
34
carrying
transgenic
fetuses.”118 [1]
recovered
oocytes”118,119
Sus
scrofa
(wild
boar)
human cells in
pigs [3]
✓
“hematopoie
tic stem
cells were
engrafted in
pigs.”120
N/A
N/A
✓
120
“injecting fetal
pigs with 5 × 10^7
human T
cell-depleted bone
marrow cells”120
“long-term
engraftment (>1
year) of human
cells in pigs.”120
N/A
Acinonyx
jubatus
(cheetah)
accepts allograft,
“14 reciprocal skin
grafts between
unrelated cheetahs
were accepted”,
skin grafts from
domestic cats were
rapidly rejected121
[2]
N/A
✓
121
N/A
✓
121
“antibiotics were
administered and
the surgical area
was bandaged”121
“allograft and
autograft were
virtually
indistinguishable 2
weeks after
surgery”, “studies
were terminated
early (day 23)”121
N/A
Canis
lupus
familiari
s
(domesti
c dog)
“Bone marrow
allografts
performed in pairs
of dogs bearing
identical DLA
haplotypes”122;
fetal
microchimerism103
; skin allograft
tolerance123 [2]
✓
103
✓
young
adult
dogs123;
adult
beagles
122
✓
103
✓
122,12
3
“nonmyeloablativ
e conditioning
(200 cGy TBI)
and transient
immunosuppressi
on”123; “Radiation
chimeras given
bone marrow”122
“4 weeks
posttransplantation”,
for at least 76
weeks, “There was
long-term (>5 yrs)
renal allograft
survival”123; 10-25
days122
“T and B cells contained
donor-type cells”123; “The actual
rate of microchimerism may be
higher, as the assay performed
here would not detect female
microchimerism.”122
35
Papio
hamadry
as
(Hamadr
yas
baboon)
xenotransplantatio
n, “No hyperacute
rejection de-
veloped”124;
“Seven
pig-to-baboon
orthotopic liver
xeno-
grafts are
reported.”125 [3]
✓
“piglets
were used as
donors to
recipient
baboons”125
N/A
N/A
✓
124,12
5
“treated with a
chronic
immunosuppressi
ve regimen.”,
“Thymic
irradiation (700
cGy)”, “Three
baboons were
treated with
aspirin”124; “The
remaining three
animals, which
were given human
fibrinogen, did not
bleed.”125
“one graft survived
up to 6 months after
transplantation.”, 78
and 179 days124;
survival of
xenografts up to 3 ½
days125
N/A
Homo
sapiens
(human)
allogeneic organ
transplantation126;
blood group
chimerism in
twins and
triplets104;
xenograft from
baboon to
human127 [3]
✓
microchimer
ism22,23,104,105
✓
recipie
nt was
a
35-year
old
human,
the
donor
was a
“15-yea
r-old
male
baboon
”127;
microc
himeris
m
betwee
✓
natural
microc
himeri
sm22,23,
104,105,12
6
✓
126,127
immunosuppressi
on “to mitigate
preformed
antigraft antibody
syndromes and
cellular rejection”,
“20 units of blood
were given during
the 11 h
operation.”127
70 days127; “fetal
cells have been
found to persist for
years, probably for a
lifetime, in the
circulation of
healthy women”105;
“Twins share cells in
the womb and can
harbor these cells
into adulthood”22
fetal cells in the blood of
mothers with scleroderma
disease: “an average of seven
male cells per 10 milliliters of
blood.”, “the level of fetal cells
is about one in 1 million cells in
maternal circulation,”, “a
48-year-old
mother who had a goiter
removed. To her
surprise upon examining the
removed goiter
tissue, Bianchi discovered that
one whole section of the
woman’s thyroid was
predominantly male,
presumably from her son.”22
36
n
mother
and
fetus
during
pregna
ncy22,23,
105
Callithri
x jacchus
(commo
n
marmose
t)
blood chimerism
between twins and
triplets106 [1]
✓
tissues from
“animals
that had died
at birth or
had to be
euthanized
for reasons
of bad
health.”106
N/A
✓
“from
a
captiv
e
colony
kept at
the
Psych
ologic
al
Institu
te,
Univer
sity of
Zurich
”106
N/A
N/A
age at birth or age
when euthanized106
N/A
Didelphi
s
virginian
a
(Virginia
opossum
)
maternal allograft,
“none of the 24
young less than 12
days of age
rejected the
maternal
allografts”128 [1]
✓
grafts “were
taken from
the maternal
ear skin and
placed on
the dorsum
of the pouch
✓
grafts
from
matern
al
skin128
N/A
✓
128
“the mother (was)
anesthetised with
sodium
pentobarbital”128
“at least 80 days in
most cases”128
N/A
37
young at
ages varying
from 3-17
days”128
Oryctola
gus
cuniculu
s
(Europea
n rabbit)
xenografted rat
tissue129 [3]
N/A
N/A
N/A
✓
129
N/A
“rat hippocampal
grafts developing for
8 weeks in the rabbit
septum”129
“grafts significantly increased in
their volume (600 to 800% of
the initial value). Typical
pyramidal neurons were present
in the grafted hippocampus,
though their organization into a
typical layer was absent”129
Equus
ferus
(wild
horse)
microchimerism, “
blood chimerism
occurred in 4 out
of 5 cases with
identical blood
group”107 [1]
✓
107
N/A
✓
107
N/A
N/A
"fusion of the
chorial sacs was
found to have
developed in two
cases of mummified
foetus which were at
about the 6th month
of gestation.”107
N/A
Dasypus
novemci
nctus
(armadill
o)
skin grafts
accepted between
monozygotic
littermates130 [1]
✓
“each
animal both
donated a
graft to and
received a
graft from
each of its
littermates”1
30
N/A
N/A
✓
130
“grafts were
redressed at the
early inspections
to protect them
from trauma.”130
20 days, 50 days
after grafting, “on
the 85th
postoperative day,
when the
observation period
was terminated.”130
N/A
38
Mesocric
etus
auratus
(Syrian
hamster)
skin homografts
between
individuals of
completely
unrelated stocks131
[2]
N/A
✓
exchan
ging
grafts
betwee
n adult
animal1
31
N/A
✓
131
“each animal
received a single
graft fitted into an
appropriately
sized bed on the
lateral thoracic
wall.”131
200 days, 24 to 140
days131
N/A
Macaca
mulatta
(rhesus
macaque
)
maternal
microchimerism108
[1]
✓
fetal blood
from the
early third
trimester108
✓
“mater
nal
blood
sample
s were
collecte
d
during
gestatio
n and at
pregna
ncy
termina
tion.”108
✓
108
N/A
N/A
1–1.5 years of age108
“maternal microchimerism in at
least one compartment (thymus,
liver, spleen, lymph nodes,
and bone marrow) (range:
0.001–1.9%
chimeric cells).”108
39
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Accepts cells from close
relative (e.g. twin, mother)
Highest level of tumour
invasiveness observed
Highest level of
chimerism observed
Cancer
No chimerism
Accepts cells from
same species
Accepts cells from
different species
red algae
filamentou s fungi
sac fungi
comb jellies
tunicates
vertebrates
cnidarians
echinoderms
protostomes
placozoans
sponges
land plants
400 MY
Highest level of chimerism observed
Highest level of tumour invasiveness observed
Transmissible cancer
Cancer-like growth
No cancer/cancer-like growth
Virginia opossum
cheetah
sheep
wild boar
European rabbit
common rat
house mouse
cotton-top tamarin
common marmoset
Hamadryas baboon
human
armadillo
Highest level of
chimerism observed
No chimerism
Accepts cells from
same species
Accepts cells from
different species
40 MY
Malignancy prevalence
Highest level of chimerism observed
Benign prevalence
1.00
0.75
0.50
0.25
0.00
Neoplasia prevalence
Accepts cells from close
relative (e.g. twin, mother)
Highest level of tumour invasiveness observed
Accepts cells from close
relative (e.g. twin, mother)
Accepts cells from
same species
Accepts cells from
different species
Highest level of chimerism observed
Transmissible cancer
Cancer
Cancer-like growth
No cancer/cancer-like growth
No chimerism
P-value = 0.03*
R² = 0.37
