Chimerism in Callitrichids

The immersion Internship at Pacific Primate Sanctuary (PPS) provides Interns with an opportunity to expand their knowledge of New World Primate species, both through hands-on experience as well as making use of educational resources. The monthly Talk Story- Special Topic encourages Interns to gain a more in depth understanding of each of the species here at PPS.  Following is a Special Topic written by Intern Holly.

 Special Topic by Intern Holly

Genetic chimerism occurs when an organism has cells that are genetically distinct from one another. This organism may have blood cells of differing blood types, or two differently-colored eyes, or they may even have a mix of male and female reproductive organs. In most animals, chimerism develops when multiple fertilized eggs merge together while in utero, creating a single organism with multiple cells that are different from rest of the body’s genetic structure; thus, chimerism is often the result of a zygote absorbing their twin. Chimerism can also occur in plants, but the chimera is often the result of a mutation during cell division, rather than two early-developed organisms merging together.

All species of Callitrichidae, aside from the Goeldi’s marmoset (Callimico goeldii), routinely give birth to fraternal twins. Early in development, the placentas of the twin embryos fuse together. This placental fusion allows the twins to exchange stem cells, specifically germ cells and hematopoietic cells. Germs cells are the cells that will give rise to gametes (ovum and sperm) in reproductive organs, while hematopoietic cells will become blood cells. These mixed hematopoietic cells can be found in blood, bone marrow, and internal organs, which means that every tamarin and marmoset is a composite of cells from multiple individuals: a chimera. 

Callitrichids also routinely engage in germline chimerism, a trait that is unique to them specifically due to its regular occurrence, rather than by chance. As mentioned, germ cells will become gametes, the cells that an organism uses to reproduce. During cell exchange through the fused placenta the twins’ germ cells exchange alleles. Alleles are the expressed variants of genes, i.e.: whether an animal will have blue or green eyes. Not only is every tamarin and marmoset a chimera, they also carry reproductive alleles of their fraternal twin, in addition to their own. 

One implication of germline chimerism is that a tamarin or marmoset could sire or give birth to offspring that are more genetically related to their twin than to them. Evidence suggests that chimerism is a contributor for the cooperative breeding system found in Callitrichids. In a cooperative breeding system, all members of a family group help care for offspring, not just parents. Personal reproductive fitness is the number of offspring that an individual bears, while inclusive reproductive fitness is the number of offspring equivalents that an individual raises. The non-reproducing “helpers” in a cooperative breeding system are engaging in inclusive fitness: by taking care of their siblings or nieces/nephews, they are ensuring that their family line lives on, even if they do not reproduce themselves. Due to germline chimerism and the resulting interrelatedness of the family group, it becomes very important for a tamarin or marmoset to help care for their siblings/niece/nephew, because that offspring may actually be more closely related to them than to their biological parents. 

Many of the tamarins and marmosets at Pacific Primate Sanctuary come from the same family groups. With the knowledge of chimerism, it becomes evident that the level of interrelatedness is more complicated than a standard family tree diagram would suggest. When attempting to pair a male and female, we have had the most success with pairing two completely unrelated individuals. Not only does this kind of pairing have the most resemblance to wild Callitrichid breeding pairs, but it is possible that tamarins and marmosets may recognize family traits, even in an individual they have never met before, and prefer not to partner with a closely related individual, although, more evidence is needed to support this hypothesis. (Male Callitrichids at PPS have been vasectomized.)

References

• Rutherford, J. N. & Tardif, S. “Mother’s Little Helper? The Placenta and Its Role in Intrauterine Maternal Investment in the Common Marmoset (Callithrix jacchus).” 
• The Smallest Anthropoids: The Marmoset/Callimico Radiation, edited by Susan M. Ford, Leila M. Porter, & Lesa C. Davis, Springer, 2009, pp. 301-329.