Delving into the world of chimeras

The term chimera has been borrowed from Greek mythology, transcending ancient tales to

become a captivating concept within the fields of biology and genetics. In mythology, the

chimera was a monstrous hybrid creature. However, in the biological context, a chimera refers to an organism with cells derived from two or more zygotes. While instances of natural

chimerism exist within humans, researchers are pushing the boundaries of genetics via the

intentional creation of chimeras, consequentially sparking debates and breakthroughs in various fields, spanning from medicine to agriculture.

Despite the theory that every cell in the body should share identical genomes, chimeras

challenge this notion. For example, the fusion of non-identical twin embryos in the womb is a

way chimeras can emerge. While visible cues, such as heterochromia or varied skin tone

patches, may provide subtle hints of its existence, often individuals with chimerism show no

overt signs, making its prevalence uncertain. In cases where male and female cells coexist,

abnormalities in reproductive organs may exist.

Furthermore, advancements in genetic engineering and CRISPR genome editing have also

allowed the artificial creation of chimeras, which may aid medical research and treatments. In

2021, the first human-monkey chimera embryo was created in China to investigate ways of

using animals to grow human organs for transplants. The organs could be genetically matched by taking the recipient’s cells and reprogramming them into stem cells. However, the process of creating a chimera can be challenging and inefficient. This was shown when researchers from the Salk Institute in California tried to grow the first embryos containing cells from humans and pigs. From 2,075 implanted embryos, only 186 developed up to the 28-day time limit for the project.

Chimeras are not exclusive to the animal kingdom; plants exhibit this genetic complexity as well. The first non-fictional chimera, the “Bizzaria” discovered by a Florentine gardener in the

seventeenth century, arose from the graft junction between sour orange and citron. Initially

thought to be an asexual hybrid formed from cellular fusion, later analyses revealed it to be a

chimera, a mix of cells from both donors. This pivotal discovery in the early twentieth century

marked a turning point, shaping our understanding of chimeras as unique biological

phenomena. Chimera is a common form of variegation, with parts of the leaf appearing to be green and other parts white. This is because the white or yellow portions of the leaf lack the green pigment chlorophyll, which can be traced to layers in the meristem (areas found at the root and shoot tip that have active cell division) that are either genetically capable or incapable of making chlorophyll.

As we conclude this exploration into the world of chimeras, from the mythological realm to the scientific frontier, it’s evident that these entities continue to mystify and inspire, broadening our understanding of genetics, development, and the interconnectedness of organisms. Whether natural wonders or products of intentional creation, chimeras beckon further exploration, promising a deeper comprehension of the fundamental principles that govern the tapestry of life.

Written by Maya El Toukhy

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