In a cavernous room inside a low-slung glass building in south Delhi, the lights turn on and off automatically in tune with the biological rhythms of fish. About 10,000 zebrafish—each half the length of a human finger when fully grown—swim daintily in 2,000 small tanks stacked up on shelves, all kept at a controlled 28 degrees Celsius. The lights come on at 9 am, and go off at 11 pm—an ideal light-dark cycle for the fish, creating an indoor night and day. Zebrafish like to breed in the morning, and getting them to breed is the point. At the room’s double-door entrance, a sign says visitors are not allowed beyond 7 pm. Otherwise, “the fishes get agitated,” Adita Joshi, a genetic researcher, told me.
I visited the fish, at the labs of the Institute of Genomics and Integrative Biology, on a morning in early September. Parents are kept in smaller tanks at the top, I was told, while their progeny proliferate below. The IGIB houses numerous strains of zebrafish, some of which occur naturally, while others are produced through gene manipulation. Next door to the tank room, zebrafish embryos are kept refrigerated on Petri dishes, with tens or even hundreds on each one. Researchers tweak the embryos’ genetics through various processes, including “gene knockdown,” which inhibit the expression of particular genes through the use of chemical substances.
Sridhar Sivasubbu, who heads the lab, likened the zebrafish to a sandbox for genetic experimentation. In scientific parlance, zebrafish are model organisms—species fast to breed and easy to keep in laboratory conditions, and whose characteristics allow certain kinds of experimentation. Famous examples include mice and fruitflies. The seemingly unremarkable zebrafish, native to the Indian subcontinent and widely available in pet stores, is particularly interesting since a large portion of the genes that code for protein are conserved in it, despite its genome being half the size of that of humans. Its embryos are also transparent, meaning that “right from one cell to a functional, free-swimming organism,” Sivasubbu said, its development can be observed in detail under a microscope. This allows researchers to test the physiological effects of manipulations of the zebrafish genome, and so to correlate particular genes to particular physical traits and defects efficiently—within three days the researchers are done with the embryos. Widespread research on zebrafish across the world in the last decades has advanced the understanding of basic genetics, vertebrate development and human disease.