The nematode C. elegans is an important model organism to study aging, genetics, neuroscience, innate immunity, development, apoptosis (programmed cell death) and drug response.

It is an ideal model organism because it is small, inexpensive, its genome is sequenced, it is well studied and easy to propagate (short generation periods).


In C. elegans the sexes are defined by the number of sex chromosomes: An embryo with only one X chromosome (XO genotype) will develop into a male worm, while an embryo with two X chromosomes (XX genotype) will become a hermaphrodite.

Under normal conditions, males only account for around 0.1% of the population which is caused by nondisjunction of X chromosomes. However, under certain stress situations such as food scarcity or heat, this rate is higher.

Hermaphrodites can produce progeny by self-fertilization or sexual reproduction with males. During sexual reproduction, the number of males is much higher (50%) because male sperm outcompetes the sperm from hermaphrodites. Furthermore, a hermaphrodite inseminated by a male can lay up to 1000 eggs as opposed to 300 during self-fertilization.

Life cycle

Life cycle diagram, a series of life cycle stages that form a circle with arrows indicating the direction of the stages. The embryo phase is depicted as a small oval. An arrow points to L1. After L1, the cycle splits into two possible paths. One arrow points to L2, depicted as a small nematode. The other arrow points to L2d, and the arrow is labelled with limited food, dense population, and high temperature. From L2, an arrow points to L3, and then an arrow points to L4, depicted as a mature nematode. From L2d, an arrow points to the Dauer phase. From the Dauer phase, an arrow that is labelled with plentiful food, sparse population, and temperate environment points to L4, the mature nematode. From L4, an arrow points to reproductive adult, which is depicted as a larger mature nematode. From reproductive adult, an arrow points back to embryo and the cycle continues on as previously described.

Figure 1: C. elegans life cycle

After hatching, the worms go through four larval stages (L1-L4) before becoming adults. Completing one cycle takes approximately 36h. Under unfavorable conditions (overcrowding, insufficient food, or heat stress), L2 worms may enter the dauer stage (L2d) where the development is paused until the conditions improve.