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.
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.