As we have seen, RNA interference was first demonstrated in the nematode Caenorhabditis elegans when geneticists discovered that they could silence specific genes in this species by injecting the animals with double-stranded RNA that was complementary to the genes. The geneticists were studying gene expression in C. elegans because this species had proved to be an excellent model genetic organism, particularly for studies of how genes influence development. For reasons that are not completely understood, RNAi is particularly effective in this species.

You may be asking what a nematode is and why it is a model genetic organism. Although rarely seen, nematodes are among the most abundant organisms on Earth, inhabiting soils throughout the world. Most of these worms are free living and cause no harm, but a few are important parasites of plants and animals, including humans. Although C. elegans has no economic importance, it has become widely used in genetic studies because of its simple body plan, ease of culture, and high reproductive capacity (Figure 10.26). First introduced to the study of genetics by Sydney Brenner, who formulated plans in 1962 to use C. elegans for the genetic dissection of behavior, this species has made important contributions to the study of development, cell death, aging, and behavior.

An ideal genetic organism, C. elegans is small, easy to culture, and produces large numbers of offspring. The adult C. elegans is about 1 mm in length. Most investigators grow C. elegans on agar-filled petri plates that are covered with a lawn of bacteria, which the nematodes devour. Thousands of worms can be easily cultured in a single laboratory. Compared with most multicellular animals, they have a very short generation time, about 3 days at room temperature. And they are prolific reproducers: a single female produces 250–1000 fertilized eggs in 3 to 4 days.

Another advantage of C. elegans, particularly for developmental studies, is that the worm is transparent, allowing easy observation of internal development at all stages. It has a simple body structure, with a small, invariant number of somatic cells: 959 cells in a mature hermaphroditic female and 1031 cells in a mature male.

Most mature adults are hermaphrodites, with the ability to produce both eggs and sperm and undergo self-fertilization. A few are males, which produce only sperm and mate with hermaphrodites. The hermaphrodites have two sex chromosomes (XX); the males possess a single sex chromosome (XO). Thus, hermaphrodites that self-fertilize produce only females (with the exception of a few males that result from nondisjunction of the X chromosomes). When hermaphrodites mate with males, half the progeny are XX hermaphrodites and half are XO males.

Eggs are fertilized internally, either by sperm produced by the hermaphrodite or by sperm contributed by a male (see Figure 10.26). The eggs are then laid, and development is completed externally. Approximately 14 hours after fertilization, a larva hatches from the egg and goes through four larval stages—termed L1, L2, L3, and L4—that are separated by molts. The L4 larva undergoes a final molt to produce the adult worm. Under normal laboratory conditions, worms will live for 2 to 3 weeks.