Genetic screens are useful for tracking down the genes involved in almost any biological process, and they have been used to search for mutations that alter developmental timing. Such screens were performed in the nematode Caenorhabditis elegans (Figure 21–41). This worm is small, relatively simple, and precisely structured. The anatomy of its development is highly predictable and has been described in extraordinary detail, so that one can map out the exact lineage of every cell in the body and see exactly how the developmental program is altered in a mutant. Genetic screens in C. elegans revealed mutations that disrupt developmental timing in a particularly striking way: in these so-called heterochronic mutants, certain cells in a larva at one stage of development behave as though they were in a larva at a different stage of development, or cells in the adult carry on dividing as though they belonged to a larva (Figure 21–42).
Genetic analyses showed that the products of the heterochronic genes act in series, forming regulatory cascades. Unexpectedly, two genes at the top of their respective cascades, called Lin4 and Let7, were found to code not for protein but instead for microRNAs (miRNAs)—short, untranslated, regulatory RNA molecules, 21 or 22 nucleotides long. These act by binding to complementary sequences in the noncoding regions of mRNA molecules transcribed from other heterochronic genes, thereby repressing their translation and promoting their degradation, as discussed in Chapter 7. Increasing levels of Lin4 miRNA govern the progression from first-stage larva cell behaviors to third-stage larva cell behaviors.
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Figure 21–41 Caenorhabditis elegans.
a side view of an adult hermaphrodite is shown. (From J.e. Sulston and h.r. horvitz, Dev. Biol. 56:110–156, 1977. With permission from academic press.)
Figure 21–42 Heterochronic mutations in the Lin14 gene of C. elegans. Only the effects on one of the many altered lineages are shown. a loss-of-function (recessive) mutation in Lin14 causes premature occurrence of the pattern of cell division and differentiation characteristic of a late larva, so that the animal reaches its final state prematurely and with an abnormally small number of cells. the gain-of-function (dominant) mutation has the opposite effect, causing cells to reiterate patterns of cell divisions characteristic of the first larval stage, continuing through as many as five or six molt cycles. the cross denotes a programmed cell death. Green lines represent cells that contain Lin14 protein (which binds to DNa), red lines those that do not. (adapted from V. ambros and  h.r. horvitz, Science 226:409–416, 1984. With permission from the authors; and p. arasu, B. Wightman and G. ruvkun, Genes Dev. 5:1825–1833, 1991. With permission from the authors.)
Increasing levels of Let7 miRNA govern the progression from late larva to adult. In fact, Lin4 and Let7 were the first miRNAs to be described in any animal: it was through developmental genetic studies in C. elegans that the importance of this whole class of molecules for gene regulation in animals was discovered.
More generally, in many animals, miRNAs help regulate the transitions between different stages of development. For example, in flies, fish, and frogs, the maternal mRNAs that are loaded into the egg in the mother are removed during early development when the genome of the embryo begins to be transcribed; at this stage, the embryo begins to express specific miRNAs that target many maternal mRNAs for translational repression and degradation.