An Introduction to the Invertebrates, Second Edition - tiera.ru

HOW ARE NEMATODES RELATED TO OTHER ANIMALS?99decisions in individual cells. Mutations may result in discretetransformations of cell fate at any point in a lineage, or may switchdevelopment between lineages. Powerful probing of C. elegansrevealed ten times more mutations than had been predicted, thefirst clear demonstration that mutations are not as rare as theoryhad dictated. This finding proved to have general application andinteresting causes, as will be explained in Chapter 20. Genomes asa whole, in contrast to individual genes, have greatly changed in thecourse of evolution.The extent to which genes, and therefore fundamental biologicalprocesses, have been conserved throughout evolution is the mostsurprising and important discovery to emerge from comparativework on nematodes, fruit flies and mammals. Even comparison ofthe genome of C. elegans with that of yeast (a unicellular fungus)reveals a considerable degree of resemblance; the differences mayhelp to give us a genetic definition of a multicellular animal. At thesame time, about 400 genes are found that are specific to nematodes,for example those concerned with biochemically unique structuressuch as the cuticle. Such genes could be useful to control nematodepests without damaging other animals. C. elegans has equivalentsof about half of known human genes, some of which offer medicalopportunities: the principle is that when a gene is known to causean illness, that gene can be inserted into the worm to find exactlywhat goes wrong. To give a few examples of the potential use ofopportunities offered by work on the nematode:1. Genes controlling programmed cell death in C. elegans maybecome important in treating cancer (where cells fail to die).2. One gene is found to alter the rates of metabolism and ofageing throughout the nematode, and this gene has a humancounterpart that makes a receptor for insulin.3. The recently discovered form of RNA called RNAi has beenmade accessible by work on C. elegans, where it is found tocontrol the patterning of neurones. RNAi consists of smallpieces of double-stranded RNA that can inhibit the expressionof the genes from which they are derived, by destroyingtheir mRNA. An insertion that can ‘shut genes off ’ is clearlya wonderful tool for investigating gene function: RNAi hasbeen used to silence nematode genes that correspond tomammalian disease-causing genes. The way is open to medicalsuppression of such genes without damage to the living humanbody in which they occur.8.6 How are nematodes related to other animals?Morphological evidence has failed to answer this question, andmolecular comparisons have been hampered by the unusually rapid