[
1990]
The biological processes collectively called aging are being dissected in our laboratory using classic genetic analyses akin to those used in the dissection of other fundamental biological processes, e.g., development or metabolism. Many pitfalls are inherent in the genetic analysis of components of fitness; many result from effects of inbreeding. These inbreeding effects have been avoided by the use of the small free-living nematode Caenorhabditis elegans. The hermaphroditic life-style of this animal facilitates the analysis of life span and senescence by permitting the direct isolation and genetic analysis of long-lived mutants and recombinant inbred (RI) lines without complications resulting from inbreeding problems. Both approaches to obtaining long-lived genotypes have been used effectively in the analysis of the aging processes of C. elegans and the reader will find a brief summary of
[
1983]
In the preparation of this review, I have made the basic assumption that the desire of the reader is to understand the biological basis of organismic aging. Given this premise, the organism of choice should be one that offers the most immediate hope of arriving at such an understanding. An ideal organism should have a short lifespan; be inexpensive to maintain; be experimentally malleable by a variety of techniques including molecular, morphological, genetic, and biological approaches; and be the object of study in a sufficient number of different laboratories to assure the accumulation of a critical mass of data. The nematode, Caenorhabditis elegans, admirably fulfills all of these basic requirememts. Researchers in the field of aging are faced with a large number of different theories which purport to explain the molecular basis of organismic aging. There are two major reasons for this proliferation of theoretical views. First, aging is an extremely complex phenomenon involving changes in a number of different physiological systems; these physiological changes are often detected, but proof that any one of the changes is responsible for aging is lacking. Second, the focus of a great deal of the research in the field has not been so much on understanding the biological basis of the entire aging process as on understanding one or another of the consequences of this process, particularly in humans and other mammals. The mammalian model systems may often be quite inappropriate for addressing the more basic, long-term questions about the
[
1989]
Transposable elements have recently been described in several species: Caenorhabditis elegans, Caenorhabditis briggsae, Ascaris lumbricoides, and Panagrellus redivivus. Because of the intense interest in C. elegans as an experimental organism for developmental genetic studies and the availability of sophisticated genetics, most is know about transposons in this species. This review focuses principally on Tc1 (Tc=transposon) of C. elegans, the best understood element in nematodes. Other elements in C. elegans and also elements in other species of nematodes will be briefly surveyed. The interested reader should also see two recent related reviews. The genome of C. elegans is 8 x 10(7) base pairs (bp) in extent, the smallest known for any metazoan. There are six chromosomes per haploid set, and about 83% of C. elegans DNA behaves as single-copy sequence in renaturation experiments. The repeated sequences are of several types, including functional genes, inverted or "foldback" sequences, and short repeated sequences of a few hundred nucleotides. The global arrangment of these short repeats is of the "short-period-interspersion" or "Xenopus" pattern. Some of the repetitive sequences consist of transposable elements, and at least five distinct families have been identified in C. elegans, Tc1 through Tc5. The sequence of one Tc1 element has been determined and shows that Tc1 resembles bacterial insertion sequence elements with terminal inverted repeats and a central open reading frame. The complete sequences for any members of the other transposon families have not been determined, but the data suggest that Tc2, Tc3, and Tc5 are also insertion sequence-like in structure and that Tc4 is foldbacklike in structure. No "retrotransposon-like" elements have been identified in C. elegans, although such elements have been described in A.