Natural Variation and Copulatory Plug Formation in Caenorhabditis eleguns Jonathan Hodgkin and Tabitha Doniach

Most of the available natural isolates of the nematode Caenorhabditis elegans have been examined and compared with the standard laboratory wild type (Bristol N2). Molecular markers, in particular transposon restriction fragment length polymorphisms, were used to assign these isolates to 22 different races, for which brood size and spontaneous male frequency were determined. Several distinctive traits were observed in some of these races. One example is m a b 2 , in a race from Vancouver, which leads to severe distortion of male genitalia and prevents male mating. Another is gro-1, segregating in a Californian race, which is associated with slow growth, heat resistance and longevity. Many races differ from N2 in carrying a dominant allele at the plg-1 locus, causing copulatory plug formation by males. Properties and possible advantages of the plugging trait have been investigated. The dominant plg-1 allele does not lead to increased male mating efficiency, but males from a Stanford race (CB4855), in which the plugging trait was first observed, are much more virile than N2 males. Crosses between N2 and CB4855 indicate that the higher virility is due to multiple factors. Size differences between N2 and CB4855 are associated with factors mapping to LGV and LGX. T HE nematode Caenorhabditis eleguns has now become established as a standard experimental system for the investigation of a great variety of phenomena in biology: cell lineage, differentiation, cell death, aging, sex determination, myogenesis, neuronal guidance and specification, to name only a few. One reason for the usefulness of this species is that its structure, development and genome are sufficiently simple to permit exhaustive descriptions. The complete cell lineage, cellular anatomy and neuronal wiring were determined some time ago; more recently, an essentially complete physical map of the genome has been assembled, and this is now being exploited to generate complete sequence coverage (HODGKIN et al. 1995; WATEFSTON and SULSTON 1995). By late 1996, more than half of the 100 million bp of the genome had been sequenced. This wealth of molecular information is complemented by detailed genetic studies on many hundreds of genes, defined by thousands of mutations. Almost all of the research carried out on C. elegans has addressed the properties of this animal in a single laboratory environment, growth on the surface of agar plates using Escherichia coli as a food source. However, understanding an organism in depth must also entail intimate knowledge of its natural ecology, population genetics and evolutionary history. There exists a large contrast between extensive knowledge of C. elegans as an experimental system, and great ignorance about its natural environment and ecology. The genome seCorresponding authmrJonathan Hodgkin, MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, United Kingdom. E-mail: [email protected] San Francisco, CA 94143. 'Pre.wnt address: Department of Physiology, University of California, Genetics 146: 149-164 (May, 1995) quencing project is revealing the existence of thousands of predicted genes of entirely unknown function. It is possible that many of these enigmatic genes have roles that may only become apparent in a natural situation. Moreover, all organisms that are alive today exist only as a consequence of several billion years of evolutionary history, and inevitably carry some of that history with them. C. elegans and its ancestors probably occupied various different habitats and ecological niches in the past, and some parts of its genome may reflect specializations that are no longer relevant or assayable today. Examination of the natural history of C. elegans and comparisons with related species should be informative