  Background: Caenorhabditis elegans is a major model system in biology, yet
very little is known about its biology outside the laboratory. Especially, its
unusual mode of reproduction with self-fertile hermaphrodites and facultative
males raises the question of its frequency of outcrossing in natural
populations. Results: We describe the first analysis of C. elegans individuals
sampled directly from natural populations. C. elegans is found predominantly in
the dauer stage and with a very low frequency of males compared with
hermaphrodites. While C. elegans was previously shown to display a low
worldwide genetic diversity, we find by comparison a surprisingly high local
genetic diversity of C. elegans populations; this local diversity is
contributed in great part by immigration of new alleles rather than by
mutation. Our results on heterozygote frequency, male frequency and linkage
disequilibrium furthermore show that selfing is the predominant mode of
reproduction in C. elegans natural populations, yet that infrequent outcrossing
events occur, at a rate of approximately 1%. Conclusions: Our results give a
first insight in the biology of C. elegans in the natural populations. They
demonstrate that local populations of C. elegans are genetically diverse and
that a low frequency of outcrossing allows for the recombination of these
locally diverse genotypes.
