[
International Worm Meeting,
2005]
The normal distribution of recombination events on meiotic bivalents depends on homolog recognition, pairing, and interference. In C. elegans, homolog recognition is required for all crossovers, while pairing and interference are thought to control crossover distribution and number. To determine how pairing and interference affect crossover placement, we developed a method for precisely locating all crossovers on C. elegans chromosomes. We confirmed previous reports that in the wild type each bivalent has one and only one crossover. Thus, wild-type C. elegans chromosomes have complete interference. Previous reports of double crossovers are likely to have been instances of gene conversion.Interference requires two intact homologs, since a gap in one homolog disrupts interference(1). However, a break in homology, rather than a physical break, could account for this requirement. To distinguish between these models, we measured interference in animals heterozygous for a large insertion. The homologs in these animals are intact but have a megabase-scale break in homology. We found that insertions disrupt recombination locally, and thus appear to disrupt homolog pairing. However, every bivalent still experiences essentially one and only one crossover, demonstrating that interference is still complete. Thus, the mechanism for interference is independent of homolog identity.Although insertions did not affect crossover number, they did have an effect on crossover distribution. We found that recombination is redistributed in a polar fashion in insertion heterozygotes. Recombination was consistently higher on one side of the insertion, and lower but not eliminated on the other side. Significantly, in all cases high recombination correlated with the homolog recognition region. Thus, a break in homology disrupts recombination not only in its vicinity, but also in a large region on the side of the break distal from the homolog recognition region. This result is distinct from observations of crossover disruption in translocation heterozygotes, because recombination is not completely eliminated distal to insertions. These data suggest that pairing can sometimes reinitiate after a megabase scale break in sequence homology. However, reinitiation can only partially compensate for initial pairing defects.1. K. J. Hillers, A. M. Villeneuve, Curr Biol 13, 1641 (Sep 16, 2003).
[
International C. elegans Meeting,
1999]
Many rhabditid nematodes use other soil invertebrates for transport between microenvironments. These phoretic associations form as dauer larvae and typically have no ill-effects on the host animal. Caenorhabditis remanei has been reported as a phoretic associate of terrestrial isopods (Armadillidium vulgare and A. nasatum) and of snails (Oxychilus cellarius). However, these associations were reported from anthropogenic environments (compost heaps) and may not accurately reflect the natural history of C. remanei. Here, associations of C. remanei and with woodland isopods are reported. Five species of isopods, Trachelipus rathkii, Cylisticus convexus, Porcellio scaber, Porcellio spinicornis, and A. nasatum, have been obtained from woodland habitats in and around Dayton Ohio. C. remanei has been found in association with four of these species, T. rathkii, C. convexus, P. scaber, and A. nasatum. In all cases, associated nematodes were dauer larvae. The absence of infesting nematodes in in P. spinicornis is significant. These isopods were found intermingled with infested isopods of other species and the probability of not detecting infesting nematodes in P. spinicornis due to sampling error is less than 0.01. Experiments to identify the cause of this host preference are under way as are additional collections to determine the geographic extent of C. remanei - isopod associations. et tu elegans? In the course of these studies, various isopods were obtained from commercial sources. From two different sources, C. elegans-infested populations of P. scaber were obtained. Local populations of P. scaber are being examined for C. elegans infestations.