Methods Cell Biol,
The clone-based physical map of the 100-Mb Caenorhabditis elegans genome has evolved over a number of years. Although the detection of clone overlaps and construction of the map have of necessity been carried out centrally, it has been essentially a community project. Without the provision of cloned markers and relevant map information by the C. elegans community as a whole, the map would lack the genetic anchor points and coherent structure that make it a viable entity. Currently, the map consists of 13 mapped contigs totaling in excess of 95 Mb and 2 significant unmapped contigs totaling 1.3 Mb. Telomeric clones are not yet in place. The map carries 600 physically mapped loci, of which 262 have genetic map data. With one exception, the physical extents of the remaining gaps are not known. The exception is the remaining gap on linkage group (LG) II. This has been shown to be bridged by a 225-kb Sse83871 fragment. Because the clones constituting the map are a central resource, there is essentially no necessity for individuals to construct cosmid and yeast artificial chromosome (YAC) libraries. Consequently, such protocols are not included here. Similarly, protocols for clone fingerprinting, which forms the basis of the determination of cosmid overlaps and the mapping of clones received from outside sources and has to be a centralized operation, and YAC linkage are not give here. What follows is essentially a "user's guide" to the physical map. Details of map construction are given where required for interpretation of the map as distributed. The physical mapping has been a collaboration between the MRC Laboratory of Molecular Biology, Cambridge, United Kingdom (now at The Sanger Centre, Cambridge, UK) and Washington University School of Medicine, St. Louis, Missouri. Inquiries regarding map interpretation, information, and materials should be addressed to email@example.com or firstname.lastname@example.org.
Work in our laboratory over the past several years has focused on the nature of early determinative decisions in embryos of the free-living nematode Caenorhabditis elegans. Two of these decisions regard determination of sex and determination of the level of X-chromosome expression. C. elegans has two sexes, self-fertilizing hermaphrodites and males. Hermaphrodites normally have two X chromosomes, and males have only one (there is no Y chromosome). Genetic and molecular evidence suggest that C. elegans compensates for this difference in X dosage, not by X inactivation as in mammals, but rather by global regulation of the X chromosome as in Drosophila; that is, X-linked genes are expressed at a higher level per chromosome in 1X than 2X animals, so that levels of X expression are similar in the two sexes. Also as in Drosophila, the primary signal that dictates both sex determination and level of X expression in C. elegans is the ration of the number of X chromosomes to the number of sets of autosomes (X/A ratio) rather than the absolute number of X chromosomes.|