Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 37

although stable, present some technical difficulties in their characterization.
Genetically, they generate rearranged linkage between markers. Molecularly, detec-
tion of the two breakpoints is needed to provide information about the rearrange-
ment. Methods for specifically isolating and characterizing inversions have not been
well developed and this in part explains the paucity of available ones for use.
C. Deletions (Deficiencies)
Chromosomes that have lost a region of DNA are often used in C. elegans
research. These chromosomes are known as deletions, a term that normally refers
to the DNA that is missing (deleted) or deficiencies, referring to the failure to
complement two or more adjacent genetic loci (phenotypic deficiency). The
nomenclature for both is Df. Dfs range in size from small deletions that disrupt
a few genes to those that remove megabases (Mb) of sequence involving hundreds
of genes. They can arise spontaneously but are more usually generated in muta-
genesis screens. Deletions are one of the most useful mapping tools and as such a
large resource of Df strains covering over 70% of the genome has been generated
( Ahnn and Fire, 1994; Chanal and Labouesse, 1997 ). Specific information about
deletions in a defined region of the genome can be obtained from WormBase
( www.wormbase.org ).
1. Deletions as Mapping Tools
Probably, the most common use of deletions is for mapping uncloned mutations
( Clark et al., 1990; Fay, 2006; Johnsen and Baillie, 1991; Meneely and Herman,
1979; Rogalski et al., 1982; Sigurdson et al., 1984; Stewart et al., 1998 ). A typical
mapping scheme would involve crossing each mutant strain to a Df strain and
assaying (scoring) for rescue (complementation) of the mutant phenotype. If the
mutation is rescued by the deficiency strain, the mutation does not fall within the
region covered by the deficiency. If the mutant phenotype is observed in the F1
generation, the mutation is not rescued (failure to complement) and maps within the
extent of the deficiency. If deficiencies overlap, it is possible to limit the position of
the mutation to a relatively small region defined by the overlap of two or more
deficiencies ( Fig. 4 ).
2. Deletions for the Study of Meiotic Recombination
Animals hemizygous for portions of the X-chromosome were used to identify the
left end as critical for proper X-chromosome disjunction in XX hermaphrodites
( Herman et al., 1982 ). Subsequently, deletions that affected the pairing properties of
the X-chromosome identified the X-chromosome pairing center ( Villeneuve, 1994 ).
Deletions of this region that can pair, have reduced levels of crossing over
( Broverman and Meneely, 1994 ).