Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 24

chromosomes as well as transgenic constructs. In this chapter, we describe the spe-
cialized chromosomes that are available in C. elegans, their origins, practical consid-
erations, and methods for generation and evaluation. We will summarize their uses for
biological studies, and their contribution to our knowledge about chromosome biology.
I. Introduction
Caenorhabditis elegans researchers have at their disposal a large collection of
mutant strains and specialized chromosomes, constituting an extensive genetic toolkit
for genetic analyses. Specialized chromosomes are chromosome rearrangements that
have either been recovered in genetic screens or specifically constructed and adapted
for research purposes. They range in size from large reciprocal translocations to
single-gene deletions and in structure from major chromosomal rearrangements to
transgenic arrays. They may be inherited either as chromosomal insertions or extra-
chromosomal fragments. The collection includes a variety of chromosomal rearran-
gements, several types of transgenic arrays containing plasmid, cosmid, fosmid, or
reporter fusion constructs, inserted marker sequences, and naturally occurring var-
iants. Modifying chromosomes in order to address biological questions or obtain tools
for better research methodology is standard practice for C. elegans researchers,
who have adopted specialized chromosomes for a variety of purposes, including
maintenance of mutations, mapping, investigating gene expression, and the study
of specialized processes such as meiosis.
Traditionally, the best known and most commonly used type of rearrangement is
generically called a balancer, a term inherited from Drosophila genetics where chro-
mosomal rearrangements have been used to maintain lethal mutations ( Muller, 1918 ).
Lethal mutations are so-called because they cannot be propagated as homozygotes.
The category includes those that arrest development as embryos, larvae, sterile adults,
or that produce progeny that cannot be maintained over successive generations. To
maintain these mutants, they must be kept as heterozygous strains. The wild-type copy
of the gene may be present on the homolog or provided by gene duplication.
Balancers can be applied to a variety of tasks including strain construction,
maintaining existing mutations, and screening for newmutations. The best balancers
have reduced crossover frequencies in the balanced region, phenotypes that are
distinguishable as heterozygotes and are genotypically stable with low spontaneous
mutation frequencies.
It is possible to balance (maintain a genotype) using tightly linked markers ( Rose
and Baillie, 1979 ) or by selecting for a dominant phenotype of the heterozygote
( Moerman and Baillie, 1979, 1981; Rogalski et al., 1982; Rogalski and Riddle,
1988 ) but in general chromosomal rearrangements are easier to use and more
effective ( Herman, 1978 ). Chromosomal rearrangements that can be used as bal-
ancers are generally of two types. A major class comprises those that reduce or
eliminate recombination between a mutation-bearing chromosome and its wild-type
homolog. These include translocations, inversions, and some deletions. A second