Biology Reference
In-Depth Information
but strains carrying two copies have been documented ( Herman et al., 1976;
Rogalski and Riddle, 1988 ). Those that are ''free'' elements segregate in a non-
Mendelian fashion and can be lost both mitotically and meiotically during gameto-
genesis and somatic cell divisions ( Herman et al., 1976 ). The exceptions are those
that carry the meiotic pairing regions (HRRs). Two such duplications have been
characterized, sDp1 (I;f) ( Rose et al., 1984 ) and mDp1 (IV;f) ( Rogalski and Riddle,
1988 ). The stability of nonpairing free duplications varies with size and other
characteristics.
Some duplications have been recovered as exceptional segregants from transloca-
tion strains ( Edgley et al.,2006 ), or as products of a rare recombination event in an
inversion strain ( Zetka and Rose, 1992 ). Additionally, duplication derivatives pro-
duced by the spontaneous shortening of sDp2 (I;f) have been characterized ( McKim
and Rose, 1990 ). This shortening occurred at variable rates, and produced duplica-
tions that were relatively stable in mitotic cell division (e.g., sDp2, hDp5, and hDp20)
and duplications that were quite unstable, and continued to shorten more frequently
than the stable ones (e.g., hDp2 and hDp23). In the germ line, larger duplications were
more stable and transmitted at a higher frequency. Amore recent analysis using aCGH
also found considerable variability in the length of sDp3 (III;f) ( Jones et al., 2007 ),
which was used to isolate and balance lethals on chromosome III ( Stewart et al.,
1998 ). In addition to the length variability, discrete duplications internal to sDp3 were
observed, revealing the genetic complexity of some duplications, which can compro-
mise the usefulness of duplications for accurate gene mapping.
1. Duplications as Balancers
Genomic duplications that do not crossover with the wild-type chromosomes
make effective balancers. Mutations can be maintained as homozygotes while the
duplication provides a wild-type copy of the mutated gene ( Fig. 6 ). Unlike with
translocation balancers aneuploid progeny do not result from mutations balanced in
this manner. This can be advantageous for certain applications such as phenotypic
characterization of lethal mutations that may be difficult to distinguish from the
aneuploid progeny.
2. Duplications as Tools for Mosaic Analysis
Another application of duplication balancers is mosaic analysis ( Hedgecock and
Herman, 1995; Hunter and Wood, 1992; Yochem et al., 2000 ). The first mosaicism
studies in C. elegans employed X-irradiation of embryos heterozygous for the flu-3
mutation. However, the frequency of mosaicism was very low (less than 0.1%
mosaic worms) ( Siddiqui and Babu, 1980 ). An approach that took advantage of
the spontaneous loss of free duplications during somatic cell division was proposed
by Herman (1989) . In this approach, a free duplication that rescues a mutation will
produce mosaic animals that comprise of cells retaining the duplication that will be
phenotypically normal in addition to cells having lost the duplication displaying the
Search WWH ::




Custom Search