Polytene chromosomes in Drosophila
Polytene
chromosomes are produced by endoreplication,
in which chromatids replicate without separating, so that each
chromosome is seen as more than 210
> 1000 copies side by side. Polytene chromosomes occur in the
salivary glands of larval Drosophila
and other flies,
where the multiple gene copies assist with the large-scale production
of the
glue protein that allows
them to stick to the walls of fly vials. The banding patterns are
useful in the study of Drosophila cytogenetics,
since they allow ready identification of physical modifications
including segmental inversion, duplication, and deletion. Study of
these changes is in turn useful in the study of Drosophila evolution, since it
allows relationships among different species to be tracked.