Biology Reference
In-Depth Information
Minichromosomes have been found in
D. melanogaster
that apparently origi-
nated from the transposable element TE1 (
Block et al. 1990
). The minichromo-
some contains two structural genes,
white
and
roughest
, from the
Drosophila
X chromosome and part of chromosome 2. This minichromosome was relatively
stable and inherited by 33-47% of progeny, indicating that it contains a centro-
mere. Centromere-like elements lacking chromosome arms have been found in
the phorid
Megaselia scalaris
(
Wolf et al. 1991
). The function of these elements
is unknown, but they could be B chromosomes that have been reduced to a min-
imal size.
4.6.1 Horizontal Gene Transfer from Microorganisms to Insects
Moran and Jarvik (2010)
documented that horizontal or lateral transfer of carot-
enoid-production genes from fungi to aphids has occurred. The aphid carot-
enoid genes are related to fungal genes, and the fungal genes were found to
have integrated into the aphid genome and duplicated. As a result, aphids are
the only insects known to produce carotenoids. How the fungal genes trans-
ferred into the aphid genome is unknown. Contamination of the sample with
fungal DNA was ruled out, and sequence analysis suggests the genes transferred
as a single event, preserving the gene arrangement observed in certain fungi.
The genes were found in both
Aphis pisum
and
Myzus persicae
, suggesting the
transfer preceded their shared ancestor. In
A. pisum
, the presence of red color-
ation from the carotenoids is correlated with different susceptibilities to natural
enemies.
Horizontal transfer of genes from bacterial endosymbionts to arthropod
genomes also has been found.
Hotopp (2011)
suggested transfer of endosym-
biont DNA to animals is probably more common than transfer of genes from
free-living bacteria, especially if the endosymbiont is present in the germ line.
Several examples are known in which portions of the genome of the symbi-
ont
Wolbachia
have been transferred into the host genome. The bean beetle
Callosobruchus chinensis
contains
≈
30% of a
Wolbachia
genome inserted into
the X chromosome. Some of these genes are transcribed at a low level, but it is
not known whether the genes are functional (
Hotopp 2011
).
Drosophila ananas-
sae
has nearly the entire 1.4-Mb
Wolbachia
genome in the 2L chromosome. At
least 28
Wolbachia
genes are transcribed at low levels (
Hotopp et al. 2007
).
The parasitoid wasp
Nasonia vitripennis
genome contains 13 proteins pre-
viously only found in poxviruses, but the proteins also are found in multiple
strains of
Wolbachia
, and analysis indicated that the wasps acquired these pro-
teins from
Wolbachia
(
Werren et al. 2010
). Once
Nasonia
had the proteins, the
genes were duplicated and diverged over time; the genes are transcribed in
