Biology Reference
In-Depth Information
males and females through various life stages.
Hotop et al. (2007)
concluded
that up to 70% of
Wolbachia
-infected hosts might have
Wolbachia
genes
inserted into their nuclear genomes and that horizontal gene transfer from
microorganisms may have a significant impact on insect evolution.
Analysis of the genome of the pea aphid,
Acrthosiphon pisum
, indicates that
only 12 genes or gene fragments are of bacterial origin, with only two from
Buchnera aphidicola
, their obligatory endosymbiont, both of which seem to be
inactive (
Nikoh et al. 2010
). The other genes seem to be related to genes from
relatives of
Wolbachia
. At least eight of the genes seem to be functional, and
expression of seven of these genes is highly up-regulated in bacteriocytes. Thus,
Nikoh et al. (2010)
concluded that reduction of the genome in
Buchnera
, the
ancient endosymbiont in aphids with a highly reduced genome (1/7th the size
of related bacteria), did not result in transfer of
Buchnera
genes to the aphid
nuclear genome. Rather, it seems that aphids use a set of duplicated genes
acquired from other bacteria to aid the
Buchnera
-aphid mutualism.
Acuna et al. (2012)
found a bacterial gene was transferred into the coffee
berry borer,
Hypothenemus hampei
, a beetle pest of coffee. The gene encodes
a mannanase that hydrolyzes the major storage polysaccharide in the coffee
bean, that is the beetle's presumed food. The gene is found in
H. hampei
but
not in the closely related
Hypothenemus obscurus
, indicating the gene has pos-
sibly allowed adaptation to a new host plant. This is the first time this gene has
been found in an insect. A phylogenetic analysis of diverse mannanase genes
from prokaryotes and eukaryotes indicated the beetle gene was most closely
related to prokaryotic mannanases, grouping with the
Bacillus
clade. The gene
is flanked by two eukaryotic transposons of the hAT and Tc1/mariner superfami-
lies, further supporting the hypothesis that the gene is integrated into the bee-
tle genome and not present in gut symbionts. Furthermore, the presence of the
TEs suggests a possible mechanism for the horizontal gene transfer event.
4.7 B Chromosomes
B chromosomes are a heterogeneous class of, often heterochromatic, chromo-
somes sometimes referred to as accessory or supernumerary chromosomes that
occur in plants and animals. B chromosomes may have little effect on the phe-
notype, differ in number from one cell type to another, and may occur only in
some individuals of the species. B chromosomes may not segregate normally
in mitosis and meiosis (
Wagner et al. 1993
). A B chromosome in the parasitic
wasp
Nasonia vitripennis
causes the compaction and loss of paternally derived
chromosomes in fertilized eggs, leading to the production of all male progeny
