Biology Reference
In-Depth Information
has
<
600 genes on a short, linear chromosome and a circular plasmid, which
contains genes for synthesizing pantothenate, an essential vitamin.
The twospotted spider mite,
Tetranychus urticae
(Arthropoda: Chelicerata:
Acari: Tetranychidae), is not an insect at all, but an important chelicerate agricul-
tural pest, feeding on
>
250 plant species. It produces silk, hence the name “spi-
der mite.” Its genome was sequenced and provides another example (other than
the pea aphid) of a genome of a plant-feeding pest (
Grbic et al. 2011
). This tiny
genome (90 Mbp) contains novel genes for silk production, and expanded detox-
ification-gene families that are likely associated with feeding on so many differ-
ent plants. Interestingly, carotenoid biosynthesis genes were found in
T. urticae
's
genome (Altincicek et al. 2012), as they were in the pea aphid (
Moran and Jarvik
2010
), suggesting that the carotenoid-synthesis genes were horizontally trans-
ferred from fungi to the ancestor of both aphids and the twospotted spider
mite. The carotenoids found in
T. urticae
were thought to have been obtained
from their host plants and are important in diapause induction. The spider
mite, which has a greatly reduced segmentation, also has only eight of the 10
expected
Hox
genes, with
Hox3
+
and
abdominalA
+
missing. Silk is produced by
paired glands connected to the pedipalps (mouthparts) of spider mites, and it is
stronger and thinner than the silk produced by the spider
Nephila clavipes
.
The pea aphid,
Acyrthosiphon pisum
, genome was sequenced and found
to be very large (464Mbp) (
Godfray 2010, International Aphid Genomics
Consortium 2010
). The genome has extensive gene duplications in
>
2000 gene
families, and it has lost some evolutionarily conserved genes. Some gene dupli-
cations seem to belong to TEs and encode reverse transcriptases and trans-
posases. Other duplications involve sugar-transporter proteins that may help the
aphid feed on phloem, which is rich in carbohydrates but low in nitrogenous
nutrients. Comparisons of all the 34,600 proteins of the aphid with the proteins
of the 13 other fully sequenced arthropods indicated aphids have had multiple
gene expansions specific to aphids, with the highest protein-gene content of
all species sequenced so far (
Huerta-Cepas et al. 2010
). Examples of gene fami-
lies that have greatly expanded include amino-acid transport, anti-apoptosis,
response to oxidative stress, sensory perception of smell, carbohydrate trans-
port, and olfactory behavior. A number of gene losses have occurred (defense
response, immune response, detection of bacterium, antimicrobial humoral
response, and sensory perception of taste).
Moran and Jarvik (2010)
found that
the pea aphid has multiple enzymes involved in carotenoid biosynthesis that
were derived from fungal genes that transferred horizontally and became inte-
grated into the aphid genome and duplicated. The pea aphid was the first ani-
mal known to make its own carotenoids.
