Biology Reference
In-Depth Information
reverted to the behavior of unmated sexual wasps, laying eggs in hosts appropri-
ate for male eggs.
Cardinium
-infected thelytokous wasps distributed their eggs
almost equally between host types, similarly to mated sexual females.
Nakamura et al. (2011)
compared the effects on the immune system of
Bombyx mori
by using
Wolbachia-
and
Cardinium
-infected cells using a silk moth
microarray.
Wolbachia
infection did not change gene expression or induce or
suppress immune responses but
Cardinium
infection induced many immune-
related genes. The differences were attributed to the fact that the two micro-
organisms have different cell wall structure, with
Wolbachia
lacking genes
encoding lipopolysaccharide components and other cell wall components.
Much remains to be learned about the role(s)
Cardinium
plays in the biology
of their arthropod hosts.
4.12.3 Polydnaviruses in Parasitoids
A particularly interesting example of an intimate relationship between insects
and symbionts is illustrated by the relationship between polydnaviruses
and parasitoids. The
polydnaviruses
are found only in the Braconidae and
Ichneumonidae among the parasitic Hymenoptera (
Krell 1991, Fleming 1992,
Stoltz and Whitfield 1992
). Polydnaviruses are symbiotic proviruses that have
double-stranded circular DNA genomes; they are literally “poly DNA-viruses,”
having segmented genomes composed of several circular DNA molecules. For
example, the viral genome within
Campoletis sonorensis
consists of 28 DNA mol-
ecules ranging from
≈
5.5 to 21 kb, with the total genome size
≈
150 kb.
Polydnaviruses ensure that some species of braconids and ichneumonids
(
=
parasitoids) are able to successfully parasitize their lepidopteran hosts. At
least 50 species of parasitoids are known to contain polydnaviruses (
Stoltz and
Whitfield 1992
) and
>
30,000 species are thought to carry them (
Shelby and
Webb 1999
). Genera of parasitoids containing polydnaviruses seem to be more
speciose and have broader host ranges than sibling groups lacking them, sug-
gesting that the viruses contribute to the evolutionary success of their parasitoid
hosts (
Shelby and Webb 1999
). The two polydnaviral groups, Ichnoviridae and
Brachoviridae, are phylogenetically and morphologically distinct and use differ-
ent mechanisms to inhibit the immunity and development of the lepidopteran
hosts (
Webb 1998
). The association between braconid parasitoids and their
viruses seems to have lasted at least 60 million years (
(Whitfield 1997
).
Polydnaviruses replicate only in braconid or ichneumonid wasp ovaries and
are secreted into the oviducts from where, during oviposition, they are injected
