Biology Reference
In-Depth Information
in the cytoplasm of neotropical
Drosophila
species. They showed that in some
Drosophila
popu-
lations, females infected with SRO do not give birth to males by killing their sons selectively at
an early stage of development, which leads eventually to an all-female population. They also showed
that the SR trait could be transferred interspeciÝcally by microinjection of hemolymph to other
individuals (Sakaguchi and Poulson, 1963). Later, the SRO was identiÝed as a spiral-shaped
bacterium that lacked a cell wall and was thus designated
Spiroplasma
. Because of its spiral shape
and mobility,
Spiroplasma
was once mistakenly thought to be a member of spirocetes. Also, because
of its membrane Ýlterability, it was sometimes mistaken for a virus.
Spiroplasma
species belong to the order Mycoplasmatales, and more than 50 strains have been
identiÝed to date from more than 30 species (Williamson, 1998). Whereas the known genome sizes
of
Mycoplasma
species are less than 1 Mb, those of
Spiroplasma
have been estimated to be around
1.5 Mb (Dybvig, 1990). Unlike mutualistic symbionts, which are usually localized in speciÝc cells
such as bacteriocytes,
Spiroplasma
are almost ubiquitous, both extracellular and intracellular, in
the tissues, including hemolymph, of an infected host.
Male killing caused by
Spiroplasma
is an early killing, so called because male mortality occurs
typically during embryogenesis or the Ýrst larval instar. The early male killing has commonly been
studied using
Drosophila
species infected with
Spiroplasma
. Among the progeny produced by the
infected female Þies, males are selectively killed early in embryogenesis. The mechanisms under-
lying this son killing are largely unknown, whereas some observations may provide clues to
understand them. For instance, in some
Drosophila
species and lineages, it is known that early and
later broods will scarcely receive the bacteria from their mother. However, this incomplete trans-
mission is not associated with the appearance of sons. The daughters that are scored as uninfected
may in fact be infected with very low numbers of
Spiroplasma
, which may be sufÝcient, however,
to kill their brothers (Ebbert, 1993). However, an alternative explanation for this phenomenon
suggests that male-lethal spiroplasmas produce a diffusible toxin or virus that actually kills males
(Williamson, 1965). Indeed, there are some suggestive molecular differences in the number and
type of virus they harbor (Cohen et al., 1987).
It is known that
Spiroplasma
species harbor arrays of extrachromosomal genetic elements.
Among them, SVC3, a polyhedrosis virus with a short tail, is shared by all the known species of
Spiroplasma
. Interestingly, it has been demonstrated that SVC3 isolated from a strain induces cell
lysis of other strains of
Spiroplasma
(Whitcomb, 1980). Since every strain harbors its own SVC3,
the infection of the virus itself obviously cannot be responsible for the cell lysis. It has long been
known that there is a competition between strains of
Spiroplasma
that cause the SR trait and those
that do not with respect to the infection in the
Drosophila
hosts. It is highly likely that this
phenomenon is due to the lytic action of the SVC3 they harbor. It is a matter of conjecture whether
or not the lytic action of SVC3 is somehow implicated in the male killing by spiroplasmas.
In addition to viruses
such as SVC3, spiroplasmas harbor various sized, closed-circular DNA
whose functions have yet to be discovered. These are collectively known as latent plasmids. Some
of them are on occasion integrated into the chromosomal DNA of spiroplasmas (Whitcomb, 1980).
M
ICROSPORIDIA
,
A
L
ATE
M
ALE
-K
ILLER
Microsporidia are eukaryotes that are classiÝed as either protists or fungi. They are well known as
one of the few eukaryotes that lack mitochondria. Because of this, microsporidia were once
considered to be direct descendants of a primitive eukaryote before capturing an ancestral bacterium
of mitochondria. Now, however, many lines of evidence suggest that they have secondarily lost the
cell organelle because of a prolonged life in anaerobic milieus.
Microsporidia are also known as male-killing agents of certain insects. More precisely, they
are late killers, so called because male mortality occurs late in development, typically in late larval
instars. Whereas microsporidia can be transmitted both vertically and horizontally in many insects,
late male killing has so far only been recorded in mosquitoes. When microsporidia are transmitted
