Biology Reference
In-Depth Information
10.11.1
Spiroplasma
Strains
The sex-ratio condition of
Drosophila willistoni
, and at least 10 other
Drosophila
species, is due to a
Spiroplasma
strain (
Ashburner 1989, Williamson et al. 1999,
Anbutsu and Fukatsu 2003
).
Spiroplasma
strains are maternally inherited, trans-
ovarially transmitted, and lethal to male embryos. Male eggs die at early devel-
opmental stages before gastrulation.
Anbutsu and Fukatsu (2003)
compared the
titers of two strains of
Spiroplasma
by a quantitative PCR method; one strain
killed males and one did not but was otherwise very similar. They compared the
ability of the
Spiroplasma
strains to proliferate within their hosts to determine
whether their titer altered the ability of the benign strain and discovered that,
indeed, the benign strain did not replicate as well. Thus, the
titer
of the strain
was important in resulting in male killing.
Spiroplasma
strains can be transmitted between species by injecting hemo-
lymph, but
Spiroplasma
strains from different species are different, and a dif-
ferent virus is associated with each. When
Spiroplasma
strains from different
species are mixed, they clump because the viruses lyse the
Spiroplasma
of the
other species.
Why would male-killing bacteria persist in insect populations?
Martins et al.
(2010)
tested the hypothesis that death of males results in increased resources
for sibling females. This suggests that infected females should be larger than
uninfected females, or have higher viability, or shorter development times.
Tests involving infected and uninfected
D. melanogaster
females collected
from the wild indicated that
infected
females produced more daughters than
uninfected females, and there was a decrease in development time of infected
females, perhaps as a result of reduced competition with sibling males (which
died).
10.11.2 L-Form Bacteria
The
Drosophila paulistorum
complex contains six semispecies (subgroups derived
from a single species that are thought to be in the process of speciation) that do
not normally interbreed. When they are crossed in the laboratory, fertile daugh-
ters and sterile sons are produced. Streptococcal L-form bacteria were isolated
and cultured in artificial media that are associated with the sterility (
Somerson
et al. 1984
). The L-forms are transferred through the egg and each semispe-
cies appears to have a different microorganism. L-forms can be microinjected
into females and can produce the expected male sterility. This suggests that an
L-form normally has a benign relationship with its own host; however, if it is
transferred to a closely related host, male sterility is induced.
