Genome Sequence of the Drosophila melanogaster Male-Killing
Spiroplasma
Strain MSRO Endosymbiont
Authors: Paredes Juan C. , Herren Jeremy K. , Schüpfer Fanny , Marin Ray , Claverol Stéphane , Kuo Chih-Horng , Lemaitre Bruno , Béven Laure 
Published: April 2015

ABSTRACT

Spiroplasmas are helical and motile members of a cell wall-less eubacterial group called
Mollicutes
. Although all spiroplasmas are associated with arthropods, they exhibit great diversity with respect to both their modes of transmission and their effects on their hosts; ranging from horizontally transmitted pathogens and commensals to endosymbionts that are transmitted transovarially (i.e., from mother to offspring). Here we provide the first genome sequence, along with proteomic validation, of an endosymbiotic inherited
Spiroplasma
bacterium, the
Spiroplasma poulsonii
MSRO strain harbored by
Drosophila melanogaster
. Comparison of the genome content of
S. poulsonii
with that of horizontally transmitted spiroplasmas indicates that
S. poulsonii
has lost many metabolic pathways and transporters, demonstrating a high level of interdependence with its insect host. Consistent with genome analysis, experimental studies showed that
S. poulsonii
metabolizes glucose but not trehalose. Notably, trehalose is more abundant than glucose in
Drosophila
hemolymph, and the inability to metabolize trehalose may prevent
S. poulsonii
from overproliferating. Our study identifies putative virulence genes, notably, those for a chitinase, the H
2
O
2
-producing glycerol-3-phosphate oxidase, and enzymes involved in the synthesis of the eukaryote-toxic lipid cardiolipin.
S. poulsonii
also expresses on the cell membrane one functional adhesion-related protein and two divergent spiralin proteins that have been implicated in insect cell invasion in other spiroplasmas. These lipoproteins may be involved in the colonization of the
Drosophila
germ line, ensuring
S. poulsonii
vertical transmission. The
S. poulsonii
genome is a valuable resource to explore the mechanisms of male killing and symbiont-mediated protection, two cardinal features of many facultative endosymbionts.

IMPORTANCE
Most insect species, including important disease vectors and crop pests, harbor vertically transmitted endosymbiotic bacteria. These endosymbionts play key roles in their hosts’ fitness, including protecting them against natural enemies and manipulating their reproduction in ways that increase the frequency of symbiont infection. Little is known about the molecular mechanisms that underlie these processes. Here, we provide the first genome draft of a vertically transmitted male-killing
Spiroplasma
bacterium, the
S. poulsonii
MSRO strain harbored by
D. melanogaster
. Analysis of the
S. poulsonii
genome was complemented by proteomics and
ex vivo
metabolic experiments. Our results indicate that
S. poulsonii
has reduced metabolic capabilities and expresses divergent membrane lipoproteins and potential virulence factors that likely participate in
Spiroplasma
-host interactions. This work fills a gap in our knowledge of insect endosymbionts and provides tools with which to decipher the interaction between
Spiroplasma
bacteria and their well-characterized host
D. melanogaster
, which is emerging as a model of endosymbiosis
.