Hans Zauner

**[This week in GigaScience we published research revealing a previously
hidden diversity of symbiotic
bacteria](https://academic.oup.com/gigascience/article/10/3/giab021/6187866?searchresult=1)
from the genus *Rickettsia*, spanning a wide range of arthropod hosts.
Using accidentally amplified sequence data from a barcoding database as
a starting point, the results will help to better understand the
co-evolution of these intimate symbioses. The new data are also relevant
for studying human and animal diseases that are caused by some types of
*Rickettsia*.**

When certain insects or other arthropods produce more daughters than
sons, or no sons at all, a group of unusual bacteria are often to blame.
Certain symbiotic bacteria, particularly some members of the genera
*Wolbachia* and *Rickettisa*, evolved tricks to mess with the sex ratio
of their hosts' offspring, which makes sense from an evolutionary point
of view: These symbiotic bacteria are transmitted from host mothers (but
not fathers) to their offspring. Bacteria that manage to shift the
host's sex ratio towards daughters, for example by killing male larvae,
thereby increase their own chances to have lots of descendants.

<figure class="wp-block-image size-large is-resized">
<img
src="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-1024x577.jpg"
class="wp-image-3845" loading="lazy" decoding="async"
srcset="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-1024x577.jpg 1024w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-300x169.jpg 300w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-768x433.jpg 768w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-1536x866.jpg 1536w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/sohail-sarwar-DwAsNMOM0hQ-unsplash-2048x1155.jpg 2048w"
sizes="(max-width: 768px) 100vw, 768px" width="768" height="433"
alt="Rickettisa host" />
</figure>

However, a surplus of daughters is only the most visible hallmark of
these special relationships between arthropods and symbiotic bacteria,
which may be described as fluctuating somewhat between being parasitic
and mutualistic.

While often capable of manipulating the host for their own advantage,
*Rickettsia* and *Wolbachia* can also provide services that are of
mutual benefit for host and bacteria alike

For example, they can defend their host against fungal pathogens or help
out with digestion and metabolic needs.

Thus, the lifestyles of many symbiotic bacteria and their arthropod
hosts became closely entangled over the course of evolution. And while
*Wolbachia* is the textbook example for this type of symbiosis,
*Rickettsia* are just as fascinating.

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<figure class="wp-block-image size-large is-resized">
<img
src="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/330px-Ricketts_Howard_Taylor_1871-1910.jpg"
class="wp-image-3825" loading="lazy" decoding="async"
srcset="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/330px-Ricketts_Howard_Taylor_1871-1910.jpg 330w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/330px-Ricketts_Howard_Taylor_1871-1910-181x300.jpg 181w"
sizes="(max-width: 330px) 100vw, 330px" width="330" height="548" />
<figcaption>Howard Taylor Ricketts</figcaption>
</figure>
:::
:::

However, historically *Rickettsia* have first been identified not
because for their host-manipulating, symbiont way of life, but as agents
of human disease. The associated diseases --
[Rickettsioses](https://en.wikipedia.org/wiki/Rickettsiosis) -- come in
a variety of forms, including typhus, African tick-bite fever, and Rocky
Mountain spotted fever (actually, "*Rickettsia*" are named after Howard
Taylor Ricketts \[1871--1910\], who investigated Rocky Mountain spotted
fever).

Different bacteria are responsible for distinct types of Rickettsioses,
and, somewhat confusingly, not all of them are still considered to be in
the genus *Rickettsia* proper.

In any case, *Rickettsia* are a large group and their host range is
amazingly diverse. [In their new *GigaScience*
paper](https://doi.org/10.1093/gigascience/giab021), Jack Pilgrim and
colleagues from the Universities of Liverpool (UK) and Guelph (Canada)
show that members of a subgroup of the genus, the so-called Torix group,
are associated with a wide host range of arthropods.

Their work started with a serendipitous finding in the barcode of life
database ([BOLD](https://www.boldsystems.org/)). BOLD stores a
collection of short mitochondrial DNA sequences that are used for
identifying species by means of their genetic fingerprint, a method
called "barcoding" ([see
GigaBlog](http://gigasciencejournal.com/blog/new-in-gigascience-the-squishome/)
for a previous post on the topic). One gene that is regularly used for
barcoding studies in arthropods is COI, which resides in the
mitochondrial genome and encodes the enzyme *cytochrome oxidase I.*

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<figure class="aligncenter size-large is-resized">
<img
src="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/BOLD.jpg"
class="wp-image-3828" loading="lazy" decoding="async"
srcset="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/BOLD.jpg 759w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/BOLD-300x244.jpg 300w"
sizes="(max-width: 569px) 100vw, 569px" width="569" height="464" />
<figcaption>Screenshot from the BOLD system ( “Trace
viewer”)</figcaption>
</figure>
:::
:::
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Mitochondria are descendants of bacteria themselves, and so it is not
surprising that bacterial COI genes are similar to the mitochondrial
versions in arthropods. Therefore, there's a certain risk that by
probing for, say, a beetle's mtDNA-COI, the COI of a bacterial symbiont
is picked up instead.

BOLD contains more than 8 million COI sequences, and "hundreds of hits
were observed with high sequence similarity to Torix group
*Rickettsia*", the authors note in the paper. They used these
"non-target" amplicons to get a glimpse into Torix diversity and host
range. To this end, they analyzed a bin of BOLD data from a project that
covered 184,585 arthropod specimens in total.

Jack Pilgrim and his colleagues looked specifically for COI sequences
that don't match the morphological assignment and could then be shown to
be of bacterial origin. "The dominant genus was *Rickettsia* with 753
(66.9%) amplifications, compared to *Wolbachia* with 306 (27.2%)", the
authors note. Within the *Rickettsia* subset, the Torix group dominated
with 95 % of the amplicons. In terms of hosts, *Rickettsia* are found
all over the place in the arthropod world based on the BOLD data,
including, more prominently, in *Hymenoptera* (a group including wasps,
bees and ants), *Diptera* (flies), and *Hemiptera* (true bugs).

The authors supplemented their BOLD-derived findings with two more
analyses: They experimentally probed for the presence of *Rickettsia*
DNA in 1,612 arthropod individuals from 169 hand-picked species (both
aquatic and terrestrial), and they also analyzed data from the Short
Reads Archive (SRA) for contaminants pointing towards *Rickettsia* as
the source.

The overall picture that emerges is that of a previously hidden
diversity of symbiotic Torix-group *Rickettsia*. The potential symbionts
appear in a number of host taxa that were previously not known to harbor
*Rickettsia*, also including gastropods and several parasitoid wasp
families, to name just two examples.

<figure class="wp-block-image size-full">
<img
src="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/arthropodgallery.jpg"
class="wp-image-3831" loading="lazy" decoding="async"
srcset="http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/arthropodgallery.jpg 742w, http://gigasciencejournal.com/blog/wp-content/uploads/2021/03/arthropodgallery-300x290.jpg 300w"
sizes="(max-width: 742px) 100vw, 742px" width="742" height="717" />
</figure>

How exactly all these different Torix-group bacteria interact with their
respective hosts, how many are capable of influencing sex ratios for
example, remains to be seen.

One host group of special interest are blood feeders such as bed bugs,
sand flies, ticks and biting midges. It will be interesting to explore
how bacteria of the Torix group influence the ecology and life cycles of
these hematophagous ("blood-eating") hosts that are relevant for human
and animal diseases.

The new work and the host diversity revealed in the analyses also
support the "aquatic hot spot" hypothesis, with aquatic arthropods
apparently more often associated with Torix *Rickettsia* than
terrestrial ones.

Last but not least there's a technical take-home message for scientists
performing mtDNA barcoding studies: "Although *Rickettsia* will only
interfere with barcoding in a minority of cases (\~0.4%), it is likely
that alternate screening primers for some studies will need to be
considered", the authors emphasize. As an example, they cite a known
case where accidental *Rickettsia* COI amplification was treated as if
it were the COI of the freshwater amphipod *Paracalliope fluvitalis*,
thereby confounding the conclusions of a phylogeographic analysis.

Not only do *Rickettsia* distort the sex ratio of arthropod hosts, they
also can mislead scientists!

With the diversity data now available, research of *Rickettsia* and
their hosts will move forward, to increase our understanding of their
co-evolution with arthropods, but also to get a better grip on various
human and animal diseases.

**Read the research article:**

Jack Pilgrim, Panupong Thongprem, Helen R Davison, Stefanos Siozios,
Matthew Baylis, Evgeny V Zakharov, Sujeevan Ratnasingham, Jeremy R
deWaard, Craig R Macadam, M Alex Smith, Gregory D D Hurst, *Torix
Rickettsia* are widespread in arthropods and reflect a neglected
symbiosis, *GigaScience*, Volume 10, Issue 3, March 2021, giab021,
<https://doi.org/10.1093/gigascience/giab021>

**Supporting open** **data in GigaDB:**

Pilgrim J; Thongprem P; Davison HR; Siozios S; Baylis M; Zakharov EV;
Ratnasingham S; deWaard JR; Macadam CR; Smith MA; Hurst GDD (2021):
Supporting data for "*Torix Rickettsia* are widespread in arthropods and
reflect a neglected symbiosis" GigaScience Database.
[http://dx.doi.org/10.5524/100873](http://dx.doi.org/10.5524/100873.)

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all](http://gigasciencejournal.com/blog/rickettsia-bacteria-to-rule-them-all/){rel="nofollow"}
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<strong>
 This week in GigaScience we published research revealing a previously hidden diversity of symbiotic bacteria from the genus
 <em>
  Rickettsia
 </em>
 , spanning a wide range of arthropod hosts. Using accidentally amplified sequence data from a barcoding database as a starting point, the results will help to better understand the co-evolution of these intimate symbioses.
</strong>

Messaggi di Rogue Scholar

One group of bacteria to rule them all