Symbiotic microorganisms are omnipresent in nature, ubiquitously associated with animals,
plants, fungi, protists, and all other life forms including humans, ranging from having
parasitic through commensalistic to mutualistic associations, and affecting every
biological aspect of innumerable organisms living on earth (McFall-Ngai et al., 2013;
Webster, 2014). Insects represent the majority of macroscopic biodiversity described
thus far (Grimaldi and Engel, 2005), and their ubiquitous interactions with microbes
underpin their diversity and adaptability in the ecosystem (Bourtzis and Miller, 2003;
Zchori-Fein and Bourtzis, 2011).
Studies on microbe-insect symbioses have a long history. During the early to mid twentieth
century, microbe-insect symbioses were regarded as a focal research area in microbiology.
Before the era of molecular biology, high-resolution light microscopy developed by
Carl Zeiss and other optic companies was the cutting-edge technology in biology. Using
high-quality microscopes, a number of German and other European microbiologists enthusiastically
surveyed diverse insects, terrestrial arthropods, and other organisms for their internal
microbiota. The enormous number of microscopic observations were compiled by the outstanding
German microbiologist, Paul Buchner (Sapp, 2002), in the monumental book “Endosymbiosis
of Animals with Plant Microorganisms” (Buchner, 1965). Subsequently, however, advancements
in this research area were for decades very slow, mainly because of the general uncultivability
of the symbiotic microorganisms—researchers could observe some bacteria residing in
and associated with insect cells and tissues cytologically but were unable to characterize
or even identify the microbes. In the 1980s, the invention of PCR and the development
of DNA sequencing technologies brought about an epoch-making breakthrough in microbiology—environmental
microorganisms became identifiable without cultivation on the basis of 16S rRNA gene
sequencing. I remember that, in 1989, just before I began my graduate work at the
University of Tokyo in the lab of Hajime Ishikawa, who was among the pioneers of the
molecular and genomic aspects of aphid endosymbiotic bacteria (Fukatsu, 2006), the
first 16S-based molecular phylogenetic identification of an uncultivable aphid endosymbiont
[later named Buchnera aphidicola (Munson et al., 1991)] was published by Paul Baumann's
group at the University of California, Davis (Unterman et al., 1989). The idea that
this extremely specialized insect symbiont is allied to Escherichia coli was a big
surprise at that time and served as strong motivation for the field to reach a better
understanding of the fastidious microorganisms that are closely allied with insects.
In 2000, when I had already started to run my own lab, Ishikawa's group determined
the first complete genome of an uncultivable microbial mutualist of the pea aphid,
Buchnera aphidicola (Shigenobu et al., 2000), which opened a new era of powerful genomic
approaches to microbe-insect symbiosis studies. At that time, Sanger-based DNA sequencing
technology was so time-, labor- and cost-intensive that the microbial genomics was
not easily accessible for the majority of individual microbiologists. However, from
2007 onward, high-throughput DNA sequencing technologies, initially 454 and Solexa
and then Illumina, PacBio, Nanopore, and others, became available, which propelled
the explosive accumulation of microbial genome data. I was amazed to see interesting
insect genomes coming up one after another from Nancy Moran's group at the University
of Arizona (McCutcheon and Moran, 2007; McCutcheon et al., 2009; Moran et al., 2009),
and I soon also jumped into the excitement. Sequencing insect symbiont genomes is
like opening treasure boxes, uncovering a variety of astonishing evolutionary aspects
such as initial massive accumulation of junk DNA elements (Wu et al., 2004; Toh et
al., 2006) and subsequent size reduction, often approaching organelle-like sizes (Nakabachi
et al., 2006; Pérez-Brocal et al., 2006), catastrophic genome erosion finally leading
to symbiont replacements (Campbell et al., 2017; Matsuura et al., 2018), metabolic
complementarity between co-evolving reduced symbiont genomes (McCutcheon and Moran,
2007, 2010), extremely tiny symbiont genomes streamlined for specific biological functions
(Anbutsu et al., 2017; Salem et al., 2017), dynamic lateral gene transfer and functional
fusion across symbiont and host (Dunning-Hotopp et al., 2007; Husnik et al., 2013),
and others. The number of publications on microbe-insect symbioses from 1985 to 2018
can be seen in Figure 1, in which the above-mentioned historical trajectory and development
are impressively illustrated in relation to the technological innovations.
Figure 1
Number of publications on microbe-insect symbioses from 1985 to 2018.
Inherently, studies on symbiosis are destined to be interdisciplinary, encompassing
the fields of ecology, evolution, genomics, and cellular and molecular biology. Reflecting
this, despite the large number of scientific journals published in the world, few
focus on the publication of microbe-insect symbiosis studies. To my knowledge, Symbiosis
(https://link.springer.com/journal/13199) is the only journal whose mission is to
publish papers on animal-microbe, plant-microbe, microbe-microbe, and other forms
of symbiotic associations. Molecular Plant-Microbe Interactions (https://apsjournals.apsnet.org/loi/mpmi)
publishes a considerable number of symbiosis-related papers, though restricted to
plant-microbe symbioses. The Invertebrate Microbiology section of Applied and Environmental
Microbiology (https://aem.asm.org/) and the Microbe-Microbe and Microbe-Host Interactions
field of the ISME Journal (https://www.nature.com/ismej/) are the suitable outlets
for papers on microbe-insect symbiotic associations. Journal of Invertebrate Pathology
does publish papers on insect symbionts, but the journal mainly focuses on parasites
and pathogens (https://www.journals.elsevier.com/journal-of-invertebrate-pathology).
Environmental Entomology (https://academic.oup.com/ee) highlights Insect-Symbiont
Interactions as a subject area.
Personally, it has been a challenging task for me to find suitable journals in which
to publish my own microbe-insect symbiosis studies. For example, dating back to the
early 1990s for my master's thesis at the University of Tokyo, I wrote up three papers
on a variety of bacterial and fungal symbionts of aphids. I finally published them
in entomological journals, Insect Biochemistry and Molecular Biology and Journal of
Insect Physiology (Fukatsu and Ishikawa, 1992a,b,c) but could not help feeling that
those were not actually a perfect fit. The absence of suitable outlets for insect
symbiosis studies has been a long-lasting frustration for me, and this frustration
must be shared generally by those who are working on such interdisciplinary research
fields related to symbiosis.
In this context, it was a laudable decision of Frontiers in Microbiology to launch
the Microbial Symbioses section in 2013. Since then, the Microbial Symbioses section
has served as a home to numerous innovative, high-quality research studies, and commentaries
on symbiosis. As a newly-appointed Specialty Chief Editor, I am highly motivated to
build up an ideal platform for publishing microbe-insect symbiosis studies in the
Microbial Symbioses section of Frontiers in Microbiology. To that end, I have contacted
the world's most qualified experts who are actively working on microbe-insect symbiotic
associations and interactions and have organized a launching Editorial Board for microbe-insect
symbioses consisting of 14 Associate Editors as of September 2019. To my knowledge,
no other scientific journals specialize in the publication of microbe-insect symbiosis
studies. Though not as a journal but as a part of the journal section, I am aiming
at manifesting such a publishing body of high quality and visibility, which offers
prompt handling, fair and robust evaluation, and qualified publication of submitted
manuscripts covering diverse aspects of microbe-insect symbiotic associations and
interactions. The open access policy of Frontiers in Microbiology, together with the
notable impact factor of 4.259, provides a good reason for consideration. I look forward
to the very best works on microbe-insect symbioses being submitted to Frontiers in
Microbiology.
Author Contributions
The author confirms being the sole contributor of this work and has approved it for
publication.
Conflict of Interest
The author declares that the research was conducted in the absence of any commercial
or financial relationships that could be construed as a potential conflict of interest.
The reviewer JM declared a past collaboration with the author to the handling editor.