Microhabitat-specificity of the hindgut microbiota in higher termites
Termites are a group of eusocial insects in the superorder Dietyaptera, believed to have evolved from a lineage of ancient cockroach-like ancestors 150 million years ago. They play an important role in the breakdown of dead plant material, with the help of microorganisms harboured in the gut. Th...
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|Summary:||Termites are a group of eusocial insects in the superorder
Dietyaptera, believed to have evolved from a lineage of ancient
cockroach-like ancestors 150 million years ago. They play an important
role in the breakdown of dead plant material, with the help of
microorganisms harboured in the gut. The termites can be classified into
flagellate-harbouring lower termites and flagellate-free higher termites. In
comparison to the lower termites, the higher termites have undergone
immense phylogenetic and dietary diversification, that has led to major
changes in their gut structure. This diversification in the host is reflected in
differences in their gut communities.
To understand how host phylogeny and diet help shape bacterial
communities in higher termites, I conducted an extensive pyrosequencingbased
community survey of the gut communities of the major higher
termite subfamilies, Macrotermitinae, Termitinae, and Nasutitermitinae.
First, I constructed clone libraries and calculated phylogenetic trees for
relevant bacterial taxa found in a variety of higher termites. The node
information in these trees was used to provide a robust phylogenetic
backbone for the accurate taxonomic assignment of the shorter
pyrosequences. The analysis revealed that phylogenetically related
termites in general, have similar community structure. However, one of the
wood-feeding termites showed a greater similarity in gut community
structure to other wood-feeders, in spite of not being phylogenetically
related to them. The results suggest that although host phylogeny appears
to be the major driving force in the determination of gut community
membership, host diet can significantly contribute to community structure.
However, far from being a homogenous environment, the higher
termite gut is a highly structured habitat and shows the presence of
spatially separated and physicochemically distinct compartments.
Conditions unique to each compartment, playa significant role in shaping
distinct compartment-specific communities. I used pyrotag sequencing to
conduct an in-depth analysis of the communities of gut compartments
from termites belonging to the families Termitinae and Nasutitermitinae. I
found that homologous compartments from closely related termites are
more similar in their community structure than adjacent compartments
from the same termite. Based on our results, we hypothesize that similar
ecological conditions such as increased alkalinity in the anterior gut, drive
community structure in the gut compartments, and are reflected in overall
hindgut community structure as well.
The paunch (or P3 compartment) is the most voluminous of all
hindgut compartments in wood-feeding higher termites, and is densely
colonized by bacteria. Studies have shown that cellulase activity in the
hindgut is particle-associated and possibly of bacterial origin. By
fractionation of particles in the paunch lumen, using density-dependent
centrifugation , I was able to show that the fraction enriched in wood fibers
contributes substantially to the total cellulase activity in the hindgut. Using
pyrosequencing, I examined the bacterial communities associated with the
wood fibers in two wood-feeding members of the Nasutitermitinae. The
results revealed the presence of a distinct cellulolytic fiber-associated
community, primarily composed of the phyla TG3, Fibrobacteres and
Spirochaetes. This fiber-associated community appears to have filled the
niche for cellulose digestion, vacated by the flagellates.
Lastly, the gut wall in termites is one of the major habitats in the gut,
and home to an endospore-forming filamentous bacterium called
'Candidatus Arthromitus'. Due to the lack of a cultured isolate, the phylogenetic identity of 'Arthromitus' was disputed, and often confused
with similar filamentous bacteria from mammalian guts. Phylogenetic
analysis of picked filaments reveals 'Candidatus Arthromitus' to be a
diverse clade of bacteria, found widely among arthropods, that is distinct
from the segmented filamentous sequences recovered from mammalian