Results
We identified 134 CHC compounds in total from our representative termite
and cockroach species (Tab. S1). The six major CHC compound classes
detected were n -alkanes, n -alkenes, alkadienes as well as
mono-, di-, and tri-methyl-branched alkanes (Fig. 2). The different
compound classes greatly vary in relative amounts across all species and
not all classes were observed in each species. On average,n -alkanes show higher relative abundancies in termites (29.1 %)
than in cockroaches (15.43 %), whereas di-methyl-branched alkanes show
the reversed pattern with much higher average abundancies in cockroaches
(25.96 %) than in termites (0.027 %). Tri-methyl alkanes, on the other
hand, only occur in detectable quantities in B. germanica .
Alkadienes were only found in Rf, M. darwiniensis and N.
castaneus , with minimal trace occurrences also in B. orientalis .
Generally, mono-methyl-branched alkanes were the most abundantly
detected compound class across the tested cockroach and termite species,
however, they occurred in comparably low quantities in M.
darwiniensis and N. castaneus . Unsaturated compounds, generally
considered to be among the structurally more complex CHCs indicating
higher chemical complexity together with methyl-branched alkanes (Martin
and Drijfhout 2009; Kather and Martin 2015), occur inconsistently in two
high (ST) and one low (OPT) social complexity termite species, but only
in traces in the cockroaches. However, the most structurally complex
methyl-branched alkanes with three methyl branches occur exclusively in
just the cockroach species B. germanica . Moreover, the
structurally most simple CHC profile, consisting almost exclusively ofn -alkanes and mono-methyl-branched alkanes, was found in the high
social complexity termite C. formosanus .
The molecular phylogeny of our tested termite and cockroach species
mostly mirrors their respective levels of social complexity except forM. darwiniensis and C. formosanus , which represents the
most basal termite group despite displaying a high level of social
complexity (Fig. 3). This is not at all reflected in their chemical
phylogeny based on the average CHC divergence between the species.
Namely, the most highly supported cluster (99 Bootstrap) encompasses a
solitary (B. orientalis ), highly social (C. formosanus )
and lowly social (K. flavicollis ) species. Moreover, all levels
of social complexity that cluster together in the molecular phylogeny
are basically broken off in the chemical phylogeny, with no recognizable
pattern. Unsurprisingly, a Mantel test found no significant correlation
between the molecular and chemical phylogeny (r = 0.3971, P = 0.1291).
Comparing overall CHC biosynthesis gene transcript counts with the total
number of CHC compounds detected in each species, we found no
significant correlation between the quantities
(χ2 – test,
r=0.12, p=0.79; Fig. 4). R. flavipes has the highest transcript
counts (191) but the third lowest CHC compound count (37), conversely,M. darwiniensis has the lowest transcript count (111) but the
second highest CHC compound count (59). Again, no trend towards the
different levels of social complexity could be detected, which is best
exemplified in the three high social complexity termite species, where
both the highest (R. flavipes ) and the lowest (M.
darwiniensis ) number of transcripts as well as the second highest
(M. darwiniensis ) and the lowest number (C. formosanus ) of
CHC compounds was detected. Across all investigated CHC biosynthesis
gene transcripts (Tab. 1), counts did not vary systematically by social
complexity level (χ2= 1.11, df = 1, p = 0.29), which
is also apparent in a heat map representing the individual transcript
counts per species normalized by their average relative abundancies
(Fig. 5).