Fungus farming has evolved at least ten times in Scolytinae, in contrast to the single origin of fungus farming in attine ants and macrotermitine termites
[27, 28]. Although limited resolution in tree topology was found in all types of analyses, the fungus farming taxa and sister lineages were well resolved. We found that all origins of fungus farming in Scolytinae were derived and a reversal to a non-fungal diet could not be traced on any of the tree topologies examined. It is noteworthy that although these findings are concordant with previous studies
, our data are more complete in terms of taxon sampling, inclusive of a higher number of fungus farming taxa per clade. The only possible reversal indicated by some of the Bayesian topologies related to Camptocerus with respect to Cnemonyx. However, a more complete taxon sampling for these two genera showed that Camptocerus is indeed monophyletic
; Smith and Cognato, unpublished molecular data]. Our data thus corroborate the hypothesis that fungus farming is indeed a non-reversible evolutionary transition.
The many origins of fungus farming did not correlate strongly with some of the biological factors that benefit from a symbiotic relationship between fungi and beetles. Although we did observe a trend in fungus farming evolving more often in lineages with close inbreeding, the reverse transition rate (from outbreeding to inbreeding in fungus farming lineages) were negligibly low and the association between specific reproductive modes and fungus farming was not significantly correlated (see Table
3). This is perhaps most clearly illustrated by the complete lack of regular inbreeding in four ambrosia beetle lineages (Scolytodes unipunctatus, the genera Camptocerus and Scolytoplatypus, and the entire subtribe Corthylina). Furthermore is regular inbreeding the norm in true bark beetles such as in some Dendroctonus and Araptus [see e.g.
. Fungus farming is therefore a trait that at least sometimes evolves relatively independent of reproductive biology. Repeated origins of fungus farming must therefore be explained by additional ecological factors such as the frequent facultative association between bark beetles and fungi that grow in the phloem and bark of the host trees. Based on this perspective, it is notable that termites have only evolved fungus farming on one occasion even though fungus is an important food component for many other termite groups
Compared with the timing of the origin of Scolytinae, more than 100 Ma, the development of obligate symbiotic fungus farming occurred relatively late. In all groups where an estimate of crown age was reliable, they revealed origins younger than 50 Ma, with 95% confidence interval ± 12 myr (Table
1). Xyloterini and Scolytoplatypodini had stem ages older than the crown age for Corthylina, which could potentially indicate a slightly older origin of fungus farming in these groups. However, it is equally likely that close relatives of these taxa were not included in our study which would overestimate the age of these fungus farmers. Regardless of these uncertainties, the Ophiostomales fungi have certainly existed much longer than the ambrosia beetles as shown by the multiple independent origins of the symbiotic fungi
[18, 30], and thus have likely been nutritionally advantageous to the early lineages of bark beetles that preceded the first ambrosia gardeners. In light of the ubiquitous presence of ambrosia beetles in pantropical forests, and the likely early availability of ambrosia fungi, one may wonder why such a successful adaptation should have taken so long to evolve. There are two particularly relevant factors that may not have been optimal at the earliest stage of bark beetle evolution – tropical forest diversity and climate.
About 98 percent of the known ambrosia beetle fauna is tropical or subtropical
, which emphasizes that fungal symbiosis is largely dependent on moist conditions in warm climates
[8, 31]. Thus, the timing of modern moist tropical forests expansion may be relevant to the origin of fungus farming beetles. Elements of angiosperm-dominated tropical forests developed during the mid-Cretaceous, but did not radiate extensively until the Palaeocene or early Eocene era
[32–35]. This time period experienced a thermal maximum (PETM) of some 5-8 degrees warmer climate from 58 to 45 Ma
. Several groups of animals and plants showed increased diversification associated with the increasing angiosperm dominance
, in particular during or just after PETM
[33, 38–41] when tropical elements dominated floras and faunas from the equator to mid-latitudes e.g.
[35, 42, 43]. Corthylina, Xyloterini, Scolytoplatypus and Camptocerus originated during or immediately after PETM and had likely taken advantage of the large tropical angiosperm forests emerging during this time period.
The only group of fungus cultivating insects that may have occurred in the Cretaceous period is a related group of weevils in the subfamily Platypodinae. Recent studies are inconsistent about the phylogenetic position of these beetles, but they are definitely part of the advanced weevil radiation
[13, 44, 45]. Although the timing of this group seems problematic as a consequence of a generally higher substitution rate at independent genetic loci see
, a late Cretaceous origin at 100-80 Ma seems realistic based on molecular data
[13, 45] and a fossil from Burmese amber (Grimaldi, pers. comm). Climate during this time period is less well understood, but was probably quite warm, dominated by Magnoliales and the early expanding Malphigiales
[35, 46]. However, the greatest part of the platypodine radiation took place much later, with more than 90 percent of the diversity originating in the Eocene and later time periods
It is interesting that fungus farming in ants and termites have similarly late origins as in most Scolytinae beetles. Attine ants first originated around 50 Ma
, similar to Corthylina beetles. However, the major radiation of these ants occurred later, around 20 Ma, which corresponds to our estimates for Bothrosternus-Eupagiocerus and the great Xyleborini radiation. During this intermediate ‘Antarctic thawing’ period
, which lasted some 10 million years, tropical climates again dominated near mid-latitudes
. This is also the time period when the fungus gardening termites (Macrotermitinae) diversified
, after their origin in tropical rainforests of Africa
The late origin of the greatest ambrosia beetle radiation in Xyleborini is well supported by our data. Stem age was only 23 Ma (Additional file
S2) for a clade that is closely related to bark beetles in the genera Coccotrypes, Dryocoetiops and Ozopemon[49–52]. The species diversity in Xyleborini is therefore unparalleled by any other ambrosia beetle lineage or other scolytine lineage. A recent origin of Xyleborini fits well with their absence from Dominican amber, a fossil source otherwise rich on older ambrosia beetle groups such as Corthylina and Platypodinae
. The great diversity of Xyleborini stands in contrast to a relatively modest diversity in the other clades of scolytine ambrosia beetles, particularly so in perspective of time. The reason for their great diversity is unclear. There are at least nine other scolytine clades of ambrosia beetles and only three of these are marginally more diverse than their sister group (see Table
2). Xyleborini are also characterized by regular inbreeding by sibling mating which is generally a great success factor in scolytine evolution, including Hypothenemus and related genera in Cryphalini. However, among the seven origins of regular inbreeding, only Xyleborini and inbreeding Cryphalini are more diverse than their sister group, so evidently there is no direct connection between inbreeding and diversification. There is therefore nothing overtly unique with this group of beetles compared with other ambrosia beetle lineages. To conclude, Xyleborini is most likely diverse because of chance effects, evolving at the right time (global warming in the ‘Antarctic thawing’ period) in the most productive parts of the globe, in the tropical regions.