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The origin of agricultural insects

For millions of years, some termites, ants and cockroaches have the ability to cultivate fungi in order to feed themselves. This mutualism between insects and fungi (one of the best studied symbioses in the natural world) is an evolutionary strategy analogous to the agricultural activity of the human species, which has existed since the Neolithic Revolution.

A beetle pierces the trunk of a tree to make a gallery in the wood to protect its setting. As it digs the tunnel, it disperses ragweed fungal spores that will feed the larvae. When they pierce another tree, the adult beetles will be vectors for the transmission of the fungus spores to a new habitat. This mutualism between ragweed insects and fungi could be much older than previously thought.

According to the conclusions reached in a new study, that age could be more than 100 million years.

The research analyzes for the first time the symbiotic relationship and coevolution between ragweed beetles and fungi from a paleontological perspective, and does so through the Cretaceous fossil record of these biological groups. The study authors are experts David Peris and Xavier Delclòs, from the Faculty of Earth Sciences and the Biodiversity Research Institute (IRBio) of the University of Barcelona (UB), and Bjarte Jordal, from the University of Bergen ( Norway).

Understanding the origin of the symbiosis between insects and fungi is a field of interest in various scientific disciplines. At present, the mutualism between beetles and symbiotic ragweed fungi is the cause of pests in forests and crops that generate serious ecological and economic losses. However, “the ecological factors that led to this feeding strategy are still not well understood, nor how it was transformed into a symbiotic relationship with forced dependence,” explains David Peris, co-author of the study.

The ragweed beetle Crossoterasus externedentatus (Curculionidae: Platypodinae). (Photo: Bjarte Jordal)

Traditionally, phylogenetic studies suggest that fungiculture for beetles began more than 50 million years ago – much earlier than other insects – and some studies even date it to about 86 million years ago. “It is likely that the symbiotic relationship between fungi and cockroaches originated more than 100 million years ago, during the Lower Cretaceous, in groups of cockroaches that had previously gone unnoticed,” David Peris now reveals.

As part of the research, experts have studied several specimens of biological groups captured in Cretaceous amber from around the world. Thus, the origin of the ragweed fungi is older than the main groups of beetles of the subfamilies Scolytinae and Platypodinae – family Curculionidae – that today grow fungi on tree trunks, the authors point out.

“This indicates that these fungi used some other group of insects to disperse millions of years ago,” says Peris. “In addition, other groups of beetles with a behavior similar to that of ragweed beetles – family Bostrichidae and especially Lymexylidae – present an abundant older fossil record that would coincide with the appearance of ragweed fungi, as revealed by other previous studies.”

“The most interesting of all – continues the expert – is that some works cite the ability to cultivate fungi in some of these current species.”

The mushroom cultivation process begins when the beetles colonize a new tree trunk or branch. During the Cretaceous, the abundance of wood fungi and cockroaches led to an initial domestication of some groups of fungi. Initially, fungal spores were accidentally transported from tree to tree by borer beetles, ‘until this mutually beneficial association evolved into a more intimate symbiosis in which fungi were transported into a tree, fungal mycelia grew and larvae of the beetles used to feed on the fungus, ”says Bjarte Jordal.

This combination of factors, added to the high capacity for adaptation and change of the symbionts, facilitated the morphological and ecological adaptations of the biological groups that converged evolutionarily in an obligatory mutualism. That is, a symbiotic relationship between insects and fungi of a beneficial nature for both organisms that still exists.

“However, it will be necessary to deepen the knowledge about the ecology of the species of the families Lymexylidae and Bostrichidae to obtain more concrete conclusions. Therefore, the discovery of new fossils in Cretaceous amber of these biological groups will undoubtedly help to better interpret the evolutionary history of this symbiotic relationship that has survived to this day ”, concludes Professor Xavier Delclòs.

The study is entitled “Origin and evolution of fungus farming in wood-boring Coleoptera – A palaeontological perspective”. And it has been published in the academic journal Biological Reviews. (Source: UB)

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