Tuesday 13 March 2012

Evidence of fungal parasites modifying the behavior of ants from the Eocene Messel Shale.

Fungal parasites often manipulate the behavior of insects in a variety of ways, but in particular by causing insects to move to a location beneficial to the fungi prior to death. One notable example of this is the fungus Ophiocordyceps unilateralis from Southeast Asia, which causes Carpenter Ants to leave their home in the tree canopy and bite into the leaf veins of leaves in the forest understory before lying. This is a cooler, moister environment than the canopy, an ideal place for the fungus to hatch its spores.

In a paper in the journal Biology Letters David Hughes of the Department of Organismic and Evolutionary Biology at Harvard University, School of Biosciences at the University of Exeter and the Department of Entomology at Penn State University, Torsten Wappler of the Steinmann Institute at the University of Bonn, and Conrad Labandeira of the Department of Paleobiology at the National Museum of Natural History at the Smithsonian Institution, and the Department of Entomology at the University of Maryland discuss an example of leaf damage from the Eocene Messel Shale in Germany apparently caused by behavior similar to that seen in ants targeted by Ophiocordyceps unilateralis.

(a) Leaf from the Messel Shale with 29 marks interpreted as death-grip scars by fungus infected ants. (b) Drawing showing detail of (a). (c) Photograph showing detail of (b) showing a single grip-mark. (d) Another grip mark. (e) Detail of (a) showing (L) unaffected leaf tissue, (H) hyperplasic cell files on a callused area, (S) area of fungal infection, (C) cut area. (f) Modern Carpenter Ant, infected with Ophiocordyceps unilateralis in death grip. (g) Death grip scar on modern leaf. (h) Drawing of modern leaf with death grip scars. (i) Photograph of same leaf as in (h).

The earliest known example of a fossil insect with a parasitic fungal infection is from a scale insect in 105 million year old (Cretaceous) amber from Burma; a 21 million year old (Miocene) termite with a similar infection is known from amber from the Dominican Republic, but neither show whether the fungus was capable of modifying the behavior of the insects. Modern Ophiocordyceps fungi only infect ants belonging to the tribe Camponotini, which are not known from the Messel Shale, but are known from Eocene amber from Germany and the Ukraine.

The Messel Shale fossils originate from a (now disused) quarry in southwestern Germany, about 35 km southeast of Frankfurt. The quarry produced bituminous shale which is noted for a large variety of well preserved Eocene fossils (about 47 million years old), particularly birds, mammals, insects and plants. At the time of formation the site was a lake with a stagnant anoxic bottom, into which the bodies of animals could sink and become preserved, often in excellent detail, with soft tissues not actually preserved but mapped by the bacterial films that covered them. It is thought that the lake may have periodically released toxic gas in a similar way to modern Lake Nyos in Cameroon, due to the large number of apparently intact bird and bat fossils found in the deposits. The ecology and climate of the site is considered to have been roughly analogous to that of modern Thailand.

The geological setting of the Messel Pit. From Lenz, Wilde & Riegel (2007).

The site was nearly used for landfill after quarrying ended in the 1970s, but fortunately was eventually preserved. It is now a UNESCO World Heritage Site.