Sunday 30 August 2015

Preservation of haemoglobin-derived chemicals in a fossil Mosquito from the Eocene of Montana.

The 1993 film Jurassic Park speculated that Dinosaur DNA might be extracted from blood preserved inside Mosquitoes, Culcidae, trapped in amber during the Mesozoic. Needless to say this is highly improbable for a number of reasons, most notably that fossils preserved in amber, while retaining physical shape, are known to be chemically altered during the preservation process. A less obvious flaw in this story is that Mosquitoes do not typically favour woodland environments, and are therefore rather rare as fossils preserved in amber, though a number of Biting Midges, Ceratopogonidae, are known from amber, and some of these have been found in association with preserved Trypanosome parasites, which are spread by blood-sucking Insects, implying that these fossil Midges were blood-feeders.

In a paper published in the Proceedings of the National Accademy of Sciences of the United States of America on 12 November 2013, Dale Greenwalt of the Department of Paleobiology at the National Museum of Natural History, Yulia Goreva of the Department of Mineral Sciences at the National Museum of Natural History, Sandra Siljeström, also of the Department of Mineral Sciences at the National Museum of Natural History, as well as the Geophysical Laboratory at the Carnegie Institution and the Department of Chemistry, Materials, and Surfaces at the SP Technical Research Institute of Sweden, Tim Rose, again of the Department of Mineral Sciences at the National Museum of Natural History and Ralph Harbach of the Department of Life Sciences at the Natural History Museum in London describe the presence of haemoglobin-derived chemicals in a fossil Mosquito from the Middle Eocene Kishenehn Formation of Montana.

The Kishenehn Formation comprises finely bedded shales laid down in anoxic conditions at the bottom of a lake about 46 million years ago. These shales have produced a number of very well preserved Mosquitoes, which have been assigned to two species, Culiseta kishenehn and Culiseta lemniscata, both interpreted to be small Bird-feeding species. One of these appears to be a blood-engorged female (only female Mosquitoes suck blood, and then only before producing eggs), which Greenwalt et al. chose for testing for haemoglobin derived products. This specimen was well enough preserved to assign it to the genus Culiseta but not to a specific species, though the abdomen region was well preservesd.

Culiseta species (Diptera: Culicidae), a blood-engorged female from the Middle Eocene Kishenehn Formation of northwestern Montana. Note the distended and opaque dark-coloured abdomen. Scale Bar is 2 mm. Greenwalt et al. (2015).

The most obvious indicator of haemoglobin is iron; however iron is a very common element in fossil preservation, particularly is fossils such as those from the Kishenehn Formation which were preserved in anoxic waters, where sulphur-iron Bacteria typically play a role in preservation. In order to establish whether the engorged female Mosquito contained more iron than would be expected, Greenwalt et al. also tested a male Mosquito from the same formation (male Mosquitoes do not suck blood). Samples of material from the abdomens of both specimens were tested for iron with mass spectrometry, revealing that the female did indeed contain raised levels of iron, suggesting the presence of haemoglobin derived chemicals.

Culiseta species (Diptera: Culicidae), a male from the Middle Eocene Kishenehn Formation of northwestern Montana. The white dots indicate the areas of that were analysed. Scale Bar is 2 mm. Greenwalt et al. (2015).

Next Greenwalt et al. tested the specimens for porphyrins, a group of proteins from which haemoglobin molecules are constructed, and which it breaks back down to as it decays. These could be used to definitively indicate the presence of vertebrate blood in the Mosquito at the time of death. Porphyrins are not exclusively produced by vertebrates; chlorophyll is composed of porphyrins as are the haemocyanin molecules used to transport oxygen in some invertebrates (but not Mosquitoes), and many microorganisms also use porphyrins. However no known organism produces porphyrins in any appreciable quantities in an anoxic environment, so any porphyrins found in the specimens are likely to have got there before the Mosquitoes died.

Using time-of-flight secondary ion mass spectrometry Greenwalt et al. were able to detect haemoglobin-derived porphyrin molecules in the female Mosquito, but not the male, suggesting that this specimen was indeed gorged with vertebrate blood at the time of death. This is the first detection of blood derived chemicals in a blood-feeding Insect known in the fossil record.

See also…

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