40
occur, the body of the isopod will reveal iridescent spots. When infections are heavy,
the entire animal will turn deep violet, purple or blue. An isopod thus colored can be
assured of having an iridovirus infection. An isopod in mid-Cretaceous Myanmar
amber with iridescent blue body regions provides indirect evidence of an ancient
strain of the isopod iridescent virus (Poinar 2014c). This is another example of
identifying fossil viruses using indirect, but reliable evidence based on detailed
studies of extant cases.
2.3 Fossil Pathogenic Bacteria
While reports of fossil bacteria are not rare, the ubiquitous presence of saprophytic
bacteria makes it difficult to separate the latter from pathogenic or symbiotic bacteria unless other evidence is available (e.g., the location of the infected cells and the
condition of the host). Again, there is direct and indirect evidence of the presence of
fossil pathogenic bacteria. Direct evidence involves finding fossil bacteria closely
associated with infected animal and plant tissue while indirect evidence is based on
signs or symptoms indicating that a possible bacterial infection is present—based
on extant investigations, when physical evidence of bacteria is lacking.
2.3.1 Direct Evidence of Fossil Pathogenic Bacteria
There are several examples of direct evidence of pathogenic bacteria in amber. One
is a Bacillus sporangium inside the pseudocoel of the fossil mycetophagous nematode, Oligaphelenchus atrebora (Fig. 2.15) in Mexican amber. This bacterium was
most likely a parasite that developed in the body cavity of the nematode (Poinar
1977). Members of Bacillus sp. are known to infect present day nematodes
(Dollfus 1946).
Another example of direct fossil evidence involves insect-parasitic heterorhabditid
nematodes. Several infective stages of Proheterorhabditis burmanicus were
preserved as they emerged from their beetle host in mid-Cretaceous Myanmar
amber. Extant species of the related genus Heterorhabditis carry and release spores
of the luminescent bacterium, Photorhabdus luminescens, into the body cavity of
insects. The bacteria multiply and kill the insects within 24 h. The nematodes then
feed on the combined bacteria and decomposing host tissue and during development, newly formed infective stage juvenile nematodes acquire bacterial cells in the
lumen of their alimentary tract. These juvenile nematodes then introduce the bacteria into new developmental hosts to continue the cycle. Bacterial cells that had
emerged from an infected fossil beetle parasitized by Proheterorhabditis burmanicus are adjacent to the infected host (Poinar 2011) (Fig. 2.16).
G. Poinar
occur, the body of the isopod will reveal iridescent spots. When infections are heavy,
the entire animal will turn deep violet, purple or blue. An isopod thus colored can be
assured of having an iridovirus infection. An isopod in mid-Cretaceous Myanmar
amber with iridescent blue body regions provides indirect evidence of an ancient
strain of the isopod iridescent virus (Poinar 2014c). This is another example of
identifying fossil viruses using indirect, but reliable evidence based on detailed
studies of extant cases.
2.3 Fossil Pathogenic Bacteria
While reports of fossil bacteria are not rare, the ubiquitous presence of saprophytic
bacteria makes it difficult to separate the latter from pathogenic or symbiotic bacteria unless other evidence is available (e.g., the location of the infected cells and the
condition of the host). Again, there is direct and indirect evidence of the presence of
fossil pathogenic bacteria. Direct evidence involves finding fossil bacteria closely
associated with infected animal and plant tissue while indirect evidence is based on
signs or symptoms indicating that a possible bacterial infection is present—based
on extant investigations, when physical evidence of bacteria is lacking.
2.3.1 Direct Evidence of Fossil Pathogenic Bacteria
There are several examples of direct evidence of pathogenic bacteria in amber. One
is a Bacillus sporangium inside the pseudocoel of the fossil mycetophagous nematode, Oligaphelenchus atrebora (Fig. 2.15) in Mexican amber. This bacterium was
most likely a parasite that developed in the body cavity of the nematode (Poinar
1977). Members of Bacillus sp. are known to infect present day nematodes
(Dollfus 1946).
Another example of direct fossil evidence involves insect-parasitic heterorhabditid
nematodes. Several infective stages of Proheterorhabditis burmanicus were
preserved as they emerged from their beetle host in mid-Cretaceous Myanmar
amber. Extant species of the related genus Heterorhabditis carry and release spores
of the luminescent bacterium, Photorhabdus luminescens, into the body cavity of
insects. The bacteria multiply and kill the insects within 24 h. The nematodes then
feed on the combined bacteria and decomposing host tissue and during development, newly formed infective stage juvenile nematodes acquire bacterial cells in the
lumen of their alimentary tract. These juvenile nematodes then introduce the bacteria into new developmental hosts to continue the cycle. Bacterial cells that had
emerged from an infected fossil beetle parasitized by Proheterorhabditis burmanicus are adjacent to the infected host (Poinar 2011) (Fig. 2.16).
G. Poinar