Biology Reference
In-Depth Information
Box 8.1 Amplifying Ancient DNA from Insects in Amber: Controversial Results?
Why the controversy? Is amber a special form of preservative that allows DNA to persist for unusually
long periods (millions of years)? Amber entombs insect specimens completely, after which they completely
dehydrate so the tissue is effectively mummified. The terpenoids, major constituents of amber, could
inhibit microbial decay (
Austin et al. 1997a
). Certainly, preservation of amber-embedded insects seems to
be exceptional and insect tissues in amber appear comparable in quality to the tissues of the frozen wooly
mammoth (which is “only” 50,000 years old). But is the DNA in these tissues preserved and can it be
amplified by the PCR?
Claims have been made that DNA can be extracted from insects in amber, including a fossil termite,
Mastotermes electrodominicus
, estimated to be 25-30 million years old (
DeSalle et al. 1992
); a 120- to
130-million-year-old conifer-feeding weevil (Coleoptera: Nemonychidae) (
Cano et al. 1993a
); and a 25- to
40-million-year-old bee (
Cano et al. 1993b
). These are extraordinary ages for DNA!
The DNA sequences obtained from all amber-preserved insects meet several, but not all, criteria of
authenticity; the fossil DNA sequences “make phylogenetic sense” and DNA has been isolated from more
than one specimen in several cases (although the weevil DNA was derived from a single specimen) (
Austin
1997a
).
Yet extraction and amplification of fossil DNA sequences from amber-preserved insects must be
reproduced in independent laboratories, to meet quality criteria. This has cast doubt on the authenticity
of the reports (
Austin 1997a,b; Sykes 1997; Walden and Robertson 1997; Gutierrez and Marin 1998;
Hofreiter et al. 2001
).
One of the most controversial claims involved the isolation of a “living” bacterium from the abdomen of
amber-entombed bee. Bacterial DNA from a 25-million-year-old bee was obtained and sequenced.
Cano
and Boruki (1995)
reported reviving a bacterial spore, culturing it, and identifying it. The classification of
the bacterium is controversial (
Beckenbach 1995, Priest 1995
) because the bacterium could have come
from a currently undescribed species of the
Bacillus sphaericus
complex. The modern
B. sphaericus
complex
is incompletely known, so the “ancient” sequence obtained could be that of a modern, but previously
unidentified, bacterium. These bacteria often are isolated from the soil (
Yousten and Rippere 1997
).
Other claims of amplifying ancient DNA have been disproved. For example, the mitochondrial cytochrome
b
sequence of an 80-million-year-old dinosaur from the Upper Cretaceous in Utah was discovered to be,
most probably, of human origin (
Hedges and Schweitzer 1995
). Likewise, a 20-million-year-old magnolia
leaf produced sequences that were similar to those of modern magnolias. The authenticity of the magnolia
sequences were cast into doubt because they were exposed to water and oxygen during preservation and DNA
is especially vulnerable to degradation under such conditions. The jury is out on the authenticity of ancient
DNA in insects embedded in amber. Fortunately, the scientific criteria for resolving the controversy are clear.
High-throughput sequencing machines (NextGen sequencing) work well with small DNA fragments in
ancient bones, hair, and teeth. Next-generation sequencing has resulted in the entire genomes of an
ancient cave bear, a mammoth, and the Neanderthal (
Gibbons 2010
).
difficulties are enormous (
Rollo 1998, Cooper and Poinar 2000
). Theoretical
considerations indicate that maximal DNA survival of 50 thousand to 1 million
years could be possible (
Hebsgaard et al. 2005
). In fact, most DNA decays rel-
atively rapidly and amplifiable DNA rarely is found in fossils one hundred to
a few thousand years old, and these are rarely longer than 100bp unless the
cadaver has been frozen.
