Conservation genetics – molecular detectives at work
Box 3 | Molecular detectives at work
Researchers at Macquarie University set out to compare the similarity of the genetic makeup of the tammar wallaby populations on Kangaroo Island and Kawau Island. Genetic theory tells us that closely related animals are genetically more similar than those that are more distantly related. One way of determining genetic similarity is by comparing the DNA from different individuals (Box 1).
The technique used by the research team involved taking DNA samples from the animals and searching for segments that contained small sequences of base pairs that are repeated numerous times. These segments are called microsatellite markers. Markers that have a different number of repeat sequences are called alleles – alternative forms of the same genetic sequence.
The investigators took DNA samples from about 30 individuals in both the Kawau and Kangaroo Island populations. Back in the laboratory, they looked at the DNA of these individuals for sequences that would be useful as markers to distinguish between the two populations. Once these were obtained, the researchers used a technique called polymerase chain reaction (PCR), in conjunction with electrophoresis, to genotype – or determine the alleles of – individual wallabies.
Polymerase chain reaction
Scientists use PCR to make multiple copies of DNA markers so that they have sufficient quantities with which to work. The first step is to make primers – these have a sequence of bases that is complementary to the base pairs at either end of the marker. The DNA is separated into its two strands and the primers are allowed to bind to these complementary sequences. An enzyme (called polymerase) is then added, which helps a chemical reaction take place in which the marker is replicated many times – a process called amplification.
Millions of copies of the DNA marker are produced in this way. These are then used in the next step, electrophoresis.
Electrophoresis
The technology associated with genetics is changing rapidly – a technique used widely 5 years ago might well be outdated today. But electrophoresis remains one of the most tried and true techniques. It involves placing the amplified DNA markers onto a gel (a thin slice of jelly-like material). The gel is then attached to electrodes so that an electric current flows through it. Since the DNA markers have a negative charge, they migrate along the gel towards the positive electrode. After a number of hours, the researcher removes the electrodes and analyses the position of the markers. Shorter markers (those with the base sequence repeated fewer times) move faster than longer ones and travel further in the same time, so the markers are separated by size.
By comparing the distance travelled by the markers, conservation geneticists can detect differences in their length. Using a DNA sample from each wallaby, they can determine the alleles of each animal. From this, they can estimate the genetic differences between individual animals and between populations.
In the case of the tammar wallaby, the results of this procedure were exciting. Of the alleles located in the Kangaroo Island population, 37 were absent from the Kawau Island population, suggesting that the two populations were quite distinct genetically and, therefore, that the Kawau Island population had originated on the mainland of Australia.
Boxes
Box 1. What is genetics?
Box 2. The problem of small populations
Related sites
Microsatellites identify introduced New Zealand tammar wallabies (Macropus eugenii) as an 'extinct' taxon (Animal Conservation, UK)
The
polymerase chain reaction (University of Sheffield, UK)
Polymerase
chain reaction – xeroxing DNA (Access Excellence, USA)
Polymerase chain
reaction (PCR) (National Human Genome Research Institute, USA)
Use
of PCR in forensic science (Graphics Gallery, Access Excellence,
USA)
Page updated October 2004.