In 1997, Michael C. SCHOLL started taking the biopsy samples of Great White Sharks at Dyer Island. This project was lead by Les R. NOBLE, Catherine S. JONES and Amanda T. PARDINI at the University of Aberdeen in Scotland, and soon resulted in the first paper in Molecular Ecology in June 2000 «Isolation and characterization of dinucleotide microsatellite loci in the great white shark, Cacharodon carcharias».
Soon many other collaborators around the world joined the project, and samples from Australia, New Zealand and South Africa were all pooled together to analyse inter-continental variations. In July 2001, Nature published the results of this extensive collaboration orchestrated by the team at the University of Aberdeen: «Sex biased dispersal of great white sharks»: dispersal of great white sharks is sex-biased, with philopatric (non-roving) females and roving males.

This project has thus shed some light on one important aspect of the biology of the Great White Shark. A lot more can be learned when more samples are analysed from these regions, but especially when other areas are added to the pool of data.

Sampling of tissue samples is continuing in South Africa at Dyer Island by the White Shark Trust, in Mossel Bay by Marine and Coastal Management, and by the Natal Sharks Board in Durban.

Read a little more about the molecular side of this project:

The relatively new era of genetic analysis of natural populations has enabled biologists to investigate a field which could previously only be answered by extensive behavioural observation. Some genetic markers have proven to be useful: mtDNA (mitochondrial DNA), Major Histocompatibility Complex loci, allozyme loci, and Variable Number of Tandem Repeats (VNTR). VNTR markers are characterised by a core sequence which consists of a number of identical repeated sequences. They can be divided into two categories based on the repeat length: minisatellite with 15-70 base pairs (bp) and microsatellite with 2-6 bp. Microsatellites are increasingly used as the marker of choice for a variety of population genetic studies.

Currently, minisatellites - sometimes called DNA fingerprints - are used to identify individuals from forensic samples. The DNA is cut into fragments, separated by size on a gel, and transferred to a filter, which is probed with a radioactive labelled sequence complementary to the minisatellite repeat to produce ladder of fragments represented by a series of bands on a piece of photographic film placed over the filter. Unlike microsatellite, however, minisatellites are sometimes clustered in partcular regions of the genome and this may reduce the amount of information obtained from them. Because microsatellites are widespread in the eukaryotic genome, this problem is lessened once primers for the flanking regions are found.

Microsatellites, or SSRS - Simple Sequence Repeats - are tandemly repeated and highly polymorphic for length, providing a number of alleles in any one population, DNA sequences which represent a source of markers for gene linkage, mapping and individual identification - or a genetic tag. Microsatellites most commonly take the form of dinucleotide repeats such as [CA]n, but can also be tri- and tetra-nucleotide repeat sequences that appear to be non-coding. Estimated to occur every 20 to 30Kb on average they are highly variable - hypervariable - and relatively easy to detect. Because they are non-functional, they are not subjected to strong selection so the presence or absence of a particular marker would not affect an individual’s survival. Microsatellites have been traditionally classified according to the nature of the repeat as perfect (without interruptions), imperfect (with one or more interruptions) or compound (adjacent tandem repeats of a different sequence).

Microsatellites are useful for a number of analyses. They were originally utilised for gene mapping and have been extensively used in linkage analyses in the association with disease susceptibility genes. In addition they have proven useful in the analysis of paternity and kinship and in the probability of sample identity at both the individual and population levels, and in the study of entire populations and hybridisation of closely related species. Comparison of levels of variation between species and populations have also proven useful in the assessment of overall genetic variation. They can be used to estimate effective population size and to gain insight into the degree of population substructure including both the amount of migration between subpopulations and genetic relationships among the various subpopulations.

Knowledge of population structure is essential for the conservation of genetic resources and for an adequate analysis of population dynamics. For natural populations under exploitation, an incorect interpretation of the genetic structure can lead to over-exploitation and to the erosion of genetic resources via depletion of some, or all, of the population’s spawning components. This problem is particularly relevant to marine fisheries, which for political or administrative convenience are often managed under assumptions of single, large, homogeneous breeding populations. If indeed there are separate breeding components, parameters such as population growth rate and fishing mortality should be estimated separately for each component. When the assumptions of homogeneous and large breeding populations are not met and there are number of discrete or semidescrete spawning components within management units, those components most easily captured are likely to be eliminated, often with detrimental effects on the stock as a whole.

The contrast between mitochondrial DNA and microsatellite DNA analysis in resolving population structure may be related to the fact that mtDNA behaves as a single locus owing to the lack of recombination, whereas many independent loci can be analyzed using microsatellite DNA. In addition, microsatellite loci can show extremely high levels of allelic variation. Furthermore, microsatellite loci are codominant markers inherited in a Mendelian fashion, in contrast to the haploid mitochondrial DNA, which is predominantly maternally inherited. The former can be tested for Hardy-Weinberg expectations if sample sizes are larger than currently expedient to deal with, and can thus provide additional information about population structure. The difference in the mode of inheritance between mitochondrial and microsatellite DNA suggests that the combination of the two techniques may be very powerful in resolving questions of population structure for species with differential migration or dispersal rates between sexes.