We selected three contrasting S. pupula agg. species in which to investigate the extent of gene flow, which will reflect the effectiveness of dispersal between ponds or lakes within and between countries. The three species we chose possess different breeding systems and so will enable us to assess the extent to which breeding system affects the genetic structure of diatom populations. Two of these three S. pupula species were described as separate species by Mann et al. (2004)
- S. capitata – a heterothallic diatom, i.e. within this species, just as within humans, there are two sexes, essentially male and female.
- S. pupula “elliptical” – a homothallic diatom, i.e. it likes to mate with itself! Genetic recombination is therefore less than in a heterothallic species.
- S. blackfordensis – a diatom with a mating system in some ways intermediate between S. capitata and “elliptical”, i.e. it prefers to mate with a cell of the opposite sex, but can also mate intraclonally.
With the help of Ecogenics, a Swiss company and Dr Satoshi Nagai in Japan, we have so far developed microsatellite markers for S. capitata and S. blackfordensis. Microsatellites are repetitive regions of “junk” DNA (e.g. CACACACACACA) that are found in all organisms. The length of the repeat can vary between individuals of a species and so can act as part of a fingerprint to distinguish one individual from another. Indeed, microsatellite markers are used to determine paternity in humans.
Developing microsatellite markers can be hard work, but is usually worth the effort, because they provide a lot more information than sequencing genes, which are much more conserved, i.e. similar between individuals. Microsatellites can inform us of the degree of gene flow between populations of diatoms and hence the extent to which diatoms are dispersed around the world (e.g. by birds or blown by the wind). So far we have found S. capitata in ponds in Scotland, England, Belgium and Australia, but S. blackfordensis only in Scotland; we need to work harder to find it elsewhere! We are almost ready to start conducting large-scale studies of the genetic structure of diatom populations.