Stickleback projects
These are the projects I am doing in Dolph Schluter's lab with the stickleback fish populations found all round coastal British Columbia.
Assortative mating between differentially adapted sticklebacks
Benthic (top) and limnetic (bottom) sticklebacks from Paxton Lake. Both are females, the benthic species feeds on invertebrates on the lake bed, whereas the limnetic eats plankton in the surface waters.
A male stickleback from the limnetic-like population in North Lake (on the Sechelt peninsular).
A female stickleback from the benthic-like population in Brannen Lake, Vancouver Island. Both fish are asleep after a brief exposure to Perrier.
Changes in mate preferences may be driven by natural selection, and these can lead to assortative mating between differentially adapted populations. For example, mate choice may be based on characters under divergent natural selection, or mate choice criteria may shift onto traits that are more conspicuous in that environment. In either case, potential mates from other environments will not fit the mate choice criteria of the local population, leading to assortative mating.
I am testing this idea with stickleback fish from British Columbia. Lake dwelling sticklebacks sometimes specialise either on eating plankton in the surface waters or on catching invertebrates on the lake bed. Both types coexist in several lakes around BC (see picture below), which suggests that adapting to either niche can also generate reproductive isolation.
Since assortative mating can evolve between coexisting species in response to maladaptive hybridisation, the high level of assortative mating between the benthic/limnetic species pairs might not be entirely driven by adaptation the their respective niches. I am therefore testing for assortative mating between solitary stickleback populations that are adapted to either the limnetic or the benthic niche.
Solitary populations are often reluctant to nest in tanks, so I allowed 3 benthic-like and 3 limnetic-like males to build their nests in 4m x 4m enclosures in the UBC artificial ponds. This setup also maximises the opportunities for mate choice, as both male and female sticklebacks can choose their mate. Because this precludes direct observation of matings, the mate choice is inferred from the location of the females eggs.
If adaptation can generate assortative mating, we expect significantly more than half of the matings to involve individuals from the same environment. If this is not the case, the effect of local adaptation must be negligible compared to other forces acting on mate choice criteria. We did actually find strong assortative mating, suggesting that mate preferences really do change as a by-product of adaptation to different environments.
Finding the genes for fitness
Researchers have now found a number of chromosome regions and even some genes responsible for the morphological differences between stickleback populations, but are these regions the most important in adapting to new environments. Kerry Marchinko and I are currently testing this idea by forcing a hybrid population between a lake and a marine population to readapt to freshwater in the UBC artificial ponds. Once the fish have overwintered and reproduced, we will see which parts of the genome are over-or uner-represented in both the surviving adults and their offspring. We will then check whether these regions are the same or different from those involved in phenotypic changes.
Does parallel phenotypic evolution use the same genes?
Does parallel speciation (the independent evolution of similar groups of species) involve divergence at the same genes? Some genes may be more ‘useable’ in evolution, perhaps because they have a high mutation rate or low levels of pleiotropy, and this should increase their probability of contributing to adaptive evolution. I have tried to test this idea by locating the major genomic regions responsible for the differences between stickleback species pairs (see above). In collaboration with [Katie Peichel]{3] and her lab at the Fred Hutchison Cancer Research Centre in Seattle, we have constructed a Quantitative Trait Locus map for the morphological differences between two of the species pairs (Priest Lake and Paxton Lake), and tested how many QTL are shared between the two maps.