Evidence for adaptive variation at the genes coding for cytosolic phosphoglucose isomerase (PGIC) in Festuca ovina L

Sammanfattning: The gene (Pgi) encoding the enzyme phosphoglucose isomerase (PGI) which plays a central role in the main pathways of carbon metabolism has been shown to be of adaptive significance in a wide range of different species. Earlier studies of enzyme electromorph variation in the grass Festuca ovina suggest that variation in cytosolic PGI (PGIC) may be involved in the adaptive response of F. ovina to the alvar grassland microhabitat variation on the Baltic island of Öland, Sweden. PGIC is encoded by two genes in F. ovina: PgiC1 is “native” to F. ovina, whereas the PgiC2 gene has been horizontally acquired from the genus Poa. In the present thesis, different selection tests identified two codon (amino acid) sites (173, 200) as candidates for positive selection within the PgiC1 gene. Sites 173 and 200 being under positive selection is supported by the predicted PGI structural consequence of the observed amino acid changes at these two sites in F. ovina, and by the experimentally determined functional consequence of amino acid changes at several human PGI sites that have 3-D structural locations similar to those of the two selected sites in F. ovina. The fact that the single nucleotide polymorphism (SNP) and amino acid variation at the two selected sites is significantly associated with fine-scale microhabitat variation in the alvar grasslands provides further support for a scenario of positive selection on sites 173 and 200 in F. ovina. Weak balancing selection, possibly targeting codon sites 173 and 200 (located within the 5’ “portion” of PgiC1) may have contributed to the high level of polymorphism within this portion of the PgiC1 sequence, which is significantly more polymorphic than the 3’ portion of the sequence. The transgenic PgiC2 locus is not present in all F. ovina individuals, and the presence/absence of the functional version of this locus was also found to be associated with the fine-scale alvar grassland microhabitat variation. In summary, the present thesis represents the first study that has investigated the adaptive variation at more than one cytosolic Pgi gene within a single species, and makes F. ovina the most well-studied plant species for the Pgi adaptive variation in natural populations.