Genetic Analysis of Cytosolic PGI in Festuca ovina

Abstract: Festuca ovina, sheep's fescue is a widely distributed perennial outbreeder that belongs to the grass family Poaceae and is very common in the northern temperate zone. The genus Festuca s.l. is partitioned into fine-leaved fescues to which F. ovina belongs, and broad-leaved fescues to which the genus Lolium is a close relative. Wide crosses within the Festuca-Lolium complex often yield fertile hybrids. My thesis describes a genetic analysis of the variation for cytosolic phosphoglucose isomerase (PGI) in Festuca ovina. The investigation was initiated since an isozyme study demonstrated that some plants had multibanded electrophoretic patterns that proved genetically uninterpretable. Chromosome analysis showed that the plants were not polyploid, but diploid with the standard number of 14 chromosomes. Classical genetic analysis based on artificial crosses revealed that a second PGI locus segregated in south-Swedish populations of F. ovina. The second locus (PgiC2) and the standard locus (PgiC1) assorted independently, i.e. were not linked. Sequence data revealed that the two loci were highly diverged. Moreover, PgiC2 proved to be more complex than PgiC1, since all three PgiC2 variants analysed were composed of two genes. One of these genes may be a pseudogene. The results demonstrate that PgiC2 is not a simple, duplicated version of PgiC1. The sequences present today at these two loci must have started to diverge a long time ago, perhaps as far back as the time of the differentiation of the genus. I suggest that PgiC2 entered F. ovina recently via an introgression event from another species in the Festuca-Lolium complex, where it normally functions as the "standard" locus for cytosolic PGI. This model is supported by the fact that alleles from PgiC1 and PgiC2 produce enzyme subunits that easily function together despite their wide sequence divergence. Thus, my thesis brings into focus a possible but not much discussed way for eukaryot organisms to add new genes to their genomes.