Lena Ghatnekar

The introgression of a functional nuclear gene from Poa to Festuca ovina.

In sheeps fescue, Festuca ovina, genes coding for the cytosolic enzyme phosphoglucose isomerase, PGIC, are not only found at the standard locus, PgiC1, but also at a segregating second locus, PgiC2. We have used PCR-based sequencing to characterize the molecular structure and evolution of five PgiC1 and three PgiC2 alleles in F. ovina. The three PgiC2 alleles were complex in that they carried two gene copies: either two active genes or one active and one pseudogene. All the PgiC2 sequences were very similar to each other but highly diverged from the five PgiC1 sequences. We also sequenced PgiC genes from several other grass species. Phylogenetic analysis of these sequences indicates that PgiC2 has introgressed into F. ovina from the distant genus Poa. Such an introgression may, for example, follow from a non-standard fertilization with more than one pollen grain, or a direct horizontal gene transfer mediated by a plant virus.

Selfing and outcrossing but no apomixis in two natural populations of diploid Potentilla argentea

It has been suggested that diploid Potentilla argentea, hoary cinquefoil, is a facultative apomict. We have investigated the way such plants produce seeds under natural conditions in two populations from southern Sweden. About one hundred plants were studied cytometrically to ascertain their ploidy level. A standard population investigation based on two informative isozyme loci was then performed on the diploid plants. Finally, seeds were taken from heterozygous plants to determine the genetic constitution of their offspring. From the offspring analysis it followed that apomixis did not occur, at least not to any noticeable degree. From the population investigation it could be deduced that most seeds were produced by selfing.

Origin and timing of the horizontal transfer of a PgiC gene from Poa to Festuca ovina.

A segregating second locus, PgiC2, for the enzyme phosphoglucose isomerase (PGIC) is found in the grass sheeps fescue, Festuca ovina. We have earlier reported that a phylogenetic analysis indicates that PgiC2 has been horizontally transferred from the reproductively separated grass genus Poa. Here we extend our analysis to include intron and exon information on 27 PgiC sequences from 18 species representing five genera, and confirm our earlier finding. The origin of PgiC2 can be traced to a group of closely interrelated, polyploid and partially asexual Poa species. The sequence most similar to PgiC2 is found in Poa palustris with a divergence, based on synonymous substitutions, of only 0.67%. This value suggests that the transfer took place less than 600,000 years ago (late Pleistocene), at a time when most extant Poa and Festuca species already existed.

Structure of the Natural Transgene PgiC2 in the Common Grass Festuca ovina

Background A horizontal gene transfer has brought an active nuclear gene, PgiC2, from a polyploid Poa species (P. palustris or a close relative) into the common grass sheeps fescue (Festuca ovina). The donor and the receptor species are strictly reproductively separated, and PgiC2 occurs in a polymorphic state within F. ovina. The active gene copy is normally closely linked to a very similar pseudogene. Methodology/Principal Findings By genome walking we have obtained the up- and downstream sequences of PgiC2 and of corresponding genes in the donor and recipient species. Comparisons of these sequences show that the complete upstream region necessary for the genes expression is included in the transferred segment. About 1 kb upstream of PgiC2 a fragment with transposition associated properties has been found (TAF). It is present in P. palustris and its polyploid relatives, though not at the homologous position, and is absent from many other grasses, including non-transgenic F. ovina plants. It is possible that it is a part of a transposing element involved in getting the gene into a transferring agent and/or into the recipient chromosome. Conclusions/Significance The close similarity of the up- and downstream regions with the corresponding regions in P. palustris excludes all suggestions that PgiC2 is not a HGT but the result of a duplication within the F. ovina lineage. The small size of the genetic material transferred, the complex nature of the PgiC2 locus, and the associated fragment with transposition associated properties suggest that the horizontal transfer occurred via a vector and not via illegitimate pollination.

Geographic and molecular variation in a natural plant transgene

A PCR based survey of Festuca ovina plants from populations around the southern part of the Baltic Sea demonstrates both geographic and molecular variation in the enzyme gene PgiC2, horizontally transferred from a Poa-species. Our results show that PgiC2—a natural functional nuclear transgene—is not a local ephemeral phenomenon but is present in a very large number of individuals. We find also that its frequency is geographically variable and that it appears in more than one molecular form. The chloroplast variation in the region does not indicate any distinct subdivision due to different colonization routes after the last glaciation. Our data illustrate the geographic and molecular variation that may occur in natural populations with a polymorphic, unfixed transgene affected by diverse kinds of mutational and evolutionary processes.

A horizontally transferred nuclear gene is associated with microhabitat variation in a natural plant population.

Horizontal gene transfer involves the non-sexual interspecific transmission of genetic material. Even if they are initially functional, horizontally transferred genes are expected to deteriorate into non-expressed pseudogenes, unless they become adaptively relevant in the recipient organism. However, little is known about the distributions of natural transgenes within wild species or the adaptive significance of natural transgenes within wild populations. Here, we examine the distribution of a natural plant-to-plant nuclear transgene in relation to environmental variation within a wild population. Festuca ovina is polymorphic for an extra (second) expressed copy of the nuclear gene (PgiC) encoding cytosolic phosphoglucose isomerase, with the extra PgiC locus having been acquired horizontally from the distantly related grass genus Poa. We investigated variation at PgiC in samples of F. ovina from a fine-scale, repeating patchwork of grassland microhabitats, replicated within spatially separated sites. Even after accounting for spatial effects, the distributions of F. ovina individuals carrying the additional PgiC locus, and one of the enzyme products encoded by the locus, are significantly associated with fine-scale habitat variation. Our results suggest that the PgiC transgene contributes, together with the unlinked ‘native’ PgiC locus, to local adaptation to a fine-scale mosaic of edaphic and biotic grassland microhabitats.

A DNA marker for the duplicated cytosolic PGI genes in sheep's fescue ( Festuca ovina L.)

Active duplicate PgiC genes in sheeps fescue, Festuca ovina, are associated with a PCR marker of specific length (about 370 bp, of which 231 are in an intron). Using this marker, the frequency of plants with duplicate genes is estimated to be about 10% in a population from southern Sweden. The close molecular similarity between the electrophoretically different duplicated genes is in accordance with the conclusion reached earlier that they are indeed alleles at the same locus.

Contrasting patterns of nucleotide polymorphism suggest different selective regimes within different parts of the PgiC1 gene in Festuca ovina L.

BackgroundPhosphoglucose isomerase (PGI, EC 5.3.1.9) is an essential metabolic enzyme in all eukaryotes. An earlier study of the PgiC1 gene, which encodes cytosolic PGI in the grass Festuca ovina L., revealed a marked difference in the levels of nucleotide polymorphism between the 5’ and 3’ portions of the gene.MethodsIn the present study, we characterized the sequence polymorphism in F. ovina PgiC1 in more detail and examined possible explanations for the non-uniform pattern of nucleotide polymorphism across the gene.ResultsOur study confirms that the two portions of the PgiC1 gene show substantially different levels of DNA polymorphism and also suggests that the peptide encoded by the 3’ portion of PgiC1 is functionally and structurally more important than that encoded by the 5’ portion. Although there was some evidence of purifying selection (dN/dS test) on the 5’ portion of the gene, the signature of purifying selection was considerably stronger on the 3’ portion of the gene (dN/dS and McDonald–Kreitman tests). Several tests support the action of balancing selection within the 5’ portion of the gene. Wall’s B and Q tests were significant only for the 5’ portion of the gene. There were also marked peaks of nucleotide diversity, Tajima’s D and the dN/dS ratio at or around a PgiC1 codon site (within the 5’ portion of the gene) that a previous study had suggested was subject to positive diversifying selection.ConclusionsOur results suggest that the two portions of the gene have been subject to different selective regimes. Purifying selection appears to have been the main force contributing to the relatively low level of polymorphism within the 3’ portion of the sequence. In contrast, it is possible that balancing selection has contributed to the maintenance of the polymorphism within the 5’ portion of the gene.