Aneura mirabilis is a parasitic liverwort that exploits an existing mycorrhizal association between a basidiomycete and a host tree. This unusual liverwort is the only known parasitic seedless land plant with a completely nonphotosynthetic life history. The complete plastid genome of A. mirabilis was sequenced to examine the effect of its nonphotosynthetic life history on plastid genome content. Using a partial genomic fosmid library approach, the genome was sequenced and shown to be 108,007 bp with a structure typical of green plant plastids. Comparisons were made with the plastid genome of Marchantia polymorpha, the only other liverwort plastid sequence available. All ndh genes are either absent or pseudogenes. Five of 15 psb genes are pseudogenes, as are 2 of 6 psa genes and 2 of 6 pet genes. Pseudogenes of cysA, cysT, ccsA, and ycf3 were also detected. The remaining complement of genes present in M. polymorpha is present in the plastid of A. mirabilis with intact open reading frames. All pseudogenes and gene losses co-occur with losses detected in the plastid of the parasitic angiosperm Epifagus virginiana, though the latter has functional gene losses not found in A. mirabilis. The plastid genome sequence of A. mirabilis represents only the second liverwort, and first mycoheterotroph, to have its plastid genome sequenced. We observed a pattern of genome evolution congruent with functional gene losses in parasitic angiosperms but suggest that its plastid genome represents a genome in the early stages of decay following the relaxation of selection pressures.

Abstract:The plastid genome (plastome) is a rich source of phylogenetic and other comparative data in plants. Most land plants possess a plastome of similar structure. However, in a major group of plants, the ferns, a unique plastome structure has evolved. The gene order in ferns has been explained by a series of genomic inversions relative to the plastome organization of seed plants. Here, we examine for the first time the structure of the plastome across fern phylogeny. We used a PCR-based strategy to map and partially sequence plastomes. We found that a pair of partially overlapping inversions in the region of the inverted repeat occurred in the common ancestor of most ferns. However, the ancestral (seed plant) structure is still found in early diverging branches leading to the osmundoid and filmy fern lineages. We found that a second pair of overlapping inversions occurred on a branch leading to the core leptosporangiates. We also found that the unique placement of the gene matK in ferns (lacking a flanking intron) is not a result of a large-scale inversion, as previously thought. This is because the intron loss maps to an earlier point on the phylogeny than the nearby inversion. We speculate on why inversions may occur in pairs and what this may mean for the dynamics of plastome evolution.Key words: Osmunda, Gleichenia, Lygodium, Vandenboschia, Dicksonia, Marsilea, Adiantum, genome evolution, inversion.Résumé : Le génome plastidique (plastome) est une riche source de données phylogénétiques et autres chez les plantes. La plupart des plantes terrestres possèdent un plastome de structure semblable. Cependant, au sein d'un groupe majeur de plantes, les fougères, a évolué un plastome à la structure unique. L'ordre des gènes chez les fougères a été expliqué par une série d'inversions génomiques par rapport à l'organisation du plastome rencontré chez les plantes à graines. Dans ce travail, les auteurs examinent pour la première fois la structure du plastome à travers tout le groupe des fougères. Les auteurs emploient une stratégie PCR pour cartographier et partiellement séquencer les plastomes. Les auteurs ont observé qu'une paire d'inversions se chevauchant partiellement dans la région inversée répétée était présente chez l'ancêtre commun à la majorité des fougères. Cependant, la structure ancestrale (des plantes à graines) est encore observée chez les premières branches à diverger du tronc, lesquelles mènent aux osmundacées et aux hymenophyllacées. Une seconde paire d'inversions chevauchantes est présente dans l'embranchement qui mène aux principales fougères leptosporangiées. Les auteurs ont également trouvé que la position unique du gène matK chez les fougères (dépourvu d'un intron flanquant) n'est pas le résultat d'une inversion de grande taille comme cela a été suggéré antérieurement. Elle découle plutôt du fait que la perte de l'intron serait survenue plus tôt dans la phylogénie que l'inversion voisine. Les auteurs spéculent sur les causes possibles de l'occurrence des inversions en paires et ce que cela s...

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Locoism is a toxic syndrome of livestock caused by the ingestion of a subset of legumes known as locoweeds endemic to arid and semiarid regions of the western United States. Locoweeds contain the toxic alkaloid swainsonine, which is produced by the endophytic fungi Undifilum species. Two chemotypes of plants can coexist within toxic populations of locoweeds: chemotype 1 plants are defined as individuals containing swainsonine concentrations greater than 0.01% and quantitatively greater amounts of Undifilum, while chemotype 2 plants are defined as individuals containing less than 0.01% swainsonine and quantitatively smaller amounts of Undifilum. To elucidate the mechanisms that govern chemotypes, the amount of Undifilum in seeds/embryos was manipulated, thus altering subsequent swainsonine concentrations in three locoweed species: Astragalus mollissimus, Astragalus lentiginosus, and Oxytropis sericea. Chemotype 1 seeds that were fungicide-treated or had the seed coat removed resulted in plants with swainsonine concentrations comparable to those in chemotype 2 plants. Conversely, embryos from seeds of chemotypes 1 and 2 that were inoculated with the endophyte resulted in plants with swainsonine concentrations comparable to those of chemotype 1 plants. This reproducible interconversion between the two swainsonine chemotypes suggests that the quantity of endophyte present in the seed at the time of germination is a key determinant of the eventual chemotype. Additionally, this is the first report of the inoculation of locoweeds with the endophyte Undifilum species.