Haplomitrium

Abstract: Some group II introns in the organelle genomes of plants and algae are disrupted and require trans-splicing of the affected exons from independent transcripts. A peculiar mitochondrial nad5 gene structure is universally conserved in flowering plants where two trans-splicing introns frame a tiny exon of only 22 nucleotides, and two additional conventional group II introns interrupt the nad5 reading frame at other sites. These four introns are absent in the liverwort Marchantia polymorpha, which carries a group I intron at an unrelated site in nad5. To determine how intron gains and losses have sculptured mitochondrial gene structures in early land-plant evolution, we have investigated the full nad5 gene structures in the three bryophyte classes and the fern Asplenium nidus. We find the single Marchantia group I intron nad5i753 present as the only intervening sequence in both closely (Corsinia and Monoclea) and distantly related (Noteroclada, Bazzania, and Haplomitrium) liverwort genera. In a taxonomically wide spectrum of mosses (Sphagnum, Encalypta, Timmia, Ulota, and Rhacocarpus); however, we additionally identify the angiosperm-type group II introns nad5i230 and nad5i1455. The latter is a cis-arranged homolog to one of the two angiosperm trans-splicing introns, notably the first of its kind in mosses. In the hornwort Anthoceros, the ‘‘moss and liverwort–type’’ group I intron nad5i753 is absent, and, besides nad5i230 and nad5i1455, intron nad5i1477 is present as the second ancestral group II intron which has evolved into a trans-splicing arrangement in angiosperms. The influence of highly frequent RNA editing, most notably in the genera Haplomitrium, Anthoceros, and Asplenium, on phylogenetic tree construction is investigated and discussed. Taken together, the data (1) support a sister group relationship of liverworts as a whole to all other embryophytes, (2) indicate loss of a group I and serial entries of group II introns in the nad5 gene during early evolution of the nonliverwort lineage, and (3) propose a placement of hornworts as sister group to tracheophytes.

Abstract: Gene transfer from the mitochondrion into the nucleus is a corollary of the endosymbiont hypothesis. The frequent and independent transfer of genes for mitochondrial ribosomal proteins is well documented with many examples in angiosperms, whereas transfer of genes for components of the respiratory chain is a rarity. A notable exception is the nad7 gene, encoding subunit 7 of complex I, in the liverwort Marchantia polymorpha, which resides as a full-length, intron-carrying and transcribed, but nonspliced pseudogene in the chondriome, whereas its functional counterpart is nuclear encoded. To elucidate the patterns of pseudogene degeneration, we have investigated the mitochondrial nad7 locus in 12 other liverworts of broad phylogenetic distribution. We find that the mitochondrial nad7 gene is nonfunctional in 11 of them. However, the modes of pseudogene degeneration vary: whereas point mutations, accompanied by single-nucleotide indels, predominantly introduce stop codons into the reading frame in marchantiid liverworts, larger indels introduce frameshifts in the simple thalloid and leafy jungermanniid taxa. Most notably, however, the mitochondrial nad7 reading frame appears to be intact in the isolated liverwort genus Haplomitrium. Its functional expression is shown by cDNA analysis identifying typical RNA-editing events to reconstitute conserved codon identities and also confirming functional splicing of the 2 liverwort-specific group II introns. We interpret our results 1) to indicate the presence of a functional mitochondrial nad7 gene in the earliest land plants and strongly supporting a basal placement of Haplomitrium among the liverworts, 2) to indicate different modes of pseudogene degeneration and chondriome evolution in the later branching liverwort clades, 3) to suggest a surprisingly long maintenance of a nonfunctional gene in the presumed oldest group of land plants, and 4) to support the model of a secondary loss of RNA-editing activity in marchantiid liverworts.

Abstract: Summary • Haplomitrium, a primitive liverwort taxon with only remote affinities to other liverwort groups, develops root-like subterranean axes harbouring fungal endophytes. Here we report on the fungal association in H. gibbsiae and H. ovalifolium, using light and electron microscopy. • The epidermal cells of subterranean axes secrete abundant mucilage that harbours aseptate fungal hyphae. The fungus penetrates the epidermal cells and forms intracellular arbuscules invested by the host cytoplasm. Infection is restricted to epidermal cells in H. gibbsiae, whereas in H. ovalifolium the fungus also infects the cortical cells immediately adjacent, where it forms prominent swellings (‘lumps’). In H. gibbsiae similar fungal swellings are formed in the epidermal cells along with arbuscules. In both species the lumps undergo cytoplasmic degeneration and collapse, showing a shorter lifespan than the arbuscules. • The fungal infection in Haplomitrium presents affinities with symbiotic associations with glomeromycotean fungi in higher plants (arbuscular mycorrhizas) and thalloid liverworts. However, the pattern of fungal morphogenesis in Haplomitrium has no precedent in bryophytes nor in higher plants. • Considering the Glomeromycota as the most ancient lineage of mycorrhizal fungi, and Haplomitrium as basal in land plant phylogenies, the association described here may be the most primitive land plant–fungal symbiosis documented to date.