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opencc-zeroDec 2017View details →

Data from: Comparative genomics of the nonlegume Parasponia reveals insights into evolution of nitrogen-fixing rhizobium symbioses

Nodules harboring nitrogen-fixing rhizobia are a well-known trait of legumes, but nodules also occur in other plant lineages, with rhizobia or the actinomycete Frankia as microsymbiont. It is generally assumed that nodulation evolved independently multiple times. However, molecular-genetic support for this hypothesis is lacking, as the genetic changes underlying nodule evolution remain elusive. We conducted genetic and comparative genomics studies by using Parasponia species (Cannabaceae), the only nonlegumes that can establish nitrogen-fixing nodules with rhizobium. Intergeneric crosses between Parasponia andersonii and its nonnodulating relative Trema tomentosa demonstrated that nodule organogenesis, but not intracellular infection, is a dominant genetic trait. Comparative transcriptomics of P. andersonii and the legume Medicago truncatula revealed utilization of at least 290 orthologous symbiosis genes in nodules. Among these are key genes that, in legumes, are essential for nodulation, including NODULE INCEPTION (NIN) and RHIZOBIUM-DIRECTED POLAR GROWTH (RPG). Comparative analysis of genomes from three Parasponia species and related nonnodulating plant species show evidence of parallel loss in nonnodulating species of putative orthologs of NIN, RPG, and NOD FACTOR PERCEPTION. Parallel loss of these symbiosis genes indicates that these nonnodulating lineages lost the potential to nodulate. Taken together, our results challenge the view that nodulation evolved in parallel and raises the possibility that nodulation originated ∼100 Mya in a common ancestor of all nodulating plant species, but was subsequently lost in many descendant lineages. This will have profound implications for translational approaches aimed at engineering nitrogen-fixing nodules in crop plants.

opencc-zeroDec 2015View details →

Data from: Influence of oxidative homeostasis on bacterial density and cost of infection in Drosophila–Wolbachia symbioses

The evolution of symbioses along the continuum between parasitism and mutualism can be influenced by the oxidative homeostasis, i.e. the balance between reactive oxygen species (ROS) and antioxidant molecules. Indeed, ROS can contribute to the host immune defence to regulate symbiont populations, but are also toxic. This interplay between ROS and symbiosis is notably exemplified by recent results in arthropod-Wolbachia interactions. Wolbachia are symbiotic bacteria involved in a wide range of interactions with their arthropods hosts, from facultative, parasitic associations to obligatory, mutualistic ones. In the present study, we used Drosophila-Wolbachia associations to determine whether the oxidative homeostasis plays a role in explaining the differences between phenotypically distinct arthropod-Wolbachia symbioses. We used Drosophila lines with different Wolbachia infections and measured the effects of pro-oxidant (paraquat) and antioxidant (glutathione) treatments on the Wolbachia density and the host survival. We show that experimental manipulations of the oxidative homeostasis can reduce the cost of the infection through its effect on Wolbachia density. We discuss the implication of this result from an evolutionary perspective and argue that the oxidative homeostasis could underlie the evolution of tolerance and dependence on Wolbachia.

Figure 4 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 4 - Photobiont-mediated guilds in lichens (modified from Rikkinen 2003).The lichen-forming fungi belong to three different guilds, one centring around cyanobacterial symbionts (A), the second around green algal symbionts (B), and the third around another genus of green algal symbionts (C). The lichen in the middle of the picture houses both green algae and cyanobacteria (in cephalodia), meaning that its fungal symbiont can operate in two different guilds (C and A). As the symbiotic propagules of this lichen only contain the fungus and green algal photobiont, the fungus is a core species in guild C and a fringe species in guild A. Under certain conditions this fungus may give rise to cyanobacterial morphotypes (A4) and/or green algal thallus lobes. The core species of the lichen guilds produce innumerable symbiotic propagules, most of which will never develop into mature thalli of that lichen species. Germinating spores of fringe species (A1–A4, B1–B5) may commonly acquire their photobionts from small free-living populations that originate from disintegrating symbiotic propagules of the core species. At the latest when the thallus of a fringe species dies and disintegrates (A1), some of the photobionts are released back to the local environment for the common benefit of all fungi of the same guild. However, without the ability to produce symbiotic propagules, the fringe species cannot effectively disperse appropriate photobionts into new habitats. Some fringe species are aggressive enough to steal photobionts from juvenile stages or weakened thalli of other lichen species (A3), or live as lichenicolous lichens (B2) on other lichens of the same guild. The juvenile stages of some green algal lichens establish loose cyanotrophic associations with free-living cyanobacteria (B5) and/or cyanolichens (B4). Some lichenicolous fungi (B3) have evolved from lichen-forming ancestors and in many cases also their host ranges still appreciate guild boundaries.

Figure 1 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 1 - Bipartite and tripartite cyanolichens. A In the bipartite cyanolichen Peltigera scabrosa the cyanobacterial symbiont (Nostoc) forms a continuous layer just below the upper cortex of the lichen thallus B Nephroma bellum is another example of bipartite cyanolichens C In the tripartite cyanolichen Peltigera aphthosa the Nostoc symbiont is restricted to wart-like cephalodia (shown magnified) on the upper surface of the thallus, while the green algal symbiont (Coccomyxa) forms the photobiont layer D Nephroma arcticum is another example of tripartite cyanolichens. The large cephalodia of this species are internal, but clearly visible through the upper cortex of the hydrated thallus.

Figure 3 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 3 - Examples of cyanolichens examined in molecular studies of cyanobacterial diversity. A Cephalodial symbionts of the tripartite cyanolichen Lobaria pulmonaria remain poorly known (e.g. Rikkinen et al. 2002, Myllys et al. 2007) B Nostoc symbionts of Pseudocyphellaria species have been analyzed in several studies (e.g. Summefield et al. 2002, 2006, Rikkinen et al. 2002, Stenroos et al. 2006) C Peltigera venosa may have different Nostoc genotypes in different cephalodia (Paulsrud et al. 2000) D Nostoc symbionts of bipartite Peltigera species have been identified in many studies (e.g. Paulsrud and Lindblad 1998, O'Brien et al. 2005, Kaasalainen et al. 2012) E Otalora et al. (2010) analyzed genetic diversity of Nostoc in Collema and related cyanolichens F Many tropical Leptogium specimens were screened by Kaasalainen et al. (2012) G A Nostoc strain isolated from Pannaria pezizoides produces potent hepatotoxins in culture (Oksanen et al. 2004, Kaasalainen et al. 2009) H The cyanobacterial symbionts of Coccocarpia species are only distantly related to Nostoc (Lücking et al. 2009).

Figure 2 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 2 - Free-living and lichen-symbiotic Nostoc strains. A Free-living Nostoc colonies and gelatinous cyanolichens (Collema sp.) growing on mineral soil in northern Spain B Free-living Nostoc colony on limestone in northern Italy C Lichenized Collema thallus (containing Nostoc symbionts) on limestone in northern Italy D–F Morphological variation of Nostoc symbionts inside two Leptogium thalli in southwestern Kenya. The large clear cells are nitrogen-fixing heterocysts, the smaller translucent structures are fungal hyphae in optical cross-section.

Figure 6 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 6 - Plant hosts of symbiotic cyanobacteria; some of which can share Nostoc symbionts with cyanolichens. A The liverwort Blasia pusilla has Nostoc symbionts in auricles (Rikkinen and Virtanen 2008) B Hornworts house Nostoc symbionts in slime cavities (Costa et al. 2002) C The cyanobacterial symbiont of the water fern Azolla is not closely related to lichen symbiotic cyanobacteria (Ran et al. 2010) D All cycads associate with cyanobacteria, mainly Nostoc (Costa et al. 1999) E The cyanobacterial symbionts of cycads are housedin specialized roots (Costa et al. 2004, Gehringer et al. 2010, Yamada et al. 2012) F Gunnera species have endosymbiotic Nostoc in creeping rhizomes (Nilsson et al. 2000).

Figure 5 from: Rikkinen J (2013) Molecular studies on cyanobacterial diversity in lichen symbioses. MycoKeys 6: 3-32. https://doi.org/10.3897/mycokeys.6.3869

Figure 5 - Environments sampled in molecular studies of cyanobacterial diversity. A Arctic tundra on Svalbard; so far only two studies have included cyanolichen specimens from polar environments (Wirth et al. 2003, Kaasalainen et al. 2012) B Boreal forest in central Finland; cyanolichens from boreal forests have been examined in several studies (e.g. Paulsrud and Lindblad 1998, Myllys et al. 2007, Fedrowitz et al. 2011) C Temperate forest in western North America; also cyanolichens from temperate forests have been analyzed in several studies (e.g. Rikkinen et al. 2002, Summerfield et al. 2002, Fedrowitz et al. 2011) D Tropical montane forest in East Africa; so far only two studies have included cyanolichen specimens from tropical ecosystems (Lücking et al. 2009, Kaasalainen et al. 2012).

publicAug 2013View details →

Data from: Characterizing the plasticity of nitrogen metabolism by the host and symbionts of the hydrothermal vent chemoautotrophic symbioses Ridgeia piscesae

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publicOct 2013View details →

Data from: Effects of parasitism on aphid nutritional and protective symbioses

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publicMar 2016View details →

Data from: Influence of oxidative homeostasis on bacterial density and cost of infection in Drosophila–Wolbachia symbioses

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