Utex—the culture collection of algae at the university of texas at austin 1993 list of cultures1

It has previously been proposed that sex is selected for in organisms living in antagonistic biotic environments. Here, the contrasting case of mutualistic biotic environments is considered. In these environments, it is argued that there should be selection against sex and an accompanying low rate of genetic change. This proposition is tested on mutualistic symbioses with substantial histories of co-evolution in which one partner, the inhabitant, lives partly or wholly inside the other, the exhabitant. This asymmetry between the partners means that inhabitants should exhibit a reduction in sex in comparison to related free-living taxa and a lower rate of genetic change than exhabitants.



The patterns that emerge from an analysis of 10 kinds of mutualistic symbiosis strongly support these predictions. Where adequate information is available, sex is usually reduced in inhabitants in comparison to related free-living taxa, although it remains widespread in exhabitants. Inhabitants are also represented by a much smaller taxonomic diversity than exhabitants. Various alternative reasons for these patterns are considered but it is concluded that they are best explained by the mutualistic environment in which the inhabitants live.

Biotic environments and the maintenance of sex–some evidence from mutualistic symbioses

Lichens, encompassing 20,000 known species, are symbioses between specialized fungi (mycobionts), mostly ascomycetes, and unicellular green algae or cyanobacteria (photobionts). Here we describe the first parallel genomic analysis of the mycobiont Cladonia grayi and of its green algal photobiont Asterochloris glomerata. We focus on genes/predicted proteins of potential symbiotic significance, sought by surveying proteins differentially activated during early stages of mycobiont and photobiont interaction in coculture, expanded or contracted protein families, and proteins with differential rates of evolution. A) In coculture, the fungus upregulated small secreted proteins, membrane transport proteins, signal transduction components, extracellular hydrolases and, notably, a ribitol transporter and an ammonium transporter, and the alga activated DNA metabolism, signal transduction, and expression of flagellar components. B) Expanded fungal protein families include heterokaryon incompatibility proteins, polyketide synthases, and a unique set of G-protein α subunit paralogs. Expanded algal protein families include carbohydrate active enzymes and a specific subclass of cytoplasmic carbonic anhydrases. The alga also appears to have acquired by horizontal gene transfer from prokaryotes novel archaeal ATPases and Desiccation-Related Proteins. Expanded in both symbionts are signal transduction components, ankyrin domain proteins and transcription factors involved in chromatin remodeling and stress responses. The fungal transportome is contracted, as are algal nitrate assimilation genes. C) In the mycobiont, slow-evolving proteins were enriched for components involved in protein translation, translocation and sorting. The surveyed genes affect stress resistance, signaling, genome reprogramming, nutritional and structural interactions. The alga carries many genes likely transferred horizontally through viruses, yet we found no evidence of inter-symbiont gene transfer. The presence in the photobiont of meiosis-specific genes supports the notion that sexual reproduction occurs in Asterochloris while they are free-living, a phenomenon with implications for the adaptability of lichens and the persistent autonomy of the symbionts. The diversity of the genes affecting the symbiosis suggests that lichens evolved by accretion of many scattered regulatory and structural changes rather than through introduction of a few key innovations. This predicts that paths to lichenization were variable in different phyla, which is consistent with the emerging consensus that ascolichens could have had a few independent origins.

/pdf/the-lichen-symbiosis-re-viewed-through-the-genomes-of-45gvedboag.pdf

The lichen symbiosis re-viewed through the genomes of Cladonia grayi and its algal partner Asterochloris glomerata

Lichenized fungi of the genus Lepraria lack ascomata and conidiomata, and symbionts codisperse by soredia. Here, it is determined whether algal symbionts associated with Lepraria are monophyletic, and whether fungal and algal phylogenies are congruent, both of which are indicative of a long-term, continuous association between symbionts. The internal transcribed spacer (ITS) and part of the actin type I locus were sequenced from algae associated with Lepraria, and the fungal ITS and mitochondrial small subunit (mtSSU) were sequenced from fungal symbionts. Phylogenetic analyses tested for monophyly of algal symbionts and congruence between algal and fungal phylogenies. Algae associated with Lepraria were not monophyletic, and identical algae associated with different Lepraria individuals and species. Algal and fungal phylogenies were not congruent, suggesting a lack of strict codiversification. This study suggests that associations between symbionts are not strictly maintained over evolutionary time. The ability to switch partners may provide benefits similar to genetic recombination, which may have helped this lineage persist.

http://www.symbiology.com/pdf/Nelsen264.pdf

Dissociation and horizontal transmission of codispersing lichen symbionts in the genus Lepraria (Lecanorales: Stereocaulaceae).

A comparative review of 58 Lepraria species and 2 varieties is provided together with species descriptions and a key. Lecanora leuckertiana is transferred to Lepraria. The following taxa are included: Lepraria achariana, L. adhaerens, L. alpina var. alpina, L. alpina var. zeorinica, L. atlantica, L. atrotomentosa, L. aurescens, L. bergensis, L. borealis, L. caesiella, L. caesioalba var. caesioalba, L. caesioalba var. groenlandica, L. celata, L. coriensis, L. crassissima, L. diffusa, L. eburnea, L. ecorticata, L. elobata, L. friabilis, L. gelida, L. goughensis, L. granulata, L. humida, L. impossibilis, L. incana, L. isidiata, L. jackii, L. lanata, L. lecanorica, L. leprolomopsis, L. leuckertiana, L. lobata, L. lobificans, L. membranacea, L. multiacida, L. neglecta, L. neojackii, L. nigrocincta, L. nivalis, L. normandinoides, L. nylanderiana, L. obtusatica, L. pallida, L. pulchra, L. rigidula, L. salazinica, L. santamonicae, L. santosii, L. sipmaniana, L. squamatica, L. straminea, L. sylvicola, L. texta, L. toensbergiana, L. toilenae, L. umbricola, L. usnica, L. vouauxii, L. xerophila. Additionally some previously published but unnamed taxa by different authors are discussed.

Taxonomic and ecologic problems in the genus Lepraria (Stereocaulaceae, lichenised Ascomycota)

The phycobionts of 20 different lichens were studied in culture, described and assigned to species of Pseudotrebouxia and Trebouxia . Identical phycobiont species were isolated from widely different lichens which showed that these algal symbionts were not specific to particular mycobionts. There was much variation in cellular detail within individual algal species indicating that the criteria currently used to separate species of these phycobionts may be too restrictive.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0024282981000078

A Study of Trebouxia and Pseudotrebouxia Isolates from Different Lichens

Zoosporogenesis was observed in the algal symbiont (Trebouxia gelatinosa) of Parmelia caperata. This is the first detailed report of this phenomenon in a Trebouxia species and, more importantly, in lichens, where zoosporogenesis in this alga is usually suppressed and the arrested zoospores form aplanospores. Observations of zoospore formation by algal cells within the natural thallus and in thallus fragments incubated on a mineral medium suggest that zoospores are released within thalli and may escape from the thallus to form free-living microcolonies. These colonies potentially could unite with hyphae derived from spores of either the same, or genetically different mycobionts (fungal symbionts) to establish new lichen associations, as has been shown in laboratory studies. Such natural resyntheses could be one cause for the observed heterogeneity characterizing fungi in many lichen populations. Other implications of free-living Trebouxia populations are discussed.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0024282980000151

Zoosporogenesis In Trebouxia Gelatinosa: Ultrastructure Potential For Zoospore Release And Implications For The Lichen Association

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A Study of Trebouxia and Pseudotrebouxia Isolates from Different Lichens

Zoosporogenesis In Trebouxia Gelatinosa: Ultrastructure Potential For Zoospore Release And Implications For The Lichen Association