Sleeping Beauties: Horizontal Transmission via Resting Spores of Species in the Entomophthoromycotina

Many of the almost 300 species of arthropod-pathogenic fungi in the Entomophthoromycotina (Zoopagomycota) are known for being quite host-specific and are able to cause epizootics. Most species produce two main types of spores, conidia and resting spores. Here, we present a review of the epizootiology of species of Entomophthoromycotina, focusing on their resting spores, and how this stage leads to horizontal transmission and persistence. Cadavers in which resting spores are produced can often be found in different locations than cadavers of the same host producing conidia. Resting spores generally are dormant directly after production and require specific conditions for germination. Fungal reproduction resulting from infections initiated by Entomophaga maimaiga resting spores can differ from reproduction resulting from conidial infections, although we do not know how commonly this occurs. Reservoirs of resting spores can germinate for variable lengths of time, including up to several months, providing primary infections to initiate secondary cycling based on conidial infections, and not all resting spores germinate every year. Molecular methods have been developed to improve environmental quantification of resting spores, which can exist at high titers after epizootics. Ecological studies of biological communities have demonstrated that this source of these spores providing primary inoculum in the environment can decrease not only because of germination, but also because of the activity of mycopathogens.

Induction of Conjugation and Zygospore Cell Wall Characteristics in the Alpine Spirogyra mirabilis (Zygnematophyceae, Charophyta): Advantage under Climate Change Scenarios?

Extreme environments, such as alpine habitats at high elevation, are increasingly exposed to man-made climate change. Zygnematophyceae thriving in these regions possess a special means of sexual reproduction, termed conjugation, leading to the formation of resistant zygospores. A field sample of Spirogyra with numerous conjugating stages was isolated and characterized by molecular phylogeny. We successfully induced sexual reproduction under laboratory conditions by a transfer to artificial pond water and increasing the light intensity to 184 µmol photons m-2 s-1. This, however was only possible in early spring, suggesting that the isolated cultures had an internal rhythm. The reproductive morphology was characterized by light- and transmission electron microscopy, and the latter allowed the detection of distinctly oriented microfibrils in the exo- and endospore, and an electron-dense mesospore. Glycan microarray profiling showed that Spirogyra cell walls are rich in major pectic and hemicellulosic polysaccharides, and immuno-fluorescence allowed the detection of arabinogalactan proteins (AGPs) and xyloglucan in the zygospore cell walls. Confocal RAMAN spectroscopy detected complex aromatic compounds, similar in their spectral signature to that of Lycopodium spores. These data support the idea that sexual reproduction in Zygnematophyceae, the sister lineage to land plants, might have played an important role in the process of terrestrialization.

Differentiation of Basidiobolus spp. Isolates: RFLP of a Diagnostic PCR Amplicon Matches Sequence-Based Classification and Growth Temperature Preferences

The genus Basidiobolus, known since 1886, is primarily associated with reptiles and amphibians. Although globally distributed, rare infections caused by members of this genus mainly occur in tropical and subtropical regions. Morphological and physiological characteristics were used in the past for the description of species. However, some of these characteristics vary depending on culture conditions. Therefore, most species names are regarded as synonyms of B. ranarum as the only pathogenic species. Yet, not all environmental isolates are necessarily pathogenic. This study aimed to analyze if environmental Basidiobolus isolates can be distinguished reliably based on morpho-physiological and molecular characteristics. Eleven isolates originally obtained from feces of south African reptiles and one type strain, Basidiobolus microsporus DSM 3120, were examined morpho-physiologically. Sequence analysis of the 18S and partial 28S rRNA gene and restriction analysis of a diagnostic amplicon (restriction fragment length polymorphism, RFLP) were performed for all 12 strains. Based on the results obtained, morphological features and the 18S rRNA sequence proved insufficient for the reliable differentiation of isolates. However, isolates were distinguishable by growth temperature profiles, which matched isolate clusters established by partial 28S rRNA gene sequence and restriction analysis of a Basidiobolus specific diagnostic PCR amplicon. Our results indicate that RFLP analysis can be used as a fast screening method to identify Basidiobolus isolates with similar physiological characteristics.

Diversity of Backusella (Mucoromycotina) in south-eastern Australia revealed through polyphasic taxonomy

Here we explore the diversity of one morphologically distinguishable genus in the Mucoromycotina, Backusella, in south-eastern Australia. We isolated more than 200 strains from locations across the states of Victoria and Tasmania. Characterization of these strains using a combination of approaches including morphology, sucrose utilization and whole genome sequencing for 13 strains, revealed 10 new species. The genetic basis for interspecies variation in sucrose utilization was found to be the presence of a gene encoding an invertase enzyme. The genus Backusella is revised and a new key for species identification produced. Given that we have more than doubled the number of species in this genus, this work demonstrates that there may be considerable undiscovered species diversity in the early diverging fungal lineages. Citation: Urquhart AS, Douch JK, Heafield TA, et al. 2021. Diversity of Backusella (Mucoromycotina) in south-eastern Australia revealed through polyphasic taxonomy. Persoonia 46: 1-25. https://doi.org/10.3767/persoonia.2021.46.01.

Aposymbiosis of a Burkholderiaceae-Related Endobacterium Impacts on Sexual Reproduction of Its Fungal Host

Bacterial endosymbionts inhabit diverse fungal lineages. Although the number of studies on bacteria is increasing, the mechanisms by which bacteria affect their fungal hosts remain unclear. We herein examined the homothallic isolate, Mortierella sugadairana YTM39, harboring a Burkholderiaceae-related endobacterium, which did not produce sexual spores. We successfully eliminated the bacterium from fungal isolates using ciprofloxacin treatment and asexual spore isolation for germinated asexual spores. Sexual spore formation by the fungus was restored by eliminating the bacterium from isolates. These results indicate that sexual reproduction by the fungus was inhibited by the bacterium. This is the first study on the sexual spore infertility of fungal hosts by endofungal bacteria.

Chlamydomonas reinhardtii, Volvox carteri and related green algae accumulate ketocarotenoids not in vegetative cells but in zygospores

Zygospores of green alga such as Chlamydomonas reinhardtii, Volvox carteri or Dunaliella salina display a bright orange color indicative of carotenoids, yet there have been no reports on their pigment composition. The genomes of these algae contain genes for homologs of the β-carotene ketolase (BKT) from the well-known astaxanthin producer Haematococcus pluvialis, that were assumed to be pseudogenes, because none of these species has been reported to accumulate astaxanthin or other ketocarotenoids. Here, we show that C. reinhardtii and V. carteri synthesize ketocarotenoids specifically in zygospores. Contrary to the vegetative aplanospores of H. pluvialis, the major ketocarotenoid in zygospores of C. reinhardtii is not astaxanthin but 4-ketolutein. Moreover, the ketocarotenoids in maturing zygospores are not synthesized de novo but from carotenoids of the photosynthetic apparatus liberated by a massive breakdown of thylakoid membranes. In line with this conclusion, incubation of zygospores at 9°C instead of 22°C resulted in a reduced thylakoid breakdown and only low amounts of ketocarotenoids, while the accumulation of storage lipids was less affected. Furthermore, we show the full-length BKT from C. reinhardtii to catalyze the ketolation of both α-carotene and lutein in carotenogenic bacteria. We also detected putative BKT genes in the genomes of various other green algae not yet known to synthesize ketocarotenoids, suggesting a zygospore-specific accumulation of ketocarotenoids to be common among Chlamydomonadales. Our observation that zygospores of C. reinhardtii accumulate ketocarotenoids together with storage lipids sheds light on the physiology of a largely unexplored algal life stage crucial for survival and propagation.