Gigaspora margarita forms globose spores in the soil.Each spore is 260-480um in diameter, formed singly and terminally on subtending hypha, which are often septate below the suspensor-like cell in age (Becker and Hall 1976) though cytoplasmic streaming was observed between the suspensor-like cell and the spore body when young (Sward 1981a). Spore bearing suspensor-like cells are 27-58 um broad, thickening at the attachment point with the spore, with 1-5um thick cell walls and are hyaline to light brown and smooth. The walls of the spores are smooth and hyaline with 4-10 fused laminations each 1.5-4um thick giving the wall a total thickness of 5-24 um when hydrated. To age spores of G. margarita, generally the greater the number of laminations the greater the age of the spore. The internal structure of the spore consists of many small oil-like droplets, which coalesce near the germination region with age and generally appear white in color (Becker and Hall 1976). The outer wall of the spores was found to be absent of chitin and consist mainly of polysaccharides, lipids, and protein, which stained dark-purple or blue with Toluidine blue (Sward 1981a).
The closest species to resemble G. margarita is Gigaspora gilmorei (Becker and Hall 1976). Both have white azygospores and laminated spore walls while other species of the genus Gigaspora have colored azygospores and often with a single layered spore wall. For example: G. margarita is distinct from G. gilmorei by having white vesicles, rather than brown as in G. gilmorei. Additionally, G. gilmorei has a cell wall with five layers of non-uniform thickness, which easily separate, while G. margarita has up to ten layers, which do not readily separate when the spore is damaged. G. margarita is distinguished from G. calospora by having generally larger spores and laminated walls (Becker and Hall 1979).
Gigaspora margarita has been shown to enhance the growth of a number of plants by forming arbuscular mycorrhizal (AM) associations with plant roots. Additional growth enhancement by inoculation with G. margarita has been shown in tobacco (Csinos 1981) and cotton where it essentially reversed the stunting caused by the nematode Meloidogyne incognita (Roncadori and Hussey 1977). Additionally, it has been shown to form AM associations with corn (Zea maysL.), sudangrass (Sorghum sudanense(PIper) Staph.), and onion (Allium cepaL.). In the eudicots, it has been shown to form AM associations with soybean (Glycine maxL.) and tomato (Solanum lycopersicumL.) among many others (Becker and Hall 1976).
Gigaspora margarita is known to harbor an endosymbiotic bacteria. Originally classified in the genus Burkholderia (Bianciotto et al. 1996) the bacterium has now been officially described as the novel species ‘Cadidatus Glomeribacter gigasporarum’ (Bianciotto et al. 2003), this bacterium was first observed in the late-1970s to early-1980s, as a “type of unusual organelle” but recognized as a ‘bacteria-like organism’ that had been identified in other species in the then-termed Endgonaceae (Sward 1981a). Attempts were made to isolate and culture this bacterium outside of its host spore but were unsuccessful until relatively recently when Jargeat et al. (2004) developed a protocol to isolate and keep these bacteria alive for up to 4 weeks. Though the bacteria have been isolated and kept alive, efforts to culture these bacteria and have them reproduce remain unsuccessful. ‘Ca. Glomeribacter gigasporarum’ is a rod-shaped, gram-negative bacteria which contains no flagella or pili and numerous ribosomes (Jargeat et al. 2004). No isolates of G. margarita have been found without endosymbiotic bacteria populations in nature (Ruiz-Lozano and Bonfante 2001), however, isolates of G. margarita ‘cured’ of the bacteria have been generated in vitro (Alessandro Desiro, personal communication). It is unknown how this association came to be, however certain bacterial genes have been shown to be involved in host cell colonization. One such gene, vacB, was first isolated in Shigella flexneri (Tobe et al. 1992) and has been found in the Burkholderia strain that is symbiotic with G. margarita (Ruiz-Lozano and Bonfante 2000).
Bacteria of many different families have been shown to be associated with the spore surface of G. margarita more than the substrate within which these spores grow. These bacterial families, discovered through sequence and clustering analysis belong to two main phyla, the Proteobacteria and the Actinobacteria (Long et al. 2008).
While G. margarita does not reproduce sexually, diversity in the genome has been shown with multiple genomes present in the organism at one time. This is proposed to be due to the accumulation of mutations over time in individual nuclei, then segregation of these mutated nuclei in a single spore or many spores, leading to genetic variability over time (Kuhn et al. 2001).
The genome of G. margarita has yet to be sequenced, though the construction of a genomic DNA library was reported (Van Buuren et al. 1999). Additionally, the mitochondrial genome was sequenced using 454 and Illumina technologies and was the second AM fungi organellar genome to be published (Pelin et al. 2012).
Gigaspora margarita grows in a variety of soil types in arbuscular mycorrhizal association with a variety of plants including soybean, tomato, and onion (Becker and Hall 1979).
An identification key for this species was published by Becker and Hall (1976) at the time of discovery and is reproduced below:
Key to the species of Gigaspora
1a.) Azygospores white… (2)
1b.) Azygospores not white… (3)
2a.) Spore wall with distinct laminations of near equal width; spores germinate without forming peripheral compatments; vesicles white… G. margarita
2b.) Spore wall with inner and outer layers of unequal thickness; spores germinate from peripheral compartment; vesicles brown… G. gilmorei
3a.) Azygospores yellow, smooth…(4)
3b.) Azygospores brown, with warts or minute spines…(5)
4a.) Globose spores less than 300um in diameter, pale yellow; vesicles smooth to knobby, formed singly… G. calospora
4b.) Globose spores greater than 300um in diameter, bright yellow to greenish-yellow; vesicles echinulate, formed in clusters… G. gigantea
5a.) Globose spores greater than 300um in diameter, dark brown with hyaline warts; vesicles with coralloid projections… G. coralloides
5b.) Globose spores less than 300um in diameter, light brown with minute spines; vesicles smooth… G. heterogama
In 1981, the morphology of G. margarita spores and their development was the topic of a three part series of articles in the journal New Phytologist (Sward 1981a-c). Part one focused on the dormant spore morphology including the spore wall structure and composition, part two focused on the changes in morphology and structure as the spore starts to germinate while part three is focused on the emergence of the germ tube and its growth.
Using both light and electron microscopy, as well as cytological methods, Sward (1981a) showed that the composition of the lamination in the cell wall were not uniform, but were composed of four main layers: a chitinous layer, a ‘cementing’ layer, as well as an inner and outer protein/lipid/polysaccharide layer. Sward (1981b) used electron and light microscopy to show that surface sterilization with sodium hypochlorite can force germination. At the initiation of germination, a large redistribution of the cytoplasm occurs near the region where the germ tube will emerge. Sward (1981c) used light and electron microscopy to further follow the development of the germ tube and recognized that, given the differences in structure of the outer and inner wall, the germ tube seemed to utilize different strategies to penetrate these layers. It was also noted, unsurprisingly, that the primary cell wall is deposited at the apex of the germ tube as it grew out of the spore followed soon by deposition of the secondary wall.
The genus Gigaspora was once classified in the Endogonaceae by Gerdemann and Trappe (1974). In 1990 it was then moved into a new family, Gigasporaceae in the suborder Gigasporinae of the order Glomales (Morton and Benny 1990). The division Glomeromycota was then revised in 2004 by Walker and Schussler and Gigasporaceae was then placed in the order Diversisporales (Walker and Schussler 2004). G. margarita is now placed in the phylum Mucuromycota, subphylum Glomeromycotina, order Glomeromycetes, class Diversisporales, family Gigasporaceae, genus Gigaspora (Spatafora et al. 2016).
Taxonomy of Gigaspora spp. is distinguished by the morphology of both the spore (namely the spore wall structure) and how they form septa at the base of the newly derived spores (Khade 2011).
The type specimen of G. margarita was isolated by W.N. Becker on February 9, 1976 from the University of Illinois in Champaign County. It was discovered in a pot culture of soil from the Agronomy South Farm, and found in soil from field #1101 (at the time a soybean field) after this soil was planted with soybean and used to inoculate autoclaved soil (Becker and Hall 1976).
Gigaspora margaritahas been shown to form arbuscular mycorrhizal (AM) associations associations with a wide range of plant hosts. In the monocots, it has been shown to form AM associations with corn (Zea mays L.), sudangrass (Sorghum sudanense (PIper) Staph.), and onion (Allium cepa L.). In the eudicots, it has been shown to form AM associations with soybean (Glycine max L.) and tomato (Solanum lycopersicum L.).
Gigaspora margarita was first described as a new species in 1976 by W.N. Becker from the University of Illinois and I.R. Hall from the Invermay Agricultural Research Station, Mosgiel, New Zealand. It was identified in soil sample from a soybean fieldas white spores amongst a population of yellow-colored spores typical ofanotherGigasporaspecies,G. gigantea. Originally thought to be the immature stage of the yellow-colored G. gigantea spores, these white spores were soon discovered to be morphologically distinct from the spores of G. gigantea. Additionally, it was also discovered that the immature spores of G. gigantea were also yellow in color, further confirming G. margarita as a separate species with characteristic white spores. The specific epithet margarita- or 'pearl' - was chosen by Becker and Hall to distinguish this species taxonomically.
Gigaspora margaritawas first described in 1976 by W.N. Becker from the University of Illinois and I.R. Hall from the Invermay Agricultural Research Station, Mosgiel, New Zealand. It was identified in soil sample from a soybean fieldas white spores amongst a population of yellow-colored spores typical ofanotherGigasporaspecies,G. gigantea. Originally thought to be the immature stage of the yellow-colored G. gigantea spores, these white spores were soon discovered to be morphologically distinct from the spores of G. gigantea. Additionally, it was also discovered that the immature spores of G. gigantea were also yellow in color, further confirming G. margarita as a separate species with characteristic white spores. The specific epithet margarita- or 'pearl' - was chosen by Becker and Hall to distinguish this species taxonomically.Since its discovery, G. margarita has been the subject of a variety of studies spanning a number of disciplines.
