Although made up of an increasing number of species, the phylum Glomeromycota presents broad structural diversity as all 289 described species have phenotypic characters related to the glomerospores that differentiate them (Goto & Maia 2006; Goto 2009). However, even after recent advances, species identification remains a difficult task, requiring a lot of experience from the observer, as highlighted by Morton (1988) more than 20 years ago. Apparently this scenario has not changed much despite there being more professionals involved with the taxonomy and systematics of FMA.

Schenck & Pérez published, in 1990, a manual that compiled 147 species, with a key for identification. However, numerous descriptions of new species, genera, families, orders, and even classes have since become available (Blaszkowski 2012, Blaszkowski et al. 2012, 2014; Goto et al. 2012; Oehl et al. 2013, 2014). Furthermore, the keys included in the manual do not present, for many species, the structure of the spore wall, considered one of the most important morphological criteria for AMF identification (Morton 1988; Goto 2009).

Currently, the largest morphological databases, using studies from fungi kept in jars of live cultures, are made available by Morton on the International Culture Collection of Vesicular Arbuscular Mycorrhizal Fungi INVAM page (http://invam.caf.wvu.edu ). and by Blaszkowski ( http://www.zor.zut.edu.pl/Glomeromycota/ ). However, no identification key is available for consultation on these electronic pages.

In the literature, some restricted keys are found, dealing only with genera (Silva et al. 2005) or species referred to in ecological studies, or comparisons between new species and others, morphologically close (Tandy 1975; Nicolson & Schenck 1979; Gerdemann & Trappe 1973 ; Walker & Trappe 1981; missing broader keys, such as those formulated by Hall (1984), Schenck & Pérez (1990) and Blaszkowski (2012), which are outdated. The identification key published by Hall (1984) involves many genera that do not form arbuscular-type mycorrhiza, and basically uses criteria that group morphologically similar fungi. Recent efforts have been made by Janusz Blaszkowski and Fritz Oehl in several publications to reduce these taxonomic limitations, providing keys for identifying AMF species, but there are still no broad keys for this group of fungi that have grown in both new taxa and groups of research around the world.

Thus, problems in identifying AMF species persist, mainly due to advances in molecular techniques. However, we provide here AMF identification keys based exclusively on morphological characters related to glomerospores to try to reduce these taxonomic limits, especially for those research groups that do not have linked taxonomists.

However, we emphasize that although identification keys are good guides to direct species identifications, it does not exclude a partnership and confirmation of these species by a taxonomist.

ACAULOSPORA

All Acaulospora species are treated in detail in an identification key in: Lin TC, Silva GA, Oehl F. 2019. Acaulospora tsugae, a new species in the Glomeromycetes from Taiwan, and a key to species in Acaulosporaceae. Nova Hedwigia (in press).

ALBAHYPHA

1 – Spores reacting to melzer in the first evanescent layer....................................... .................................................... ..................................... A. walkeri

1' – Spores reacting to melzer in the third flexible layer............................................ .................................................... ....................................... A. drummondii

AMBISPORA (acaulosporoid synanomorph)

1 – Spores with ornamented wall................................................. .................................................... .................................................... ................................................. two

1' – Spores without decorated wall................................................. .................................................... .................................................... ................................................. 4

2 – Ornamentation on the external wall................................................. .................................................... .................................................... .................................................... .3

2'- Ornamentation on the middle wall................................................. .................................................... .................................................... .................................................... ..... 5

3 - Cerebriform ornamentation.................................................. .................................................... .................................................... ........................... A. jimgerdemannii

3'- Irregular pustular ornamentation................................................. .................................................... .................................................... ............................. A. brasiliensis

3'' – Warty ornamentation................................................. .................................................... .................................................... ....................................... A. granatensis

4 – Light yellow to brown spore................................................. .................................................... .................................................... .............................. A. gerdemannii

4'- White hyaline spore................................................. .................................................... .................................................... .................................................... ..... A. fennica

5 – Reticular ornamentation.................................................. .................................................... .................................................... .............................................. A. reticulata

5'- Ornamentation on the two layers of the middle and complementary walls (concave and convex)................................. ........................................ A. appendicula

ARCHAEOSPORA

1 – Acaulosporoid spores with only one scar and three layers on the wall................................................. .................................................... ................................... two

1' – Entrophosporoid spores with three layers in the wall....................................... .................................................... ............................... Intraspora schenckii

2 - Spores produced singly in the soil................................................. .................................................... .................................................... ...................................... 3

2'- Spores produced in aggregates................................................. .................................................... .................................................... ........................... A. myriocarpa

3 – Ornamented spores with large perforations on the surface................................................. .................................................... ..................................... A. undulata

3'- Spores without ornamentation on the external surface................................................. .................................................... .................................................... .......... A. trappei

BULBOSPORA/ORBISPORA

2 – Small, greenish-yellow spores (<100µm), reaction to melzer on the inner wall, acquiring a dark purple color...... B. minima

2' – Dark yellow to light brown spores (>100µm), reaction to Melzer on the inner wall, acquiring a reddish color....... O. Pernambucana

CETRASPORA

2 – Pale yellow hyaline spores................................................. .................................................... .................................................... .................................................... .. 5

2' – Pigmented spores................................................. .................................................... .................................................... .................................................... ................... 6

3 – Pigmented spores.............................................. .................................................... .................................................... ................................................ W. armeniaca

3' – Hyaline spores................................................. .................................................... .................................................... .................................................... ............................. 4

4 – Melzer reaction in the germinal component................................................. .................................................... .................................................... ............................. 7

4' – Melzer reaction in the structural and germinative component....................................... .................................................... ........................................... C. pellucida

5 – Spores decorated with small warts and a strong reaction to Melzer, acquiring a purple color................................. ..................... C. helvetica

5' – Spores decorated with buds (3.5 – 6.5µm high and 7.0 – 10µm wide)................................ .................................................... ............ C. nodosa

6 – Spores decorated with spines................................................. .................................................... .................................................... ......................... C. spinosissima

6' – Spores decorated with striations................................................. .................................................... .................................................... ................................. C. striata

7 – Dark yellow to golden yellow spores....................................... .................................................... .................................................... .................... C. auronigra

7' – Hyaline cream spores................................................. .................................................... .................................................... ................................................ W. gilmorei

CLAROIDEOGLOMUS

1 – Spores with four layers on the wall................................................. .................................................... .................................................... ......................................... two

1' – Spores with two or three layers on the wall....................................... .................................................... .................................................... ................................. 3

2 – Second and third layers of thick wall (1.3 – 6.3 µm and 5.0 µm) respectively................................ .................................................... .... C. luteum

2' – Second and third thin wall layers (0.6 – 2.0 µm and 3.8 µm) respectively................................ .................................................... .. C. claraideum

3 – Spores with three layers on the wall................................................. .................................................... .................................................... ..................... C. lamellosum

3' – Spores with two layers on the wall................................................. .................................................... .................................................... ............................................... 4

4 – Spores hyaline to opaque................................... ................................................ ................................................ ............................................. C. white

4' – Pigmented spores................................................. .................................................... .................................................... .................................................... ................... 5

5 – Light yellow to dark yellow spores and more intense reaction to Melzer, acquiring a frequent purple color................................. C. etunicatum

5' – Olive, dark spores and frank reaction to Melzer, acquiring a pinkish and infrequent coloration................................. ..................................... C. hanlinii

CORYMBIGLOMUS

1 – Spores with peridiospore................................................. .................................................... .................................................... .................................................... ............. two

1' - Spores peridiosporum............................................. ................................................ ................................................ ........................................... C. pacificum

2 - Spores with globose vesicles on the surface of the peridium....................................... .................................................... ............................................... C. globiferum

2'- Spores without vesicles................................................. .................................................... .................................................... .................................................... ..................... 3

3 – Spores with three layers making up the wall................................................. .................................................... .................................................... .. C. corymbiforme

3'- Spores with only one layer making up the wall....................................... .................................................... ................................................ W. tortuosum

teeth brushed

2 – Pigmented spores.............................................. .................................................... .................................................... .................................................... ................... 3

2' – Hyaline spores................................................. .................................................... .................................................... .................................................... ..... D. cerradensis

3 – Spores showing reticular ornamentation................................................. .................................................... .................................................... ........................... 4

3' – Spores without reticular ornamentation................................................. .................................................... .................................................... ............................................. 5

4 – Polygonal lattice with small spines inside the lattice....................................... .................................................... ............................... D. reticulata

4' – Globose to subglobose reticulum with ball-like projections inside the reticulum.................................. .................................................... .... D. nigra

5 – Spores with double ornamentation................................................. .................................................... .................................................... ............................... D. biornata

5' – Spores with only one ornamentation................................................. .................................................... .................................................... ....................................... 7

6 – Pigmented spores with a chafranulated wall layer....................................... .................................................... .......................... D. hawaiensis

6' – Large hyaline spores (>350μm). .................................................. .................................................. .................................. D. scuta

7 – Dark red spores with collicular ornamentation on the inner surface of the spore wall................................. ........................... D. colliculosa

7' – Light brown to dark brown spores with spiny ornamentation on the outer surface of the wall................................. .............. D. heterogamy

FUNNELIFORMIS

1 – Ornamented spores................................................. .................................................... .................................................... .................................................... ................ two

1' – Spores without ornamentation................................................. .................................................... .................................................... .................................................... ......... 5

2 – Spores decorated with projections................................................. .................................................... .................................................... .......................................... 3

2' – Spores with depressions................................................. .................................................... .................................................... .......................................... F. multiforum

3 – Ornamentation on the external surface of the second laminated layer of the spore wall................................................. .................................................... .............. 4

3' – Ornamentation on the inner surface of the third layer of the spore wall................................................. .................................................... ...... F. verruculosum

4 – Yellowish spores, decorated with spines and three layers composing the wall, external thick evanescent, second laminated and a third flexible hyaline...................... .................................................... .................................................... .................................................... ......................... F. halonatum

4' – Dark brown spores, decorated with equinulations and two layers composing the wall, external hyaline evanescent following the laminate................................ .................................................... .................................................... .................................................... ........................................... F. monosporus

5 – Spores with five layers making up the wall................................................. .................................................... .................................................... .......... F. caesaris

5' – Spores with two or three layers making up the wall................................................. .................................................... .................................................... ................. 6

6 – Spores with two layers making up the wall................................................. .................................................... .................................................... ...... F. coronatum

6' – Spores with three layers making up the wall................................................. .................................................... .................................................... .............................. 7

7 – Second layer of the laminated and pigmented wall................................................. .................................................... .................................................... .................... 8

7' – Second layer of the semi-persistent and hyaline wall............................................ .................................................... .................................................... ................. 10

8 – Second layer with perforations (spongy) in mature spores....................................... .................................................... ..................... F. kerguelense

8' – Spores without ornamentation................................................. .................................................... .................................................... .................................................... ......... 9

9 – Spores formed singly in the soil (ectocarpics) and the first layer of the evanescent wall reacting to Melzer, acquiring a pink color................................ .................................................... .................................................... .................................................... ............................................. F. geosporum

9' – Spores formed in aggregates in the soil and first layer of the evanescent wall without reaction to Melzer................................ ............ F. dimorphicus

10 – Spores formed in aggregates in the soil with the first and second layer of the wall reacting to Melzer, acquiring a pink color................................. .................................................... .................................................... .................................................... .................................................... . F. mosseae

10' – Spores formed singly in the soil with a laminated layer reacting to the Melzer reaction, acquiring a brownish-red color................................ .................................................... .................................................... .................................................... .............................. F. fragilistratum

FUCKED

1 – Spores with wall decorated with thorns....................................... .................................................... .................................................... .......................... two

2 – Yellowish spores with a reaction to Melzer on the external wall................................................. .................................................... ........................................... F. aurea

2' – Reddish brown spores without reaction to Melzer....................................... .................................................... ............................................. F. heterogamy

GIGASPORA

1 – Creamy white, yellow or greenish colored spores in water.................................. .................................................... .............................................. two

1' – Pinkish spores in water....................................... .................................................... .................................................... ................................. G. rosea

2 – Large spores (240 – 800 mm) with walls <14 mm thick................................. .................................................... .............................. G. gigantea

2' – Smaller spores (143 – 480 mm) and wall thickness > 14 mm................................ .................................................... .................................................... .. 3

3 – Thick spore wall reaching up to 45 mm thick and slightly flexible................................. .................................................... ...... G. decipiens

3' – Spore wall varying from 15 – 26 mm....................................... .................................................... .................................................... ............................................... 4

4 – Two or more suspensory bulbs attached to the spore....................................... .................................................... .......................................... G. ramisporophora

4' – Only a suspensory bulb attached to the spore....................................... .................................................... .................................................... .................................... 5

5 – Cream to green colored spores, best seen together under a magnifying glass.................................. .................................................... ..................... G. albida

5' – Creamy white to dark yellow spores, best seen together under a magnifying glass................................. .................................................... ... G. margarita

KAMIENSKI

1 – Small spores <30µm, two layers on the wall and reaction to melzer in the second layer................................. ........................................ K. perpusilla

1' – Larger spores >30µm, two layers without reaction to melzes................................. .................................................... ........................................ K. bistrata

KUKLOSPORA

2 – Spores decorated with depressions on the external surface................................................. .................................................... ............................... K. kentinensis

2'- Spores ornamented with spiny projections on the external surface....................................... .................................................... ..................... K. spinosa

PACISPORA

1' – Spores with smooth wall................................................. .................................................... .................................................... .................................................... ................. 4

2 – Ornamentation with small warts 3-5 µm....................................... .................................................... .................................................... ......... P. coralloidea

2' – Ornamentation with different projections of warts................................................. .................................................... .................................................... ................... 3

3 – Ornamentation with thin and long tubes (> 5 µm)................................... .................................................... ........................................... P. chimonobambusae

3' – Ornamentation with thin and short tubes (< 5 µm)................................................. .................................................... .................................................... ........... P. dominikii

3'' – Ornamentation with buttons of similar width and length............................................ .................................................... .............................. P. scintillans

3''' – Ornamentation similar to long pustules. .................................................. .................................................. .......... P. patagonica

4 – Pigmented spores.............................................. .................................................... .................................................... .................................................... .. P. robigina

4' – Hyaline spores................................................. .................................................... .................................................... .................................................... ..... Franciscan P.

PARADENTISCUTATA / INTRAORNATOSPORA

1 – Spores with an ornamented outer and inner surface of the spore wall.................................................. .................................................... .................................. two

1' – Spores only with an ornamented internal surface................................................. .................................................... ............................................... I. intraornata

2 – Spores decorated with warts densely distributed on the external surface....................................... .................................................... P. bahiana

2'- Spores decorated with papillae widely distributed on the external surface............................................ .................................................... ..... P. maritime

PARAGLOMS

2 - Hyaline to yellowish spores and small depressions (0.5-1.0 µm) in the second layer................................ ............................................... P. brasilianum

2'- Pigmented spores with wider depressions (1.5-4.0 µm) in the second layer................................ .......................................... P. Pernambucanum

3 - Spores with three layers making up the wall................................................. .................................................... .................................................... ............................... 4

3'- Spores with two layers making up the wall................................................. .................................................... .................................................... ........... P. laccatum

3 - Yellowish spores at maturity and laminated third layer ........................................ .................................................... ................................. P. occultum

4' - Hyaline spores at maturity and flexible third layer....................................... .................................................... ........................................... P. majewskii

RACOCETRA

2 – Ornamented with projections................................................. .................................................... .................................................... .................................................... ........ 3

2' – Ornamented with depressions................................................. .................................................... .................................................... .................................... R. undulata

3 – Black, opaque spores................................................. .................................................... .................................................... .................................................... .................. 4

3'- Yellow to reddish brown spores....................................... .................................................... .................................................... ..................................... 5

4 – Warty ornamentation with a blunt surface................................................. .................................................... .................................................... ........ R. gregaria

4' – Irregular ornamentation on a flat surface................................................. .................................................... .................................................... ............ R. coralloidea

5 – Warty ornamentation.................................................. .................................................... .................................................... .................................................... ............. 7

5' – Spiny ornamentation................................................. .................................................... .................................................... .................................................... ............ 6

6 – Thorns on the surface of the first layer of the wall................................................. .................................................... .................................................... ..... R. persica

6' – Spines originating in the second layer of the wall, perforated at the tip.................................. .................................................... .......................... R. draft

7 – Spores with small warts (0.5–1.5 × 0.5–1.5 µm)………………................................ .................................................... ........................................... R. verrucosa

7'- Spores with larger, irregular warts (0.9–3.4 × 0.9–2.5 µm)................................. .................................................... .................................. R. beninensis

8 – Hyaline to pinkish glomerospores................................................ .................................................. .................................................. .................................................. 9

8' - Pigmented glomerospores................................................. .................................................... .................................................... .................................................... ..... 10

9 – First layer of the evanescent and expanding wall................................................. .................................................... .................................................... R. alborosea

9' – First unitary layer................................................. .................................................... .................................................... ........................................ R. weresubiae

10 – Weak reaction to Melzer, spore acquiring a yellowish to orange color.................................................. .................................................... ........................... 11

10' – Reaction to Melzer dextrinoid, spore acquiring reddish color............................................ .................................................... ................ R. tropicana

10 – First layer of the wall easily detaching from the second layer, which is thick (10 – 35µm)................................. .................... R. castanea

10' – First layer of the wall strongly adhered to the second, which is generally thin (6-8 µm)................................. ................................................. R .

REDECKERA

1 – Spores with one or two layers on the wall....................................... .................................................... .................................................... ................................ two

1' - Spores with three or four layers on the wall....................................... .................................................... .................................................... ............................... 3

2 – Spores with only one layer on the wall................................................. .................................................... .................................................... ........ R. megalocarpa

2' – Spores with two layers on the wall................................................. .................................................... .................................................... ................................. R. fulva

3' - Spores with four layers on the wall................................................. .................................................... .................................................... ..................... R. avelingiae

SCLEROCYSTIS

1 – Peridium with sinuous hyphae arrangement................................................. .................................................... .................................................... ............................... S. winding

1' – Period without sinuous arrangement................................................. .................................................... .................................................... .................................... S. coremioides

SCUTELLOSPORA

2 – Ornamented spores with small tubular projections (0.5 - 1.0µm) and larger bacilliform columns (2 – 10µm)................................ ... S. dipapilosa

2' – Spores with rough papillae heterogeneously distributed on the external surface................................................. .................................................... .S. alterata

2'' - Spores with crenulata ornamentation, with sub-polygonal projections (tetra-penta-hexagonal) with a dome-shaped surface........ S. crenulata

3 – Reaction to Melzer in the structural and germinative component................................................. .................................................... .................................................... ........... 4

3' – Reaction to Melzer only in the germinal component................................................. .................................................... .................................................... .................. 5

4 – Structural component acquiring a yellowish to orange color................................................. .................................................... .................... S. calospora

4' – Structural component acquiring a dark reddish brown color................................................. .................................................... .............. S. arenicola

5 – Reaction to Melzer in the two layers of the germinal component....................................... .................................................... .................... S. dipurpurescens

5' – Reaction to Melzer only in the innermost layer of the germinal component................................................. .................................................... ......... S. aurigloba

SEPTOGLOMUS

(See also the identification key available in Oehl F., Sánchez-Castro I., Silva DKA, Palenzuela J., Silva GA (2019) Septoglomus nigrum, a new arbuscular mycorrhizal fungus from France, Germany and Switzerland. Nova Hedwigia 109 in press)

1 – Spores with two layers on the wall................................................. .................................................... .................................................... ............................................ two

2 – Light yellow to dark yellow spores....................................... .................................................... .................................................... .......................................... 3

2' – Dark, orange, dark brown or black spores....................................... .................................................... .................................................... ................. 4

3 – Spores reacting to Melzer in the evanescent layer................................................. .................................................... ............................................... S. jasnowskae

3' - Spores without reaction to Melzer....................................... .................................................... .................................................... ...................................... S. africanum

4 – Dark reddish brown to black spores....................................... .................................................... .................................................... .......................... 7

4' - Orange to dark brown spores....................................... .................................................... .................................................... ........................................... 8

5 – Spores with three layers on the wall................................................. .................................................... .................................................... .............................................. 6

5' - Spores with four layers on the wall................................................. .................................................... .................................................... ..................... S. turnauae

6 – Third laminated layer (5-11µm) pigmented dark brown to black................................. .................................................... ..................... S. furcatum

6' – Third laminated layer (12-19µm) pigmented dark orange to light brown................................. .................................................... ................... S. titan

6'' – Third laminated layer (0.7-2.2µm) pigmented light yellow to ochre................................. .................................................... .......................... S. xanthium

7 – Dark brown to black spores, thick evanescent layer (1-8.5µm) and dark brown to black laminated second layer....... S. constrictum

7' - Reddish brown to dark red spores, thin evanescent layer (2.5-3.0µm) and laminated orange to dark brown second layer...................... .................................................... .................................................... .................................................... .............................................. S. altomontanum

8 – Persistent outer layer................................................. .................................................... .................................................... .................................................... ......... 9

8' – Evanescent outer layer................................................. .................................................... .................................................... .................................. S. deserticola