Acaulospora splendida

Sieverd., Chaverri & I. Rojas

SPORES single in the soil; formed laterally on the neck of a sporiferous saccule; hyaline; smooth, glistening; globose to subglobose; 190-250 µm diam; ovoid, oblong, broadly ellipsoid, broadly rectangular or triangular with round edges to irregular; (147-)209-317 x (133-)157-264 µm.

SUBCELLULAR STRUCTURE OF SPORES consists of a spore wall and one inner germination wall.

In PVLG
In PVLG+Melzer's
Spore wall composed of two hyaline layers (swl1 and 2).

Layer 1 evanescent, 0.3-0.7 µm thick, continuous with the wall of the sporiferous saccule, always tightly adherent to layer 2 and difficult to observe.

Layer 2 laminate, 1-2 µm thick.

Germination wall composed of three hyaline layers (gwl1-3).

Layer 1 membranous, 0.2-0.5 µm thick, usually adherent to layer 2.

Layer 2 semi-flexible, 0.5-1.0 µm thick.

Layer 3 semi-flexible, 0.5-1.0 µm thick in water, 2-3(-6) µm thick in spores stored in formalin or lactophenol, occasionally separating from layer 2.

In Melzers' reagent, only layer 3 of the germination wall stains reddish-brown.

GERMINATION ORB. Not found.
In PVLG+Melzer's reagent

SPORIFEROUS SACCULE. According to Sieverding et al. (1988), the sporiferous saccules of A. splendida are globose, broadly ellipsoid or irregular, 160-300 x 150-200 µm and slightly constricted at the neck. The neck narrows towards the place of spore attachment where it is 25-52 µm wide. The wall of the sporiferous saccules is 0.3-0.7 µm thick. The sporiferous saccules of mature spores become translucent and may collapse, but usually remain attached to the spore.

 
In PVLG

CICATRIX. 6-11 µm diam, closed by the spore wall layer 2.

 

 

MYCORRHIZAE. Spores of A. splendida have been found associated with roots of Quercus costaricensis Liebm. and weeds of the families Asteraceae and Poaceae (Sieverding et al. 1988). Attempts to grow this fungus in one-species cultures failed.

DISTRIBUTION. The holotype of A. splendida comes from spores recovered from around roots of Q. costaricensis cultivated in a nursery of Conceptión de San Rafael, Heredia, Costa Rica. The only other so far known site of the occurrence of A. splendida is the Volcano la Malintzin, Tlaxcala, Mexico where this fungus has been associated with roots of Vicia faba L. (Estrada-Torres et al. 1992).

NOTES. The description of the morphological and biochemical properties of spores of A. splendida presented above was almost entirely prepared based on the original description of this fungus (Sieverding et al. 1988). The specimens of A. splendida (spores crushed in PVLG and PVLG + Melzer's reagent; slides no. 5075-5078) provided by Dr. E. Sieverding, Institute for Plant Production and Agroecology in the Tropic and Subtropics, University of Hohenheim, Germany, preserved exceptionally poorly and made impossible to verify most of the properties described. The only corrections incorporated here regard the phenotypic characters of the second spore wall layer and the second and the third layers of the germination wall. According to Sieverding et al. (1988), the second layer of the spore wall is unit, and its observation under high magnifications of the microscope showed it to consist of very thin sublayers and thus it is a laminate layer. Layers 2 and 3 of the germination wall have originally been considered unit. However, their marked elasticity (not fragility that distinguishes a unit layer) visible in the specimens examined suggest them to be of the type of semi-flexible layers.

Apart from A. splendida, other species of arbuscular fungi whose acaulosporioid spores are hyaline through at least a part of their life cycle are A. delicata, A. morrowiae, A. myriocarpa, A. nicolsonii, A. polonica, and Archaeospora trappei. Of them, spores of only A. nicolsonii are within the size range of spores of A. splendida (Walker et al. 1984). The mean size of spores of the other species is from 2.2-fold (A. delicata; Morton 2002) to 7.6-fold (A. myriocarpa; Schenck et al. 1986) lower than the mean size of spores of A. splendida. Additionally, spores of A. splendida always remain hyaline, similarly as those of A. myriocarpa and Ar. trappei (Błaszkowski 2003; Schenck et al. 1986), whereas spores of A. delicata, A. morrowiae, A. nicolsonii and A. polonica become pale yellow (Morton 2002), pale yellow (3A3; Błaszkowski 2003) to pale yellow-brown (Morton 2002), pale yellow brown (Walker et al. 1984), and white (1A1; Błaszkowski 2003), respectively, with age.

However, the most important differences between A. splendida and the other species listed above are hidden in the subcellular structure of their spores, as well as in the phenotypic and biochemical properties of the components of this structure. Considering the subcellular structure of spores, the species compared here may be divided into two groups. In the first group, there are A. splendida, A. myriocarpa, and Ar. trappei, whose subcellular structure of spores consists of a spore wall and only one inner germinal wall (Błaszkowski 2003; Morton 2002; Morton and Redecker 2001; Schenck et al. 1986; Sieverding et al. 1988). However, although the spore wall of the two species is 2-layered, the inner germinal wall of spores of A. splendida comprises three layers, and that of the two other species is 2-layered and none of them stains in Melzer's reagent. In contrast, the innermost layer of the germinal wall of spores of the species discussed here stains reddish brown in this reagent.

The fungi representing the second group are A. delicata, A. morrowiae, A. nicolsonii, and A. polonica. In contrast to spores of A. splendida having only one inner germinal wall (Sieverding et al. 1988), spores of the other three species have two such walls (Błaszkowski 2003; Morton 2002; Walker et al. 2007).

REFERENCES

Błaszkowski J. 2003. Arbuscular mycorrhizal fungi (Glomeromycota), Endogone, and Complexipes species deposited in the Department of Plant Pathology, University of Agriculture in Szczecin, Poland. http://www.agro.ar.szczecin.pl/~jblaszkowski.

Estrada-Torres A., Varela L., Hernandez-Cuevas L., Cavito M. E. 1992. Algunos hongos micorrizicos arbusculares del estado de Tlaxcala, México. Rev. Mex. Mic. 8, 85-110.

Morton J. B. 2002. International Culture Collection of (Vesicular) Arbuscular Mycorrhizal Fungi. West Virginia University: http://www.invam.caf.wvu.edu/.

Morton J. B., Redecker D. 2001. Two families of Glomales, Archaeosporaceae and Paraglomaceae, with two new genera Archaeospora and Paraglomus, based on concordant molecular and morphological characters. Mycologia 93, 181-195.

Schenck N. C., Spain J. L., Sieverding E. 1986. A new sporocarpic species of Acaulospora (Endogonaceae). Mycotaxon 25, 111-117.

Sieverding E., Chaverri A., Rojas I. 1988. Acaulospora splendida, a new species in the Endogonaceae from Cosa Rica. Mycotaxon 33, 251-256.

Walker C., Reed L. E., Sanders F. E. 1984. Acaulospora nicolsonii, a new endogonaceous species from Great Britain. Trans. Brit. Mycol. Soc. 83, 360-364.

Walker C., Vestberg M., Demircik F., Stockinger H., Saito M., Sawaki H., Nishmura I., Schüßler A. 2007. Molecular phylogeny and new taxa in the Archaeosporales (Glomeromycota): Ambispora fennica gen. sp. nov., Ambisporaceae fam. nov., and emendation of Archaeospora and Archaeosporaceae. Mycol. Res. 111, 137-13.