Morton et al. 1995

Terminology by Morton, Bentivenga and Bever (1995)

The terminology proposed by Morton et al. (1995) is a reflection of several studies on comparative spore ontogeny in several genera within the Glomeromycota, considering that morphology can be defined empirically by patterns and processes during development. These studies were performed to (1) determine the discrete phenotypic structure of spores based on their spatial and temporal origin, (2) define the states of transformation of these structures from their origin to maturity, and (3) relationships with neighboring structures.

These studies resulted in the proposal of a spore development model that demonstrates that the differentiation and transformation of subcellular characters follows a hierarchically ordered sequence (primary, secondary and tertiary characters). The primary characters refer to the spore wall, germinative walls and pre-germination structures. The secondary characters refer to the distinct layers (which correspond to the walls in the terminology of Walker, 1983) differentiated in the spore wall and in the germinal walls and the characteristics present in the germination structures (color, shape, margin). Finally, the tertiary characters refer to the characteristics of each layer, such as color, plasticity, thickness, reaction to Melzer, type and dimension of ornamentation, etc.

Evidence from ontogenetic studies to date (Franke & Morton 1994, Stürmer & Morton 1997, Stürmer & Morton 1998, Bentivenga & Morton 1995, among others) indicates that this model is universal within the Glomeromycota. Hierarchical development implies the presence of restrictions in the morphological variation of characters. The model also indicates that the differences between species reside mainly in the characteristics found in the spore wall. For example, the morphological differences between Gigaspora species are due to the color and size of the spore, characteristics that are associated with the spore wall. Another example is the genus Racocetra, whose species share a single germinal wall with two layers and the difference between them lies in the presence or absence of ornamentation, type of ornamentation, color and size of the spore, etc.

Thus, this terminology consider the following characters forming the AMF spores:

1) SPORE WALL: structure originated from the fertile hypha and differentiated during spore growth. Made up of 2 to 4 layers that differentiate properties such as color, rigidity and thickness, reaction to Melzer, etc.

2) GERMINAL WALLS: discrete structure formed by layers that are synthesized after the spore wall has differentiated and the spore has reached its final size. Formed by 2 or 3 layers that differentiate properties such as color, rigidity, thickness, reaction to Melzer, etc.

3) GERMINATION STRUCTURE: structures formed as a requirement for the formation of the germ tube. It differs in design and position according to each family.

The terminology presented by Morton et al. (1995) is based on a relevant biological process, namely, the ontogeny of spores and based on comparative studies of different genera within the phylum. This way it is adopted for the description of the species in the pages of the CICG.

References

Bentivenga, S.P. & Morton, J.B. 1995. A monograph of the genus Gigaspora, incorporating developmental patterns of morphological characters. Mycologia 87: 720-732.

Franke, M. & Morton, J.B. 1994. Ontogenetic comparisons of the endomycorrhizal fungi Scutellospora heterogama and Scutellospora pelllucida: revision of taxonomic character concepts, species description, and phylogenetic hypothesis. Can. J. Bot. 72:122-134.

Morton, J.B., Bentivenga, S.P. & Bever, J.D. 1995. Discovery, measurement, and interpretation of diversity in arbuscular mycorrhizal fungi (Glomales, Zygomycetes). Can. J. Bot 73:

Stürmer, S. L. & Morton, J. B. 1997. Developmental patterns defining morphological characters in spores of four species in Glomus. Mycologia 89:72-8

Stürmer, S. L. & Morton, J. B. 1999. Taxonomic reinterpretation of morphological characters in Acaulosporaceae (Glomales) based on developmental patterns in two Acaulospora and one Entrophospora species.. Mycologia 91:849-857.