New Terminology

Developmental Concepts of Morphological Characters

The developmental concepts presented on this page were generated from exhaustive comparative studies of fungal isolates of selected species in all genera grown in pot cultures. Only recently have these data provided a pattern which is predictable for all species encountered in the collection. This pattern is being used to construct a model universal for all Glomeromycota which defines the level of resolution of each discrete morphological character. The model is complex, and so it will not be explained in its entirety until all the details have been published in the primary literature. Publications of this work done at INVAM are:

The universal pattern for differentiation of subcellular structures in glomalean spores is distinctly hierarchical, in that there are structures that are made up of component parts and each component part has unique phenotypic properties. It is this hierarchy which starkly contrasts with previous taxonomic concepts. Once it is understood, however, interpretations of taxonomic structures are more intuitive and predictable. The focus on this page is morphological properties of spores to “keep it simple” and because a majority of the evolutionary conserved features are present therein.

1. Spore Wall

Each spore, regardless of species, produces one spore wall which originates from the fertile hypha on which the spore is borne and either remains continuous with the hyphal wall or becomes detached (sessile). This spore wall grows and differentiation's mostly while the spore is expanding in size. When the spore ceases to expand, then all layers of the spore wall have formed, and they may continue to differentiate properties such as color, rigidity, and some thickening to a limited extent. The spore wall as a single entity does not exist in taxonomic concepts now generally accepted. Below are photographs depicting the continuity between layers of the hyphal wall and that of the spore wall. In Glomus, the link between the hypha and spore wall is most obvious during spore development because both hyphal and spore walls differentiate together and at the same rate (see Morton, 1996; Stürmer and Morton, 1997).

Various Spore Walls

A. Layers of the Spore Wall

Within each spore wall, two or more (although rarely more than four) layers are synthesized (see labels in photos above). Position is the most important information amongst layers (or where is one located relative to all others present). In the pages on this website, therefore, these layers are numbered from outermost to innermost (L1, L2, L3, etc.). Layers are not defined by the order in which they are formed (although most develop from outer to inner surface of the spore) because developmental studies should not need to be conducted by anyone trying to make an identification or to describe a new species. Some of the more commonly distributed layers are synonymous with various “wall” types of present-day taxonomic concepts; others are new and have been included in descriptions but not formally defined as discrete “walls”.

Question: What distinguishes a wall from a layer?

Answer: A wall has a unique origin in the development of the spore, whereas layers represent component parts that share a common origin. The distinction is determined empirically by temporal and spatial appearance of each structure relative to the one before it and the one after it in developmental comparisons by the taxonomist defining the species.

B. Properties of Each Layer in the Spore Wall

It is here that developmental concepts also deviate from the established paradigm. Phenotypes represent divergent properties of layers and represent most of the evolutionary innovation that has occurred. The range of phenotypes is much less predictable than the number of layers, and so we are likely to find new phenotypes as additional species are found. There is prediction, however, among some layers with certain phenotypes with regard to their position in the spore wall and the extent to which they are permanent or degrade. Structures previously termed “unit walls” are actually layers of the spore wall with many different phenotypes, such as mucilagenous, slate-like, granular, swollen in acid (= “expanding wall”) and so on. They also may be permanent or sloughing (= “evanescent wall”). A layer which forms few to numerous sublayers (common origin and of similar properties) as it grows and differentiates is synonymous with the “laminated wall”. We often refer to this layer as a “laminate layer” with a variable number of “sublayers” for continuity in terminology. In some species a thin layer on the inner surface of the spore wall separates, which because it is both thin and flexible (both traits are linked, understandably), resembles a flexible inner wall (called previously a “membranous wall”). Such a layer is present in species of most genera, although it is not always obvious. It is recognized as a part of the spore wall because it originates as part of the wall of the subtending hypha in all genera except Acaulospora and Entrophospora.

Various properties of spore wall layers recognized conventionally as "unit or evanescent walls".