Form versus function

FORM VERSUS FUNCTION, WITH A FOCUS ON LEVELS OF
TAXONOMIC RESOLUTION

With the revision in interpretation of morphological characters to reflect their origins and their relationships with neighboring characters, we now are able to more naturally define taxonomic groups in Glomales (plus some possible groupings that have yet to be formally recognized) within the context of relationships between form and function. These relationships are summarized in the table below, together with a hypothesis of "level of resolution" of each major character. Level of resolution refers to the level of the taxonomic hierarchy in which the character in question determines group membership (unique to the group and shared by all members of that group).

STRUCTURE FUNCTION and LEVEL OF RESOLUTION

INTRACELLULAR
ARBUSCULES Design, structural relationship with host plasma membrane, and biochemical interactions regulate exchange of carbon, energy, and nutrients between fungus and host plant. Despite considerable variation in structure, there are unique aspects of arbuscule architecture shared by members of each suborder (Glomineae and Gigasporineae).

INTRARADICAL
VESICLES Designed to compartmentalize and store energy-rich lipids during mycorrhizal development, possibly for maintenance and regrowth of the fungal organism after roots have ceased metabolic function. Evidence for infectiveness of these structures is questionable intraradical spores have been mistaken for vesicles). Found only in the suborder Glomineae. Structural properties of vesicles appear to be unique to each family in which they are formed (Glomaceae and Acaulosporaceae)

EXTRARADICAL
AUXILIARY CELLS Fragile cells which compartmentalize lipids. Their abundance before sporulation and decline thereafter suggests they provide carbon macromolecules independent of the host during formation of many large spores. There is no evidence these structures are infectious. Found only in the suborder Gigasporineae, family Gigasporaceae. Surface ornamentation patterns are unique to each genus (Gigaspora and Scutellospora).

INTRARADICAL
HYPHAE Provides the means for the fungal organism to establish "infection units" in roots, source for arbuscules (offshoots) and vesicles (terminal or more rarely intercalary). Signficant differences in branching architecture, size, structure (projections, swellings, etc.) are evident between suborders and also between families in each suborder.

EXTRARADICAL
HYPHAE Exists in various forms with possible different functions: (i) absorbative hyphae colonizing rhizosphere soil, (ii) runner hyphae growing along root surfaces to establish new entry points, and (iii) fertile hyphae forming new spores. Too little comparative evidence to determine level of resolution. Unique patterns are predicted to occur at the family level or above (given the distribution of other conserved vegetative structures).

ASEXUAL
SPORES Designed for long-term survival in soil or roots; as vehicles for short and long range dispersal. As important, they are equivalent to somatic germ cells by germinating independent of the parent and establishing individuals in a new mycorrhiza. Mode of formation is unique to each family and to each genus (Acaulospora and Entrophospora) in the family Acaulosporaceae.

SPORE
WALL Probably important in spore survival in soil and in facilitating short or long-range dispersal. Outer layers may be important in facilitating interactions with other microbes and microfauna. Each layer (two or more usually present) may have a distinct phenotype (color, thickness, ornanmentation pattern). Spore size also is a function of the duration of spore wall differentiation. In all genera of Glomales, the unique properties of spore wall layers group discontinuous geographic populations into species.

FLEXIBLE GERMINAL
WALLS Linked directly to germination events in all genera that possess them. Conserved enough so that the same phenotypes are shared by groups of species that currently have no formal level of classification (perhaps subgenus?).

PREGERMINATION
STRUCTURE Provides the structural base from which the germ tube can grow and penetrate walls and layers to emerge outside the spore. Differences most evident between each family with the exception of Gigasporaceae, where unique differences occur between each genus (Gigaspora and Scutellospora).

We emphasize here that the functional roles of characters may increase usefulness of interpreting a taxonomy within an ecological context, but functional properties rarely can be used in classification for several reasons.

They usually vary with ecological circumstances and thus are not stable enough to providing consistent grouping criteria.

They often are the result of convergent evolution, especially when genetically different organisms occupy the same niche (as is the case for all taxonomic groups of arbuscular mycorrhizal fungi).

STRUCTURE	FUNCTION and LEVEL OF RESOLUTION
INTRACELLULAR ARBUSCULES	Design, structural relationship with host plasma membrane, and biochemical interactions regulate exchange of carbon, energy, and nutrients between fungus and host plant. Despite considerable variation in structure, there are unique aspects of arbuscule architecture shared by members of each suborder (Glomineae and Gigasporineae).
INTRARADICAL VESICLES	Designed to compartmentalize and store energy-rich lipids during mycorrhizal development, possibly for maintenance and regrowth of the fungal organism after roots have ceased metabolic function. Evidence for infectiveness of these structures is questionable intraradical spores have been mistaken for vesicles). Found only in the suborder Glomineae. Structural properties of vesicles appear to be unique to each family in which they are formed (Glomaceae and Acaulosporaceae)
EXTRARADICAL AUXILIARY CELLS	Fragile cells which compartmentalize lipids. Their abundance before sporulation and decline thereafter suggests they provide carbon macromolecules independent of the host during formation of many large spores. There is no evidence these structures are infectious. Found only in the suborder Gigasporineae, family Gigasporaceae. Surface ornamentation patterns are unique to each genus (Gigaspora and Scutellospora).
INTRARADICAL HYPHAE	Provides the means for the fungal organism to establish "infection units" in roots, source for arbuscules (offshoots) and vesicles (terminal or more rarely intercalary). Signficant differences in branching architecture, size, structure (projections, swellings, etc.) are evident between suborders and also between families in each suborder.
EXTRARADICAL HYPHAE	Exists in various forms with possible different functions: (i) absorbative hyphae colonizing rhizosphere soil, (ii) runner hyphae growing along root surfaces to establish new entry points, and (iii) fertile hyphae forming new spores. Too little comparative evidence to determine level of resolution. Unique patterns are predicted to occur at the family level or above (given the distribution of other conserved vegetative structures).
ASEXUAL SPORES	Designed for long-term survival in soil or roots; as vehicles for short and long range dispersal. As important, they are equivalent to somatic germ cells by germinating independent of the parent and establishing individuals in a new mycorrhiza. Mode of formation is unique to each family and to each genus (Acaulospora and Entrophospora) in the family Acaulosporaceae.
SPORE WALL	Probably important in spore survival in soil and in facilitating short or long-range dispersal. Outer layers may be important in facilitating interactions with other microbes and microfauna. Each layer (two or more usually present) may have a distinct phenotype (color, thickness, ornanmentation pattern). Spore size also is a function of the duration of spore wall differentiation. In all genera of Glomales, the unique properties of spore wall layers group discontinuous geographic populations into species.
FLEXIBLE GERMINAL WALLS	Linked directly to germination events in all genera that possess them. Conserved enough so that the same phenotypes are shared by groups of species that currently have no formal level of classification (perhaps subgenus?).
PREGERMINATION STRUCTURE	Provides the structural base from which the germ tube can grow and penetrate walls and layers to emerge outside the spore. Differences most evident between each family with the exception of Gigasporaceae, where unique differences occur between each genus (Gigaspora and Scutellospora).