Ultrastructural characters

Septal pores

The sequence from „simple“ septal pores to highly elaborate dolipores in Basidiomycota reflects their main evolutionary lineages. Septal pores without rim-like swellings (dolipores) and without parenthesomes are only known from Puccinimycotina and certain Ustilaginomycotina. Dolipores without parenthesomes have evolved several times convergently. The most complex ultrastructure of septal pores is found in dolipores with various types of parenthesomes.

Fig. 14. Some basidiomycetous septal pores. Pucciniomycotina: a Cryptomycocolax (W = Woronin-like bodies), b Gymnosporangium. Ustilaginomycetes: c Doassansia, d Tilletia. Agaricomycotina: e Tremella, f Tulasnella, g Ceratobasidium, h Schizophyllum. In the background a lightmicroscopic photo of Uthatobasidium with a doliporous septum. TEM photos R. Bauer.

Evolutionary trends in basidiomycetous septal pores:

Septal pore simple > dolipore lacking parenthesomes > dolipore with parenthesomes

Parenthesomes lacking > tubulate

Parenthesomes continuous > perforate

Several basidiomycetous monophyla are well characterized by their septal pore structures. In the simple pored Pucciniomycotina, microbodies are typical for Cryptomycocolacales, Saccoblastiaceae, and Classiculomycetes, and cystosomes for Cystobasidiales. Agaricostilbomycetes and Pucciniomycetes are devoid of such bodies. In the Ustilaginomycotina dolipores without parenthesomes occur in the Tilletiales and Entorrhizales. In the Agaricomycotina dolipores with continuous parenthesomes are characteristic for the Dacrymycetales, Sebacinales, Auriculariales, and Tulasnellales. Continuous parenthesomes are also reported for Geastrales, Gomphales, and Trechisporales (van Driel et al. 2009). The mixed occurence of dolipores with either continuous parenthesomes or perforated ones in the Cantharellales and Hymenochaetales is confusing (van Driel et al. 2009). All higher evolved Agaricomycotina have dolipores with perforate parenthesomes. However, the documentation of septal pore types in Agaricomycotina is still fragmentary.

Spindlepole bodies

Spindelpole bodies (SPBs) are essential cell organelles, functionally involved in cell division. Only few studies have been carried out to clarify the ontogenetic cycles of SPBs. The data available from these investigations document considerable differences between Ascomycota and Basidiomycota (Fig. 15).

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Fig. 15. Left: Spindle pole cycle of Cryptomycocolax abnormis. Modified after Oberwinkler & Bauer (1990) and Oberwinkler (1995), TEM orig. R. Bauer. Right: Meiosis and pindle pole cycle of Ustilago maydis. Redrawn and strongly modified after O’Donnell & McLaughlin (1984a,b).

Differences of SPBs in Ascomycota and Basidiomycota and evolutionary trends

SPB disk-like > knob-like

SPB with middlepiece > without middlepiece

SPB position during nuclear division extranucleate > extranucleate – intranucleate – extranucleate

SPBs are microtubule-organizing centers, as basal bodies, nucleus-associated bodies and centrosomes. Bornens & Azimzadeh (2007) considered the connection of the flagellum and the nucleus as a prerequisite for the common origin of a centriole-based centrosome in the metazoa and the SPB in fungi.

Studying the ultrastructure of meiosis in the hollyhock rust fungus, Puccinia malvacearum, O’Donnell & McLaughlin (1981a,b,c), and in Ustilago maydis O’Donnell & McLaughlin (1984a,b), drew also attention to the SPB cycle (Fig. 15). Meiosis and SPB cycle have been analyzed in Pachnocybe ferruginea (Bauer & Oberwinkler 1990), in Cryptomycocolax abnormis (Oberwinkler & Bauer 1990), in Microbotryum violaceum (Berbee et al. 1991), in Sphacelotheca polygoni-serrulati (Bauer et al. 1991), and in Agaricostilbum pulcherrimum (Bauer et al. 1992). These studies contributed to ​​ the understanding that SPBs in basal clades of the Pucciniomycotina are disk-like, as in Ascomycota. In contrast, in derived taxa of the Puccinimycotina SPBs are subglobose, globose ones are typical for Ustilaginomycotina and Agaricomycotina.

Symplechosomes

Based on transmission electron microscopic studies of Saccoblastia farinacea, Bauer & Oberwinkler (1991b) reported on a unique cell organelle, the symplechosome (Fig. 16). Further investigations have shown that the symplechosome is typical for and restricted to the Atractiellomycetes (Bauer et al. 2006). Species of Helicogloea and Saccoblastia have resupinate basidiocarps, Atractiella and Phleogena contain stilboid fungi. The pycnidial members, Basidiopycnis hyalina and Proceropycnis pinicola, were introduced by Oberwinkler et al. (2006). – Since origin and function of symplechosomes are unknown and their structural composition is uniform, evolutionary trends cannot be discussed. – The Atractiellomycetes are mentioned here because they were reported as mycobionts of orchids recently (Kottke et al. 2009).

Fig. 16. Symplechosomes consist of stacks of plate-like cisternae that are connected by hexagonally arranged bars. Such bars often link symplechosomes with mitochondria (M). Orig. R. Bauer.