Proc. Indiaa Acad. Sci. (Ptant Sci.), Vot. 94, Nos 2 & 3, April & May 1985, pp. 319-339. 9 Printed in India. Problems and prospects in the systematics of the Ascomycotina D L HAWKSWORTH Commonwealth Mycological Institute, Ferry Lane, Kew, Surrey TW9 3AF, UK Abstrar The classification of the Ascomycotina remains one of the major problems in systematic mycology. A survey of the va¡ systems proposed over the last tire decades is presented identifying their key aspects. A table including outlines of 26 systems is provided. Constraints to developing a generaUy acceptable system are identi¡ as arising from the size of the group, its geological age, assumption of phylogenetic concepts, data availability. convergence, delimitation of genera, and taxa remaining to be discovered. Progress towards an acceptable system must pay due regard to the users of taxonomy and have strong elements of stability. Data collection could be approached by concerted efforts of groups of specialists tackling particular orders in turn. It seems most appropriate to progress by building up the hierarchy from maximum information content groups, concentrating on delimiting mono- phyletic units in the ranks of orden"and family, and restricting hypothetical overall systems to specialist debate rather than recommending them for general use. Keywords. FungŸ Ascomycotina; licbens; systematics; classification. 1. Introduction The last three decades have seen the systematics of the Basidiomycotina and Deuteromycotina placed on an increasingly secure base, with substantial areas of consensus as to higher categories meriting recognition and generic limits. In contrast, the casual observer might with some justification have the impression that the situation in the Ascomycotina has become increasingly unstable. Symptomatic is the title of the meeting "Ascomycetes in Disarray' organized by the British Mycological Society on 17 November 1984. Luttrell's (1951) remark that "The classification of the Ascomycetes constitutes perhaps the most difficult taxonomic problem in mycology" is unfortunately still valid over 30 years after ir was made. The object of this contribution is to both examine how the present situation has arisen, identify the constraints, and establish guidelines as to how we might proceed towards a generally acceptable system. 2. Proposed systems A selection of systems for the Ascomycotina used during the last five decades is compiled in table 1, from which the extent of variation even amongst current workers will be clear. Table 1 only includes ranks down to order; had space permitted the incorporation of family names, the discrepancies would be even more marked. Clements and Shear (1931) aimed at a practical system with ten orders and no higher categories within the group. Their concepts of some orders were, however, extremely broad. For example, their Sphaeriales even included the Cyttariaceae, and their Pezizales the Caliciaceae, Helotiaceae, Parmeliaceae and Patellariaceae. The main characters these authors used to separate orders were the appearance of the ascus- containing structure (ascoma), particularly whether it was stromatic, more of less 319 320 D L Hawksworth "l'~tble I. C]assificatiorl~ proposed fo~ ~ Ascomycofina L93!-84. Clements and Nannfeldt (1932) MflIer (1949) LuttreiI (i951} ron Arx & M'~ller Chadefaud (1960) Shr (1931) (1954, t962) Laboulbeniale~ DIPLOBIONTICAE HEMIA5COMYCETES SYNASCOM YCETES Endomycetales LABOULBENIOM YCETES Gymnascalr HAPLOBIONTICAE Endomycetales Protomycetales Taphrinalcs Laboulbeniales Pcrisporiales Endomyr Exoa~ates DISCOMYCETES Sphacrialcs ~ ftEMIASCI EUASCOMYCETES HEMIASCOMYCETES UNITUNICATAE L› Dothideales* EUASCOMYCETES Plr162 Dipodascalr Plecta~ales Lecanorales* Microthy ¡ ale..~ Plectascales Eurouale~ Endomycetales Erysiphales Hš Phacidiales" Ascoloculares Ery~iphales Taphrinalr Clavicipitales Hypodermales PezlTAt|es" My¡ M yrian gaales Sphaenates Helotiales Agyriates Pseudosphaeriates Pyrenomycetes EUASCOMYCETES Phacidiales L› Tuberales Hemisphaerialcs Pseudosphaeriates Bitunictt~e Helotiales Tympanidales Trichothyrialr Dothidr Myriangiales Ostropales Ostropales Ascohymeniales Microthynales Dothideales Pezizales Leotialr Pe'91 Hyslcrialr Pseudosphaerial~ Tuberales Sarcoscyphales Osttopales Hypocreales Hysteriales Laboulbr Calir Helotiales Lophi91 Trichothyriales Peziz› Lecanofalcs* Sphar162 Ul~itnmiclt~tr BITUNICATAE Pezizales Sphar Laboutbeniales Plectomycetes Myrmngiales "aberrant'" Diaponhales Diacomycetes Aspergillales Pseudosphaeriales Tuberales Valsal~ Ostropa[es Microascales Hystefiates Elaphomycetal~ Coronophorales Hr Onygena[es Dothioralr PYRENOMYCETES Clavieipitales Pr Pyvenomycetes Dothtd› lncertae sedis Tuberales Xylariales Pleosporales Exoascales Hypocreales Dothiorales Laboulbeniales Diaponhales My riangiales Er3,'siphales Cor onophoriens Coronophorales Coronophorales Coryneliales Nectriales Laboulbeniomycetes Sphaerlac› Laboulbr Diaporthales Discomycetes Sordariale~ Peztzalr Nr162 Tuberales Diatrypales Cyttarialcs Xylariales Ostropa[es Hypont.~ct nales Helotiales Glomerellales Lecaaorales* Clat,ir Caticiales* Clavicipitales "aberrant" Erysiphales Eurotiales H• Endomycctales Dipodascales * = Orders inctuding at teaM some lichenJormiag fungŸas cirgumscribr by the authors indicated. absent, ostiolate, disc-like, scale-like or elongate. Although such features are valuable in diagnostic keys, more critical studies of their development and structure were clearly required. Clements and Shear (1931) recognized that knowledge at that time was grossly inadequate, but stressed one point which should not be overtooked by present workers, that "'to follow a conservative course in the recognition and limitations of orders and famities seems to... best serve the.., interests of users" and that a repeatedly changing system "constantly shifts the foundations of mycology to the disadvantage of practically everyone". A major attempt to place ascomycete systematics on a firmer base was that of Nannfeldt (1932), who while not developing a complete system, endeavoured to distinguish two major groups on the basis of the ontogeny of the ascoma; the Ascoloculares in which the asci developed in cavities in a pre-formed stroma, and the Systematics of the Ascomycotina 321 Ggumann (1964) Hale (1967) Kreisel (196q) Ainsworth et at I 97 ~, Henssen & Jalms (1973) PROTOTUNICATAE ASCOMYCET[DAE ENDOMYCETES HEM[ASCOMYCETES ASCOHYMENIALE PILZE Endomycetale$ Lecanorales* Endomycetales Endomycetales Cakiciales* Protomycetates Sphaeriales ~ Taphrinales Protomycetales Lee,ahora|es* Ascosphar162 Caliciales ~ ASCOMYCETES Taphrinales Leeanormeae| Aspergilla|es Euascomycetidae Liehinineae ~ Microasr Asr Peltigr Onygr LOCULOASCOMYCETIDAE Eurotiales PLECTOMYCETES Teloschistineae* Meliotalr M yriangiales ~ Gymnascales Ascosphaerales Physctineae* Laboulbr Pleosporales ~ Tuberales Eurotiales Per tusariineae* Hystedales* Pezizales Microascales Gyalectales* EUTUNICATAE Helotiates Erysiphales Ostropates~ Unitunlcatae M icroasc,a|es Meliolales Ostropineae * Erysiphales Laboulbeniales Graphidineae ~ Clavicipitalr Included only Xylariales Sphar Sphaeriales orders with lichen- Erysiphales PYRENOMYCETES Verrucariales" Diaporlhales forming fungt H ypocreales Hypocreales Xylariales Spkaerialr Ctavicipitales ZWISCHENGRUPPE Coronophorales Locu!oascomycetidae Sphaeriales Arthoaiales w Phacidiales Myriangiales Corynr Helotiales Capnodiales Coronophorales Pczizales Dot hidcales" ASCO LOCULARE PtLZE Tuberalcs Coronophorales Pleosporales" Taphrinales Phacidiales DISCOMYCETES Dothideales* Bltunicatae Lec~norales* Phacidiales Mydangial~ Hysteriales" Hetotiales Pseudosphaeriales Calic~ales* Ostrol~ates Inclttded only orders Hysteriales Pezizalcs with lichert-forming Dothiorales Tuberales fungi. LABOULBENIOMYCETES Laboulbeniales LOCULOASCOMYCETES Myriangiales Pteosporales Hysteria]es Dothideales Capnodiales Microlhynales Table 1 (contd) Ascohymeniales where the asci develop as a hymenium and not in a pre-formed stroma. Nannfeldt's concepts were quickly followed by some authors (e.g. Bessey 1935), but did not then receive any general acceptance. At this time highly pertinent observations on the ontogeny of pyrenomycetous fungi were being made by Chesters (1938), the full significance of which only started to be appreciated in the 1970's. Miller (1949) recognized that "important fundamental characters such as those of the ascus, or those of the ascocarp [ascoma] centrum, have been largely ignored in establishing relationships" and that "this has resulted in artificial groupings and keys of little practical use". In this paper he developed his earlier ideas and endeavoured to construct a system "based primarily on characters of the ascus correlated with ascocarp [ascoma] centrum characters". However, in contrast to Nannfeldt (1932), his major groups ~vere based on rather superficial features; whether a specialized ascoma 322 D L Hawksworth TiMe 1 (conEmued) Ainsworth et a/. (I 973) Wehmeyer (J975) Barr ~1976) von Arx (1979) E¡ (1982) HEMIASCOMYCETES PROTOASCOMYCETES HEMiASCOMYCETES Gymnoascalr EUASCOM YCETIDAE Endomycetalr Endomyr162 Protomyeetalr Eurotiales Group A Protomycctalcs Hemiaseales Taphrinales Sphaeriales Lecanorales" Taphrinal~ Taphrinale* Spermophthorales Pkacidiales Gyal~ctales ~ Cephaloascales Hclotiales Pr241 PLECTOMYCETES CARPOASCOMYCETES Dipodaseales Pezizales Peltigerales" Eurotialr Loculoascomycetcs Ascoideales Erysiphale5 Microthyrialr Dothidr PYRENOMYCETES Hysteriales EUASCOMYCETES Group B Erysiphale$ Myriangiales Plectoascoraycetidae