On the Classification of Lichen Photomorphs Author(s): Jack R. Laundon Reviewed work(s): Source: Taxon, Vol. 44, No. 3 (Aug., 1995), pp. 387-389 Published by: International Association for Plant Taxonomy (IAPT) Stable URL: http://www.jstor.org/stable/1223409 . Accessed: 01/06/2012 03:20

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http://www.jstor.org TAXON 44 - AUGUST 1995 387

POINTS OF VIEW

Edited by M. Roos'

On the classification of lichen photomorphs

Jack R. Laundon2

Introduction

Thanks to the excellent work of Dughi (1937) and James (1975; James & Henssen, 1976), later confirmed by DNA analysis (Armaleo & Clerc, 1991), it is now established that certain single lichen fungi combine with both a green alga and a blue-green cyanobacterium to form two lichens of strikingly different appearance. In a number of examples joined thalli of both lichens are known. Unfortunately there is neither an agreed term to denote an entity of such a union, nor is there agreement on its classification. These matterscan only be resolved by discussion and suggestion.

Terminology

The words chimera (James, 1975), morphotype (James & Henssen, 1976), phyco- type (Swinscow, 1977), chimeroid association (Brodo & Richardson, 1978), phyco- symbiodeme (Renner & Galloway, 1982), photosymbiodeme (Galloway, 1988), phototype (J0rgensen, 1991), and morph (Purvis & al., 1992) have all been deployed to describe one of the lichens involved in this relationship. Unfortunately none of these terms is entirely satisfactory. A chimera is an organism combining two geneti- cally distinct tissues, and the term should therefore be regarded as misapplied in the present context, despite its popular appeal. Words ending in type should be reserved in botany, as far as possible, for reference to type specimens (e.g. lectotype). The term "deme" denotes closely related organisms from a single locality, and is there- fore also misapplied in our case. On the other hand the word morph, which indeed refers to the form of an organism, is by itself imprecise. It is therefore proposed here that the term "photomorph" be used to denote such lichens, because it combines reference both to the photobiont of the lichen and its distinct structure.A photomorph is defined as an organism whose form is determined by the nature of its photosyn- thesis. It appears unnecessary to follow Armaleo & Clerc (1991: 2) in adopting differing terms for the single component ("morphotype")and the dimorphic complex ("photosymbiodeme").

1 Rijksherbarium/HortusBotanicus, P.O. Box 9514, NL-2300 RA Leiden, Netherlands. 2 14 Victory Avenue, Morden,Surrey SM4 6DL, U.K. 388 TAXON 44 - AUGUST 1995

Classification Most photomorphs were originally described as distinct species before their true nature was established. Some lichenologists continue in this traditioneven now when the relationship is known. Thus pairs of lichen species are recognized, each repre- senting the same fungus when forming a symbiosis with either of two disparate photobionts. This classification was adopted in Pseudocyphellaria Vain. by Gallo- way (1988: 25) who remarked that, since "both chloro- and cyanosymbiodemes appear to be capable of independent existence and have characterswhich permit their taxonomic separation, they are given independent species names", claiming that "the International Code of Botanical Nomenclature makes no allowance for the existence of joined thalli differing only in the nature of the photobiont present". In fact the Code explicitly rules against such treatment by stating that "for nomenclatural pur- poses names given to lichens shall be considered as applying to their fungal compo- nent" (Art. 13.1), so that Galloway's species concept is in evident conflict with the rules of botanical nomenclature. In his account of 48 species, five such species pairs were listed: P. coriacea (Hook. f. & Taylor) D. J. Galloway & P. James and P. allanii D. J. Galloway; P. durietzii D. J. Galloway and P. hookeri (C. Bab.) D. J. Galloway & P. James; P. lividofusca (Kremp.) D. J. Galloway & P. James and P. knightii D. J. Galloway; P. rufovirescens (C. Bab.) D. J. Galloway and P. murrayi D. J. Galloway; and P. pubescens (Mull. Arg.) D. J. Galloway & P. James and P. margaretiae D. J. Galloway; the green algal photomorph being cited first in each case. White & James (1988: 104-105) similarly classified the photomorphs in Nephroma as distinct species, in spite of their known nature. Opposing the "traditionalists"are the "modernists" who unite the names of both photomorphs under the oldest name at species rank. There is then only one name for both. Unfortunately, no indication is left of which photomorph is implied when the name alone is used. This classification was adopted for Lobaria amplissima (Scop.) Forssell, Peltigera britannica (Gyeln.) Holt.-Hartw. & T0nsberg, and Sticta cana- riensis (Florke) Bory ex Delise by Purvis & al. (1992: 362, 442, 583), and in the genus Sticta (Schreb.) Ach. by Santesson (1993: 213). The former remarked that since "all lichen names are based on the mycobiont alone a single name has to be applied to two seemingly very disparatemorphs sharing the same mycobiont". I agree with the "modernists", which include Armaleo & Clerc (1991: 8), that the same species name should be applied to both photomorphs. Their classification as distinct species is not warrantedby their biology and should therefore cease. How- ever, it would be advantageous to distinguish between them by the use of different infraspecific names, e.g. at the rank of forma. In this way the disparate photomorphs could be recognized by name, which is often important,i.a., because one usually has a much wider distribution than the other. Some botanists will doubtless oppose this view, because there is no evidence of any genetic difference between the fungi in correlated photomorphs. The observed differences relate to nutrition, the different photobionts providing the nutrients for the fungus partner. J0rgensen (1991) also comments that it is "necessary to allow separate names for different phototypes of lichens. This is best done under Art. 11.1." - the article that allows for the use of separate names for the form-taxa of fungi. The use of the category of forma for mere modifications (i.e. biotypes, units of a purely phenotypical nature) is traditional in plant classification; it has a long history and is well established (Du Rietz, 1930: TAXON 44 - AUGUST 1995 389

342-348) although that rank is now rather rarely adopted. However, it can still sometimes be useful in taxonomy, as in the present case, and should then be used as appropriate.

Acknowledgements I thankthe RoyalBotanic Gardens, Kew, for the use of the library.Daniele Armaleo kindly provideda photocopyof his valuablepaper.

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