The Species Pair Concept in the Lichen Dendrographa leucophaea (Arthoniales): Analyses Based on ITS Sequences

The internal transcribed spacers (ITS) of the nuclear ribosomal DNA were sequenced for 16 specimens of the lichen Dendrographa leucophaea (Tuck.) Darb. (Roccellaceae) in order to determine whether the sterile and fertile specimens within the (also called a species-pair) form two distinct monophyletic groups. Forty-four informative sites were obtained. Phylogenetic trees were calculated from aligned sequence data, using Dendrographa alectoroides Sundin & Tehler as an outgroup. The cladistic analysis produced two most parsimonious trees, from which a consensus tree was calculated. Well supported clades containing both sexual and asexual specimens were found in the tree, confirming the theory that asexual specimens of socalled are not monophyletic. The concept of species-pairs (Poelt 1970, 1972) is well known and widely used in lichenology. It refers to closely related, morphologically indistinguishable lichens that differ from each other by their dispersal strategies only. The so-called species produces fruiting bodies and sexual spores, while its counterpart, the is vegetatively dispersed by soredia, isidia, or fragmentation. It has also been suggested that microconidia could function as asexual dispersal propagules to establish pycnidial anamorphs (Tehler 1988). Aside from the divergent dispersal strategies, such counterparts differ little if at all, morphologically, but may have different geographical distributions and ecological tendencies (e.g., Culberson 1973; Culberson & Culberson 1973). The taxonomic treatment of differs from one lichenologist to another, but the two counterparts are traditionally treated and named as separate species. For thorough reviews, see Tehler (1982) and Mattson and Lumbsch (1989). Poelt (1972) suggested that the counterparts of speciespairs should be given rank. Moberg (1977) used the terms primary and secondary in the case of the genus Physcia. According to Culberson (1986), the asexual condition among lichens cannot have been constantly regenerated and further, the asexual morphs show such a uniform morphology that they can not have polyphyletic origin. Mattson and Lumbsch (1989) suggested that the sexual and asexual parts of should not be treated as taxonomically different when sexual and asexual morphs are sympatric and intermediate forms occur. When having mostly allopatric distribution and intermediate individuals only at the overlapping distribution areas they would be regarded as separate subspecies. Allopatric that do not have intermediate forms would not represent a real pair. Mattson and Lumbsch also suggested that sterile lichens could be ancestors to new, sterile taxa. Other workers (e.g., Robinson 1975; Tehler 1982) suggested that the asexual forms of speciespairs may have a polyphyletic origin and should not be treated as taxa of their own. Robinson (1975) assumed that sympatric sorediate and fertile morphs could recombine with each other, because microconidia of the sorediate lichens might fertilize the ascocarp forming ones. Tehler (1982) suggested that the sorediate morphs of a species-pair could frequently arise from the fertile ones. This would explain the (e.g., chemical) variation between the 0007-2745/98/404-41 1$0.95/0 This content downloaded from 157.55.39.45 on Thu, 01 Sep 2016 05:51:38 UTC All use subject to http://about.jstor.org/terms 1998] LOHTANDER ET AL.: THE SPECIES PAIR CONCEPT 405 sterile counterparts of some species-pairs. According to Tehler (1982) the only case when an asexual morph could be considered as a of its own, is when the clone has completely lost its ability to reproduce sexually, and no longer has living sexual relatives that might give rise to new asexual clones. Both Poelt (1970) and Tehler (1982) considered asexual morphs as evolutionary dead ends. Our goal in this study is to determine whether the asexual specimens of the species-pair Dendrographa leucophaea (Tuck.) Darb. evolved as a single evolutionary lineage separate from their corresponding sexual morphs, and thus are monophyletic; or if they have originated on several independent occasions and thus are polyphyletic. If the asexual stage had a single evolutionary history, asexual specimens would form a monophyletic clade in the phylogenetic tree. Until now, it has not been possible to experimentally verify any hypotheses concerning lichen species-pairs. This is the first attempt to examine them by applying cladistic methods to DNA sequence data. We have also examined whether gross morphological features were congruent with our molecular results. The genus Dendrographa Darb. contains two species, Dendrographa leucophaea and D. alectoroides Sundin & Tehler (Roccellaceae). Both are fruticose lichens that occur on trees, shrubs, and north-facing vertical rocks in the coastal habitats of California, U.S.A. (Sundin & Tehler 1996). Dendrographa leucophaea has a white, byssoid medulla; short microconidia; and complanate branches, while D. alectoroides has a brownish, coalescent medulla; long microconidia; and terete branches. A recent morphological study of the genus Dendrographa has been carried out by Sundin and Tehler (1996). Sexual specimens of both Dendrographa have apothecia, while the asexual specimens disperse by thallus fragmentation. Neither soredia or isidia have been found (Sundin & Tehler 1996). Sundin and Tehler (1996) treated the asexual morphs as forma, but suggested that the terms anamorph and teleomorph as used in fungal taxonomy would be more appropriate (see also Tehler 1988). We have used the term forma for convenience, although it does not correspond to groups in a phylogenetic sense. Besides lacking ascomata, D. leucophaea f. minor differs from its sexual counterpart D. leucophaea f. leucophaea by having short internodes and more frequent lateral branchlets. It also occurs in more shaded microhabitats, and has a more restricted distribution than the sexual morphs. The sterile counterpart of D. alectoroides f. alectoroides is D. alectoroides f. parva. Both Dendrographa leucophaea and D. alectoroides sometimes have transitional morphs with ascomata and Pt. Reyes *