BUNGARTZ, FRANK* and THOMAS H. NASH III. Department of Plant Biology, Arizona State University, PO Box 87 1601, Tempe, AZ 85287 - 1601, USA. - Is Buellia subalbula (Nyl.) Müll. Arg. a "Diplotomma" with one-septate ascospores?
The lichen genus Diplotomma introduced in 1849 by Flotow has
frequently been subject to taxonomic debate. Originally proposed as a
segregate of Lecidea because of a double exciple, it soon
became more widely accepted when Massalongo emphasized the
pluriseptate ascospores as the main character of the genus. Modern
checklists and floras are divided on whether to treat the group as
part of Buellia which has been characterized in the strict
sense by one-septate ascospores. Nordin criticized the classical
concept and recently confined Diplotomma to a group of species
with calcium oxalates in the thallus, moderately pruinose apothecia
and a conspicuously thickened perispore. Material of Buellia
subalbula from the Sonoran Desert Region was examined with the
light microscope, and the scanning and transmission electron
microscope. Calcium oxalate content of the thallus was studied with
x-ray diffractometry and specimens were analyzed using thin layer
chromatography. The species has not previously been reported from
North America and was commonly misidentified as B. retrovertens
Tuck., a taxon recently synonymized with B. dispersa A. Massal.
Buellia amabilis, a taxon described by de Lesdain from Mexico
is most likely a later synonym of B. subalbula. The species is
characterized by a thickly crystalline-pruinose thallus with abundance
of calcium oxalates, moderately pruinose apothecia and a thickened
perispore. These characters are consistent with Nordin's concept of
Diplotomma s. str. even though the perispore is only moderately
thickened. However, B. subalbula has one-septate ascospores and
only a proper, not a thalline exciple.
Key words: Buellia amabilis, Buellia subalbula, Diplotomma, North America, Sonoran Desert Region, Taxonomy