Melanohalea exasperatula, commonly known as the lustrous camouflage lichen or lustrous brown-shield, is a species of corticolous (bark-dwelling) foliose lichen in the family Parmeliaceae. It has a widespread global distribution and is common in both Europe and northern North America. Its thallus can grow up to 5 cm (2 in) in diameter, with marginal lobes up to 5 mm (3⁄16 in) broad. The upper surface is pale olive-green to red-brown, with isidia (propagules of vegetative reproduction) that are unbranched, inflated, and hollow. It can be distinguished from similar species by the shape and structure of these isidia. The lower surface of the thallus is pale tan to pale brown with scattered, pale rhizines. Apothecia (sexual fruiting bodies) are uncommon, while pycnidia (asexual fruiting bodies) and secondary metabolites are not found in this species. The lack of defensive chemicals makes it vulnerable to grazing by slugs and snails. The evolutionary history of M. exasperatula is linked to major climatic events during the Miocene and Pliocene epochs.
Taxonomy The Finnish lichenologist William Nylander first scientifically described the lichen in 1873, classifying it in the genus Parmelia. He briefly described the lichen as: "Perhaps a subspecies of P. exasperata, but thinner, with a smaller thallus and densely packed small linear lobes." He further noted that the type specimen, collected by Johan Petter Norrlin, was found growing on bark in Etelä-Häme (Hollola, Finland). In 1978, Theodore Esslinger transferred it to the newly circumscribed genus Melanelia as part of a revision of the cetrarioid lichens. In 2004, a molecular phylogenetics-informed restructuring of Melanelia into more natural monophyletic groups led to the reclassification of the species. Two new genera were proposed: Melanelixia and Melanohalea. The taxon was transferred to Melanohalea, along with 18 other brown parmelioid species that are primarily distributed on bark and wood in the Northern Hemisphere. The taxon Parmelia papulosa, first described by Martino Anzi in 1868 (as Imbricaria olivacea f. papulosa) and later transferred to Parmelia by Edvard Vainio in 1888, is a synonym of Melanohalea exasperatula. Although it was published a few years earlier than Nylander's Parmelia exasperatula, it is nevertheless not a validly published name, as Camille Montagne had already used the name Parmelia papulosa for a different taxon in 1842. The species epithet exasperatula derives from the Latin word exasperatus ('roughened') with the diminutive suffix -ula, meaning 'somewhat roughened', referring to the roughness of the thallus surface (exasperatis). Common names used to refer to the Melanohalea exasperatula include "lustrous camouflage lichen" and "lustrous brown-shield".
Evolutionary history The genus Melanohalea, including M. exasperatula, underwent significant diversification primarily during the Miocene and Pliocene epochs, much earlier than the Pleistocene glacial cycles. This diversification is linked to major climatic changes and the formation of new habitats. Bayesian skyline plot analyses—a method that uses genetic data to estimate historical population sizes by analyzing patterns of DNA differences between individuals—indicate that population expansions for M. exasperatula occurred during the Pleistocene but predated the Last Glacial Maximum. The population growth for this species began approximately 200,000 years before present, during the Saalian Pleniglacial cycle. Melanohalea exasperatula's current distribution and population structure are the result of both ancient diversification events and more recent ecological adaptations.
Genetic diversity and population structure Genetic diversity in Melanohalea exasperatula is assessed through two main metrics: haplotype diversity and nucleotide diversity. Haplotype diversity measures the uniqueness of a particular genetic sequence (haplotype) within a population. High haplotype diversity means there are many different genetic sequences present, indicating a high level of genetic variability. Nucleotide diversity measures the average differences at the nucleotide level between pairs of DNA sequences in a population. It gives an indication of how much genetic variation there is within the population. Moderate levels of both haplotype and nucleotide diversity in M. exasperatula suggest that the species has sufficient genetic variation to adapt to various environments, supporting its wide distribution. A genealogical concordance method used in the study found strong evidence for previously unrecognized species-level lineages within traditional morphology-based species, including M. exasperatula. This implies that there may be cryptic diversity within what is currently recognized as M. exasperatula, necessitating further morphological and genetic studies to fully resolve the species boundaries.
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