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What’s in a name? Emergent strains, admixtures and fuzzy species in Ceratocystis

Thomas Harrington: Iowa State University

<div>Phylogenetic analyses may identify lineages, but are they strains or species? Strains of <em>Ceratocystis </em>are readily dispersed by humans, and Ceratocystis wilt is a major emerging disease on fruit and forest trees in plantations, cities, and native ecosystems around the world. In the last 25 years, the species <em>C. fimbriata</em> has been elevated to its own genus and split into 36 species, mostly circumscribed by phylogenetic analyses alone, without diagnostic phenotypic characters or comparisons to natural populations. Most South American isolates and some of the new species form a monophyletic group that is sexually compatible with the original <em>C. fimbriata</em>, a sweet potato strain that may have originated in Ecuador. This biological species includes five recently named species, which appear to be strains. At least two of these strains were introduced to Asia and are fully interfertile in controlled crosses. Admixtures of the two are evident in Asia, where major losses are seen on pomegranate, mango, <em>Acacia</em>, and <em>Eucalyptus</em>. Purely molecular species concepts may inappropriately designate strains as species and suggest that they are genetically isolated. However, application of the biological species concept suggests that introduced strains of <em>C. fimbriata</em> have the potential to recombine. A good species name conveys a wealth of information and may lead to a better understanding of the epidemiology and management of new outbreaks, as well as better quarantine restrictions.</div>