Abstract
Diacrodon is a monotypic genus of the tribe Spermacoceae (Rubiaceae), endemic to northeastern Brazil. Diacrodon compressus is frequently misidentified with a two lobed calyx species of Borreria, B. verticillata. Traditionally, in Spermacoceae the fruit type was considered a diagnostic character among the genera. In this sense, D. compressus presents a strongly compressed, one seeded and indehiscent fruit (vs. globose, two seeded and dehiscent fruit in B. verticillata). In this work, we address two objectives: evaluate the systematic position and determine the identity of Diacrodon in respect to other taxa. Molecular analyses using ITS and ETS indicate that D. compressus is strongly related to other species of Borreria. The morphological results revealed that D. compressus, despite of its type of fruit, is identical to Borreria in floral and palynological features. As conclusion, the new combination Borreria diacrodonta is made and a lectotype is designated. An updated description of the species and a key to the Borreria species with a two lobed calyx are provided. The distribution of B. diacrodonta is extended to Brazilian states Goiás and Minas Gerais, and Paraguay. By this taxonomical change it has become clear that the dehiscence of the fruits lack taxonomic value in the delimitation of Borreria.
Highlights
Diacrodon is a monotypic genus that is endemic to Brazil
Diacrodon compressus falls into a clade together with American Borreria species and is strongly supported (Clade I, ML-BS=87)
The first subclade shows Borreria spinosa and Diacrodon as sister group, which are closely related to B. dasycephala (Cham. & Schltdl.) Bacigalupo & E.L
Summary
Diacrodon is a monotypic genus that is endemic to Brazil. Its sole species Diacrodon compressus Sprague was described from two dried specimens that were collected by Bolland in Ceará, Brazil (Sprague 1928). Diacrodon was recorded in species lists only for the original locality (Robbrecht 1988, Andersson 1992). Delprete and Jardim (2012) and Souza (2015) included D. compressus in new Brazilian floristic works. Diacrodon is scarcely treated in other bibliographic manuscripts probably due to its very restricted distribution and its large similarity with Borreria verticillata (L.) G. Mey., with which it is often confused
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