Molecular‐based phylogenetic placement and revision of Micrasepalum ( Spermacoceae‐Rubiaceae )

NUÑEZ FLORENTIN, MARIELA; SALAS, ROBERTO M.; JANSSENS, STEVEN B.; DESSEIN, STEVEN; CARDOSO, DOMINGOS

TAXON

INT ASSOC PLANT TAXONOMY

Año: 2021 vol. 70 p. 1300 - 1316

0040-0262

Micrasepalum is a taxonomically and phylogenetically poorly understood genus of Rubiaceae endemic to the global biodiversityhotspot of the Caribbean islands. It has traditionally been associated with species in the Spermacoce clade, but neverproperly evaluated in a phylogenetic context. Also, little information about seed and fruit morphology of Micrasepalum specieswas provided in their original descriptions and the few subsequent taxonomic treatments.We performed a molecular phylogeneticstudy based on nuclear (ITS, ETS) and plastid (atpB-rbcL, petD, rps16, trnL-trnF) DNA sequence data to resolve the enigmatictaxonomic identity of Micrasepalum. Our analyses show a largely unresolved or poorly supported backbone in both the nuclearand plastid phylogenies of the Spermacoce clade, so that the sister relationship of Micrasepalum is still uncertain. MergingMicrasepalum with any lineage of the polyphyletic Borreria and Spermacoce or the phylogenetically well-defined Hexasepalum,Mitracarpus or Richardia would result in a morphologically heterogeneous collection of genera difficult to recognize within theSpermacoce clade. The unresolved relationships within the species-rich Spermacoce clade are suggestive of a rapid radiation inwhich Micrasepalum is one of several phylogenetically recalcitrant lineages, but that can be clearly defined morphologically.The genus is distinctive in having a caducous, 2-lobed calyx and indehiscent, one-seeded fruit. An updated identification key for allCaribbean genera of the Spermacoce clade and a full taxonomic revision of Micrasepalum are presented, including descriptions focusedon reproductive characters, palynological analysis, a distribution map and new lectotypifications, as well as a new synonym forM. eritrichoides.