CONICET | Buscador de Institutos y Recursos Humanos

The PCK Clade, represented by at least six genera (Brachiaria, Urochloa, Eriochloa, Megathyrsus, Melinis and Chaetium), is a strong monophyletic group within the Paniceae tribe (Poaceae). Despite the effort made by many authors, the internal resolution of this group remains poorly solved. To better understand of the phylogeny of PCK clade, we analyzed the structure and development of the inflorescence, spikelet and floret of that group, to determine morphological homologies and find new characters to combine with molecular data previously published into an integrated phylogenetic analysis. The most important results are: (1) twenty-one adult inflorescence types were identified, (2) an extensive variation of branch system and spikelet development pattern was found (some of them new for Paniceae and even for the grass family), (3) nine different floral developmental patterns were described and, (4) some results support the possibility of more than one sexual expression pattern. The phylogenetic analysis combining previously published molecular data and our results on inflorescence morphology and development, showed that Brachiaria s.s and Melinis are the Basal Clade, while Urochloa is paraphyletic and associated with Megathyrsus and Eriochloa. Nevertheless, the phylogeny of the PCK Clade remains unsolved. The lack of a phylogenetic signal and the weak resolution of evolutionary relationship among species may be interpreted as a signal of explosive speciation or rapid radiation. Evidences for this hypothesis are: (1) coincidence in the lack of internal resolution and politomies in the consensus trees presented until today, (2) weakness of characters used, alone or combined, to understand the entire history of the group, (3) the wide morphological diversity among species in terms of inflorescence structure and development.

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