Straightening out the screw-pines: A first step in understanding phylogenetic relationships within Pandanaceae

The Paleotropical monocot family Pandanaceae includes ca. 700 species assigned to four genera: Pandanus (ca. 500 spp.), Freycinetia (ca. 200 spp.), Martellidendron (6 spp.) and Sararanga (2 spp.). The most speciose genus, Pandanus, was classically subdivided into eight subgenera. Previous cladistic analyses revealed that several key morphological characters might have evolved independently several times, thus highlighting the need for a robust molecular phylogenetic framework to elucidate phylogenetic relationships and infrafamilial and in frageneric classification within this group. In this study, three plastid DNA regions (matK, trnQ-rps16, trnL-trnF) and 200 individuals (representing 134 species and 609 newly produced sequences) spanning the taxonomic and biogeographic diversity of the family are analyzed to test the monophyly at the familial and generic levels, and to infer phylogenetic relationships within the family. Particular emphasis is devoted to Pandanus with the aim of recognizing key morphological characters that reflect the evolutionary history of the genus. Phylogenetic inferences support the monophyly of Pandanaceae and establish Sararanga as sister to the rest of the family, with Freycinetia as sister to the Pandanus-Martellidendron pair. Although relationships are not well-resolved within the latter clade, three supported lineages are retrieved: (1) the Acrostigma clade comprising taxa of P. subg. Acrostigma, (2) the Martellidendron clade including taxa assigned to the genus Martellidendron and (3) the core Pandanus clade including taxa of all other subgenera of Pandanus. Morphological and biogeographic evidence supporting clade definitions are discussed in detail. This study provides the first phylogenetic backbone for Pandanaceae, which is sufficiently robust to serve as a springboard for future research into the evolutionary history of this neglected family.