The phylogenetic position of milkweeds (Apocynaceae subfamilies Secamonoideae and Asclepiadoideae): Evidence from the nucleus and chloroplast

The nuclear gene phytochrome A (PHYA) from 71 species of crown clade Apocynaceae (subfamilies Asclepiadoideae, Secamonoideae, Periplocoideae, and four lineages of Apocynoideae) and outgroups is used to (1) test the chloroplast phylogeny that places the African tribe Baisseeae, with solitary pollen grains, as sister to the pollinia‐bearing milkweeds (Secamonoideae plus Asclepiadoideae); (2) resolve the position of tetrad‐bearing Periplocoideae, the proposed milkweed sister group based on morphology; (3) place the enigmatic Dewevrella ; and (4) clarify relationships of the three other primary crown clade lineages: Rhabdadenia , New World clade, and Asian clade. Separate analyses of PHYA and chloroplast sequences agree in placing an African monad‐bearing clade (Baisseeae plus Dewevrella ) as the sister group of the milkweeds. Combined PHYA and chloroplast datasets under parsimony and maximum likelihood reject Periplocoideae as the milkweed sister group with statistical significance. Rhabdadenia is placed as sister to the rest of the crown clade, consistent with aspects of its floral morphology and wood anatomy, but without statistical support. Other relationships among the primary crown clade Apocynaceae lineages are also not supported statistically, but power analysis indicates that four to eight times as many characters (26,376–52,752 aligned positions) will be sufficient for a robust estimate.