Revision of Oenothera sect. Oenothera subsect. Emersonia (Onagraceae)

Oenothera sect. Oenothera subsect. Emersonia consists of four species, 0. stubbei sp. nov., O maysillesii, 0. macrosceles, and 0. organensis, which have scattered relictual distributions in central and northern Mexico and the Organ Mountains, New Mexico, United States. Diagnostic features of subsect. Emersonia include: perennial habit; stems decumbent, ascending or weakly erect, rooting at the nodes in 0. maysillesii and 0. stubbei; dentate to subentire leaves, erect buds; yellow vespertine hawkmoth-pollinated flowers; cylindrical to narrowly lanceoloid capsules; and seeds with the testa thickened distally, this thickened area with an internal cavity. Fully fertile hybrids can be made in most combinations between these four species, and it is in part because of this result that these somewhat heterogeneous species are grouped together. Based on morphology, distribution, and ability to form fertile hybrids, especially in the case of 0. maysillesii, with members of the other four subsections of sect. Oenothera, the species of subsect. Emersonia appear to represent a lineage that diverged relatively early in the evolution of the section. All four species of subsect. Emersonia are diploid, n = 7, but some of the plants of each species that we examined are heterozygous for usually one or two reciprocal translocations and up to four translocations in some populations of 0. maysillesii. Oenothera macrosceles is self-compatible, but modally outcrossing; the other three species are self-incompatible. Among the species in subsect. Emersonia, features of the seeds are heterogeneous, more so than in most other sections of Oenothera. Certain seed characters suggest relationships between species of subsect. Emersonia and those of other subsections of sect. Oenothera.