Cross compatibility between Marathrum rubrum and Marathrum schiedeanum (Podostemaceae), two closely related species of the Pacific Mexican Coast

Marathrum is a Neotropical genus of Podostemaceae that has been recognized for its elevated degree of vegetative polymorphism. Marathrum rubrum Novelo & Philbrick and Marathrum schiedeanum Cham. represent a clear example: are these distinct species or morphological variants determined by the environment? Three populations were analyzed where we carried out intra-population manual cross-pollinations and compared these to heterospecific pollinations and open-pollinations (control) to test for the presence of reproductive barriers between species. Pollen tube development, seed production, seed size, and germination were observed in legitimate and heterospecific crosses. Our results indicate that there are no reproductive barriers between these species, since we did not find differences in abundance and/or distribution of pollen tubes among pollination treatments. Significant differences among populations in the average number of ovules were found. Similarly, seed number differed among pollination treatments. In general, heterospecific crosses produced less seeds than manual cross-pollinations and open-pollinated flowers. Average seed size was larger for heterospecific crosses than manual cross-pollinations and open-pollinations, whereas no differences in seed germination among populations or pollination treatments were detected. The results support the hypothesis that these ‘species’ constitute morphological variants of the same species. These results are consistent with the proposal that M. rubrum and M. schiedeanum are conspecific with the more widespread Mesoamerican species Marathrum foeniculaceum Bonpl.

► Taxonomic barriers and reproductive characteristics remain unknown in Marathrum.
► Heterospecific cross pollinations were carried out between two Marathrum species.
► Marathrum rubrum and Marathrum schiedeanum are capable of interbreeding.
► Results support the proposal to synonymize these species under Marathrum foeniculaceum.