Correlation of self‐ and interspecific incompatibility among sympatric Hedyotis species (Rubiaceae) and consequences for hybridization

Breakdown of self‐incompatibility increases opportunities for both self‐fertilization and interspecific hybridization, although the latter is dependent on the extent of competition between heterospecific and conspecific pollen. We investigate the mating system and pollination biology of five phylogenetically closely related species within a distylous species complex in Hedyotis L. (Rubiaceae) in southern China. The complex comprises Hedyotis acutangula Champ. ex Benth., Hedyotis shiuyingiae T.Chen, Hedyotis vachellii (Hook. & Arn.) Kuntze, and two putative hybrid species, Hedyotis bodinieri (H.Lév.) Chun and Hedyotis loganioides Benth., hypothesized to result from interbreeding between these species. We test the hypothesis that the breakdown of self‐ and interspecific incompatibilities in sympatric Hedyotis species might allow interspecific hybridization in natural populations. We assessed the extent of self‐ and interspecific incompatibility in sympatric populations, including investigations of spontaneous and artificial self‐pollination, geitonogamy, inter‐ and intramorph xenogamy. Artificial interspecific crosses were undertaken between H. acutangula, H. shiuyingiae, and H. bodinieri, between H. acutangula and H. vachellii, and between H. acutangula and H. loganioides. Hedyotis acutangula is demonstrated to be self‐ and interspecific compatible, whereas H. vachellii, H. bodinieri, and H. loganioides are self‐compatible and interspecific incompatible; H. shiuyingiae, in contrast, is strictly self‐ and interspecific incompatible. Comparisons of pollen tube growth rates in hybridizing species‐pairs revealed that heterospecific pollen of H. shiuyingiae, H. vachellii, and H. bodinieri can compete with conspecific self‐pollen of H. acutangula. Our study therefore indicates that the breakdown of self‐incompatibility directly and indirectly facilitates interspecific hybridization and provides a platform for better understanding evolutionary directionality in Hedyotis.