Compatibility Relationships in Distylous Bluets: Houstonia serpyllifolia and H. longifolia (Rubiaceae)

Abstract

We performed hand-pollinations to assess the compatibility of crosses within and between long-styled (pin) and short-styled (thrum) morphs of Houstonia serpyllifolia and H. longifolia. For both species, percentage fruit-set and seed-set per capsule were significantly higher following intermorph (legitimate) than intramorph (illegitimate) crosses, as expected for distylous species. Fruit-set was significantly higher for intramorph crosses than for unpollinated controls, but seed-set did not differ significantly between the two treatments. Fruit-set in intramorph crosses was 10% to 28% of that in intermorph crosses. In the case of H. serpyllifolia, we suspect that intramorph fruit-set may have been due to the transfer of some compatible pollen by thrips. Crosses were also made between H. longifolia and H. purpurea, a co-occurring species that also belongs to subgenus Chamisme. Fruit-set and seed-set per fruit in interspecific crosses were not significantly lower than those in intraspecific crosses, but most of the seeds were small or immature and probably inviable. We conclude that H. serpyllifolia and H. longifolia express the heteromorphic genetic incompatibility typical of distylous species.