Abstract—We examine investment in male function in the context of geography and floral morphology in the plant ring species Euphorbia tithymaloides, which colonized the Caribbean from Mexico/Guatemala along two fronts that roughly correspond to the Greater and the Lesser Antilles and that meet in the vicinity of the Anegada Passage. Our results on investment in pollen relative to ovule production are consistent with E. tithymaloides relying on pollinators for reproduction across its range in the Caribbean. We document a geographic pattern of reduction in anther number and pollen:ovule ratios in populations of E. tithymaloides towards the Anegada Passage, which is consistent with a possible geographic transition towards a mixed breeding systemwhere selfing has increased importance in this area where its two expansion fronts meet. The reduction in pollen production is correlated with inflorescence morphology, which converges to relatively shorter cyathia towards the Anegada Passage along both fronts. We discuss alternative scenarios and potential drivers of the patterns we document.

Keywords—Breeding systems, convergence, divergence, Euphorbiaceae, male investment, pollen:ovule ratios, outcrossing, reproductive isolation, selfing, speciation.

Abstract:

A new species from St. Eustatius, Gonolobus aloiensis (Apocynaceae, Asclepiadoideae, Gonolobinae), is described and illustrated. This new species is endemic to St. Eustatius (northern Leeward Islands) and represents the first record of the genus for the island. It exhibits morphological similarity to continental G. albomarginatus, but can be distinguished in part by abaxial leaf surfaces glabrous, calyx and corolla lobes shorter and narrower, and abaxial corolla lobes completely lacking glandular indumentum and with eglandular indumentum restricted at most to only the top two-thirds of the lobes. An amplified key to species of Gonolobus s. s. in the West Indies is provided.