Morphology and Pollination Biology of an Intersectional Hybrid of Costus (Costaceae)

An intersectional hybridization event between hummingbird-pollinated Costus pulverulentus (sect. Ornithophilus) and large bee-pollinated C. guanaiensis var. macrostrobilus (sect. Costus) in Belize, Central America, is documented by analyses of morphology and pollination biology. Morphologically the hybrid is intermediate between the parental species and retains floral characteristics adaptive to both hummingbird and bee pollination. Nectar secretion rate and sugar concentration but not sucrose:hexose ratio in the hybrid are intermediate between those of the parental species. The hybridization event probably arose by breakdown of strict hummingbird pollination in Costus pulverulentus. Costus L. (Costaceae) occurs in moist habitats in the Neotropics, and is characterized by a spirally arranged phyllotaxy and showy terminal inflorescence. Costus is often found associated with species of Calathea, Canna, Heliconia, and Renealmia long the banks of rivers and streams and more disturbed habitats like forest trails, secondary forests, lightgaps, roadsides, and plantations. In his earlier monograph of the Neotropical Costoideae, Maas (1972) divided the genus Costus into two subgenera: Cadalvena, represented by seven species restricted to South America; and Costus, consisting of 40 species ranging from northern Central America to South America. In a more recent treatment (Maas 1977), two presumably natural sections based on labellum characteristics are recognized in subg. Costus. Section Costus is characterized by bee-pollinated flowers; the labellum is white to yellow, and has a short broad tube and an open exposed limb or landing platform often striped with red or purple. Section Ornithophilus i characterized by hummingbird-pollinated flowers; the labellum is yellow, orange, or red, and has a narrow, thick-walled, fleshy tube barely exceeding the corolla. Several species, most notably C. malortieanus Wendl. and closely related C. pictus D. Don, appear to be intermediate in character between the two sections. According to F. G. Stiles (in Maas 1977), C. malortieanus represents a transition between bee-pollinated and hummingbird-pollinated species because it is visited and pollinated by both euglossine bees and hummingbirds. Many species of Costus are sympatric and are visited by the same pollinators (Schemske 1981; pers. obs.). Hybridization has been described for species of Costus within each section (Maas 1972, 1977) but to date there are no records of hybridization between bee-pollinated and hummingbird-pollinated species (P. J. M. Maas pers. comm.). This study presents evidence for intersectional hybridization involving two Costus species, one adapted to bee-pollination and the other to hummingbird-pollination. In 1976, a new Costus was discovered on the road to San Jose, 2.5 kilometers north of the junction with the San Antonio Road, Toledo District, Belize. A single population of nine plants was found growing at the road's edge for 50 meters. This site in moist tropical forest of the Mayan mountains at elevations of 200250 meters is an area of active road construction and slash/burn agriculture. The two suspected parental species occur within 100 meters of the putative hybrid population. Costus guanaiensis var. macrostrobilus (Schumann) Maas (sect. Costus), a robust species attaining heights over 3.5 meters, is found in more exposed and drier roadside habitats. Costus pulverulentus C. B. Presl (sect. Ornithophilus) is found in shaded and usually wetter roadside habitats than either C. guanaiensis var. macro-