SELF-INCOMPATIBILITY SYSTEMS IN THE RUBIACEAE OF A TROPICAL LOWLAND WET FOREST

A program of experimental field pollinations was carried out on 14 species of woody Rubiaceae in a Costa Rican wet forest in order to test for the presence of self-incompatibility systems. Species of Cephaelis, Coussarea, Fara mea, Ha melia, Posoqueria, Psychotria, R udgea, and WarszewiczEa were investigated in the study. Ten of the species are distylous, and of these, nine were found to be self-incompatible. The site of the incompatibility barrier varied between and within species. Short style plants always had stigmatic inhibition of pollen tubes following self and intraform pollinations, but the site of the rejection response in long style plants was quite variable. In several species, pollen tubes, resulting from incompatible pollinations of the long style flowers, penetrated to the base of the style. Fruit set was followed in two of these species, and despite deep penetration of self and intraform pollen tubes in the long style morph, only interform pollinations resulted in fruit set. Four of the species tested are florally monomorphic and each was found to be self-incompatible on the basis of fruit set patterns. Pollen tubes in the styles of self-pollinated flowers of two of the monomorphic species, penetrated to the ovary, but no fruits resulted from selfpollinations. A LARGE PROPORTION of the hermaphroditic tree species tested in neotropical forests have been shown to be self-incompatible (Bawa, 1974,1979; Ruiz and Arroyo, 1978). The existence of self-incompatibility in those species has been inferred on the basis of diffierences in fruit set between selfand cross-pollinated flowers. This study examines the nature of pollen tube-pistil interactions and patterns of fruit set in 14 species of neotropical Rubiaceae. The objectives of the study were: 1) to estimate the proportion of self-compatible and self-incompatible species in a sample of Rubiaceae from one community, and 2) to determine the site in the pistil at which self-pollen is inhibited in the self-incompatible species. The Rubiaceae were chosen because the family shows a diversity of sexual systems. Both florally monomorphic and heteromorphic species exist, and sporophytic and gametophytic selfincompatibility systems have been reported as well (Bir Bahadur, 1968). The investigation was conducted at the La ' Received for publication 15 September 1982; revision accepted 8 June 1983. The authors would like to acknowledge the staS of the Organization for Tropical Studies for much administrative and logistic support of the research. B. E. Hammel, J. D. Dwyer, R. B. Foster, and G. S. FIartshorn kindly assisted with floristic information and species determinations. S. C. H. Harrett, M. H. Grayum, D. L. Mulcahy, G. B. Mulcahy and K. Searcy donated valuable advice-and criticism on earlier drafts of the manuscript. The work was supported by NSF Grants DEB 75-2 1018 and DEB 77-25558 to K. S. Bawa and DEB 79-21485 to D. L. Mulcahy. Selva Field Station of the Organization for Tropical Studies, located at the confluence of the Rio Sarapiqui and Rio Puerto Viejo, in Heredia Province, Costa Rica. The vegetation of La Selva is considered transitional between Tropical Wet Forest and Premontane Wet Forest Life Zones in the Holdridge System (Tosi, 1969) and has been described in detail by Holdridgeetal. (1971). The breeding systems ofthe following species were examined: Cephaelis elata $wartz, Coussarea sp. (voucher Beach 1424), Coussarea sp. (Beach 1467)7 Faramea sp. (Beach 1477)) F. suerrensis Donn. Smith, Hamelia patens Jacq., H. xerocarpa Kuntze, Posoqueria grandiflora Standley, Psychotria acuminata Benth., P. chiapensis Standley, P. ofDicinalis (Aubl.) Sandw., P. suerrensis Donn. Smith, Rudgea cornifolia (Humb. & Bonpl.) Standley, and Warszewiczia coccinea (Vahl) Klotzsch. Information on the ecological features of these species is provided in Table 1. Members of the Rubiaceae at La Selva occur predominantly in the understory and in light gaps of varying age as shrubs and small trees, although about 30 species are herbs, vines, or epiphytes. Our sample of 14 woody species constitutes 23% of the 60 species of trees and shrubs in the family known to occur at La Selva (B. E. Hammel, unpubl.). METHODS-Experimental pollinations were undertaken to determine if species were selfcompatible or self-incompatible. Controlled