Biogeographical and Ecological Correlates of Dioecy in the Hawaiian Flora

Abstract

The angiosperm flora of the Hawaiian Islands, with its high incidence of dioecy, has been central in development of hypotheses about the evolution of dioecy. Based on a recent taxonomic treatment of the Hawaiian angiosperm flora, we analyzed biogeographic patterns of dioecy as well as the association of dioecy with ecological traits potentially important in the evolution of dioecy for native current species and genera. We also analyzed patterns for presumed colonists of the flora to control better for phylogenetic patterns in these traits. Dioecy is associated with woodiness, especially trees; however, this pattern does not hold for gynodioecy. Within woody species, dimorphism (dioecy, gynodioecy, subdioecy, polygamodioecy) is associated with mesic habitats and occurs more frequently in species with lowland and lowland—montane distributions. In contrast, in the endemic Hawaiian Alsinoideae (Schiedea and Alsinidendron) and in Bidens (two groups with autochthonous evolution of dimorphism), dimorphism is associated with more xeric habitats, and for the former group, with wind pollination. For the entire angiosperm flora, dimorphic species are associated with flowers that are small and green. Woody dioecious species and genera are disproportionately associated with wind pollination; hermaphroditic species are disproportionately bird pollinated primarily because of one species—rich lineage in the Campanulaceae. There was no association of dimorphism and pollinator type for colonists. Because of a few species—rich lineages, dimorphism is associated with dry fruits at the specific level, but at the generic level and for colonists, dimorphism is associated with fleshy fruits. Dioecious and gynodioecious species are found more often on older islands, a result of speciation of dimorphic colonists as well as autochthonous evolution of dimorphism. Single—island endemism is not associated with dioecy in the flora as a whole or with dimorphism within lineages evolving dimorphism autochthonously; thus Baker's law is not supported within the Hawaiian Islands. Because the Hawaiian angiosperm flora originated from a minimum of only 291 colonists, many of the associations of dimorphism with ecological traits occur because of the influence of only a few species—rich lineages. Better knowledge of phylogenetic patterns and further ecological studies, particularly within those groups evolving dioecy autochthonously, are needed to determine causality.

Biologia reprodutiva e fenologia de Virola sebifera Aubl. (Myristicaceae) em mata mesofítica de Uberlândia, MG, Brasil

“…As flores pequenas e inconspícuas de V. sebifera são características muito comuns em plantas dióicas tropicais (Bawa 1980a, Sakai et al 1995, Oliveira 1996 e parecem ser regra em espécies da família Myristicaceae (Rodrigues 1980, Armstrong & Drummond 1986, Armstrong & Irvine 1989b, Momose et al 1998. A morfologia mais especializada das flores de V. sebifera, dada pela fusão das sépalas, é uma característica inerente à família (Armstrong & Drummond 1986;Armstrong & Irvine 1989b, Piña-Rodrigues 1996, Armstrong 1997, Momose et al 1998, entretanto, também difere da morfologia pouco especializada relatada freqüentemente para espécies unissexuais tropicais, incluindo espécies de cerrado (Bawa 1980a, Oliveira 1996.…”

Evolution of Insular Pacific Pittosporum (Pittosporaceae): Origin of the Hawaiian Radiation

“…Of the approximately 1000 species of native Hawaiian flowering plants, 89% are found only on these islands (Wagner et al, 1990;Sakai et al, 1995), representing the highest degree of endemism for any known flora. The insular nature of the flora is due to various factors, including a limited number of original colonizers, radiation in a unique insular environment isolated from congeners, diverse geographic origins, and the unbalanced representation of certain groups relative to their nearest mainland congeners (Fosberg, 1948;Wagner et al, 1990).…”

Phylogenetic Analysis of Dioecy in Monocotyledons

“…In broad comparative studies, correlations have also been documented between dioecy and self-incompatibility, heterostyly, insect pollination, bird dispersal, growth form, and living on islands (e.g., Bawa 1980Bawa , 1982Givnish 1982;Renner and Ricklefs 1995;Sakai et al 1995). However, the interpretation of such correlations is complicated.…”