Cryptic dioecy and leaky dioecy in endemic species of Dombeya (Sterculiaceae) on La Re´union

Abstract

The high frequency of dioecy on oceanic islands such as Hawaii and New Zealand has attracted a great deal of attention from plant evolutionary biologists. One reason suggested for the high prevalence of dioecy on oceanic islands is that taxa considered truly dioecious may have occasional hermaphrodite flowers, i.e., show leaky dioecy. In this study, we quantified the presence and distribution of leaky dioecy in a group of congeneric endemic species of the genus Dombeya (Sterculiaceae) on La Réunion island (Indian Ocean). All eight species show cryptic dioecy. Five species show strict dioecy and three species show leaky dioecy due to the presence of male trees that set fruit. Species with strict dioecy and large populations tend to occur in mid- to high-altitude moist tropical cloud forest, whereas species in smaller populations at lower altitude and in semidry tropical forest tend to show leaky dioecy. Two reasons for this differential distribution of strict dioecy and leaky dioecy are discussed. First, environmental variation along the altitudinal gradient, biotic and/or abiotic, may influence the breeding system. Second, leaky dioecy may be favored in lowland populations due to the small size and disturbed nature of such populations.

<I>Dombeya formosa</I> (Malvaceae s. l.): A New Species Endemic to La Réunion (Indian Ocean) Based on Morphological and Molecular Evidence

“…Dombeya formosa shows also flower size dimorphism between sexes (i.e. male flowers tend to be bigger than female flowers except for the gynoecium), a feature also found in the other endemic species of Dombeya on La Réunion (Humeau et al 1999a). The distinctions in both floral and vegetative features are emphasized by the striking disjunction of the flowering periods, i.e.…”

“…While the flowers of a male specimen present a reduced gynoecium without stigma and ovule, female flowers of Dombeya formosa show smaller staminodes and stamens and do not produce viable pollen (Figs. 1;2G, H;Humeau et al 1999a). …”

“…This genus plays a structuring role for both the preservation and the function of the ecosystems on these islands, from lowland to tropical montane cloud forests and particularly in the tropical wet and cloud forests found on La Réunion (Cadet 1980;Blanchard 2000). Another notable feature for the Réunion Island Dombeya is the magnitude of variations for both vegetative and reproductive characters among and within the different species (Jacob de Cordemoy 1895;Friedmann 1987;Humeau et al 1999aHumeau et al , 2000. Polymorphism of morphological characters has led to a taxonomic conundrum because of the difficulties to draw boundaries between close taxa.…”

Kadua haupuensis (Rubiaceae: Spermacoceae), a new endemic species from Kaua’i, Hawaiian Islands

“…Observations utilizing populations of nursery grown plants have revealed that Kadua haupuensis is subdioecious or "leaky dioecious," sensu Humeau et al (1999). That is, these taxa are not truly dioecious as they may produce occasional hermaphroditic flowers or individuals.…”

Leaky dioecy in Diospyros (Ebenaceae) endemic to the Island of Mauritius

“…The presence of occasional opposite sex or hermaphrodite flowers on the same individual of a dioecious species is referred as leaky dioecy. Several cases of leaky dioecy have been reported on oceanic islands, the most well-known examples being, Coprosma in the Juan Fernandez Islands, Sanctambrosia in the Desventuras Islands (see Baker and Cox 1984), Fragaria, Plantago in Hawaii (Carlquist 1974) Cotula, Clematis, Pittosporum, Dodonea, Alectryon, Anisotome, Astelia in New Zealand (Godley 1979) and Dombeya in Reunion Island (Humeau et al 1999) situated in the Indian Ocean, 150 km to the South West of Mauritius. These two islands are volcanic and form part of the Mascarene archipelago.…”