Dimorphism of the seed-dispersing organ in a pantropical coastal plant, Scaevola taccada: heterogeneous population structures across islands

摘要

Scaevola taccada is characterized by dimorphic fruits, with one type having cork and pulp (C-morph) and the other type having only pulp (NC-morph). Although within-individual dimorphism has not been observed, both morphs can occur at the same sites. The cork floats on seawater, and the pulp is eaten by birds. Thus, the morphs may have different seed dispersal abilities, via sea currents and birds, respectively. This study aimed to determine the functional characteristics of the dimorphic fruits. First, the potential seed dispersal ability of sea currents and birds was compared between the two fruits morphs by conducting a floating test and by measuring the proportions of different parts of the fruits, respectively. Next, the frequencies of the two morphs across different substrates (beaches, rocks, and cliffs) in the southern islands around Japan were analyzed. Most C-morph fruits remained floating for more than 180 days in a seawater tank, whereas all NC-morph fruits sank after approximately 1 week. The NC-morph fruits had a more pulp volume and less indigestible material than the C-morph fruits. Although both types of plants were found on many islands and all substrates, the C-morph was dominant on beaches, whereas the NC-morph was most frequent on cliffs. The frequencies of the morphs on different substrates might be influenced by the differences in their seed dispersal abilities. The two morphs may differ in fitness on different substrates. These findings improve our understanding of plant adaptations for dispersal.