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

Abstract

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.

Navigating the ‘broad freeway’: ocean currents and inland isolation drive diversification in the Pandanus tectorius complex (Pandanaceae)

“…S2.1 in Appendix S2 in Supporting Information) (Hu et al ., ). The length of time that propagules remain afloat varies among species with durations as short as a few days for some mangroves (Rabinowitz, ) to over 240 days for the strand species Scaevola taccada (Emura et al ., ) resulting in a range of dispersal potential. The persistent directionality of some currents may facilitate dispersal from one locality to the next but prevent a return trip resulting in source and sink population dynamics as was found in the Marquesas with two species of snapper (Lutjanidae) (Gaither et al ., ).…”

Fruit colour conceals endocarp dimorphism from avian seed dispersers in a tropical beach plant,Scaevola taccada(Goodeniaceae), found in Okinawa

“…within 1 wk (Emura et al . 2014). The endocarp dimorphism might serve as an alternative strategy of seed dispersal within the same species, since a considerable rate of gene flow between individuals of the two morphs has been detected (Emura et al ., unpubl.…”

“…However, Emura et al . (2014) discovered a cryptic dimorphism in the endocarp of this plant: although most plants bear fruit with the cork endocarp (C+), fruits of some individuals consistently lack such a structure (C–), even though both types of fruit have sugar-containing flesh around the seed. It is probable that these morphotypes have adapted alternatively: plants with the C+ fruit disperse seeds broadly through ocean currents (along with avian dispersal), while those with the C– fruit remain relatively localized through avian dispersal.…”

“…Despite the difference in assumed advantages of the endocarp types, the fruit colour seems not to differ between them, at least in the human vision (Emura et al . 2014). The lack of colour differentiation indicates that the fruit colour of C+ plants might relatively be a dishonest signal because of the apparently reduced nutritional contents and increased indigestible substrates.…”

Heterocarpy in Salvia roemeriana (Lamiaceae)

“…A recent review found heterospermy or heterocarpy in 292 species in 129 genera and 26 families (Wang et al 2010). Heterospermy and heterocarpy can increase variability in seed dormancy (Wang et al 2012;Bhatt & Santo 2016), dispersal (Schmitt et al 1985;Cheptou et al 2008;Ma et al 2010;Emura et al 2014) and competitive ability (Imbert et al 1997;Miguel et al 2017), as well as establishment under varying physiological conditions (e.g. salinity tolerance; Bhatt et al 2017).…”