Effects of fruit dimorphism on genetic structure and gene flow in the coastal shrub Scaevola taccada

Abstract

Background and aims: Plant propagules often possess specialized morphologies that facilitate dispersal across specific landscapes. In the fruit dimorphism of a coastal shrub, Scaevola taccada, individual plants produce either cork-morph or pulp-morph fruits. The former is buoyant and common on sandy beaches, whereas the latter does not float, is bird-dispersed, and is common on elevated sites such as slopes on sea cliffs and behind rocky shores. We hypothesized that beach populations bridge the heterogeneous landscapes by serving as a source of both fruit types, while dispersal is biased for the pulp morph on elevated sites within the islands and for the cork morph between beaches of different islands. Based on this hypothesis, we predicted that populations in elevated sites would diverge genetically over time due to isolation by distance, whereas beach populations would maintain high genetic similarity via current gene flow.

Methods: The genetic structure and gene flow in S. taccada were evaluated by investigating genome-wide single nucleotide polymorphisms in plants from 17 sampling sites on six islands (belonging to the Ryukyu, Daito and Ogasawara Islands) in Japan.

Key results: Geographical isolation was detected among the three distant island groups. Analyses within the Ryukyu Islands suggested that sandy beach populations were characterized by genetic admixture, whereas populations in elevated sites were relatively isolated between the islands. Pairwise FST values between islands were lowest between sandy beaches, intermediate between sandy beaches and elevated sites, and highest between elevated sites.

Conclusions: Dispersal across the ocean by cork morphs is sufficiently frequent to prevent genetic divergence between beaches of different islands. Stronger genetic isolation of elevated sites between islands suggests that bird dispersal by pulp morphs is restricted mainly within islands. These contrasting patterns of gene flow realized by fruit dimorphism provide evidence that fruit characteristics can strongly mediate genetic structure.

Keywords: Scaevola taccada; Coastal plants; dispersal polymorphism; fruit dimorphism; gene flow; population structure; seed dispersal.