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

Abstract

Aim To test for and describe the genetic structure of the Pandanus tectorius complex, a group of closely related ocean‐dispersed plants and members of the Indo‐Pacific coastal strand community. Location Tropical Indo‐Pacific (coastal East Africa to Polynesia). Methods We sampled 535 individuals (46 localities) from throughout the range of the complex. Fifteen microsatellite loci were used to detect and characterize population structure and estimate migration rates between island groups and broad regions. Results Hierarchical population structure was detected. Samples group into an eastern cluster (Hawaii and coastal South‐Central Pacific localities) and a western cluster [Western Pacific (WP) through Indian Ocean]. Within these two clusters, at least six regional subclusters were detected including samples from the Indian Ocean + South China Sea (SCS), Ogasawara Islands, WP, inland South‐Central Pacific, coastal South‐Central Pacific and Hawaii. Migration rates between regions are low leading to isolation and genetic differentiation while within regions, rates are much higher. In most cases, inland populations are genetically differentiated from nearby coastal counterparts. Main conclusions Substantial population structure occurs across the range of the P. tectorius complex due to dispersal limitation across stretches of open ocean and patterns of ocean currents. Low levels of asymmetric westward migration, consistent with the direction of ocean currents in the Pacific, links Hawaii and the South‐Central Pacific with populations further to the west preventing complete isolation. SCS + Indian Ocean populations are distinct from those in the Pacific due to limited dispersal between these regions. The isolation of inland populations on several islands also contributes to genetic differentiation. While population clusters have a clear geographical basis they are not completely congruent with previously recognized taxa.

Genetic diversity, structure, and demography of Pandanus boninensis (Pandanaceae) with sea drifted seeds, endemic to the Ogasawara Islands of Japan: Comparison between young and old islands

“…Furthermore, two P. tectorius , one P. kusaicolus, and three P. conicus sequences were downloaded from the NCBI database and included in the neighbour‐net analysis (Table ). These species were shown to be closely related to P. boninensis by Gallaher et al (2015), Gallaher et al (2017). Chloroplast haplotypes were determined from nucleotide substitutions, insertions, and deletions.…”

“…Gallaher et al (2017) investigated the genetic structure of the P. tectorius complex throughout its distribution range using EST‐SSRs. In the previous study, we investigated IBD in the Hawaiian Islands, which supports enough populations and is on a similar geographic scale (ca.…”

“…500 km from Kauai to Hawaiian Islands) to the Ogasawara Islands (ca. 400 km from Mukojima to Minamiiwoto Islands), using pairwise F ST values, as shown in Appendix by Gallaher et al (2017). We compared those results with data from the Ogasawara Islands obtained in the present study.…”

“…Significant IBD and correlation between island age and genetic diversity ( H E, r = 0.078, p = .829; A R , r = −0.210, p = .734) were not detected in the Hawaiian Islands, but significant IBD was detected, and A R was related to the island age in the Ogasawara Islands. From the results of Bayesian clustering analysis (Figure 3 in Gallaher et al, 2017), the Hawaiian Islands was dominated by one cluster, while the Ogasawara Islands were dominated by several clusters corresponding to geographic groups. Thus, the progressive colonization pattern of Pandanus was well conserved in P. boninensis in the Ogasawara Islands, unlike P. tectorius in the Hawaiian Islands.…”

“…boninensis is distributed instead. This is probably because the Ogasawara Islands is very isolated both geographically and genetically from other Pandanus species (Gallaher, Callmander, Buerki, Setsuko, & Keeley, 2017), probably resulting in the evolution of P . boninensis .…”