Varieties of the highly dispersible and hypervariable tree, Metrosideros polymorpha, differ in response to mechanical stress and light across a sharp ecotone

Abstract

Premise The drivers of isolation between sympatric populations of long-lived and highly dispersible conspecific plants are not well understood. In the Hawaiian Islands, the landscape-dominant tree, Metrosideros polymorpha, displays extraordinary phenotypic differences among sympatric varieties despite high dispersibility of its pollen and seeds, thereby presenting a unique opportunity to investigate how disruptive selection alone can maintain incipient forms. Stenophyllous M. polymorpha var. newellii is a recently evolved tree endemic to the waterways of eastern Hawai'i Island that shows striking neutral genetic differentiation from its ancestor, wet-forest M. polymorpha var. glaberrima, despite sympatry of these forms. We looked for evidence for, and drivers of, differential local adaptation of these varieties across the range of M. polymorpha var. newellii. 1,2,✉ 2,3 4 2,3 1 2 3 4 7/8/2019 e.Proofing https://wileyproofs.sps.co.in/eproofing_wiley_v2/printpage.php?token=7JMrMjTTqfJvX-UJ14w7DpRmdO3Dnwzi 2/19Methods For paired populations of these varieties, we compared seedling performance under contrasting light conditions and a strong water current characteristic of the riparian zone. In a reciprocal transplant experiment, adult leaf anatomy of the two varieties was contrasted. ResultsResults suggest that the riparian zone is harsh and that selection involving the mechanical stress of rushing water, and secondarily, light, led to significant reciprocal immigrant inviability in adjacent forest and riparian environments. The strongest adaptive divergence between varieties was seen in leaves and seedlings from the site with the sharpest ecotone, coincident with the strongest genetic isolation of M. polymorpha var. newellii observed previously.Conclusions These findings suggest that disruptive selection across a sharp ecotone contributes to the maintenance of an incipient riparian ecotype from within a continuous population of a long-lived and highly dispersible tree species.

Phylogeography of the highly dispersible landscape‐dominant woody species complex,Metrosideros, in Hawaii

“…newellii is a purported case of incipient sympatric speciation from the widespread M. polymorpha var. glaberrima on Hawaii Island Ekar et al, 2019). The splitstree network of individual popu- supports the hypothesis that the similarity observed between Molokai and Hawaii Island populations of this taxon is more likely due to back-colonization than to ongoing gene flow between these widespread islands.…”

“…polymorpha var. glaberrima on Hawaii Island (Stacy et al, ; Ekar et al , ). The splitstree network of individual populations showed a clear split within M .…”

“…polymorpha var. glaberrima (Stacy et al, ) through disruptive selection along forest‐riparian ecotones (Ekar et al , ). Developmentally, with a single exception (stenophyllous M .…”

“…newellii from M. polymorpha var. glaberrima through disruptive selection along forest-riparian ecotones(Ekar et al, 2019).Developmentally, with a single exception (stenophyllous M. polymorpha var. newellii), seedlings of all varieties of M. polymorpha are indistinguishable from M. polymorpha var.…”

Rapid Response to a Tree Seed Conservation Challenge in Hawai‘i Through Crowdsourcing, Citizen Science, and Community Engagement

“…There are five endemic species of Metrosideros in Hawaiʻi, originating from a single colonization followed by rapid radiation and recent speciation (Aradhya et al, 1991;Dupuis et al, 2019). Despite having wind-dispersed seeds, the rapid diversification of Metrosideros has resulted in high phenotypic diversity and local adaptation, even among subspecific taxa that sometimes occur sympatrically (Ekar et al, 2019;Stacy et al, 2014;. Metrosideros polymorpha is widely distributed across all islands, but has eight currently recognized varieties, some with single-island distributions.…”