Phylogenetic analysis supports the allochthonous origin of gynodioecy in New Zealand Gaultheria Kalm ex L.

Abstract

The origin and establishment of gender dimorphism has been postulated as a key element in the evolution of insular floras but has generally not been studied in an explicit phylogenetic framework. We tested the prior conclusion that the origin of gender dimorphism in the New Zealand species of Gaultheria Kalm ex L. is allochthonous by newly tracing the evolution of gender dimorphism on a phylogenetic tree of the Wintergreen Clade. We scored species for male sterility and breeding system type (cosexual, gynodioecious, or dioecious) based on the literature, and observations from the field and herbarium specimens. We traced the evolution of gender dimorphism, fruit evolution, and ancestral areas on the tree with parsimony, maximum likelihood, and Bayesian methods. Our analyses revealed that male sterility in New Zealand most likely emanated from a male‐sterile fleshy red‐fruited South American lineage, with reversals to cosexuality in two or three Australian lineages of the more inclusive Australasian Clade. The results support an allochthonous origin of gynodioecy in New Zealand Gaultheria and corroborate arrival of the ancestral ancestor through over‐water dispersal, likely by birds. Although gender dimorphism could have arrived to New Zealand as a preadaptation to avoid inbreeding depression in an island setting, its correlation with fleshy fruit, unspecialised flowers, and woody habit across the genus suggests that factors other than the island setting per se are responsible for its maintenance in New Zealand. Although gender dimorphism in New Zealand has usually arisen autochthonously in dry‐fruited genera, in Gaultheria the presence of gynodioecy in several dry‐fruited species is best explained by its retention from fleshy‐fruited ancestors.

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