Making the most of your host: the Metrosideros-feeding psyllids (Hemiptera, Psylloidea) of the Hawaiian Islands

Diana M Percy

doi:10.3897/zookeys.649.10213

Abstract

The Hawaiian psyllids (Psylloidea, Triozidae) feeding on Metrosideros (Myrtaceae) constitute a remarkable radiation of more than 35 species. This monophyletic group has diversified on a single, highly polymorphic host plant species, Metrosideros polymorpha. Eleven Metrosideros-feeding species included in the Insects of Hawaii by Zimmerman are redescribed, and an additional 25 new species are described. Contrary to previous classifications that placed the Metrosideros-feeders in two genera, Trioza Foerster, 1848 and Kuwayama Crawford, 1911, all 36 named species are placed in Pariaconus Enderlein, 1926; and the relationship of this genus to other Pacific taxa within the family Triozidae, and other Austro-Pacific taxa feeding on host plants in Myrtaceae is clarified. The processes of diversification in Pariaconus include shifts in galling habit, geographic isolation within and between islands, and preferences for different morphotypes of the host plant. Four species groups are recognized: the bicoloratus and minutus groups are free-living or form pit galls, and together with the kamua group (composing all of the Kauai species) form a basal assemblage; the more derived closed gall species in the ohialoha group are found on all major islands except Kauai. The diversification of Pariaconus has likely occurred over several million years. Within island diversification is exemplified in the kamua group, and within species variation in the ohialoha group, but species discovery rates suggest this radiation remains undersampled. Mitochondrial DNA barcodes are provided for 28 of the 36 species. Genetic divergence, intraspecific genetic structure, and parallel evolution of different galling biologies and morphological traits are discussed within a phylogenetic framework. Outgroup analysis for the genus Pariaconus and ancestral character state reconstruction suggest pit-galling may be the ancestral state, and the closest outgroups are Palaearctic-Australasian taxa rather than other Pacific Metrosideros-feeders.

Highlights

The colonization of remote landmasses containing few familiar host plants could be facilitated by an ability to survive on suboptimal or unfamiliar hosts (Roderick and Percy 2008, Percy 2011), and the majority of psyllids are mono- or oligophagous, the family Triozidae includes the largest number of psyllid species exhibiting atypically broad host preference (Ouvrard et al 2015)
There is strong support for the monophyly of Pariaconus (ML: 99%, NJ: 100%), and robust support for an ancestral outgroup (ML: 98%, NJ: 98%), which may be suggestive of a leaf pit-galling ancestry
Trioza eugeniae is an Australian Myrtaceae-feeder associated with Syzygium in its native Australian range, but in its introduced range in California it occurs on cultivated Metrosideros (Percy et al 2012); this species was considered another putative outgroup for Pariaconus

Summary

Introduction

The Hawaiian Islands are renowned for exemplary species radiations (e.g. Magnacca and Danforth 2006, Rubinoff 2008, Givnish et al 2009, Lerner et al 2011, Bennett and O’Grady 2012, Goodman et al 2014, Magnacca and Price 2015), as well as extraordinary and varied processes of species diversification and evolution (Rivera et al 2002, Mullen et al 2007, Rubinoff and Schmitz 2010, Wessel et al 2013, Gillespie 2016). An ancestral preadaptation to expanded host ranges (Janz et al 2006, Janz and Nylin 2008) may be an advantage allowing successful colonization that may cycle back to specialization given ecological opportunity (e.g. vacant niches, proximity of alternate hosts) (Percy et al 2004). This scenario could explain both the imbalance in colonization potential of the different psyllid families and the observed pattern of within archipelago host specialization after establishment. There are examples from introduced species of host range expansions post colonization of a new region, e.g. Trioza eugeniae Froggatt, 1901, which is associated with Syzygium (Myrtaceae) in its native Australian range, but in California where it is introduced it occurs on cultivated Metrosideros (Percy et al 2012)

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