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

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.

Revision of the Hawaiian psyllid genus Swezeyana, with descriptions of seven new species (Hemiptera, Psylloidea, Triozidae)

“…The Hawaiian psyllids in the genus Pariaconus that feed on Metrosideros polymorpha also show similar evidence of divergence in sympatry, but these Metrosideros -feeding species often exhibit clear niche partitioning of the host species by different galling and non-galling biologies and/or occupation of different parts of the plant and/or different plant morphotypes (Percy 2017a). In contrast, Swezeyana species, as far as we know, all have free-living immatures found on the leaf surface, and there are no clearly apparent biological or microhabitat shifts coincident with occupying or partitioning the host plant.…”

“…For morphological examination, ethanol-preserved material was macerated and cleared in 10% potassium hydroxide followed by clove oil, and slide mounted in Canada balsam as described in Hodkinson and White (1979). Morphological terminology follows Hodkinson and White (1979), Hollis (1984), and Percy (2003a, 2017a). The DNA barcodes provided here were sequenced from two mitochondrial gene regions, cytochrome oxidase subunit 1 (COI), and cytochrome B (cytB).…”

“…KY294142 (COI) KY294626 (cytB) (KM16-14) [previously submitted to GenBank as Swezeyana elongagena Caldwell, 1940 (in Percy 2017a)]; KY294143 (COI) KY294627 (cytB) (Hi57-03) [previously submitted to GenBank as Swezeyana elongagena Caldwell, 1940 (in Percy 2017a)].…”

“…Immatures and adults are found on the leaf surface, and all species for which the biology is known have free-living immatures. There is relatively little morphotypic variation in the host plant across the different islands, compared, for instance, to the morphotypic variation found in the host plant of Pariaconus , Metrosideros polymorpha (Percy et al 2008, Percy 2017a). The absence of observed differences in immature biologies and habitats, and the lack of host variation make it therefore difficult to interpret the drivers of the often striking morphological diversity, particularly in the immatures (Tuthill 1966).…”

“…The native species and most of the described genera are endemic with a number of exemplary cases of adaptive radiations on endemic host plants (e.g., Pariaconus Enderlein, 1926 on Metrosideros (Myrtaceae), Hevaheva Kirkaldy, 1902 on Melicope (Rutaceae), and Megatrioza Crawford, 1915 on Pritchardia (Arecaceae)) (Zimmerman 1948, Uchida and Beardsley 1988, Percy 2017a). Increasingly, molecular data on these species radiations is contributing additional information required to interpret evolutionary processes, but there remains considerably more work to do to reveal whether common dynamics are driving evolution across these different psyllid groups, as well as to determine the origins of the closest relatives outside the Hawaiian Islands (Percy 2017a). In addition, there are pressing conservation issues related to documenting psyllid diversity and species that are host specific on native plants in a rapidly eroding and threatened native flora (Percy 2017b).…”