SONG, SEX AND PSYLLID SYSTEMATICS

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Sound production in Australian psyllids

During a postdoc in Australia I investigated the sounds produced by, and the acoustic behaviour of psyllids. This research was funded by The Leverhulme Trust with an equipment grant from the Systematics Association. I also did some sound experiments with two Australian leafhoppers (Austrolopa brunneus and Stenocotus depressa), and a braconid wasp (Cotesia rubecula). With the investigation of acoustic signals I have been able to address questions relating to species concepts: specific mate recognition systems (SMRS) and reproductive barriers. These experiments represent the first time that sound production has been used to test reproductive barriers in the Psylloidea, and provides the first comparison of acoustic systems in a basal hemipteran lineage (Sternorrhyncha) to acoustics in the planthoppers and leafhoppers (Auchenorrhyncha, Hemiptera). Furthermore, the differences in sounds produced by closely related, even morphologically indistinct taxa, indicates the potential contribution of sound characters, in conjunction with morphological and molecular evidence, for resolving evolutionary relationships among psyllids.

Sound and speciation

Behavioural characters are often fast evolving and the acoustic behaviour of insects has proven to be important in insect speciation (Wells & Henry 1998). Many host specific phytophagous insects produce acoustic signals during mating (Claridge et al. 1997). The acoustic calls are specific to each species and intermediate characteristics are evident in hybrid offspring (Claridge 1985). Acoustic studies in the Hemiptera have shown how rapidly prezygotic reproductive barriers involving acoustic signals may become established (Claridge & de Vrijer 1993).

Both male and female psyllids produce species specific and sex specific acoustic signals during mating. This study shows that acoustic signals, together with data on species distribution, ecology and host plant habitat, can be used to elucidate the processes and mechanisms of speciation. A combined approach of this sort has enormous potential to explore the relative roles of natural selection related to ecology and habitat, versus sexual selection and competition related to interactions within and between species.

The use of cryptic species in acoustic studies

Cryptic species that can mainly be distinguished by mating signals are good subjects for studies of speciation. Distinct differences in the acoustic signals produced by morphologically cryptic species have been demonstrated in various insect groups (Henry 1994) including psyllids (Percy in prep.), and these may provide valuable characters for identifying incipient speciation. If acoustic signals are implemented in the early stages of psyllid speciation, sympatric sister species are expected to have more divergent call types than sister species that are not sympatric (or have different phenologies). In addition, sympatric taxa that are ecologically equivalent but acoustically divergent may suggest a more important role for sexual over natural selection in the speciation process. The rapid rate of divergence in acoustic signals suggests that there is maximum systematic information available from acoustic characters during and after recent speciation events. Subsequent divergence or convergence of song types may make distinguishing homologous characters in acoustic signals difficult (Henry et al. 1999).

Objectives:

a) to assess the role of acoustics in speciation and diversification of psyllids

b) to determine the value of sound production to phylogenetic studies

Acoustic diversity

I have recorded an astounding diversity of 'songs' from 27 species in 13 genera of Australian psyllids (examples of these can be found at Schedotrioza songs, Cardiaspina songs, other psyllid songs). The sounds range from 'honks', 'croaks' and 'burps', to 'wailing' and 'whining'.

The Recording System Acoustic signals were recorded using substrate pickup methods developed by Mike Claridge (Claridge et al. 1985) and optimized for psyllids. Recordings were made inside a transparent acrylic tube (15 cm x 4 cm diameter). A crystal gramophone cartridge was attached firmly to a cork at one end with adhesive putty and positioned where the stylus made contact with the surface of a section of stem trimmed from the host plant and placed inside the tube with the base of the stem inserted in the adhesive putty. Signals from the cartridge were amplified x10 or x 20 using an ED1241 Differential Amplifier (designed and constructed by C. Hardy, Department of Electronics and Electrical Engineering, Univ. of Glasgow, UK) and recorded at a sampling rate of 44.1 kHz on digital audio tape (Sony DAT tape recorder, model TCD-D8).

Playback experiments

I have used playback experiments to explore species boundaries in both the Schedotrioza and Cardiaspina groups. With these experiments, I attempted to test female receptivity to conspecific and heterospecific male calls from different hosts. I have used these methods to assess the likelihood of gene flow between taxa on different hosts judged by female responses to different male calls. The results suggest a possibility of unidirectional but not bi-directional gene flow between species occurring on sympatric hosts.

Host plant hybridization and psyllid acoustics

In future studies I hope to investigate psyllid diversity and acoustic behaviour across eucalypt hybrid zones in Tasmanian. The project will use a combination of morphological and acoustic data to examine the host preference and response to host hybridization by the Tasmanian psyllid species using natural and artificial eucalypt hybrids. Two hybrid systems will be examined, the first involving two Tasmanian endemic eucalypts E. risdonii and E. amygdalina, and the second involving two of the most important commercial species of eucalypts for plantation forestry in Australia, E. globulus and E. nitens. There are already several studies of the genetics and invertebrate communities in these systems (Whitham et al 1991; Whitham et al 1999; Dungey et al 2000; McKinnon et al 2001).

Acknowledgments

This research is funded by The Leverhulme Trust with an equipment grant from the Systematics Association. I have collaborated with Gary Taylor (University of Adelaide) on work with psyllids, Max Day (CSIRO Entomology) on work with leafhoppers, and Mike Keller (University of Adelaide) on work with the braconid wasp. Molecular data were generated by Geoff Clarke (CSIRO Entomology), and Martyn Kennedy (University of Otago).

References

Claridge, M.F. (1985) Acoustic behaviour of leafhoppers and planthoppers: species problems and speciation. In The Leafhoppers and Planthoppers, eds Nault, L.R. & Rodriguez, J.G. pp.103-125. John Wiley & Sons, NY.

Claridge, M.F., Den Hollander, J. & Morgan, J.C. (1985) Variation in courtship signals and hybridization between geographically definable populations of the rice brown planthopper, Nilaparvata lugens. Biological Journal of the Linnean Society 24, 35-50.

Claridge, M.F. & de Vrijer, P.W.F. (1993) Reproductive behavior: the role of acoustic signals in species recognition and speciation. In Planthoppers: Their Ecology and Management, eds Denno, R.F. & Perfect, T.J. pp.216-233. Chapman & Hall, NY.

Claridge, M.F., Dawah, H.A. & Wilson, M.R. (1997) Species in insect herbivores and parasitoids – sibling species, host races and biotypes. In Species. The Units of Biodiversity, eds Claridge, M.F., Dawah, H.A. & Wilson, M.R. pp.247-272. The Systematics Association Special Volume Series 54. Chapman & Hall, London.

Clary, D.O. & Wolstenholme, D.R. (1985) The mitochondrial DNA molecule of Drosophila yakuba: nucleotide sequence, gene organization, and genetic code. Journal of Molecular Evolution 22, 252-271.

Dungey, H.S., Potts, B.M., Whitham, T.G. & Li, H.-F. (2000) Plant genetics affects arthropod community richness and composition: evidence from a synthetic eucalypt hybrid population. Evolution 54, 1939-1946.

Henry, C.S. (1994) Singing and cryptic speciation in insects. Trends in Ecology and Evolution 9,388-392.

Henry, C.S., Wells, M.L.M. & Simon, C.M. (1999) Convergent evolution of courtship songs among cryptic species of the carnea group of green lacewings (Neuroptera: Chrysopidae: Chrysoperla). Evolution 53, 1165-1179.

Keller, M. & Percy, D.M. (submitted) The courtship sounds of Cotesia rubecula (Hymenoptera: Braconidae). Journal of Insect Behavior.

McKinnon, G.E.,Vaillancourt R.E., Jackson, H.D. & Potts, B.M. (2001) Chloroplast sharing in the Tasmanian eucalypts. Evolution 55, 703-711.

Percy, D.M. (in prep.) Acoustic signals and phylogeography of cryptic species of Eucalyptus-feeding psyllids (Hemiptera; Psylloidea; Spondyliaspidinae).

Taylor, G.S. (1990) Revision of the genus Schedotrioza Tuthill & Taylor (Homoptera: Psylloidea: Triozidae). Invertebrate Taxononomy 4, 721-751.

Taylor, K.L. (1962) The Australian genera Cardiaspina Crawford and Hyalinaspis Taylor (Homoptera: Psyllidae). Australian Journal of Zoology 10, 307-348.

Wells, M.M. & Henry, C.S. (1998) Songs, reproductive isolation, and speciation in cryptic species of insect: a case study using green lacewings. In Endless Forms: species and speciation, eds Howard, D.J. & Berlocher, S.H. pp.217-233. Oxford Univ. Press, NY.

Whitham, T.G., Martinsen, G.D., Floate, K.D., Dungey, H.S., Potts, B.M. and Keim, P. (1999) Plant hybrid zones affect biodiversity: tools for a genetic based understanding of community structure. Ecology 80, 416-428.

Whitham, T., Morrow, P. & Potts, B.M. (1991) The conservation of hybrid plants. Science 254, 779-780.

Yen, A.L. (2002) Short-range endemism and Australian Psylloidea (Insecta : Hemiptera) in the genera Glycaspis and Acizzia (Psyllidae). Invertebrate Systematics 16, 631-636.

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