Shopping for tenants: does co-evolutionary meltdown leave jumping plant lice in need of new microbial partners?

A project undertaken at the Hawkesbury Institute for the Environment, Western Sydney University, and supervised by Markus Riegler

Coinvestigators: James Cook, Martin Steinbauer

Insect science is undergoing an exciting paradigm shift with the discovery that most insects harbour microbial symbionts that drive many aspects of insect biology. Microbial symbionts of insects can define insect diets, protect insects from parasites, cause insects to speciate and, thus, contribute greatly to the ecological and economic significance of insects. Functionally, insect symbionts can be divided into obligate mutualists that insects rely upon for development and reproduction, and facultative symbionts that may provide insect hosts with fitness advantages under specific environmental conditions. Plant-sap feeding insects from the order Hemiptera (e.g. aphids) harbour intracellular, obligate mutualists, also known as primary (P-) endosymbionts (e.g. Buchnera bacteria in aphids). P-endosymbionts are maternally inherited, found in special cellular structures (the bacteriome; Figure 1), and synthesise essential amino acids that insects cannot obtain from plant-sap. Such mutualistic interactions can lead to the co-evolution of insects and endosymbionts but also to addictive dependence of insects on specialised symbiont genomes that can genetically degrade over time (symbiotic syndrome). However, plant-sap feeding insects also contain diverse secondary symbionts that play important roles in insect biology, and potentially also in host-plant, biology. Secondary (S-) endosymbionts could take up the role of dysfunctional P-endosymbionts, yet the importance and extent of such endosymbiont replacements are unknown. We used psyllids and their symbiotic microbiome to analyse and test this. Our research had the following aims:

  1. to comprehensively characterise the microbiome diversity and composition of Australian psyllid species;
  2. to investigate the co-evolutionary history of psyllids and their P-endosymbionts and S-endosymbionts;
  3. to assess the effect of endosymbiosis on endosymbiont genomes and their interactions with other endosymbionts and the host;
  4. to test the function of psyllid endosymbionts.

Jumping plantlice (or psyllids) are a group of plant-sap feeding insects. Impressively, 10% of all psyllid species are endemic to Australia. A large proportion of Australian psyllids are associated with eucalypts while another large proportion is associated with Acacia. Some Australian psyllids are known as lerp insects (Figures 2 and 3), and can cause occasional massive defoliation of trees (Figure 4) (Hall et al. 2015; Gherlenda et al. 2016; Fromont et al. 2016a).

Our research confirmed that all individuals of the superfamily Psylloidea harbour the gammaproteobacterium ‘Candidatus Carsonella ruddii’ (hereafter Carsonella) as their P-endosymbiont (Hall et al. 2016; Fromont et al. 2016b). Co-phylogenetic analyses of Carsonella and their host psyllids demonstrated strict co-divergence of psyllids and Carsonella. Such clear co-evolution has also been demonstrated for aphids and Buchnera (Gammaproteobacteria), which provides essential amino acids to aphids - as evidenced by functional physiology and comparative genomics. However, the nutritional role of Carsonella in psyllids remains to be tested. The first available whole genome sequence of Carsonella, isolated from the gall-forming hackberry psyllid, is one of the smallest known bacterial genomes, heavily characterised by gene loss and potential lateral gene transfer from Carsonella to the host genome. It lacks key genes required for vital bacterial functions and has also lost many pathways required for the synthesis of essential amino acids - suggesting it may not be able to act as a P-endosymbiont. We have sequenced the complete Carsonella and S-endosymbiont genomes from ten psyllid species and, in combination with seven other previously published Carsonella genomes our comparative analysis demonstrated gradual loss of some but not all Carsonella function across these host lineages.

We have established that all individuals of psyllid species also harbour other bacteria including bacteria that are categorised as S-endosymbionts. In aphids and whiteflies these S-endosymbionts are considered facultative, but our findings suggest that these S-endosymbionts may be more obligate to their hosts because of their universal presence within host species, i.e. they occur in all individuals, and across the entire host range (Hall et al. 2016; Fromont et al. 2016b). All individuals of each psyllid species carried at least one S-endosymbiont type belonging to the Gammaproteobacteria (e.g. Arsenophonus, Sodalis or unclassified bacteria of the family Enterobacteriaceae). We also detected Alphaproteobacteria such as Wolbachia however these Alphaproteobacteria occurred at varying prevalence across populations.

Our research has provided evidence for a number of overarching hypotheses: (a) psyllids and their P-endosymbiont Carsonella have co-diverged and are mutually addicted to each other; (b) Carsonella has experienced genetic meltdown as an outcome of the symbiotic syndrome; (c) psyllids have acquired S-endosymbionts that are obligate to psyllid species and complement the loss of function in Carsonella; (d) the acquisition of new S-endosymbionts has resulted in endosymbiont replacement in some psyllid lineages; (e) the ecological niche of psyllids has partially impacted the psyllids’ microbiomes; (e) some but not all psyllids acquire and transmit plant-pathogenic bacteria. Overall, our research has demonstrated that psyllids, besides their recognised status as pests and pathogen vectors of plants, are highly useful research models for the evolution and dynamics of insect endosymbiosis and animal-microbe interactions.

 Publications:

Fromont C, De Gabriel JL, Riegler M, Cook JM (2016a). Diversity and specificity of sap-feeding herbivores and their parasitoids on Australian fig trees. Insect Conservation and Diversity doi:10.1111/icad.12202

Fromont C, Riegler M, Cook JM (2016b). Phylogeographic analyses of bacterial endosymbionts in fig homotomids (Hemiptera: Psylloidea) reveal co-diversification of both primary and secondary endosymbionts. FEMS Microbiology Ecology 92: fiw205 http://dx.doi.org/10.1093/femsec/fiw205

Gherlenda AN, Esveld JL, Hall AA, Duursma RA, Riegler M (2016). Boom and bust: rapid feedback responses between insect outbreak dynamics and canopy leaf area impacted by rainfall and CO2. Global Change Biology 22: 3632-3641

Hall AAG, Gherlenda AN, Hasegawa S, Johnson SN, Cook JM, Riegler M (2015). Anatomy of an outbreak: the biology and population dynamics of a Cardiaspina psyllid species in an endangered woodland ecosystem. Agricultural and Forest Entomology17: 292-301

Hall AA, Morrow JL, Fromont C, Steinbauer MJ, Taylor GS, Johnson SN, Cook JM, Riegler M (2016). Codivergence of the primary bacterial endosymbiont of psyllids versus host switches and replacement of their secondary bacterial endosymbionts. Environmental Microbiology 18: 2591-2603.

 

 
Figure 1. Nymph of Cardiaspina sp. (Hemiptera: Psyllidae) from Eucalyptus moluccana. The yellow-orange cell structure in the abdominal cavity is the bacteriome that contains symbiotic bacteria such as primary the bacterial symbiont Carsonella ruddi of psyllids; psyllid species are also associated with varying secondary bacterial symbionts (Photo: M. Riegler).

Figure 2. Adult male (left) and female (right) Cardiaspina sp. (Hemiptera: Psyllidae) from Eucalyptus moluccana (Photo: M. Riegler).

Figure 3. Eggs and nymphs of Cardiaspina sp. from Eucalyptus moluccana, covered by characteristic lace lerps, carbohydrate excretions of the psyllids that act as protective cover of the nymphs feeding underneath (Photo: M. Riegler).

Figure 4. Eucalyptus moluccana defoliated by Cardiaspina sp. (Photo: M. Riegler).