Angela E. Douglas

Animals acquire photosynthetically-fixed carbon by forming symbioses with algae and cyanobacteria. These associations are widespread in the phyla Porifera (sponges) and Cnidaria (corals, sea anemones etc.) but otherwise uncommon or absent from animal phyla. It is suggested that one factor contributing to the distribution of animal symbioses is the morphologically-simple body plan of the Porifera and Cnidaria with a large surface area:volume relationship well-suited to light capture by symbiotic algae in their tissues. Photosynthetic products are released from living symbiont cells to the animal host at substantial rates. Research with algal cells freshly isolated from the symbioses suggests that low molecular weight compounds (e.g. maltose, glycerol) are the major release products but further research is required to assess the relevance of these results to the algae in the intact symbiosis. Photosynthesis also poses risks for the animal because environmental perturbations, especially elevated temperature or irradiance, can lead to the production of reactive oxygen species, damage to membranes and proteins, and ‘bleaching’, including breakdown of the symbiosis. The contribution of non-photochemical quenching and membrane lipid composition of the algae to bleaching susceptibility is assessed. More generally, the development of genomic techniques to help understand the processes underlying the function and breakdown of function in photosynthetic symbioses is advocated.

Summary: Many symbioses are widespread, abundant, and evolutionarily persistent. This is despite unambiguous evidence for conflict between the partners and the existence of cheats that use benefits derived from their partners while providing reduced or no services in return. Evidence from a diversity of associations suggests that symbioses are robust to cheating in several ways. Some symbioses persist despite conflict and cheating because of the selective advantage of cost‐free interactions (also known as byproduct mutualistic interactions), which incur no conflict. There is also evidence for the suppression of cheating by sanctions imposed by partners in some symbioses, and vertical transmission has been shown experimentally to promote traits that enhance partner performance. It is argued that these processes contribute to the apparent rarity of evolutionary transitions from symbiosis to parasitism. There is strong phylogenetic evidence for the evolutionary reversion of various symbiotic organisms to free‐living lifestyles, but at least some of these transitions can be attributed to selection pressures other than within‐symbiosis conflict. The principal conclusion is that, although conflict is common in symbioses, it is generally managed and contained. Contents Summary 849 I. Introduction 850 II. Conflict and cheats 850 III. Symbiotic interactions without cost 852 IV. Routes to conflict resolution 854 V. Discussion 856 Acknowledgements 857 References 857

The interactions between herbivorous insects and their symbiotic micro-organisms can be influenced by the plant species on which the insects are reared, but the underlying mechanisms are not understood. Here, we identify plant nutrients, specifically amino acids, as a candidate factor affecting the impact of symbiotic bacteria on the performance of the phloem-feeding aphidAphis fabae.Aphis fabaegrew more slowly on the labiate plantLamium purpureumthan on an alternative host plantVicia faba, and the negative effect ofL. purpureumon aphid growth was consistently exacerbated by the bacterial secondary symbiontsRegiella insecticolaandHamiltonella defensa, which attained high densities inL. purpureum-reared aphids. The amino acid content of the phloem sap ofL. purpureumwas very low; andA. fabaeon chemically defined diets of low amino acid content also grew slowly and had elevated secondary symbiont densities. It is suggested that the phloem nutrient profile ofL. purpureumpromotes deleterious traits in the secondary symbionts and disturbs insect controls over bacterial abundance.

Aphids ( Myzus persicae , Macrosiphum euphorbiae and Aulacorthum solani ) are reared on potato plants with phloem sucrose concentrations reduced by up to two‐fold by expressing the antisense of the sucrose‐H ⁺ symporter ( StSUT1 ) gene. The performance of My. persicae and A. solani on the antisense plants is comparable or superior to that on the wild‐type plants, but Ma. euphorbiae increases more slowly on the antisense plants than on the wild‐type and fails to feed from the antisense line with the lowest phloem sucrose concentration. Electrical monitoring by electical penetration graphs reveals that Ma. euphorbiae either do not locate the sieve elements or withdraw their stylets prematurely from the sieve elements of these plants. This difference between Ma. euphorbiae and the other aphid species may reflect interspecific variation in response both to the low phloem sucrose in transgenic plants and to pleiotropic effects of the transgene on the wider physiology of the plants. All aphid species perform well on plants with phloem sucrose concentrations that, when administered via chemically‐defined diets, support little or no aphid growth. These results illustrate the need for caution in extrapolating conclusions reached for diet‐reared aphids to aphids on plants, and demonstrate the importance of plant‐based experiments for studies of the nutritional physiology of aphids.

Facultative ‘secondary’ bacterial symbionts influence various traits of aphids, including plant utilization patterns and resistance to parasitoids. The present study is designed to test the hypothesis that these multiple effects are underlain by symbiont‐mediated changes to the aphid requirement for the dominant dietary nutrients, sucrose and amino acids. The performance of pea aphids ( Acyrthosiphon pisum ) on chemically defined diets of systematically altered sucrose and amino acid content varies among eight parthenogenetic clones, with a pattern that does not match the aphid complements of secondary symbionts, Hamiltonella defensa , Regiella insecticola and Serratia symbiotica . Aphid performance is reduced, increased and unaffected by elimination of S. symbiotica , R. insecticola and H. defensa , respectively, but with no significant effect on the range of diets on which aphids performed well. It is concluded that the impact of secondary symbionts on aphid traits is most unlikely to have a purely nutritional basis.

SUMMARY Plant phloem sap is an extreme diet for animals, partly because of its high and variable sugar content. The physiological and feeding traits of the pea aphid Acyrthosiphon pisum that define the upper and lower limits to the range of dietary sucrose concentrations utilised by this insect were determined principally using chemically defined diets containing 0.125–1.5 mol l^–1 sucrose. On the diets with 0.125 mol l^–1 and 1.5 mol l^–1 sucrose, the aphids died as larvae within 8 and 14 days of birth, respectively. On the other diets, 60–96% of aphids developed to adulthood, and the 0.5 mol l^–1 and 0.75 mol l^–1 diets supported the highest fecundity. The diet with 0.125 mol l^–1 sucrose was ingested at 36% of the rate of the 0.25 mol l^–1 sucrose diet, but >90% of ingested sucrose-carbon was assimilated on both diets. This suggests that the lower limit is dictated by the aphid feeding response, specifically, a requirement for a minimal concentration of sucrose for sustained feeding. The haemolymph osmotic pressure of aphids on diets with 0.125–1.5 mol l^–1 sucrose was up to 68% higher than on 0.125–1.0 mol l^–1 sucrose diets, but diet consumption and sucrose-carbon assimilation was not reduced on the very high sucrose diets relative to 1.0 mol l^–1 sucrose. This suggests that failure of the osmoregulatory capacity of the insects on high sucrose diets may define the upper limit to the range of dietary sucrose utilised by the aphids. The mean haemolymph osmotic pressure of aphids on plants with phloem sap containing 0.37–0.97 mol l^–1 sucrose was 1.61±0.063 MPa (mean ± s.e.m.), not significantly different from that (1.47±0.059 MPa) on diets with 0.25–1.0 mol l^–1 sucrose. It is concluded that the osmoregulatory response of aphids to diets and plants are comparable, and, more generally, that the feeding and osmoregulatory capabilities of the aphids are compatible with the phloem sugar levels commonly encountered by aphids feeding on plants.

The molecular diversity of dinoflagellate algae of the genus Symbiodinium in symbiosis with ten species of scleractinian coral, sea anemone and scyphomedusa in the Mediterreanean Sea was investigated by restriction analysis and sequencing of polymerase chain reaction-amplified fragments of SSU and LSU rRNA genes. All species bore Symbiodinium of clade A, apart from Bunodeopsis strumosa, which had clade B. The clade A sequences clustered with a previously described group, ‘temperate-A’, known only from the Mediterranean and Atlantic coasts of the UK and France. It is concluded that clade temperate-A is prevalent, but not universal, in the Mediterranean.

The incidence of phloem sap feeding by animals appears paradoxical. Although phloem sap is nutrient-rich compared with many other plant products and generally lacking in toxins and feeding deterrents, it is consumed as the dominant or sole diet by a very restricted range of animals, exclusively insects of the order Hemiptera. These insects display two sets of adaptations. First, linked to the high ratio of non-essential:essential amino acids in phloem sap, these insects contain symbiotic micro-organisms which provide them with essential amino acids. For example, bacteria of the genus Buchnera contribute up to 90% of the essential amino acids required by the pea aphid Acyrthosiphon pisum feeding on Vicia faba. Second, the insect tolerance of the very high sugar content and osmotic pressure of phloem sap is promoted by their possession in the gut of sucrase-transglucosidase activity, which transforms excess ingested sugar into long-chain oligosaccharides voided via honeydew. Various other animals consume phloem sap by proxy, through feeding on the honeydew of phloem-feeding hemipterans. Honeydew is physiologically less extreme than phloem sap, with a higher essential:non-essential amino acid ratio and lower osmotic pressure. Even so, ant species strongly dependent on honeydew as food may benefit from nutrients derived from their symbiotic bacteria Blochmannia.