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Yeast (mg/vial)

Figure 2.15 Changes in reproductive and somatic energy (joules) in female Drosophila melanogaster in response to supplementary yeast in the diet. Energy content was calculated for the eggs produced and the lipid and carbohydrate reserves in long-lived (O) and control (B) populations.

Source: Reprinted from Journal of Insect Physiology, 43, Simmons and Bradley, 779-788. © 1997, with permission from Elsevier.

reproduction comes from the supplementary yeast. Divergence in life history characters of O and B flies during selection are discussed by Simmons and Bradley (1997), and the O flies appear to have been selected for the ability to acquire nutrients. Harshman and Hoffman (2000), among others, have drawn attention to possible artefacts with laboratory selection in Drosophila, such as strong directional selection, constraints on normal behaviour such as dispersal, and an overabundance of food. The latter tends to favour adaptive responses involving the storage of energy reserves (glycogen and triacylglycerol), which are commonly measured in laboratory selection experiments, rather than conservation of energy by lowering metabolic rate, which might be more evident in the wild. Section 4.5 deals with laboratory evolution and comparative studies in relation to water balance of Drosophila.

Developmental trade-offs may not be apparent if larvae are provided with discrete resources (van Noordwijk and de Jong 1986). In mass-provisioning solitary Hymenoptera, adult body size is controlled by maternally provided resources in a protected environment (Klostermeyer et al. 1973). An energy budget for reproduction is easily constructed by collecting either intact provisions, or fully grown larvae and their faeces, from sealed brood cells. The pollen-nectar mixture provided by female solitary bees is a high-quality food, and high assimilation efficiencies have been measured in carpenter bees Xylocopa capitata (Anthophoridae) (Louw and Nicolson 1983) and leafcutter bees Megachile pacifica (Megachilidae) (Wightman and Rogers 1978). Female offspring receive larger provisions than males and are larger than males, and this greater investment has implications for the sex ratio of the offspring, males being cheaper to produce (Bosch and Vicens 2002). These authors verified the use of body size as an estimate of production costs in bees by showing that weight loss of Osmia cornuta (Megachilidae) throughout the life cycle did not differ significantly between the sexes, in spite of differences in metabolic rate, water content, cocoon construction, and development time. Male size dimorphism in bees (involving different size classes) is also controlled by female provisioning decisions (Tomkins et al. 2001). Similar relationships between provision weight and adult weight apply to solitary wasps, except that larvae are given prey such as paralysed spiders—nicely demonstrated by Marian et al. (1982) with energy budgets for a wasp nesting in the holes of electrical sockets. The digger wasp Ammophila sabulosa (Sphecidae) provisions cells with caterpillars of varying size, but offspring size is controlled by a flexible provisioning strategy which results in the same total weight of prey in each cell (Field 1992).

2.5.2 Developmental trade-offs between body parts

Metamorphosis allows resources to be redistributed among body compartments. Holometabolous insects are therefore ideal organisms for examining how resources accumulated during larval stages are allocated to reproductive or somatic tissues. Most of the growth of imaginal discs occurs in a closed system after the larva stops feeding. Removal of hind wing imaginal discs from larvae of a butterfly results in disproportionately large forewings, and use of juvenile hormone treatment to reduce horn development in male dung beetles, Onthophagus taurus (Scarabaeidae), leads to a compensatory increase in size of the compound eyes, which develop in close proximity to the horn (Nijhout and Emlen 1998). In these examples, competition between body parts is suggested by increased growth of one trait at the expense of growth in another, with no change in overall body size. Horn development is dimorphic in O. taurus, occurring only in males above a threshold size. The threshold is lower in populations subsisting on poor quality food (cow manure rather than horse dung) (Moczek 2002). In case-building caddis flies, larval resources can be manipulated by inducing them to build new cases and produce additional silk. Stevens et al. (2000) have demonstrated empirically that a short-lived caddis species preserves abdomen size (an index of reproductive allocation) at the expense of the thorax, while a long-lived species preserves thorax size in order to maintain longevity. This flexible trade-off between larval defence and adult body size again implies the partitioning of finite resources between parts of the body.

Adult feeding and reproduction in Leppidoptera Butterflies provide excellent material for examining allocation of larval nutrients to reproduction or body building, which essentially means allocation to abdomen or thorax. In two species of Nymphalidae that feed only on nectar as adults, thorax mass decreased with age but abdomen mass decreased more, so flight ability was probably not impaired. In contrast, pollen feeding by a third nymphalid, Heliconius hecale, led to increases in both thorax and abdomen mass with age (Karlsson 1994). Pollen feeding among adult butterflies is unique to Heliconius, and is associated with long adult life and eggs laid singly over a prolonged period. Boggs (1981) predicted that the ratio of reproductive reserves to soma at eclosion would vary inversely with expected adult nutrient intake, and directly with expected reproductive output (assuming that organisms are equivalent in terms of larval nutrition). Her model was supported by data on nitrogen allocation in three species of closely related heliconiines, one of which does not collect pollen. Females obtain nitrogen from larval feeding, adult pollen feeding, and male spermato-phores contributed during mating. Because the abdomen of a newly eclosed butterfly consists mainly of reserves stored in fat body, haemolymph, and developing oocytes, the ratio of abdomen total nitrogen to whole body total nitrogen can be used as an estimate of the allocation of larval resources to reproduction. Pollen feeding is unusual in butterflies, but predictions concerning larval and adult nutrient allocations to reproduction are supported by subsequent studies on other butterflies with different life histories (e.g. May 1992).

Nutrients acquired by adult foraging or from males during mating are renewable, while those accumulated during the larval stage are not. This leads to the distinction between capital breeders with non-feeding adults, and income breeders which accumulate resources for reproduction in both juvenile and adult stages. Nutrient allocation dynamics have recently been examined in more detail in nectar-feeding Lepidoptera in which proteins carried over from the larval stage play a major role in adult fecundity. Boggs (1997) used radiotracers to examine the use of glucose and amino acids acquired in larval and adult stages in two nymphalid butterflies. Glucose and amino acids labelled with 14C and 3H were painted on leaves to assess larval contributions, or included in nectar solutions to assess adult contributions. Because the adult diet is carbohydrate-rich, incoming glucose is used in preference to stored glucose (storing and then remobilizing nutrients incurs additional costs). By contrast, nitrogen is scarce in the adult diet, and juvenile reserves of amino acids are used throughout adult life. Male nutrient donations at mating, assessed by mating females with males which were labelled as larvae, are less predictably allocated, being immediately used in egg production (Boggs 1997). Stable isotopes have provided information on the dietary sources of amino acids used in egg manufacture by a day-flying hawkmoth, Amphion floridensis (O'Brien et al. 2000, 2002). These authors fed larvae on grape leaves (Vitis, C3 species) and adults on sucrose purchased as either beet sugar or cane sugar (C3 and C4 plants, respectively). C3 plants are substantially depleted in 13C relative to C4 plants, so the dietary sources of the carbons in specific egg amino acids can be identified. Essential amino acids originate entirely from the larval diet, whereas nonessential amino acids are synthesized from nectar sugar (Fig. 2.16). Amino acids in nectar contribute insignificantly to egg provisioning. After initial use of larval carbon sources, adult nectar meals provide 60 per cent of the carbon allocated to eggs, but the need for essential amino acids places an upper limit on their use in reproduction. Note that aphids also derive amino acids from dietary sucrose (Fig. 2.12), but their symbionts can synthesize the carbon skeletons of essential amino acids.

Re-allocation of larval nutritional resources can occur after metamorphosis. Flight muscle may be histolysed to provide amine groups for synthesizing non-essential amino acids in egg manufacture (Karlsson 1994). Alternatively, longevity may be favoured at the expense of reproduction, and oocytes are then resorbed when adult food is limited (Boggs and Ross 1993).

Larval performance is more important in Lepidoptera that do not feed as adults. Even as immatures, the sexes differ in food consumption and other performance criteria. Stockoff (1993)

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