Photorhabdus: towards a functional genomic analysis of a symbiont and pathogen

Pathogenicity and symbiosis are central to bacteria-host interactions. Although several human pathogens have been subjected to functional genomic analysis, we still understand little about bacteria-invertebrate interactions despite their ecological prevalence. Advances in our knowledge of this area are often hindered by the difficulty of isolating and working with invertebrate pathogenic bacteria and their hosts. Here we review studies on pathogenicity and symbiosis in an insect pathogenic bacterium Photorhabdus and its entomopathogenic nematode vector and model insect hosts. Whilst switching between these hosts, Photorhabdus changes from a state of symbiosis with its nematode vector to one of pathogenicity towards its new insect host and both the bacteria and the nematode then cooperatively exploit the dying insect. We examine candidate genes involved in symbiosis and pathogenicity, their secretion and expression patterns in culture and in the host, and begin to dissect the extent of their genetic coregulation. We describe the presence of several large genomic islands, putatively involved in pathogenicity or symbiosis, within the otherwise Yersinia-like backbone of the Photorhabdus genome. Finally, we examine the emerging comparative genomics of the Photorhabdus group and begin to describe the interrelationship between anti-invertebrate virulence factors and those used against vertebrates.

Insect lipoprotein follows a transferrin-like recycling pathway that is mediated by the insect LDL receptor homologue

The lipoprotein of insects, high-density lipophorin (HDLp), is homologous to that of mammalian low-density lipoprotein (LDL) with respect to its apolipoprotein structure. Moreover, an endocytic receptor for HDLp has been identified (insect lipophorin receptor, iLR) that is homologus to the LDL receptor. We transfected LDL-receptor-expressing CHO cells with iLR cDNA to study the endocytic uptake and intracellular pathways of LDL and HDLp simultaneously. Our studies provide evidence that these mammalian and insect lipoproteins follow distinct intracellular routes after receptor-mediated endocytosis. Multicolour imaging and immunofluorescence was used to visualize the intracellular trafficking of fluorescently labeled ligands in these cells. Upon internalization, which can be completely inhibited by human receptor-associated protein (RAP), mammalian and insect lipoproteins share endocytic vesicles. Subsequently, however, HDLp evacuates the LDL-containing endosomes. In contrast to LDL, which is completely degraded in lysosomes after dissociating from its receptor, both HDLp and iLR converge in a nonlysosomal juxtanuclear compartment. Colocalization studies with transferrin identified this organelle as the endocytic recycling compartment via which iron-depleted transferrin exits the cell. Fluorescently labeled RAP is also transported to this recycling organelle upon receptor-mediated endocytosis by iLR. Internalized HDLp eventually exits the cell via the recycling compartment, a process that can be blocked by monensin, and is re-secreted with a t(1/2) of approximately 13 minutes. From these observations, we conclude that HDLp is the first non-exchangeable apolipoprotein-containing lipoprotein that follows a transferrin-like recycling pathway despite the similarities between mammalian and insect lipoproteins and their receptors.

Digestion of nectar and insects by Palestine sunbirds

In nectarivorous birds, specialization for feeding on nectar has led to a simple gut structure with high sugar digestive efficiencies and rapid gut passage rates. These features of the digestive system may make digestion of more complex, protein-rich food sources, such as pollen or insects, less efficient. In this light, we hypothesized that sugar metabolizability in nectarivorous Palestine sunbirds (Nectarinia osea) would be high, whereas nitrogen metabolizability would be lower than typically found for birds. We measured glucose and fructose apparent metabolizabilities (*MCs) and transit times (TTs) in eight Palestine sunbirds offered either a 10% or a 50% mixed sugar diet. *MC for glucose (99.9%+/-0.1%) was significantly greater than for fructose (99.6%+/-0.4%; ANOVA; P<0.001). TT for the 10% sugar diet (26.3+/-10.1 min) was significantly shorter than for the 50% sugar diet (47.0+/-7.8 min). We measured nitrogen true metabolizability (MC) and TT in Palestine sunbirds offered a daily fruit fly intake of either 40 or 200 flies. Nitrogen MC was not significantly different between diets, and average MC for both diets was 58.5%+/-8.5% (n=8). TT was not significantly different when birds ate 10 flies (50.1+/-13.6 min) than when they ate 50 flies (48.5+/-16.5 min). The high sugar *MC and relatively rapid TT of nectar in Palestine sunbirds are similar to those found for other nectarivorous species. Transit times of insect material are longer that those found in small insectivorous species. However, MCs of insect material are lower. Thus, even though sunbirds consume easily digestible soft-bodied insects, they are less efficient at extracting protein than nonnectarivores.

High-throughput approaches to study salivary proteins and genes from vectors of disease

Blood-feeding arthropods have in their saliva a variety of molecules that affect the vertebrate host's hemostatic, inflammatory, and immune systems. The saliva of blood feeders also helps to facilitate infection of the pathogens they carry to the host, making vector saliva an attractive target to control pathogen transmission. The isolation and identification of salivary molecules from vectors of disease has been slow and difficult. Emerging technologies and new approaches in the fields of molecular biology and protein chemistry are facilitating this work. Massive sequencing of high quality, full-length cDNA libraries, coupled with proteomics and functional genomic approaches has led to the discovery of novel proteins, transcription products (genes), and biologic activities from the salivary glands of blood-feeding arthropods. This review focuses on the biologic activities identified in the saliva of various vectors of disease using classic biochemical and molecular biology approaches and new types of molecules and activities identified with high-throughput strategies.

Evolution of inhibitors of apoptosis in baculoviruses and their insect hosts

The phylogenetic relationships of inhibitor of apoptosis (IAP) from insects and insect viruses were reconstructed and compared with the phylogeny of the viruses reconstructed on the basis of DNA polymerase species. The phylogeny supported the hypothesis that there were three IAP genes in the ancestor of the viral genus Nucleopolyhedrovirus (family Baculoviridae), but that there has been differential deletion of IAP genes in different lineages within this genus. An IAP gene from the granulovirus of the lepidopteran species Cydia pomonella (CpGV) was found to be a close relative of IAP genes from species of the insect order Lepidoptera, supporting the origin of this viral gene by capture of a host gene early in the evolution of Lepidoptera. The phylogeny supported the occurrence of least one other independent event of capture of an IAP gene by a virus and suggested the possibility of at least two other such events. Contrary to the prediction that host genes with viral homologues should experience an enhanced rate of amino acid replacement, no acceleration of evolutionary rate was detectable in these lepidopteran genes, which showed particularly low rates of non-synonymous nucleotide substitution in functionally important domains.

Significance of penetration, excretion, and transovarial uptake to toxicity of three insect growth regulators in predatory lacewing adults

Topical treatment of the predatory lacewing adults, Chrysoperla carnea (Stephens) (Neuroptera: Chrysopidae) before and after the onset of oviposition with diflubenzuron (DFB) at doses based on the maximum field recommended concentration resulted in a total inhibition of egg hatch due to death of the embryo. In contrast, pyriproxyfen (PYR) and tebufenozide (TEB) did not affect fecundity and egg fertility. To explain these differences in toxicity, the patterns of penetration through the cuticle, distribution inside the insect body, and excretion were studied using [(14)C]-labeled isotopes of each insect growth regulator (IGR). Penetration of DFB and TEB reached about 16 and 26% in 7 days, whereas 88% of PYR had penetrated in 24 h. However, the rate of excretion for PYR was very high, compared to that of DFB and TEB. Low amounts of absorbed radioactivity were recovered from the female body with the exception of DFB, the ovaries and the eggs deposited during a week. DFB and PYR concentration reached a peak in the eggs deposited at the fourth and second day after treatment, respectively. The current data indicate the importance of penetration through the insect cuticle. However, other mechanisms are likely to be involved in the selectivity of the current IGRs towards this beneficial insect.

Evaluation of metabolic detoxifying enzyme activities and insecticide resistance in Frankliniella occidentalis

The western flower thrips Frankliniella occidentalis (Pergande) is a very significant pest of a number of different agricultural crops in the south-east of Spain. The importance of thrips as a pest is not due mainly to the direct damage inflicted on the plant, but to the loss in commercial value which occurs as a consequence of the development of dark spots caused by the tomato spotted wilt virus (TSWV) which they transmit. The economic threshold is therefore almost zero, which enhances the problems of resistance management. The present work is part of a global project that attempts to evaluate the status of insecticide resistance in field populations of thrips obtained from several agricultural crops. We have studied, in either individual or pooled insects, some enzyme systems classically related to detoxification of insecticides: esterase and glutathione-S-transferase (GST). The activity of these enzymes from laboratory populations selected with various classes of insecticides has also been measured using several appropriate substrates. An increase in GST mean activity was found in two field-collected strains. Differences in frequency distributions of esterase and GST activities were found for both field-collected strains and for a laboratory strain selected with acrinathrin. These activities were compared with those of a wild-type reference strain.

Structure and activity of insect cytokine GBP which stimulates the EGF receptor

Growth-blocking peptide (GBP) is an insect cytokine that possesses diverse biological activities such as larval growth regulation, cell proliferation, and stimulation of immune cells. GBP is a 25-amino acid peptide with one disulfide bond. It has been revealed that the tertiary structure of GBP consists of an N- and C-terminal disordered region and a well-structured core. Although there is only a slight similarity between the primary structures of GBP and EGF and the molecular weight of GBP is about half that of EGF, GBP directly binds and activates the EGF receptor of human keratinocyte cells. Furthermore, the tertiary structure of the well-defined region of GBP is similar to that of the C-terminal domain of EGF. This review will focus on the tertiary structure of GBP and its activities, as compared with those of EGF.

Structure and function of antifreeze proteins

High-resolution three-dimensional structures are now available for four of seven non-homologous fish and insect antifreeze proteins (AFPs). For each of these structures, the ice-binding site of the AFP has been defined by site-directed mutagenesis, and ice etching has indicated that the ice surface is bound by the AFP. A comparison of these extremely diverse ice-binding proteins shows that they have the following attributes in common. The binding sites are relatively flat and engage a substantial proportion of the protein's surface area in ice binding. They are also somewhat hydrophobic -- more so than that portion of the protein exposed to the solvent. Surface-surface complementarity appears to be the key to tight binding in which the contribution of hydrogen bonding seems to be secondary to van der Waals contacts.

[Lipid metabolism of caddisfly larvae at low pH]

The influence of low pH (5.0 and 4.0) on lipid metabolism of caddisworms Hydropsyche contubernalis L. (Trichoptera) was studied in 48 h toxicity experiments. The results were correlated with lipid composition of caddisworms directly isolated from natural water. Phospholipids, cholesterol, mono-, di-, triacylglycerols, and fatty acids were detected by thin-layer and liquid chromatography. Minimal environmental changes were shown to initiate the biochemical adaptation mechanisms strengthening the cellular membranes through their condensation due to additional phospholipid and cholesterol synthesis. In the natural medium the adaptation processes are more active than in the artificial medium. More serious changes, such as pH decrease to 4.0, suppress the adaptation processes in the first medium and terminated them in the second one.

Surface membranes, Golgi complexes, and vacuolar systems

In the absence of fossils, the cells of vertebrates are often described in lieu of a general animal eukaryote model, neglecting work on insects. However, a common ancestor is nearly a billion years in the past, making some vertebrate generalizations inappropriate for insects. For example, insect cells are adept at the cell remodeling needed for molting and metamorphosis, they have plasma membrane reticular systems and vacuolar ferritin, and their Golgi complexes continue to work during mitosis. This review stresses the ways that insect cells differ from those of vertebrates, summarizing the structure of surface membranes and vacuolar systems, especially of the epidermis and fat body, as a prerequisite for the molecular studies needed to understand cell function. The objective is to provide a structural base from which molecular biology can emerge from biochemical description into a useful analysis of function.

Analysis of methotrexate and folate transport by multidrug resistance protein 4 (ABCC4): MRP4 is a component of the methotrexate efflux system

Human MRP4 (ABCC4, MOAT-B) is a lipophilic anion transporter that is able to confer resistance to nucleotide analogues and methotrexate (MTX). We previously investigated the implications of the ability of MRP4 to confer resistance to nucleotide analogues and determined that the pump is competent in the MgATP-energized transport of cyclic nucleotides and estradiol 17beta-D-glucuronide. Here we examine the potential role of MRP4 in conferring resistance to MTX and related processes by determining the selectivity of the transporter for MTX, MTX polyglutamates, and physiological folates. In so doing, it is shown that MRP4 is active in the transport of MTX as well as the physiological folates folic acid (FA) and N(5)-formyltetrahydrofolic acid (leucovorin). MTX, FA, and leucovorin are subject to high capacity [V(max(MTX)), 0.24 +/- 0.05 nmol/mg/min; V(max (FA)), 0.68 +/- 0.14 nmol/mg/min; V(max(leucovorin)), 1.95 +/- 0.18 nmol/mg/min], low affinity [K(m(MTX)), 0.22 +/- 0.01 mM; K(m(FA)), 0.17 +/- 0.02 mM; K(m (leucovorin)), 0.64 +/- 0.23 mM] transport by MRP4. In addition, as would be expected were MRP4 a component of the MTX efflux system, its capacity to transport this agent is abrogated by the addition of a single glutamyl residue. It is also shown that glutamylation similarly affects the ability of MRP2 to transport MTX. On the basis of these transport properties, it is concluded that the efflux system for MTX includes MRP2 and MRP4, in addition to MRP1 and MRP3, and that MRP4 represents a common efflux system for both MTX and certain nucleotide analogues.

Structure-function analysis of human triacylglycerol hydrolase by site-directed mutagenesis: identification of the catalytic triad and a glycosylation site

Triacylglycerol hydrolase is a microsomal enzyme that hydrolyzes stored cytoplasmic triacylglycerol in the liver and participates in the lipolysis/re-esterification cycle during the assembly of very-low-density lipoproteins. The structure-activity relationship of the enzyme was investigated by site-directed mutagenesis and heterologous expression. Expression of human TGH in Escherichia coli yields a protein without enzymatic activity, which suggests that posttranslational processing is necessary for the catalytic activity. Expression in baculovirus-infected Sf-9 cells resulted in correct processing of the N-terminal signal sequence and yielded a catalytically active enzyme. A putative catalytic triad consisting of a nucleophilic serine (S221), glutamic acid (E354), and histidine (H468) was identified. Site-directed mutagenesis of the residues (S221A, E354A, and H468A) yielded a catalytically inactive enzyme. CD spectra of purified mutant proteins were very similar to that of the wild-type enzyme, which suggests that the mutations did not affect folding. Human TGH was glycosylated in the insect cells. Mutagenesis of the putative N-glycosylation site (N79A) yielded an active nonglycosylated enzyme. Deletion of the putative C-terminal endoplasmic reticulum retrieval signal (HIEL) did not result in secretion of the mutant protein. A model of human TGH structure suggested a lipase alpha/beta hydrolase fold with a buried active site and two disulfide bridges (C87-C116 and C274-C285).

Insects, oxygen, and iron

Konrad Bloch developed an interest in insects because they are unable to make sterols, and in yeast because these cells need oxygen to make sterols and unsaturated fatty acids. Insects, like all other organisms, must deal with the toxic effects of oxygen in the presence of iron, which itself is a vital nutrient. They do so by making proteins with high affinity for ferric or ferrous ions. Two such proteins are transferrins and ferritins. Insects produce both of these proteins, but use them in different ways from most other organisms. Insect transferrins appear to be involved in innate immunity, perhaps by sequestering ferric ions to prevent pathogens and parasites from utilizing them. Insect ferritins, unlike those of any other group of organisms, are exported into the extracellular space (hemolymph). They may be involved in iron transport and/or protection against iron overload in the diet.

Inactivation of a tachykinin-related peptide: identification of four neuropeptide-degrading enzymes in neuronal membranes of insects from four different orders

Tachykinin-related peptides (TRP) are widely distributed in the CNS of insects, where they are likely to function as transmitters/modulators. Metabolic inactivation by membrane ecto-peptidases is one mechanism by which peptide signalling is terminated in the CNS. Using locustatachykinin-1 (LomTK-1, GPSGFYGVRamide) as a substrate and several selective peptidase inhibitors, we have compared the types of membrane associated peptidases present in the CNS of four insects, Locusta migratoria, Leucophaea maderae, Drosophila melanogaster and Lacanobia oleracea. A neprilysin (NEP)-like activity cleaving the G-F peptide bond was the major LomTK-1-degrading peptidase detected in locust brain membranes. NEP activity was also found in Leucophaea brain membranes, but the major peptidase was an angiotensin converting enzyme (ACE), cleaving the G-V peptide bond. Drosophila adult head and larval neuronal membranes cleaved the G-F and G-V peptide bonds. Phosphoramidon inhibited both these cleavages, but with markedly different potencies, indicating the presence in the fly brain of two NEP-like enzymes with different substrate and inhibitor specificity. In Drosophila, membrane ACE did not make a significant contribution to the cleavage of the G-V bond. In contrast, ACE was an important membrane peptidase in Lacanobia brain, whereas very little neuronal NEP could be detected. A dipeptidyl peptidase IV (DPP IV) that removed the GP dipeptide from the N-terminus of LomTK-1 was also found in Lacanobia neuronal membranes. This peptidase was a minor contributor to LomTK-1 metabolism by neuronal membranes from all four insect species. In Lacanobia, LomTK-1 was also a substrate for a deamidase that converted LomTK-1 to the free acid form. However, the deamidase was not an integral membrane protein and could be a lysosomal contaminant. It appears that insects from different orders can have different complements of neuropeptide-degrading enzymes. NEP, ACE and the deamidase are likely to be more efficient than the common DPP IV activity at terminating neuropeptide signalling since they cleave close to the C-terminus of the tachykinin, a region essential for maintaining biological activity.

Enhanced in vivo activity of peptidase-resistant analogs of the insect kinin neuropeptide family

The diuretic/myotropic insect kinin neuropeptides, which share the common C-terminal pentapeptide core FX(1)X(2)WG-NH(2), reveal primary (X(2)-W) and secondary (N-terminal to F) sites of susceptibility to peptidases bound to corn earworm (H. zea) Malpighian tubule tissue. Analogs designed to enhance resistance to tissue-bound peptidases, and pure insect neprilysin and ACE, demonstrate markedly enhanced in vivo activity in a weight gain inhibition assay in H. zea, and strong in vivo diuretic activity in the housefly (M. domestica). The peptidase-resistant insect kinin analog pQK(pQ)FF[Aib]WG-NH(2) demonstrates a longer internal residence time in the housefly than the native muscakinin (MK), and despite a difference of over 4 orders of magnitude in an in vitro Malpighian tubule fluid secretion assay, is equipotent with MK in an in vivo housefly diuretic assay. Aminohexanoic acid (Ahx) is shown to function as a surrogate for N-terminal Lys, while at the same time providing enhanced resistance to aminopeptidase attack. Peptidaese-resistant insect kinin analogs demonstrate enhanced inhibition of weight gain in larvae of the agriculturally destructive corn earworm moth. Potent peptidase resistant analogs of the insect kinins, coupled with an increased understanding of related regulatory factors, offer promise in the development of new, environmentally friendly pest insect control measures.

Glucosinolate breakdown products as insect fumigants and their effect on carbon dioxide emission of insects

BACKGROUND

Glucosinolate breakdown products are volatile, therefore good candidates for insect fumigants. However, although they are insecticidal, the mode of action of such natural products is not clear. We studied the insecticidal effect of these compounds as fumigants, and monitored the production of carbon dioxide by the insects as a probe to the understanding of their mode of action.

RESULTS

The fumigation 24-h LC50 against the house fly (Musca domestica L.) of allyl thiocyanate, allyl isothiocyanate, allyl cyanide, and l-cyano-2-hydroxy-3-butene was 0.1, 0.13, 3.66, and 6.2 microg cm-3, respectively; they were 0.55, 1.57, 2.8, and > 19.60 microg cm-3, respectively, against the lesser grain borer (Rhyzopertha dominica Fabricius). The fumigation toxicity of some of the glucosinolate products was very close to or better than that of the commercial insect fumigants such as chloropicrin (LC50: 0.08 and 1.3 microg cm-3 against M. domestica and R. dominica, respectively) and dichlorovos (LC50: < 0.02 and 0.29 microg cm-3 against M. domestica and R. dominica, respectively) in our laboratory tests. Significantly increased CO2 expiration was found in insects exposed to the vapor of allyl isothiocyanate, allyl thiocyanate and allyl isocyanate. Allyl isothiocyanate was also found to increase the CO2 expiration of the American cockroach (Periplaneta americana L.).

CONCLUSIONS

Glucosinolate breakdown products have potential as biodegradable and safe insect fumigants. They may act on the insect respiratory system in their mode of action.

Localization of metals in cells of pterygote insects

Insects maintain the equilibrium of their internal milieu by diffusion mechanisms, but more frequently by a substantial storage of metals in the cells of numerous organs: digestive tract, Malpighian tubules, fat body, integument, and genital organs. This storage implies the precipitation, of numerous cations (Ca, Mg, K, Mn, Fe, Zn, Cu) in a structure called the spherocrystal, which originates from the endoplasmic reticulum-Golgi complex: elements precipitate on a glycosaminoglycan nucleus in thin peripheral strata. Some spherocrystals contain exclusively mineral compounds, frequently phosphates, whereas others may contain organic compounds such as urates. In some species mineralized lysosomes store Ca, Fe, Zn, and Cu. When fed additional metals found in the environment, insects such as cockroach and ant are able to stay alive and to trap the metals (Cd or Pb, for example) in the peripheral strata of spherocrystals; the cytoplasm is not altered. It seems that these insects are able to resist exposures to high levels of toxic metals. The lysosomes are able to retain toxic heavy metals (Cd or Hg, for example) within metallothionein-like proteins.

Characterization of G3BPs: tissue specific expression, chromosomal localisation and rasGAP(120) binding studies

The G3BP (ras-GTPase-Activating Protein SH3-Domain-Binding Protein) family of proteins has been implicated in both signal transduction and RNA-metabolism. We have previously identified human G3BP-1, G3BP-2, and mouse G3BP-2. Here, we report the cloning of mouse G3BP-1, the discovery of two alternatively spliced isoforms of mouse, and human G3BP-2 (G3BP-2a and G3BP-2b), and the chromosomal localisation of human G3BP-1 and G3BP-2, which map to 5q14.2-5q33.3 and 4q12-4q24 respectively. We mapped the rasGAP(120) interactive region of the G3BP-2 isoforms and show that both G3BP-2a and G3BP-2b use an N-terminal NTF2-like domain for rasGAP(120) binding rather than several available proline-rich (PxxP) motifs found in members of the G3BPs. Furthermore, we have characterized the protein expression of both G3BP-1 and G3BP-2a/b in adult mouse tissues, and show them to be both tissue and isoform specific.

Effects of various muscarinic ligands on M2AChR-Gi1alpha fusion protein expressed in Sf9 insect cells

AIM

To generate M2AChR-Gi1alpha fusion protein in baculovirus-Sf9 cells system and detect the effects of various muscarinic ligands and magnesium ion on the interaction of fused M2AChR and Gi1alpha.

METHODS

M2AChR-Gi1alpha fused DNA was generated in a two step polymerase chain reaction (PCR) and then expressed in Sf9 cells to produce fusion protein. [3H] L-quinuclidinyl benzilate (QNB) and 35S GTPgammaS binding experiments were performed to study the function of M2AChR-Gi1alpha fusion protein.

RESULTS

The expression level of M2AChR-Gi1alpha was (20.12 +/- 0.14) nmol/g protein. The affinity of GDP to Gi1alpha partner changed in the presence of different muscarinic ligands. IC50 values (95 % confidence limit) of GDP in the presence of acetylcholine (ACh), carbamylcholine, (4-hydroxy-2-butynyl)-1-trimethylammonium-m-chloro-carbanilate chloride (McN-A-343), pilocarpine, and atropine were 178 (148 - 214) micromol/L, 158 (126 - 199) micromol/L, 66 (56 - 78) micromol/L, 62 (55 -7 2) micromol/L, and 5.0 (4.6 - 5.5) micromol/L, respectively, and that in the absence of muscarinic ligand was 15.9 (14.3 - 17.6) micromol/L. Apparent affinity for GDP in the presence of carbamylcholine was markedly decreased with increasing MgCl2 concentrations, although the apparent affinity in the presence of atropine was not affected.

CONCLUSION

The M2AChR-Gi1alpha fusion protein has the pharmacological specificity of M2 receptor and the efficient signaling of the two partners. Affinity of GDP to ligand-bound fusion protein represents the species of muscarinic ligands. Mg2+ is necessary for the action of M2AChR on Gi1alpha.

Do germ line cells in Allacma fusca (Insecta, Collembola, Symphypleona) have a higher metabolic rate than somatic cells

Stereological analysis of the ultrastructure of primordial germ cells (PGCs) and the somatic (ectoderm) cells in two developmental stages of embryos and freshly hatched juveniles of Allacma fusca have shown great differences in mitochondria volume density (vd) between the two types of cells. In younger embryos (migration phase of the PGCs) the vd of mitochondria in the cytoplasm of the PGCs is 74.64% higher than in the ectoderm cells. In older embryos, (PGCs in the gonads) the vd of mitochondria is 123% higher than the corresponding value for the somatic cells cytoplasm. In the juvenile the vd of mitochondria in the ectoderm cells grows twice but is still only 2/3 of the value for the PGCs. On the basis of papers describing a direct relationship between stereological and physiological results the authors conclude that the metabolism of the primordial germ cells during embryonic development of Allacma fusca is much higher than that of the somatic ones. If the above conclusion is correct, the results presented here confirm the "disposable soma theory" (Kirkwood & Holliday 1979).