Why is there sequence similarity between insect yolk proteins and vertebrate lipases?

Lipids in Insect Reproduction: Where, How, and Why

Modern insects have inhabited the earth for hundreds of millions of years, and part of their successful adaptation lies in their many reproductive strategies. Insect reproduction is linked to a high metabolic rate that provides viable eggs in a relatively short time. In this context, an accurate interplay between the endocrine system and the nutrients synthetized and metabolized is essential to produce healthy offspring. Lipids guarantee the metabolic energy needed for egg formation and represent the main energy source consumed during embryogenesis. Lipids availability is tightly regulated by a complex network of endocrine signals primarily controlled by the central nervous system (CNS) and associated endocrine glands, the corpora allata (CA) and corpora cardiaca (CC). This endocrine axis provides hormones and neuropeptides that significatively affect tissues closely involved in successful reproduction: the fat body, which is the metabolic center supplying the lipid resources and energy demanded in egg formation, and the ovaries, where the developing oocytes recruit lipids that will be used for optimal embryogenesis. The post-genomic era and the availability of modern experimental approaches have advanced our understanding of many processes involved in lipid homeostasis; therefore, it is crucial to integrate the findings of recent years into the knowledge already acquired in the last decades. The present chapter is devoted to reviewing major recent contributions made in elucidating the impact of the CNS/CA/CC-fat body-ovary axis on lipid metabolism in the context of insect reproduction, highlighting areas of fruitful research.

Spenito-dependent metabolic sexual dimorphism intrinsic to fat storage cells

Metabolism in males and females is distinct. Differences are usually linked to sexual reproduction, with circulating signals (e.g. hormones) playing major roles. In contrast, sex differences prior to sexual maturity and intrinsic to individual metabolic tissues are less understood. We analyzed Drosophila melanogaster larvae and find that males store more fat than females, the opposite of the sexual dimorphism in adults. We show that metabolic differences are intrinsic to the major fat storage tissue, including many differences in the expression of metabolic genes. Our previous work identified fat storage roles for Spenito (Nito), a conserved RNA-binding protein and regulator of sex determination. Nito knockdown specifically in the fat storage tissue abolished fat differences between males and females. We further show that Nito is required for sex-specific expression of the master regulator of sex determination, Sex-lethal (Sxl). "Feminization" of fat storage cells via tissue-specific overexpression of a Sxl target gene made larvae lean, reduced the fat differences between males and females, and induced female-like metabolic gene expression. Altogether, this study supports a model in which Nito autonomously controls sexual dimorphisms and differential expression of metabolic genes in fat cells in part through its regulation of the sex determination pathway.

Spenito-dependent metabolic sexual dimorphism intrinsic to fat storage cells

Metabolism in males and females is distinct. Differences are usually linked to sexual reproduction, with circulating signals (e.g. hormones) playing major roles. By contrast, sex differences prior to sexual maturity and intrinsic to individual metabolic tissues are less understood. We analyzed Drosophila melanogaster larvae and find that males store more fat than females, the opposite of the sexual dimorphism in adults. We show that metabolic differences are intrinsic to the major fat storage tissue, including many differences in the expression of metabolic genes. Our previous work identified fat storage roles for Spenito (Nito), a conserved RNA-binding protein and regulator of sex determination. Nito knockdown specifically in the fat storage tissue abolished fat differences between males and females. We further show that Nito is required for sex-specific expression of the master regulator of sex determination, Sex-lethal (Sxl). "Feminization" of fat storage cells via tissue-specific overexpression of a Sxl target gene made larvae lean, reduced the fat differences between males and females, and induced female-like metabolic gene expression. Altogether, this study supports a model in which Nito autonomously controls sexual dimorphisms and differential expression of metabolic genes in fat cells in part through its regulation of the sex determination pathway.

Rethinking the ecdysteroid source during Drosophila pupal-adult development

Ecdysteroids, typified by 20-hydroxyecdysone (20E), are essential hormones for the development, reproduction and physiology of insects and other arthropods. For over half a century, the vinegar fly Drosophila melanogaster (Ephydroidea: Diptera) has been used as a model of ecdysteroid biology. Many aspects of the biosynthesis and regulation of ecdysteroids in this species are understood at the molecular level, particularly with respect to their secretion from the prothoracic gland (PG) cells of the ring gland, widely considered the dominant biosynthetic tissue during development. Discrete pulses of 20E orchestrate transitions during the D. melanogaster life cycle, the sources of which are generally well understood, apart from the large 20E pulse at the onset of pharate adult development, which has received little recent attention. As the source of this pharate adult pulse (PAP) is a curious blind spot in Drosophila endocrinology, we evaluate published biochemical and genetic data as they pertain to three hypotheses for the source of PAP 20E: the PG; an alternative biosynthetic tissue; or the recycling of stored 20E. Based on multiple lines of evidence, we contend the PAP cannot be derived from biosynthesis, with other data consistent with D. melanogaster able to recycle ecdysteroids before and during metamorphosis. Published data also suggest the PAP is conserved across Diptera, with evidence for pupal-adult ecdysteroid recycling occurring in other cyclorrhaphan flies. Further experimental work is required to test the ecdysteroid recycling hypothesis, which would establish fundamental knowledge of the function, regulation, and evolution of metamorphic hormones in dipterans and other insects.

Receptor-mediated yolk uptake is required for oskar mRNA localization and cortical anchorage of germ plasm components in the Drosophila oocyte

The Drosophila germ plasm is responsible for germ cell formation. Its assembly begins with localization of oskar mRNA to the posterior pole of the oocyte. The oskar translation produces 2 isoforms with distinct functions: short Oskar recruits germ plasm components, whereas long Oskar remodels actin to anchor the components to the cortex. The mechanism by which long Oskar anchors them remains elusive. Here, we report that Yolkless, which facilitates uptake of nutrient yolk proteins into the oocyte, is a key cofactor for long Oskar. Loss of Yolkless or depletion of yolk proteins disrupts the microtubule alignment and oskar mRNA localization at the posterior pole of the oocyte, whereas microtubule-dependent localization of bicoid mRNA to the anterior and gurken mRNA to the anterior-dorsal corner remains intact. Furthermore, these mutant oocytes do not properly respond to long Oskar, causing defects in the actin remodeling and germ plasm anchoring. Thus, the yolk uptake is not merely the process for nutrient incorporation, but also crucial for oskar mRNA localization and cortical anchorage of germ plasm components in the oocyte.

A study of the fruit fly Drosophila reveals that receptor-mediated yolk uptake is not merely a nutrient storage process for future embryogenesis, but is also required for localization of Oskar mRNA and cortical anchorage of germ plasm components in the oocyte during oogenesis.

The genome of the stable fly, Stomoxys calcitrans, reveals potential mechanisms underlying reproduction, host interactions, and novel targets for pest control

Background

The stable fly, Stomoxys calcitrans, is a major blood-feeding pest of livestock that has near worldwide distribution, causing an annual cost of over $2 billion for control and product loss in the USA alone. Control of these flies has been limited to increased sanitary management practices and insecticide application for suppressing larval stages. Few genetic and molecular resources are available to help in developing novel methods for controlling stable flies.

Results

This study examines stable fly biology by utilizing a combination of high-quality genome sequencing and RNA-Seq analyses targeting multiple developmental stages and tissues. In conjunction, 1600 genes were manually curated to characterize genetic features related to stable fly reproduction, vector host interactions, host-microbe dynamics, and putative targets for control. Most notable was characterization of genes associated with reproduction and identification of expanded gene families with functional associations to vision, chemosensation, immunity, and metabolic detoxification pathways.

Conclusions

The combined sequencing, assembly, and curation of the male stable fly genome followed by RNA-Seq and downstream analyses provide insights necessary to understand the biology of this important pest. These resources and new data will provide the groundwork for expanding the tools available to control stable fly infestations. The close relationship of Stomoxys to other blood-feeding (horn flies and Glossina) and non-blood-feeding flies (house flies, medflies, Drosophila) will facilitate understanding of the evolutionary processes associated with development of blood feeding among the Cyclorrhapha.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-00975-9.

Lipidomics reveals how the endoparasitoid wasp Pteromalus puparum manipulates host energy stores for its young

Endoparasitoid wasps inject venom along with their eggs to adjust the physiological and nutritional environment inside their hosts to benefit the development of their offspring. In particular, wasp venoms are known to modify host lipid metabolism, lipid storage in the fat body, and release of lipids into the hemolymph, but how venoms accomplish these functions remains unclear. Here, we use an UPLC-MS-based lipidomics approach to analyze the identities and concentrations of lipids in both fat body and hemolymph of host cabbage butterfly (Pieris rapae) infected by the pupal endoparasitoid Pteromalus puparum. During infection, host fat body levels of highly unsaturated, soluble triacylglycerides (TAGs) increased while less unsaturated, less soluble forms decreased. Furthermore, in infected host hemolymph, overall levels of TAG and phospholipids (the major component of cell membranes) increased, suggesting that fat body cells are destroyed and their contents are dispersed. Altogether, these data suggest that wasp venom induces host fat body TAGs to be transformed into lower melting point (more liquid) forms and released into the host hemolymph following infection, allowing simple absorption and nutritional acquisition by wasp larvae. Finally, cholesteryl esters (CEs, a dietary lipid derived from cholesterol) increased in host hemolymph following infection with no concomitant decrease in host cholesterol, implying that the wasp may provide this necessary food resource to its offspring via its venom. This study provides novel insight into how parasitoid infection alters lipid metabolism in insect hosts, and begins to uncover the wasp venom proteins responsible for host physiological changes and offspring development.

Insight into the Functional Diversification of Lipases in the Endoparasitoid Pteromalus puparum (Hymenoptera: Pteromalidae) by Genome-scale Annotation and Expression Analysis

Lipases play essential roles in digestion, transport, and processing of dietary lipids in insects. For parasitoid wasps with a unique life cycle, lipase functions could be multitudinous in particular. Pteromalus puparum is a pupal endoparasitoid of butterflies. The female adult deposits eggs into its host, along with multifunctional venom, and the developing larvae consume host as its main nutrition source. Parasitoid lipases are known to participate in the food digestion process, but the mechanism remains unclear. P. puparum genome and transcriptome data were interrogated. Multiple alignments and phylogenetic trees were constructed. We annotated a total of 64 predicted lipase genes belonging to five lipase families and suggested that eight venom and four salivary lipases could determine host nutrition environment post-parasitization. Many putative venom lipases were found with incomplete catalytic triads, relatively long β9 loops, and short lids. Data analysis reveals the loss of catalytic activities and weak triacylglycerol (TAG) hydrolytic activities of lipases in venom. Phylogenetic trees indicate various predicted functions of lipases in P. puparum. Our information enriches the database of parasitoid lipases and the knowledge of their functional diversification, providing novel insight into how parasitoid wasps manipulate host lipid storage by using venom lipases.

Targeted delivery of CRISPR-Cas9 ribonucleoprotein into arthropod ovaries for heritable germline gene editing

Cas9-mediated gene editing is a powerful tool for addressing research questions in arthropods. Current approaches rely upon delivering Cas9 ribonucleoprotein (RNP) complex by embryonic microinjection, which is challenging, is limited to a small number of species, and is inefficient even in optimized taxa. Here we develop a technology termed Receptor-Mediated Ovary Transduction of Cargo (ReMOT Control) to deliver Cas9 RNP to the arthropod germline by injection into adult female mosquitoes. We identify a peptide (P2C) that mediates transduction of Cas9 RNP from the female hemolymph to the developing mosquito oocytes, resulting in heritable gene editing of the offspring with efficiency as high as 0.3 mutants per injected mosquito. We demonstrate that P2C functions in six mosquito species. Identification of taxa-specific ovary-specific ligand-receptor pairs may further extend the use of ReMOT Control for gene editing in novel species.

Endocrine uncoupling of the trade-off between reproduction and somatic maintenance in eusocial insects

In most animals reproduction trades off with somatic maintenance and survival. Physiologically this trade-off is mediated by hormones with opposite effects on reproduction and maintenance. In many insects, this regulation is achieved by an endocrine network that integrates insulin-like/IGF-1 signaling (IIS), juvenile hormone (JH), and the yolk precursor vitellogenin (Vg) (or, more generally, yolk proteins [YPs]). Downregulation of this network promotes maintenance and survival at the expense of reproduction. Remarkably, however, queens of highly eusocial social insects exhibit both enormous reproductive output and longevity, thus escaping the trade-off. Here we argue - based on recent evidence - that the proximate reason for why eusocial insects can decouple this trade-off is that they have evolved a different 'wiring' of the IIS-JH-Vg/YP circuit.

New genetic regulators question relevance of abundant yolk protein production in C. elegans

Vitellogenesis or maternal yolk formation is considered critical to the reproduction of egg-laying animals. In invertebrates, however, most of its regulatory genes are still unknown. Via a combined mapping and whole-genome sequencing strategy, we performed a forward genetic screen to isolate novel regulators of yolk production in the nematode model system Caenorhabditis elegans. In addition to isolating new alleles of rab-35, rab-10 and M04F3.2, we identified five mutant alleles corresponding to three novel regulatory genes potently suppressing the expression of a GFP-based yolk reporter. We confirmed that mutations in vrp-1, ceh-60 and lrp-2 disrupt endogenous yolk protein synthesis at the transcriptional and translational level. In contrast to current beliefs, our discovered set of mutants with strongly reduced yolk proteins did not show serious reproduction defects. This raises questions as to whether yolk proteins per se are needed for ultimate reproductive success.

Vitellogenin family gene expression does not increase Drosophila lifespan or fecundity

One of the most striking patterns in comparative biology is the negative correlation between lifespan and fecundity observed in comparisons among species. This pattern is consistent with the idea that organisms need to allocate a fixed energy budget among competing demands of growth, development, reproduction and somatic maintenance. However, exceptions to this pattern have been observed in many social insects, including ants, bees, and termites.  In honey bees ( Apismellifera), Vitellogenin ( Vg), a yolk protein precursor, has been implicated in mediating the long lifespan and high fecundity of queen bees. To determine if Vg-like proteins can regulate lifespan in insects generally, we examined the effects of expression of ApisVg and Drosophila CG31150 (a Vg-like gene recently identified as cv-d) on Drosophilamelanogaster lifespan and fecundity using the RU486-inducible GeneSwitch system. For all genotypes tested, overexpression of Vg and CG31150 decreased Drosophila lifespan and did not affect total or age-specific fecundity. We also detected an apparent effect of the GeneSwitch system itself, wherein RU486 exposure (or the GAL4 expression it induces) led to a significant increase in longevity and decrease in fecundity in our fly strains. This result is consistent with the pattern reported in a recent meta-analysis of Drosophila aging studies, where transgenic constructs of the UAS/GAL4 expression system that should have no effect (e.g. an uninduced GeneSwitch) significantly extended lifespan in some genetic backgrounds. Our results suggest that Vg-family genes are not major regulators of Drosophila life history traits, and highlight the importance of using appropriate controls in aging studies.

The hemolymph proteome of fed and starved Drosophila larvae

The co-operation of specialized organ systems in complex multicellular organisms depends on effective chemical communication. Thus, body fluids (like blood, lymph or intraspinal fluid) contain myriads of signaling mediators apart from metabolites. Moreover, these fluids are also of crucial importance for immune and wound responses. Compositional analyses of human body fluids are therefore of paramount diagnostic importance. Further improving their comprehensiveness should increase our understanding of inter-organ communication. In arthropods, which have trachea for gas exchange and an open circulatory system, the single dominating interstitial fluid is the hemolymph. Accordingly, a detailed analysis of hemolymph composition should provide an especially comprehensive picture of chemical communication and defense in animals. Therefore we used an extensive protein fractionation workflow in combination with a discovery-driven proteomic approach to map out the detectable protein composition of hemolymph isolated from Drosophila larvae. Combined mass spectrometric analysis revealed more than 700 proteins extending far beyond the previously known Drosophila hemolymph proteome. Moreover, by comparing hemolymph isolated from either fed or starved larvae, we provide initial provisional insights concerning compositional changes in response to nutritional state. Storage proteins in particular were observed to be strongly reduced by starvation. Our hemolymph proteome catalog provides a rich basis for data mining, as exemplified by our identification of potential novel cytokines, as well as for future quantitative analyses by targeted proteomics.

Origin and evolution of yolk proteins: expansion and functional diversification of large lipid transfer protein superfamily

Vitellogenin (VTG) and apolipoprotein (APO) play a central role in animal reproduction and lipid circulation, respectively. Although previous studies have examined the structural and functional relationships of these large lipid transfer proteins (LLTPs) from an evolutionary perspective, the mechanism in generating these different families have not been addressed in invertebrates. In this study, the most comprehensive phylogenetic and genomic analysis of the LLTP superfamily genes is carried out. We propose the expansion and diversification of LLTPs in invertebrates are mediated via retrotransposon-mediated duplications, followed by either subfunctionalization or neofunctionalization in different lineages. In agreement with a previous hypothesis, our analysis suggests that all LLTPs originate from a series of duplications of a primitive yolk protein gene similar to VTG. Two early consecutive duplications of the yolk protein genes resulted in the formation of microsomal triglyceride transfer protein (MTP) and the APO gene ancestor. Gains and losses of domains and genes occurred in each of these families in different animal lineages, with MTP becoming truncated. MTP maintained only the components stabilizing the huge lipoprotein particle. Surprisingly, for the first time, two VTG-like protein families were found to independently arise in the lineages of insects. This work consolidates the reconstruction of the evolutionary roadmap of the LLTP superfamily and provides the first mechanistic explanation on the expansion of family members via retrotransposition in invertebrates.

Vertical transmission of a Drosophila endosymbiont via cooption of the yolk transport and internalization machinery

Spiroplasma is a diverse bacterial clade that includes many vertically transmitted insect endosymbionts, including Spiroplasma poulsonii, a natural endosymbiont of Drosophila melanogaster. These bacteria persist in the hemolymph of their adult host and exhibit efficient vertical transmission from mother to offspring. In this study, we analyzed the mechanism that underlies their vertical transmission, and here we provide strong evidence that these bacteria use the yolk uptake machinery to colonize the germ line. We show that Spiroplasma reaches the oocyte by passing through the intercellular space surrounding the ovarian follicle cells and is then endocytosed into oocytes within yolk granules during the vitellogenic stages of oogenesis. Mutations that disrupt yolk uptake by oocytes inhibit vertical Spiroplasma transmission and lead to an accumulation of these bacteria outside the oocyte. Impairment of yolk secretion by the fat body results in Spiroplasma not reaching the oocyte and a severe reduction of vertical transmission. We propose a model in which Spiroplasma first interacts with yolk in the hemolymph to gain access to the oocyte and then uses the yolk receptor, Yolkless, to be endocytosed into the oocyte. Cooption of the yolk uptake machinery is a powerful strategy for endosymbionts to target the germ line and achieve vertical transmission. This mechanism may apply to other endosymbionts and provides a possible explanation for endosymbiont host specificity.

Most insect species, including important disease vectors and crop pests, harbor vertically transmitted endosymbiotic bacteria. Studies have shown that many facultative endosymbionts, including Spiroplasma, confer protection against different classes of parasites on their hosts and therefore are attractive tools for the control of vector-borne diseases. The ability to be efficiently transmitted from females to their offspring is the key feature shaping associations between insects and their inherited endosymbionts, but to date, little is known about the mechanisms involved. In oviparous animals, yolk accumulates in developing eggs and serves to meet the nutritional demands of embryonic development. Here we show that Spiroplasma coopts the yolk transport and uptake machinery to colonize the germ line and ensure efficient vertical transmission. The uptake of yolk is a female germ line-specific feature and therefore an attractive target for cooption by endosymbionts that need to maintain high-fidelity maternal transmission.

Lipoproteins in Drosophila melanogaster--assembly, function, and influence on tissue lipid composition

Interorgan lipid transport occurs via lipoproteins, and altered lipoprotein levels correlate with metabolic disease. However, precisely how lipoproteins affect tissue lipid composition has not been comprehensively analyzed. Here, we identify the major lipoproteins of Drosophila melanogaster and use genetics and mass spectrometry to study their assembly, interorgan trafficking, and influence on tissue lipids. The apoB-family lipoprotein Lipophorin (Lpp) is the major hemolymph lipid carrier. It is produced as a phospholipid-rich particle by the fat body, and its secretion requires Microsomal Triglyceride Transfer Protein (MTP). Lpp acquires sterols and most diacylglycerol (DAG) at the gut via Lipid Transfer Particle (LTP), another fat body-derived apoB-family lipoprotein. The gut, like the fat body, is a lipogenic organ, incorporating both de novo-synthesized and dietary fatty acids into DAG for export. We identify distinct requirements for LTP and Lpp-dependent lipid mobilization in contributing to the neutral and polar lipid composition of the brain and wing imaginal disc. These studies define major routes of interorgan lipid transport in Drosophila and uncover surprising tissue-specific differences in lipoprotein lipid utilization.

Genetic variation in the Yolk protein expression network of Drosophila melanogaster: sex-biased negative correlations with longevity

One of the persistent problems in biology is understanding how genetic variation contributes to phenotypic variation. Associations at many levels have been reported, and yet causal inference has remained elusive. We propose to rely on the knowledge of causal relationships established by molecular biology approaches. The existing molecular knowledge forms a firm backbone upon which hypotheses connecting genetic variation, transcriptional variation and phenotypic variation can be built. The sex determination pathway is a well-established molecular network, with the Yolk protein 1-3 (Yp) genes as the most downstream target. Our analyses reveal that genetic variation in expression for genes known to be upstream in the pathway explains variation in downstream targets. Relationships differ between the two sexes, and each Yp has a distinct transcriptional pattern. Yp expression is significantly negatively correlated with longevity, an important life history trait, for both males and females.

Zebrafish lipid metabolism: from mediating early patterning to the metabolism of dietary fat and cholesterol

Lipids serve essential functions in cells as signaling molecules, membrane components, and sources of energy. Defects in lipid metabolism are implicated in a number of pandemic human diseases, including diabetes, obesity, and hypercholesterolemia. Many aspects of how fatty acids and cholesterol are absorbed and processed by intestinal cells remain unclear and present a hurdle to developing approaches for disease prevention and treatment. Numerous studies have shown that the zebrafish is an excellent model for vertebrate lipid metabolism. In this chapter, we review studies that employ zebrafish to better understand lipid signaling and metabolism.

Comparative and functional genomics of lipases in holometabolous insects

Lipases have key roles in insect lipid acquisition, storage and mobilisation and are also fundamental to many physiological processes underpinning insect reproduction, development, defence from pathogens and oxidative stress, and pheromone signalling. We have screened the recently sequenced genomes of five species from four orders of holometabolous insects, the dipterans Drosophila melanogaster and Anopheles gambiae, the hymenopteran Apis mellifera, the moth Bombyx mori and the beetle Tribolium castaneum, for the six major lipase families that are also found in other organisms. The two most numerous families in the insects, the neutral and acid lipases, are also the main families in mammals, albeit not in Caenorhabditis elegans, plants or microbes. Total numbers of the lipases vary two-fold across the five insect species, from numbers similar to those in mammals up to numbers comparable to those seen in C. elegans. Whilst there is a high degree of orthology with mammalian lipases in the other four families, the great majority of the insect neutral and acid lipases have arisen since the insect orders themselves diverged. Intriguingly, about 10% of the insect neutral and acid lipases have lost motifs critical for catalytic function. Examination of the length of lid and loop regions of the neutral lipase sequences suggest that most of the insect lipases lack triacylglycerol (TAG) hydrolysis activity, although the acid lipases all have intact cap domains required for TAG hydrolysis. We have also reviewed the sequence databases and scientific literature for insights into the expression profiles and functions of the insect neutral and acid lipases and the orthologues of the mammalian adipose triglyceride lipase which has a pivotal role in lipid mobilisation. These data suggest that some of the acid and neutral lipase diversity may be due to a requirement for rapid accumulation of dietary lipids. The different roles required of lipases at the four discrete life stages of holometabolous insects may also contribute to the diversity of lipases required by insects. In addition, insects use lipases to perform roles for which there are no correlates in mammals, including as yolk and male accessory gland proteins.

A new model system swims into focus: using the zebrafish to visualize intestinal metabolism in vivo

Many fundamental questions remain regarding the cellular and molecular mechanisms of digestive lipid metabolism. One major impediment to answering important questions in the field has been the lack of a tractable and sufficiently complex model system. Until recently, most studies of lipid metabolism have been performed in vitro or in mice, yet each approach possesses certain limitations. The zebrafish (Danio rerio) offers an excellent model system in which to study lipid metabolism in vivo, owing to its small size, genetic tractability and optical clarity. Fluorescent lipid dyes and optical reporters of lipid-modifying enzymes are now being used in live zebrafish to generate visible readouts of digestive physiology. Here we review recent advances in visualizing intestinal lipid metabolism in live larval zebrafish.

Discovering genes associated with dormancy in the monogonont rotifer Brachionus plicatilis

BACKGROUND

Microscopic monogonont rotifers, including the euryhaline species Brachionus plicatilis, are typically found in water bodies where environmental factors restrict population growth to short periods lasting days or months. The survival of the population is ensured via the production of resting eggs that show a remarkable tolerance to unfavorable conditions and remain viable for decades. The aim of this study was to generate Expressed Sequence Tags (ESTs) for molecular characterisation of processes associated with the formation of resting eggs, their survival during dormancy and hatching.

RESULTS

Four normalized and four subtractive libraries were constructed to provide a resource for rotifer transcriptomics associated with resting-egg formation, storage and hatching. A total of 47,926 sequences were assembled into 18,000 putative transcripts and analyzed using both Blast and GO annotation. About 28-55% (depending on the library) of the clones produced significant matches against the Swissprot and Trembl databases. Genes known to be associated with desiccation tolerance during dormancy in other organisms were identified in the EST libraries. These included genes associated with antioxidant activity, low molecular weight heat shock proteins and Late Embryonic Abundant (LEA) proteins. Real-time PCR confirmed that LEA transcripts, small heat-shock proteins and some antioxidant genes were upregulated in resting eggs, therefore suggesting that desiccation tolerance is a characteristic feature of resting eggs even though they do not necessarily fully desiccate during dormancy. The role of trehalose in resting-egg formation and survival remains unclear since there was no significant difference between resting-egg producing females and amictic females in the expression of the tps-1 gene. In view of the absence of vitellogenin transcripts, matches to lipoprotein lipase proteins suggest that, similar to the situation in dipterans, these proteins may serve as the yolk proteins in rotifers.

CONCLUSION

The 47,926 ESTs expand significantly the current sequence resource of B. plicatilis. It describes, for the first time, genes putatively associated with resting eggs and will serve as a database for future global expression experiments, particularly for the further identification of dormancy related genes.

Molecular aspects of viviparous reproductive biology of the tsetse fly (Glossina morsitans morsitans): regulation of yolk and milk gland protein synthesis

Tsetse fly (Diptera: Glossinidae) viviparous reproductive physiology remains to be explored at the molecular level. Adult females carry their young in utero for the duration of embryonic and larval development, all the while supplying their offspring with nutrients in the form of a "milk" substance secreted from a modified accessory gland. Flies give birth to fully developed third instar larvae that pupariate shortly after birth. Here, we describe the spatial and temporal expression dynamics of two reproduction-associated genes and their products synthesized during the first and second gonotrophic cycles. The proteins studied include a putative yolk protein, Glossina morsitans morsitans yolk protein 1 (GmmYP1) and the major protein found in tsetse "milk" secretions (Glossina morsitans morsitans milk gland protein, GmmMGP). Developmental stage and tissue-specific expression of GmmYP1 show its presence exclusively in the reproductive tract of the fly during oogenesis, suggesting that GmmYP1 acts as a vitellogenic protein. Transcripts for GmmMGP are present only in the milk gland tissue and increase in coordination with the process of larvigenesis. Similarly, GmmMGP can be detected at the onset of larvigenesis in the milk gland, and is present during the full duration of pregnancy. Expression of GmmMGP is restricted to the adult stage and is not detected in the immature developmental stages. These phenomena indicate that the protein is transferred from mother to larvae as nourishment during its development. These results demonstrate that both GmmYP1 and GmmMGP are involved in tsetse reproductive biology, the former associated with the process of oogenesis and the latter with larvigenesis.

Purification, cloning and regulation of a novel acid-lipase-like protein of hamster expressed in lacrimal glands and tears during lactation

We report a novel 48-kDa tear acid-lipase-like protein (TALLP), which is markedly induced in lacrimal glands (LG) and secreted in tears of hamster dams during lactation. TALLP is undetectable in LG and tears of normal hamsters, but is also induced after gonadectomy in both sexes and this is prevented by androgen, estrogen or thyroid hormone treatment. These observations and the obliteration of TALLP upon cessation of lactation suggest that endogenous estrogens (in females) and androgens (in males) completely repress TALLP expression. Purified TALLP is monomeric, contains approximately 18% N-glycosylation and several pI isoforms. TALLP expression was tissue-specific and immunolocalized in LG acinar cells. The cDNA deduced amino-acid sequence of TALLP precursor (398 residue, containing a 19 residues signal-peptide) showed only 43-48% identity with all known mammalian acid-lipases, including even those of other rodents, suggesting that TALLP is a prototype of a new category, within the acid-lipase family. Surprisingly, although the catalytic triad residues and other sequence features important for lipolytic activity are conserved in TALLP, it has no detectable lipase activity. However, TALLP binds the polarity sensitive hydrophobic probe, 1-aminoanthracene (K(d)=12 microM). TALLP might have a unique substrate-specificity or a lipid-binding/carrier function in tears of hamster dams. This is the first report of an acid-lipase-like protein secreted in tears of any species. Since TALLP lacks the usual lipase activity, it can be an excellent model to understand better what other structural features in acid-lipases influence their catalytic activity.

Precursor structure of egg proteins in the coral Galaxea fascicularis

In the egg of the reef coral Galaxea fascicularis, four proteins (named GfEP-1 to -4) are stored in high abundance. In the present study, a cDNA containing a full-length open reading frame for GfEP-1 was cloned, and the translated protein sequence was compared to the N-terminal sequences of GfEP-2, -3, and -4. GfEP-1 and -2 were shown to be generated by processing of a precursor of 1439 amino acids, and GfEP-3 turned out to be a partial fragment of GfEP-2. The precursor protein contained regions which exhibited similarities to vitellogenins (Vgs) in bilaterian animals (oviparous vertebrates and invertebrates including nematodes, arthropods, and molluscs). This study reports the first cloning and characterization of a full-length cDNA encoding a Vg in a non-bilaterian animal, and argues that the emergence of Vg as a precursor of egg yolk proteins predated the divergence of the cnidarian and bilaterian lineages.

vLIP, a viral lipase homologue, is a virulence factor of Marek's disease virus

The genome of Marek's disease virus (MDV) has been predicted to encode a secreted glycoprotein, vLIP, which bears significant homology to the alpha/beta hydrolase fold of pancreatic lipases. Here it is demonstrated that MDV vLIP mRNA is produced via splicing and that vLIP is a late gene, due to its sensitivity to inhibition of DNA replication. While vLIP was found to conserve several residues essential to hydrolase activity, an unfavorable asparagine substitution is present at the lipase catalytic triad acid position. Consistent with structural predictions, purified recombinant vLIP did not show detectable activity on traditional phospholipid or triacylglyceride substrates. Two different vLIP mutant viruses, one bearing a 173-amino-acid deletion in the lipase homologous domain, the other having an alanine point mutant at the serine nucleophile position, caused a significantly lower incidence of Marek's disease in chickens and resulted in enhanced survival relative to two independently produced vLIP revertants or parental virus. These data provide the first evidence that vLIP enhances the replication and pathogenic potential of MDV. Furthermore, while vLIP may not serve as a traditional lipase enzyme, the data indicate that the serine nucleophile position is nonetheless essential in vivo for the viral functions of vLIP. Therefore, it is suggested that this particular example of lipase homology may represent the repurposing of an alpha/beta hydrolase fold toward a nonenzymatic role, possibly in lipid bonding.

Cloning and expression of the yolk protein of the tsetse fly Glossina morsitans morsitans

Two major families of nutritional proteins exist in insects, namely the vitellogenins and the yolk proteins. While in other insects only vitellogenins are found, cyclorraphan flies only contain yolk proteins. Possible sites of yolk protein synthesis are the fat body and the follicle cells surrounding the oocyte. We report the cloning of the yolk protein of the tsetse fly Glossina morsitans morsitans, a species with adenotrophic viviparity. The tsetse fly yolk protein could be aligned with other dipteran yolk proteins and with some vertebrate lipases. In contrast to the situation in most fly species, only a single yolk protein gene was found in the tsetse fly. Northern blot analysis showed that only the ovarian follicle cells, and not the fat body represents the site of yolk protein synthesis.

A mammalian-like lipase gene is expressed in the female reproductive accessory glands of the sand fly Phlebotomus papatasi (Diptera, Psychodidae)

A lipase-like protein (PhpaLIP) was identified as the major protein component in the secretion of the female reproductive accessory glands of the sand fly Phlebotomus papatasi. The full-length cDNA encoding this protein was isolated and its nucleotide sequence determined. The deduced translational product of the gene contains a GFSFG motif, consistent with a GXSXG consensus, which is shared by most bacterial and eukaryotic hydrolases. Transcriptional analysis of the PhpaLIP gene showed that its expression is female-specific, and is also detectable in districts other than accessory glands, suggesting that it might play different functions. Taken together with the observation of sequence similarity shared by PhpaLIP and mammalian lipases, the demonstration of the presence of lipase activity in the accessory gland secretion suggests a possible biological role of PhpaLIP gene product.

Selective transport and packaging of the major yolk protein in the sea urchin

The major yolk protein of sea urchins is an iron-binding, transferrin-like molecule that is made in the adult gut. Its final destination though is the developing oocytes that are embedded in somatic accessory cells and encompassed by two epithelial layers of the ovary. In this study, we address the dynamics of yolk transport, endocytosis, and packaging during the vitellogenic phase of oogenesis in the sea urchin by use of fluorescently labeled major yolk protein (MYP). Incorporation of MYP into the accessory cells of the ovary and its packaging into yolk platelets of developing oocytes is visualized in isolated oocytes, ovary explants, and in whole animals. When MYP is introduced into the coelom of adult females, it is first accumulated by the somatic cells of the ovarian capsule and is then transported to the oocytes and packaged into yolk platelets. This phenomenon is specific for MYP and accurately reflects the endogenous MYP packaging. We find that oocytes cultured in isolation are endocytically active and capable of selectively packaging MYP into yolk platelets. Furthermore, oocytes that packaged exogenous MYP are capable of in vitro maturation, fertilization, and early development, enabling an in vivo documentation of MYP utilization and yolk platelet dynamics. These results demonstrate that the endocytic uptake of yolk proteins in sea urchins does not require a signal from their surrounding epithelial cells and can occur autonomous of the ovary. In addition, these results demonstrate that the entire population of yolk platelets is competent to receive new yolk protein input, suggesting that they are all made simultaneously during oogenesis.

Fat body expressed yolk protein genes in Hyphantria cunea are related to the YP4 follicular epithelium yolk protein subunit gene of pyralid moths

cDNA clones for two of the yolk proteins, YP1 and YP2, produced by the fat body of the moth, Hyphantria cunea, were sequenced and found to be homologous to the follicular epithelium yolk proteins of pyralid moths. Both cDNA clones coded for polypeptides of 290 residues and the deduced amino acid sequence identity between YP1 and YP2 was very high (79.0%). Analysis of the secondary structure of the predicted polypeptides suggests that YP1 and YP2 do not form heteromeric proteins because of differences in secondary structure due to the lack of alpha helices in YP1. Northern blot analysis showed that the transcripts for YP1 (1.2 kb) and YP2 (1.1 kb) were present primarily in the female fat body with only trace levels detectable in the ovary of the adult female. In a developmental study, the YP1 and YP2 transcripts were first detectable in 10-day-old pupae and increased into the adult stage. These results suggest that the YP1 and YP2 genes in H. cunea have been recruited to replace the vitellogenin gene as the primary source of yolk proteins. During this process they have acquired a modified pattern of expression that is different from homologous genes reported in pyralid moths. The assessment of the evolution of proteinaceous yolk in these moths should serve as an excellent model for the evolution of gene recruitment.

Drosophila yolk protein produced in E. coli is accumulated by mosquito ovaries

Despite similar functions, the yolk proteins of the higher dipteran flies and the vitellogenins found in other insects are unrelated at the sequence level and have evolved from different genes. Both are selectively endocytosed into the ovary via receptors belonging to the LDLR receptor subfamily. We cloned the Drosophila yp1 gene into an E. coli expression vector and showed that the yolk protein produced by E. coli is taken up into ovaries of both Drosophila melanogaster and the malaria mosquito Anopheles gambiae, which normally uses vitellogenin.

The major yolk protein in sea urchins is a transferrin-like, iron binding protein

The major yolk protein (MYP) in sea urchins has historically been classified as a vitellogenin based on its abundance in the yolk platelets. Curiously, it is found in both sexes of sea urchins where it is presumed to play a physiological role in gametogenesis, embryogenesis, or both. Here we present the primary structure of MYP as predicted from cDNAs of two sea urchins species, Strongylocentrotus purpuratus and Lytechinus variegatus. The sequence from these two species share identity to one another, but bear no resemblance to other known vitellogenins. Instead the sequence shares identity to members of the transferrin superfamily of proteins. In vitro iron binding assays, including both (59)Fe overlay assays of MYP enriched coelomic fluid and immunoprecipitation of native iron-bound MYP from coelomic fluid, support this classification. We suggest that one of MYP's transferrin-like properties is to shuttle iron to developing germ cells.

Artificial reproduction in a hymenopteran insect, Athalia rosae, using eggs matured with heterospecific yolk proteins and fertilized with cryopreserved sperm

Previtellogenic ovaries of Athalia rosae (Hymenoptera, Symphyta, Tenthredinidae) were transplanted into the adult female abdominal hemocoel of Athalia infumata (Symphyta, Tenthredinidae), Arge nigrinodosa (Symphyta, Argidae), and Pimpla nipponica (Apocrita, Ichneumonidae). The donor oocytes accumulated heterospecific yolk proteins and matured in the A. infumata host. On average, six mature oocytes were obtained per transplanted ovary. In contrast, the donor oocytes accumulated a limited amount of yolk but did not mature in the Ar. nigrinodosa host and did not even accumulate yolk in the P. nipponica host. The eggs that matured in the A. infumata host were injected with cryopreserved A. rosae sperm that had been taken from adult male seminal vesicles and stored at -80 degrees C. Fertilization, as confirmed by the use of visible marker mutations, was achieved and a fraction of the injected eggs developed into fertile female adults.

Ovarian function in the giant tiger prawn (Penaeus monodon) as determined by in vitro bioassay

Ovary tissue fragments of the giant tiger prawn Penaeus monodon were incubated in vitro with L-methionine[35S] plus L-cysteine[35S] as a metabolic labeling reagent. The labeled cytoplasmic and secreted proteins synthesized in vitro during incubations under various conditions were subjected to SDS polyacrylamide electrophoresis and visualized by autoradiography. Vitellogenin (Vg) was immunologically identified and shown to be actively synthesized and released into the incubation medium. The synthesis and release of Vg into the incubation medium was optimized and shown to be linear over a 16-h period. Comparisons between different ovarian regions and different stages of development revealed that the level of Vg synthesis and accumulation in the incubation media was variable depending on stage of development and region within the ovary. Coincubation of ovarian fragments with sinus gland extracts showed a dose-related inhibition of total protein and Vg synthesis. The in vitro ovarian bioassay is suitable for examining the effect of hormonal inputs of P. monodon.

Ecdysteroid titers in mated and unmated Drosophila melanogaster females

Radioimmunoassay was used to determine ecdysteroid titers in mated or unmated Drosophila melanogaster females. Whole-body ecdysteroid titers increase after mating and this response is more pronounced after 12-24 hours than it is immediately after mating. In one experiment, females were mated to transgenic males deficient in accessory gland proteins to test whether these peptides mediate the observed increase in female whole-body ecdysteroid titers. Females mated to such transgenic males do not show a pronounced increase in whole-body ecdysteroid titers. The effect of mating on female hemolymph ecdysteroid titers was also investigated. Hemolymph ecdysteroid titers decrease after mating. The ecdysteroid titer change in the hemolymph may result from yolk protein uptake of ecdysteroids into developing vitellogenic oocytes as a consequence of male accessory gland protein stimulation of female oocyte maturation and yolk protein synthesis following mating.

Structural basis for the substrate selectivity of pancreatic lipases and some related proteins

The classical human pancreatic lipase (HPL), the guinea pig pancreatic lipase-related protein 2 (GPLRP2) and the phospholipase A1 from hornet venom (DolmI PLA1) illustrate three interesting steps in the molecular evolution of the pancreatic lipase gene family towards different substrate selectivities. Based on the known 3D structures of HPL and a GPLRP2 chimera, as well as the modeling of DolmI PLA1, we review here the structural features and the kinetic properties of these three enzymes for a better understanding of their structure-function relationships. HPL displays significant activity only on triglycerides, whereas GPLRP2 displays high phospholipase and galactolipase activities, together with a comparable lipase activity. GPLRP2 shows high structural homology with HPL with the exception of the lid domain which is made of five amino acid residues (mini-lid) instead of 23 in HPL. The lid domain deletion in GPLRP2 allows the free access to the active site and reduces the steric hindrance towards large substrates, such as galactolipids. The role of the lid domain in substrate selectivity has been investigated by site-directed mutagenesis and the substitution of HPL and GPLRP2 lid domains. The addition of a large-size lid domain in GPLRP2 increases the substrate selectivity for triglycerides by depressing the phospholipase activity. The phospholipase activity is, however, not induced in the case of the HPL mutant with GPLRP2 mini-lid. Therefore, the presence of a full-length lid domain is not the unique structural feature explaining the absence of phospholipase activity in HPL. The 3D structure of the GPLRP2 chimera and the model of DolmI PLA1 reveal a higher hydrophilic/lipophilic balance (HLB) of the surface loops (beta5 loop, beta9 loop, lid domain) surrounding the active site, as compared to the homologous loops in HPL. This observation provides a potential explanation for the ability of GPLRP2 and DolmI PLA1 to hydrolyze polar lipids, such as phospholipids. In conclusion, the beta5 loop, the beta9 loop, and the lid domain play an essential role in substrate selectivity towards triglycerides, phospholipids and galactolipids.

The Drosophila melanogaster lipase homologs: a gene family with tissue and developmental specific expression

We report the molecular cloning of Drosophila genes encoding putative lipase homologs, Dm lip1, lip2 and lip3, the definition of their structure and the expression patterns during development. These Drosophila lipases are related to acid lipases, with a common GHSQG motif, within a more general consensus GXSXG, identified as the active site shared by all the members of lipase superfamily. The lip1 and lip3 genes are transcribed in different tissues and developmental stages, suggesting that they have different functions. The lip1 gene, coding for a protein similar to digestive lipases, is expressed in ovaries and early embryos and, with a different sized transcript, in all the other developmental stages. The lip3 gene, whose translation product is more similar to lysosomal acid lipases, is expressed only during the larval period. The lip2 gene seems non-functional. The Drosophila putative lipases do not show similarity with the Drosophila yolk proteins that are reported to have sequence similarity with lipoprotein lipases, but share a consistent similarity with lepidopteran proteins reported as egg specific or yolk proteins, probably corresponding to lipase homologs. The results reported here are discussed in relation to the evolution and functions of lipases within the between species.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

Intestinal uptake of lipovitellin from brine shrimp (Artemia franciscana) by larval inland silversides (Menidia beryllina) and striped bass (Morone saxatilis)

Intestinal uptake of lipovitellin (LV) from brine shrimp (Artemia franciscana) in larval inland silversides (Menidia beryllina) and striped bass (Morone saxatilis) was described using immunocytochemistry. Polyclonal antisera were raised against two subunits of LV (LV68 and LV190). When tested by immunocytochemistry, anti-LV68 showed cross-reactivity with some of the pancreatic cells especially in inland silversides. Therefore anti-LV190 was used to localize immunoreactive LV. Inland silversides at 14 days after hatching were fed Artemia nauplii and then sampled 4, 8, 12 hr after feeding. Similar experiments were carried out by using striped bass at 5 days and 15 days of age. They were sampled at 2, 4, 8, and 12 hr after feeding. Anterior enterocytes showed no evidence of uptake; however, the brush border of the cells of inland silversides reacted with the antiserum. Posterior enterocytes took up the LV and/or, possibly, their immunoreactive breakdown products. The pattern of uptake included accumulation in supranuclear vacuoles and digestion in supranuclear vacuoles, as suggested by the decay of the immunoreactivity over time. Thus, the posterior intestine of these larval fishes is the site of uptake and digestion of LV, an important nutritive component in the food of many larval fishes; this supports earlier findings using non-nutritive marker proteins.

5' coding region of the follicular epithelium yolk polypeptide 2 cDNA in the moth, Plodia interpunctella, contains an extended coding region

The 5' region of YP2 cDNA, a follicular epithelium yolk protein subunit in the moth, Plodia interpunctella, shows that the polypeptide contains an extended internal coding region. Partial cDNA clones for YP2 were isolated from a pharate adult female ovarian cDNA expression library in Lambda Zap II by screening with antigen selected YP2 antiserum. The 5' sequence of the YP2 transcript was determined by 5' RACE PCR of ovarian mRNA using YP2 sequence-specific nested primers. The combined cDNA and 5' RACE sequencing showed the YP2 transcript to be 1971 bp in length up to the poly(A) tail with a single open reading frame for a predicted polypeptide of 616 amino acids. Northern analysis showed a single YP2 transcript to be present in ovarian RNA that was approximately 2 kb in length. The predicted amino acid sequence for YP2 from P. interpunctella is most closely related to egg specific protein (ESP) from Bombyx mori and the partial YP2 sequence from Galleria mellonella. YP2 from P. interpunctella also is similar to vertebrate lipases and contains a conserved lipid binding region. However, the 5' coding region of YP2 from P. interpunctella contains an in-frame insert of approximately 438 bp that had replaced an approximately 270-bp region as compared with ESP from B. mori and YP2 of G. mellonella. This suggests that the insert occurred by a recombinational event internal to the YP2 structural gene of P. interpunctella.

Developmental regulation of yolk protein gene expression in Anastrepha suspensa

A partial cDNA clone for the 48,000 dalton yolk polypeptide gene from Anastrepha suspensa was isolated from a cDNA expression library using a yolk polypeptide antibody probe and hybridization to the Drosophila melanogaster yolk protein 1 gene. The sequenced DNA has greatest homology to the yolk protein genes from Ceratitis capitata, D. melanogaster, and Calliphora erythrocephala and, similar to these genes, shares amino acid sequence domains with those from lipases. RNA hybridization studies indicated that the yolk protein gene expression is completely female-specific and limited to the ovaries, without apparent regulation by 20-hydroxyecdysone or juvenile hormone. This is in contrast to an earlier study which suggested, based on immunological probes, that a very low level of yolk protein synthesis occurred in fat body and was not sex-specific.

Fundulus heteroclitus vitellogenin: the deduced primary structure of a piscine precursor to noncrystalline, liquid-phase yolk protein

We have cloned and sequenced a cDNA encoding a vitellogenin (Vtg) from the mummichog, Fundulus heteroclitus, an estuarine teleost. We constructed a liver cDNA library against RNA from estrogen-treated male mummichogs. Five overlapping cDNA clones totalling 5,197 bp were isolated through a combination of degenerate oligonucleotide probing of the library and PCR. The cDNA sequence contains a 5,112 bp open reading frame. The predicted primary structure of the deduced 1,704-amino-acid protein is 30-40% identical to other documented chordate Vtgs, establishing this Vtg as a member of the ancient Vtg gene family. Of the previously reported chordate Vtg sequences (Xenopus laevis, Gallus domesticus, Ichthyomyzon unicuspis, and Acipenser transmontanus), all four act as precursor proteins to a yolk which is eventually rendered insoluble under physiological conditions, either as crystalline platelets or as noncrystalline granules. The yolk of F. heteroclitus, on the other hand, remains in a soluble state throughout oocyte growth. The putative F. heteroclitus Vtg contains a polyserine region with a relative serine composition that is 10-20% higher than that observed for the other Vtgs. The trinucleotide repeats encoding the characteristic polyserine tracts of the phosvitin region follow a previously reported trend: TCX codons on the 5' end and AGY codons toward the 3' end. Whether the difference in Vtg primary structure between F. heteroclitus and that of other chordates is responsible for the differences in yolk structure remains to be elucidated. As the first complete teleost Vtg to be reported, these data will aid in designing nucleotide and immunological probes for detecting Vtg as a reproductive status indicator in F. heteroclitus and other piscine species.

The regulation of the yolk protein genes, a family of sex differentiation genes in Drosophila melanogaster

There are many obvious morphological and behavioural differences between male and female Drosophila, whose differing phenotypes are produced by a hierarchy of sex determination genes. These genes have been well characterised at the genetic and molecular level. Similarly, a number of sex-specific differentiation genes have been characterised, such as the chorion and vitelline membrane genes in females and the sex peptide and other accessory gland proteins in males. Despite the depth of these parallel studies, there is only one example of a direct link between the sex determination pathway and the downstream sex differentiation genes, namely the regulation of the female-specific yolk protein genes. The yolk proteins are synthesised in the fat body and ovarian follicle cells of the adult female and are subsequently transported to the oocyte where they are stored for utilization during embryogenesis. The expression of the yolk protein genes is not entirely controlled by the sex determination hierarchy, as several different regulatory pathways must interact to direct their correct sexual, temporal and spatial regulation during development.