Protein metabolism of Drosophila male accessory glands—II

Seminal fluid proteins induce transcriptome changes in the Aedes aegypti female lower reproductive tract

BACKGROUND

Mating induces behavioral and physiological changes in the arbovirus vector Aedes aegypti, including stimulation of egg development and oviposition, increased survival, and reluctance to re-mate with subsequent males. Transferred seminal fluid proteins and peptides derived from the male accessory glands induce these changes, though the mechanism by which they do this is not known.

RESULTS

To determine transcriptome changes induced by seminal proteins, we injected extract from male accessory glands and seminal vesicles (MAG extract) into females and examined female lower reproductive tract (LRT) transcriptomes 24 h later, relative to non-injected controls. MAG extract induced 87 transcript-level changes, 31 of which were also seen in a previous study of the LRT 24 h after a natural mating, including 15 genes with transcript-level changes similarly observed in the spermathecae of mated females. The differentially-regulated genes are involved in diverse molecular processes, including immunity, proteolysis, neuronal function, transcription control, or contain predicted small-molecule binding and transport domains.

CONCLUSIONS

Our results reveal that seminal fluid proteins, specifically, can induce gene expression responses after mating and identify gene targets to further investigate for roles in post-mating responses and potential use in vector control.

The effects of the sex chromosomes on the inheritance of species-specific traits of the copulatory organ shape in Drosophila virilis and Drosophila lummei

The shape of the male genitalia in many taxa is the most rapidly evolving morphological structure, often driving reproductive isolation, and is therefore widely used in systematics as a key character to distinguish between sibling species. However, only a few studies have used the genital arch of the male copulatory organ as a model to study the genetic basis of species-specific differences in the Drosophila copulatory system. Moreover, almost nothing is known about the effects of the sex chromosomes on the shape of the male mating organ. In our study, we used a set of crosses between D. virilis and D. lummei and applied the methods of quantitative genetics to assess the variability of the shape of the male copulatory organ and the effects of the sex chromosomes and autosomes on its variance. Our results showed that the male genital shape depends on the species composition of the sex chromosomes and autosomes. Epistatic interactions of the sex chromosomes with autosomes and the species origin of the Y-chromosome in a male in interspecific crosses also influenced the expression of species-specific traits in the shape of the male copulatory system. Overall, the effects of sex chromosomes were comparable to the effects of autosomes despite the great differences in gene numbers between them. It may be reasonably considered that sexual selection for specific genes associated with the shape of the male mating organ prevents the demasculinization of the X chromosome.

Drosophila seminal sex peptide associates with rival as well as own sperm, providing SP function in polyandrous females

When females mate with more than one male, the males’ paternity share is affected by biases in sperm use. These competitive interactions occur while female and male molecules and cells work interdependently to optimize fertility, including modifying the female’s physiology through interactions with male seminal fluid proteins (SFPs). Some modifications persist, indirectly benefiting later males. Indeed, rival males tailor their ejaculates accordingly. Here, we show that SFPs from one male can directly benefit a rival’s sperm. We report that Sex Peptide (SP) that a female Drosophila receives from a male can bind sperm that she had stored from a previous male, and rescue the sperm utilization and fertility defects of an SP-deficient first-male. Other seminal proteins received in the first mating ‘primed’ the sperm (or the female) for this binding. Thus, SP from one male can directly benefit another, making SP a key molecule in inter-ejaculate interaction.

When fruit flies and other animals reproduce, a compatible male and a female mate, allowing sperm from the male to swim to and fuse with the female’s egg cells. The males also produce proteins known as seminal proteins that travel with the sperm. These proteins increase the likelihood of sperm meeting an egg and induce changes in the female that increase the number, or quality, of offspring produced.

Some seminal proteins help a male to compete against its rivals by decreasing their chances to fertilize eggs. However, since many of the changes seminal proteins induce in females are long-lasting, it is possible that a subsequent male may actually benefit indirectly from the effects of a prior male’s seminal proteins. It remains unclear whether the seminal proteins of one male are also able to directly interact with and help the sperm of another male.

Male fruit flies make a seminal protein known as sex peptide. Normally, a sex peptide binds to the sperm it accompanies into the female, increasing the female’s fertility and preventing her from mating again with a different male. To test whether the sex peptide from one male can bind to and help a rival male’s sperm, Misra and Wolfner mated female fruit flies with different combinations of males that did, or did not, produce the sex peptide.

The experiments found that female flies that only mated with mutant males lacking the sex peptide produced fewer offspring than if they had mated with a ‘normal’ male. However, in females that mated with a mutant male followed by another male who provided the sex peptide, the second male’s sex peptide was able to bind to the mutant male’s sperm (as well as to his own). This in turn allowed the mutant male’s sperm to be efficiently used to sire offspring, at levels comparable to a normal male providing the sex peptide.

These findings demonstrate that the ways individual male fruit flies interact during reproduction are more complex than just simple rivalry. Since humans and other animals also produce seminal proteins comparable to those of fruit flies, this work may aid future advances in human fertility treatments and strategies to control the fertility of livestock and pests, including mosquitoes that transmit diseases.

On the effect specificity of accessory gland products transferred by the love-dart of land snails

Background

Sexual selection favours the evolution of male bioactive substances transferred during mating to enhance male reproductive success by affecting female physiology. These effects are mainly well documented for separate-sexed species. In simultaneous hermaphrodites, one of the most peculiar examples of transfer of such substances is via stabbing a so-called love-dart in land snails. This calcareous stylet delivers mucous products produced by accessory glands into the mate’s haemolymph. In Cornu aspersum, this mucus temporarily causes two changes in the recipient. First, the spermatophore uptake into the spermatophore-receiving organ, called diverticulum, is probably favoured by contractions of this organ. Second, the amount of stored sperm increases by contractions of the copulatory canal, which close off the tract leading to the sperm digesting organ. However, it has yet to be determined whether these effects are similar across species, which would imply a common strategy of the dart in increasing male reproductive success.

Results

We performed a cross-reactivity test to compare the in vitro response of the diverticulum and copulatory canal of C. aspersum (Helicidae) to its own and other species’ mucus (seven helicids and one bradybaenid). We found that the contractions in the diverticulum were only induced by dart mucus of certain species, while the copulatory canal responded equally to all but one species’ mucus tested. In addition, we report a newly-discovered effect causing the shortening of the diverticulum, which is also only caused by dart mucus of certain species. The advantage seems to be a distance reduction to the sperm storage organ.

Conclusions

All these findings are the first to shed light on the evolution of the different functions of accessory gland products in dart-bearing species. These functions may be achieved via common physiological changes caused by the substances contained in the dart mucus, since the responses evoked were similar across species’ mucus. Moreover, while these substances can act similarly in separate-sexed species as in simultaneous hermaphrodites, differences may occur in their evolution between the two sexual systems.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0672-6) contains supplementary material, which is available to authorized users.

Functions and analysis of the seminal fluid proteins of male Drosophila melanogaster fruit flies

The study of insect seminal fluid proteins provides a unique window upon adaptive evolution in action. The seminal fluid of Drosophila melanogaster contains over 80 proteins and peptides, which are transferred together with sperm by mating males. The functions of many of these substances are not yet known. However, those that have been characterized have marked effects on the reproductive success of males and females. For example, seminal fluid proteins and peptides can decrease female receptivity, can increase egg production and can increase sperm storage, and are necessary for sperm transfer and success in sperm competition. In this review we focus on the currently known functions of seminal fluid molecules and on new technologies and approaches that are enabling novel questions about their form and function to be addressed. We discuss how techniques for disrupting the production of seminal fluid proteins, such as homologous recombination and RNA interference, along with the use of microarrays and yeast two hybrid systems, should allow us to address ever more sophisticated questions about seminal fluid protein function. These and similar techniques promise to reveal the function of naturally-occurring variants of these proteins and hence the evolutionary significance of genetic variation for them.

Courtship in Drosophila

Courtship is a complex behavior in Drosophila that recruits a wide range of genes for its realization, including those concerning sex determination, ion channels, and circadian rhythms. Results from different experimental approaches-behavioral and genetic comparisons between species, analysis of mutants and mosaics, and identification of specific sensory stimuli-sketch the outlines of a set of pleiotropic genes acting on a distributed system in the brain to produce the species-specific sequence of responses and actions.

Assortative fertilization in Drosophila

The concept of gametic isolation has its origins in the 1937 edition of T. Dobzhansky's Genetics and the Origin of Species. Involving either positive assortative fertilization (as opposed to self-incompatibility) or negative assortative fertilization, it occurs after mating but prior to fertilization. Gametic isolation is generally subsumed under either prezygotic or postmating isolation and thus has not been the subject of extensive investigation. Examples of assortative fertilization in Drosophila are reviewed and compared with those of other organisms. Potential mechanisms leading to assortative fertilization are discussed, as are their evolutionary implications.

The accessory gland proteins in male Drosophila: structural, reproductive, and evolutionary aspects

Recent results from biochemical and molecular genetic studies of the accessory gland proteins in male Drosophila are reviewed. The most prominent feature is the species-specific variability. However, the analysis of the sex peptide in D. melanogaster shows that there is a strong homology in the molecular structure to the closely related sibling species, and that divergence increases with increasing phylogenetic distance. For this reason the sex peptide, after being transferred to the female genital tract during copulation, reduces receptivity and increases oviposition only in virgin females belonging to the same species group and subgroup. Even though studies were hitherto limited to a small number of the secretory components, it is evident that the accessory gland proteins play a key role in reproductive success of the fruit fly by changing female sexual behavior, supporting sperm transfer, storage and displacement. Thus, genes encoding the accessory gland proteins are apparently under strong evolutionary selection.

Structure and regulation of a gene cluster for male accessory gland transcripts in Drosophila melanogaster

We characterize a cluster of three genes, named Mst57Da, Mst57Db and Mst57Dc, according to their localization at polytene band 57D and their selective expression in Drosophila melanogaster males. Within a 4 kb segment of genomic DNA these genes code for poly(A)+ RNAs of about 750 nt, 650 nt and 850 nt length that are restricted to the male accessory glands. They all contain relatively long 5' and 3' untranslated regions compared to the size of their open reading frames. The 5' untranslated regions are characterized by several short open reading frames. In addition, the genes Mst57Da and Mst57Db share a 14 nucleotides long conserved sequence block in front of the putative translated regions. We propose that these common features are structural elements important for translational control of these genes. The predicted sequences of the small proteins of 55, 40 and 42 amino acids suggest that two of them (MST57Da and MST57Db) are secretory proteins and therefore components of the paragonial secretions. In P-element mediated germ line transformations of Mst57Db/lacZ fusion genes, a short gene fragment of -187/+395 is still specifically expressed in the paragonia. In addition, regions involved in negative and positive control of transcription have been identified. Development of a supernumerary paragonial gland is frequently observed in transgenic, intersexual genotypes indicating that in intersexes cells derived from the female genital disc can develop male genital structures.

Drosophila suzukii contains a peptide homologous to the Drosophila melanogaster sex-peptide and functional in both species

A peptide homologous to the Drosophila melanogaster sex-peptide (SP) was isolated from Drosophila suzukii accessory glands and its amino acid sequence determined. The D. suzukii peptide contains 41 amino acids and has a calculated molecular weight of 5100 Da. Comparison of the sequences reveals strong homologies in the N-terminal and C-terminal parts of the peptides. In the D. suzukii sex-peptide, however, five additional amino acids are inserted after amino acid 7. Based on the sequence of the peptide, a cDNA coding for the D. suzukii peptide was isolated by PCR. Sequence analysis of the cDNA confirmed the SP amino acid sequence determined by peptide sequencing. Furthermore, based on the cDNA sequence, we isolated the D. suzukii sex-peptide gene by inverse PCR. The D. suzukii sex-peptide gene contains an intron and codes for a 60 amino acid precursor. The D. melanogaster and the D. suzuki sex-peptides elicit rejection behaviour in the presence of males and an increased egg laying in virgin females of both species.

The endogenous regulation of mosquito reproductive behavior

Most female mosquitoes require a meal of blood that provides protein for egg maturation. For reproduction to occur, two behavioral sequences are essential. One is concerned with finding a host for the blood meal and the other in finding a site on which to lay the eggs that result. Stimuli from both hosts and oviposition sites initiate the reproductive behaviors of host-seeking and pre-oviposition, respectively, that are discussed in this review. After sensory receptors perceive these stimuli, the central nervous system must integrate the information and associate it with a biologically appropriate response. Host-seeking appears to be the default behavior, expressed whenever host stimuli are present. However, if the female is successful in locating a host and ingesting blood, subsequent host-seeking is inhibited when the meal distends the abdomen above a certain threshold. Host-seeking inhibition continues during egg development as a result of a humoral mechanism even after the blood volume has been reduced by digestion. At the time when eggs are maturing and host-seeking is inhibited, pre-oviposition behavior predominates if the central nervous system receives oviposition site stimuli. This behavior is also initiated by a humoral factor. Several physiological states, including insemination, age, and nutrition, can modulate both host-seeking and pre-oviposition behaviors.

Structure, cell-specific expression, and mating-induced regulation of a Drosophila melanogaster male accessory gland gene

The accessory gland of male insects is a secretory tissue of the genital tract made up of several distinct cell types. It secretes components of the ejaculatory fluid which have an important effect on the postmating behavior of the female. We have examined the sequence, structure, and expression of a gene, mst 316, expressed exclusively in the accessory glands of male Drosophila melanogaster. The mst 316 RNA encodes a small, basic protein of 52 amino acids that exhibits features common to precursors of secreted peptides, including a hydrophobic N-terminus. The tissue-specific expression of the mst 316 gene was studied using an mst 316--lacZ hybrid gene inserted into Drosophila by germ line transformation. The mst 316-lacZ fusion protein is expressed exclusively in the "main" cells of the accessory gland. It is first detected upon eclosion and exhibits a burst of synthesis in the first 3 days of adult life. The synthesis of the fusion protein is stimulated by mating, so that beta-galactosidase activity levels are two- to sixfold higher in males allowed to copulate with females compared to virgin male controls of the same age. The mating-stimulated synthesis of the mst 316-lacZ fusion protein, and by inference of the native gene product, appears to be due at least in part to increased transcript levels.

Structure and expression of a Drosophila male accessory gland gene whose product resembles a peptide pheromone precursor

The accessory gland of male insects is a genital tissue that secretes many components of the ejaculatory fluid, some of which affect the female's receptivity to courtship and her rate of oviposition. We have examined the structure and expression of two tightly linked genes that are expressed exclusively in the male accessory glands of adult Drosophila melanogaster. The two genes are transcribed from the same strand of DNA, and are separated by 20 bases. Both genes are regulated by the sex determination hierarchy and are expressed in the absence of germ cells. Immunological analysis reveals the protein products of at least one of these genes in the secretion of the accessory gland. The proteins are transferred to the female fly during copulation and are rapidly altered in the female genital tract. The predicted sequence of one protein has features of a peptide hormone precursor, and a region in which 11 of 17 amino acids are identical to egg-laying hormone (ELH) of the California sea hare, Aplysia californica.

A male accessory gland peptide that regulates reproductive behavior of female D. melanogaster

The adult male accessory glands of D. melanogaster synthesize and secrete a peptide that represses female sexual receptivity and stimulates oviposition. Normally, this peptide is transferred to females during copulation; however, the peptide shows the same biological activity after purification and subsequent injection into the abdominal cavity of female virgins. Amino acid sequencing of the purified peptide and oligonucleotide-directed cDNA cloning established that the peptide consists of 36 amino acids. It appears to be synthesized as a precursor with a hydrophobic signal sequence of 19 residues at its N-terminal end. The precursor peptide is encoded by a short mRNA that accumulates exclusively in the male accessory gland. The gene has been localized by in situ hybridization to polytene chromosomes at 70A.