In many female insects, peptides transferred in the seminal fluid induce postmating responses (PMR), such as a drastic increase of egg laying and reduction of receptivity (readiness to mate). In Drosophila, sex-peptide (SP) elicits short- and long-term PMR, but only the latter in the presence of stored sperm (sperm effect). This study elucidates the interaction between SP and sperm by immunofluorescence microscopy. Transgenic males were used to study the effects of SP modification on the PMR of females in vivo. SP is shown to bind to sperm with its N-terminal end. In females, the C-terminal part of SP known to be essential to induce the PMR is gradually released from stored sperm by cleavage at a trypsin cleavage site, thus prolonging the PMR. These findings are confirmed by analyzing the PMR elicited by males containing transgenes encoding modified SPs. SP lacking the N-terminal end cannot bind, and SP without the trypsin cleavage site binds permanently to sperm. It is concluded that by binding to sperm tails, SP prolongs the PMR. Thus, besides a carrier for genetic information, sperm is also the carrier for SP. Binding to sperm may protect the peptide from degradation by proteases in the hemolymph and, thus, prolong its half-life. Longer sperm tails may transfer more SP and thus increase the reproductive fitness of the male. It is suggested that this could explain the excessive length of sperm tails in some Drosophila species (Peng, 2005a).
Despite their amazing species diversity, some reproductive traits are common to most insects. For example, in many higher insects, mating elicits a drastic increase in egg laying and a reduction of receptivity (readiness to copulate). Thus, the postmating responses (PMR) are either evolutionarily old, or they evolve fast. They are induced by components of the seminal fluid transferred into the female during copulation and are stored in the female genital tract. Due to its well-known genetics, combined with its amenability for biochemical, physiological, and genomic analysis, Drosophila has become the best-studied species with respect to seminal peptides influencing postmating behavior in insects (Peng, 2005a).
In D. melanogaster, the PMR are elicited by three male peptides. Sex-peptide (SP) and Ovulin (Herndon, 1995) are products of the accessory glands, and the Ductus ejaculatorius peptide (DUP) is synthesized in the ejaculatory duct. They are transferred together with sperm into the female during mating. SP and DUP induce both PMR. Ovulin elicits ovulation and oviposition on the first day after mating, but does not affect receptivity. Mating with wild-type males induces the PMR for about 1 week; however, mating with males that do not transfer sperm elicits PMR only for one day (long-term and short-term PMR, respectively). (This phenomenon has been described as 'sperm effect' by Manning (1962, 1967) Males lacking functional SP (SP0 males) elicit only weak PMR lasting about one day (Chapman, 2003: Liu, 2003). Hence, in vivo SP is the major agent eliciting both the short- and the long-term PMR; Ovulin and DUP play only minor roles during the first day after copulation. Because SP0 males transfer and store sperm (Liu, 2003), sperm alone cannot induce the PMR, but stored sperm are essential for their persistence (Manning, 1962; Manning, 1967). Without sperm transfer, or injection of physiological amounts of the peptide, SP elicits only short-term PMR (Kubli, 2003, Manning, 1996 and Schmidt, 1993). How do SP and sperm interact to support the long-term PMR? A possible mechanism could be binding of SP to sperm (Kubli, 2003). This study shows that SP binds to sperm with the N-terminal end. During storage, SP is cleaved off from the tail, and the released C-terminal part elicits the long-term PMR (Peng, 2005a).
Immunofluorescence was used to visualize putative binding of peptides to sperm. Sperm were isolated from females at different time intervals after mating and were then incubated with antibodies specific for SP, DUP, or Ovulin, respectively. DUP binds only to the head and only in the first few hours, whereas Ovulin does not bind to sperm at all. Furthermore, sperm is not autofluorescent at the wavelength used for this study. Sperm isolated 5 hr after mating show binding of SP along the entire length of sperm (The antibody used for these experiments is specific for the fragment SP6-20.) During storage in the female genitalia, SP is lost from the tail. Two days after mating, the signal intensity along the entire length of the tail, in comparison with the head, is weaker and spottier than immediately after mating. Five days after mating, SP is almost absent from the tail. However, the signal is still strong on the head in all stages. A quantitative analysis reveals that SP is gradually lost from the sperm tail. Thus, SP indeed binds to sperm during several days, but it is lost from the tail while sperm are stored in the female genital tract. In contrast, DUP binds to the sperm head only for a few hours, and Ovulin does not bind at all. This is consistent with the finding that DUP and Ovulin play only minor roles in the short-term PMR. Thus, focus will be placed on the fate of SP bound to sperm (Peng, 2005a).
This study shows that SP binds to sperm with its N-terminal end and that the C-terminal part of SP is gradually cleaved from the sperm tail. Labeled SP binds with its C-terminal region to specific sites in the female nervous system and the genital tract. Binding to the central and peripheral nervous systems is dependent upon an intact C-terminal part, whereas binding to the genital tract is less demanding in terms of amino acid sequence (Ding, 2003). These findings suggest that binding in the nervous system is responsible for eliciting the PMR, whereas binding in the genital tract may reflect the presence of a peptide transporter (Kubli, 2003). This interpretation is supported by the fact that the free (non-bound) modified SPQ7Q8 of TGQQ males elicits the short-term responses, but the same SPQ7Q8 bound irreversibly to the sperm tail cannot elicit the long-term responses. Hence, replacement of R7 by Q7 and K8 by Q8 does not affect SP function, but mutant SPQ7Q8 permanently bound to the tail cannot elicit the PM responses via the SP binding proteins present in the genital tract, i.e., these SP binding proteins are probably not receptors involved in the PMR. Furthermore, the results strongly suggest that the released C-terminal part of SP enters the hemolymph to reach its targets in the nervous system. Apparently the sperm tail is the only source of SP to sustain the long-term PMR. These results are also in accord with the findings that the brain is the site of action of SP (Nakayama, 1997) and that ectopic expression of SP in the fat body of virgin females or injection of SP into the hemolymph does elicit the PMR (Schmidt, 1993; Aigaki, 1993; Peng, 2005a and references therein).
Cooperative reproductive behavior of the two sexes promotes the evolutionary success of a species, but males and females also compete to control the number and genetic diversity of their offspring. Because D. melanogaster females are polyandrous, it is in the interest of the female to eliminate surplus SP, at the latest when sperm has been used up. In contrast, it is in the male's interest to keep his mating partner monogamous. Thus, sexual conflict arises. Binding of SP to the sperm tail substantially increases the functional half-life of SP from 1 day to about 1 week, probably by hiding it from the hemolymph proteases. It is also in the male's interest to transfer as much SP as possible. Because sperm serves as a carrier to transport and stabilize SP, selection may have favored long sperm tails binding more SP and thereby increasing the reproductive fitness of the male. This could explain the excessive length of sperm tails in some Drosophila species (D. melanogaster males produce sperm of 1.8 mm, and D. bifurca males produce sperm of 58 mm!). Association of components of the male seminal fluid with sperm has also been reported for other insects, mammals, and birds. Binding of proteins that enhance male fitness to sperm may be a novel mechanism of general importance in insects and beyond. Selection experiments involving males either lacking functional SP completely (Liu, 2003; Schmidt, 1993) or producing modified SPs should enable testing of the putative influence of SP on sperm tail length in D. melanogaster. Such experiments may lead to an understanding of male-male and male-female competition at a molecular level (Peng, 2005a).
In sum, the PMR of D. melanogaster females can be divided into two phases: the short-term PMR and the long-term PMR, respectively. The short-term PMR are induced immediately after mating mainly by free SP; the long-term PMR, lasting about one week, by the C-terminal SP fragment cleaved from SP bound to the sperm tail. Both responses likely elicit the PMR by binding of SP to specific sites in the central and peripheral nervous systems. In addition, immediately after mating, the free SP probably also stimulates juvenile hormone synthesis because it contains the N terminus known to be essential for the stimulation of the corpus allatum in (Moshitzky, 1996). The elucidation of the molecular mechanism supporting the persistence of the PMR in D. melanogaster may shed light on a fundamental aspect of insect reproduction in general (Peng, 2005a).
Virgin female adult Helicoverpa armigera (Lepidoptera: Noctuidae) moths exhibit calling behaviour and produce sex pheromone in scotophase from the day after emergence; mating turns off both of these pre-mating activities. In the fruit fly Drosophila melanogaster, a product of the male accessory glands, termed sex peptide (SP), has been identified as being responsible for suppressing female receptivity after transfer to the female genital tract during mating. Juvenile hormone (JH) production is activated in the D. melanogaster corpus allatum (CA) by SP in vitro. Cross-reactivity of D. melanogaster SP has been demonstrated in the H. armigera moth: JH production in photophase virgin female moth CA in vitro is directly activated in a dose-dependent manner by synthetic D. melanogaster SP, and concurrently inhibits pheromone biosynthesis activating neuropeptide (PBAN)-activated pheromone production by isolated pheromone glands of virgin females. Control peptides (locust adipokinetic hormone, AKH-I, and human corticotropin, ACTH) do not inhibit in vitro pheromone biosynthesis. Moreover, SP injected into virgin H. armigera females, decapitated 24 h after eclosion, or into scotophase virgin females, suppresses pheromone production. In the light of these results, the existence is hypothesized of a SP-like factor among the peptides transmitted to female H. armigera during copulation, inducing an increased level of JH production and depressing the levels of pheromone produced thereafter (Fan, 2000).
Mating elicits two major changes in the reproductive behavior of many insect females. The egg-laying rate increases and the readiness to accept males (receptivity) is reduced. These postmating responses last ~1 week in Drosophila melanogaster. Males that do not transfer sperm but transfer seminal fluid during mating induce a short-term response of 1 day. The long-term response of 1 week requires the presence of sperm (sperm effect). Hence, sperm is essential for the long-term persistence of the postmating responses. Three seminal fluid peptides elicit postmating responses: ovulin (Herndon, 1995), sex-peptide (SP), and DUP99B (Saudan, 1995). Using the technique of targeted mutagenesis by homologous recombination, males have been produced with mutant SP genes. Males lacking functional SP elicit only a weak short-term response. However, these males do transfer sperm. Thus, (1) SP is the major agent eliciting the short-term and the long-term postmating responses and (2) sperm is merely the carrier for SP. The second conclusion is supported by the finding that SP binds to sperm. The 36-aa-encoding SP gene is the first small Drosophila gene knocked out with the method of homologous recombination (Liu, 2003).
After copulation without sperm transfer, the short-term responses are fully induced, but only for 1 day. Without SP transfer, oviposition is barely induced, and only on the first day. Because the expressions of ovulin and DUP99B are not affected in the SP null males, these two peptides may be responsible for the weak increase in oviposition observed on the first day after a mating with SP null males. The increase in egg laying observed in females mated to these males after day 3 corresponds to the increase in egg laying also observed over the same time period in virgin females. However, in contrast to the eggs laid by virgin females, these eggs are fertilized and produce offspring. Indeed, offspring are obtained from eggs laid many days after copulation, demonstrating that the stored sperm of the SP null males is viable and functional. Receptivity is strongly reduced only in the first few hours after mating. The short reduction of receptivity in matings with SP null males is very likely induced by DUP99B, since ovulin affects only oviposition. Because the postmating responses observed after copulation with SP null males are only weakly induced, it is believed that they are not elicited by any other, unidentified, peptides or proteins. Thus, DUP99B and ovulin act only on the first day after mating and have a weak effect in comparison with SP; i.e., SP is the crucial peptide eliciting the short-term response. Because after day 1 only SP is active, and sperm alone cannot elicit the responses, SP is also the molecular agent of the sperm effect. A mechanism is proposed for the interaction between SP and sperm (Liu, 2003).
These results are similar to those obtained by Chapman (2003), who used RNA interference to knock down SP levels. Females mated to SP knock-down males produced by RNA interference are significantly more receptive at 24 and 28 h after mating than females mated to control males. By 48 h, receptivity in mates of SP knock-down males was similar to that of virgin females. The rate of egg laying in females mated to SP knock-down males is significantly lower for 1-2 days after mating than for mates of control males, and then becomes indistinguishable from that of virgin females. The results of the RNA interference experiments showed a slightly longer initial stimulation of egg laying in mates of SP-deficient males than was found in this study. One factor contributing to these differences could be that the background rate of egg laying in virgin females was very different between the two studies. Thus, although there are slight differences in the magnitude of female postmating responses, the results of these two studies are qualitatively very similar (Liu, 2003).
It has been proposed that sperm binds SP and, upon arrival in the female genital tract, releases SP continuously. Released SP then enters the hemolymph and reaches its targets (Ding, 2003; Ottinger, 2000). Once sperm are used up, SP disappears too, and the female regains the virgin status. The hypothesis assumes that sperm acts as a carrier and that SP is the active molecule eliciting the two postmating responses. This study shows that SP is indeed the molecular agent of the sperm effect, i.e., responsible for eliciting the two postmating responses. Using immunohistochemistry. SP binds to sperm with its N-terminal region. Immediately after mating, SP binds to the head and tail of sperm. However, ~5 days after copulation, SP bound to the tail is barely detected with a polyclonal antibody specific for the middle part of SP. Hence, it is very likely released from the sperm tail and subsequently enters the hemolymph to elicit the two postmating responses (Liu, 2003 and references therein).
Reduction of the receptivity of a mated female by male compounds transferred during copulation is one way to avoid sperm competition. It has been proposed that the evolution of sperm length is a coevolved response to selection on the female reproductive tract. If the amount of SP transferred is proportional to the length of the sperm tail, long tails may have been evolutionarily favored because they carry more SP. It is suggested that SP binding to the sperm tail is an explanation for the excessive length of sperm tails in some Drosophila species. For example, the 3-mm-long male of Drosophila bifurca bears a sperm tail of 58 mm. The same reasoning may apply to other sperm-bound male substances that affect female reproductive traits in such a way that they enhance male reproductive success (Liu, 2003 and references therein).
Mating induces profound changes in female insect behavior and physiology. In Drosophila, mating causes a reduction in sexual receptivity and an elevation in egg production for at least 5 days. Injection of the seminal fluid sex peptide (SP) induces both responses in virgin females, but only for 1-2 days. The role of SP in eliciting the responses to mating remains to be elucidated. Functional redundancy between seminal fluid components may occur. In addition, mating with spermless males results in brief (1- to 2-day) post-mating responses, indicating either that there is a "sperm effect" or that sperm act as carriers for SP or other seminal fluid components. This study has used RNA interference to suppress SP expression in order to determine (1) whether SP is required to elicit full post-mating responses, (2) the magnitude of responses due to other seminal fluid components, and (3) whether SP accounts for the 'sperm effect.' Receptivity was higher and egg production lower in females mated to SP knock-down males than in controls. Comparison with virgins showed that the responses were brief. SP is therefore required for normal magnitude and persistence of postmating responses. Sperm transfer and use were normal in mates of SP knock-down males, yet their post-mating responses were briefer than after normal matings, and similar to those reported in mates of spermless son-of-tudor males. The prolonged 'sperm effect' on female receptivity and egg production is therefore entirely attributable to SP, but sperm are necessary for its occurrence (Chapman, 2003).
Females that mated with males deficient in the SP were significantly more receptive than were females mated to control males. At 24 h the receptivity of females mated to SP knock-down males was intermediate between that of females mated to control males and that of virgin females. By 48 h the receptivity of females mated to SP knock-down males was similar to that of virgins. Females mated to SP knock-down males therefore did not behave like virgin females in terms of receptivity until 48 h after mating; there was some residual reduction in receptivity caused by matings to SP knock-down males (Chapman, 2003).
On each of the 5 successive days after mating, females mated to SP knock-down males laid significantly (with one exception) fewer eggs than did females mated to control males. On days 1-2, females mated to SP knock-down males laid eggs at a level intermediate between that of females mated to control males and that of virgin females. Thereafter, the number of eggs deposited by mates of the SP knock-down males was similar to that of virgins. At 6, 24, and 48 h after mating, females mated to SP knock-down males from both lines showed significantly (with one exception) lower ovulation than mates of control males, and significantly (with one exception) higher ovulation than virgin females. Thus mates of SP knock-down males did not show a full mated response and their egg laying dropped down again to virgin levels 2-3 days after mating (Chapman, 2003).
The egg production and ovulation tests show that females mated to males deficient in the SP produce significantly fewer eggs than females mated to control males for the 5 days after mating. However, in the first 1–2 days after matings to SP knock-down males, females do not behave like virgins, although their egg production does become comparable to that of virgins after 2–3 days. Thus some residual stimulation of egg production is achieved after matings to SP knock-down males. This stimulation of egg production and ovulation is presumably caused by the transfer of other ejaculate proteins, such as Acp26Aa and Dup99B (Chapman, 2003).
There were no significant differences in egg fertility in mates of SP knock-down and control males on days 1 and 3 after mating. On day 5 the egg fertility of mates of control males was significantly lower than of mates of SP knock-down males. Egg production was significantly higher in the control females, which would have led them to run out of sperm more quickly than females mated to SP knock-down males. On day 5, all females laid equal numbers of fertile eggs, suggesting that there were no significant differences in the numbers of sperm stored across treatments. The results show that SP knock-down males transferred sperm and that these sperm were stored and used in numbers comparable to those of control males (Chapman, 2003).
The results confirm that SP is necessary for some post-mating responses of females. Two matched pairs of experimental and control lines, and the consistent findings with them indicate that the effects on the post-mating responses are attributable to the absence of the SP, rather than to some other effect of genetic background. Females mated to SP knock-down males produced by RNAi are significantly more receptive and lay and ovulate significantly fewer eggs than do mates of control males. RNAi has therefore proved to be a powerful technique for the in vivo characterization of SP function. There was some residual reduction in receptivity and stimulation of egg production in the mates of SP knock-down males. It is concluded that these residual effects in mates of SP knock-down males are due to ejaculate components and not to pheromone transfer or mating itself, because mates of DTA-E males (which mate and transfer pheromones but no Acps or sperm) show virgin levels of egg production and receptivity. The residual effects in mates of SP knock-down males are presumably due at least in part to as-yet-unidentified ejaculate component(s), because the other molecules so far shown to mediate these effects have smaller and/or shorter-lived effects (Dup99B), or affect only egg production and not receptivity (Acp26Aa) (Chapman, 2003).
These results are quantitatively similar to those of Liu (2003), who analyzed the responses of females mated to SP gene knockout males produced by homologous recombination. Mates of SP knockout males also showed some residual reduction in receptivity and stimulation of egg production. The responses were smaller than those observed in the present study, possibly attributable to differences in the fly strains used, or differences in the fly food or culturing techniques. For instance, the rate of egg laying by virgin females differed in the two studies. This trait shows substantial natural genetic variation between strains as well as clinal geographic variation (Chapman, 2003).
Sperm transfer and use appeared normal in matings with SP knock-down males, because egg fertility was unimpaired. Despite the presence of sperm, females mated to SP knock-down males showed post-mating responses similar to those of mates of son-of-tudor males, which transfer Acps and other ejaculate proteins but no sperm. These findings show that the sperm effect is in fact an effect of SP, but one that is manifest only in the presence of sperm. Sperm may act as carriers of SP, with slow release prolonging the SP response (Chapman, 2003).
These results suggest SP is unlikely to cause the increased mortality in females that is attributable to as-yet-unidentified Acps. There is no reduction in the cost of mating in mates of son-of-tudor males, which do not transfer sperm, compared with mates of wild-type males. Therefore, because sperm are necessary for full SP transfer, SP is unlikely to be responsible for the Acp-mediated cost of mating in females. Further work with these SP knock-down males is necessary to confirm this hypothesis, and to determine the net effect of SP on male and female reproductive success (Chapman, 2003).
Females of many animal species behave very differently before and after mating. In Drosophila, changes in female behavior upon mating are triggered by the sex peptide (SP), a small peptide present in the male's seminal fluid. SP activates a specific receptor, the sex peptide receptor (SPR), which is broadly expressed in the female reproductive tract and nervous system. This study pinpoints the action of SPR to a small subset of internal sensory neurons that innervate the female uterus and oviduct. These neurons express both fruitless (fru), a marker for neurons likely to have sex-specific functions, and pickpocket (ppk), a marker for proprioceptive neurons. SPR expression in these fru+ ppk+ neurons is both necessary and sufficient for behavioral changes induced by mating. These neurons project to regions of the central nervous system that have been implicated in the control of reproductive behaviors in Drosophila and other insects (Häsemeyer, 2009).
SPR was initially identified in a genome-wide pan-neuronal RNAi screen. In this screen, the panneuronal elav-GAL4 driver was crossed to a genome-wide collection of RNAi transgenes, and female progeny were scored for egg-laying defects. Mated elav-GAL4 UAS-SPR-IR females lay very few eggs and remain sexually receptive, and thus, like SPR null mutants, behave as though they were still virgins. To define the cellular requirement for SPR function, the logic of this screen was inverted, crossing the UAS-SPR-IR transgene to a collection of 998 GAL4 lines and scoring the female progeny for egg-laying defects in the same fashion. In each of these lines, the GAL4 transcriptional activator is expressed in a random but stereotyped subset of cells, in which SPR function should now be inhibited by the UAS-SPR-IR transgene (Häsemeyer, 2009).
Fifty-nine lines were identified that resulted in a strong and reproducible egg-laying defect. Many of these lines were found to be broadly expressed, as revealed with a UAS-mCD8-GFP reporter. These lines were not examined further. More restricted neuronal expression was observed in seven lines, and for each of these a series of secondary assays was performed to confirm the egg-laying defect and to assess the receptivity of both virgin and mated females. For all seven GAL4 lines, SPR knockdown resulted in reduced egg laying and increased remating of mated females, but little if any change in the receptivity of virgin females. These defects were indistinguishable from those observed upon panneuronal SPR knockdown with the elav-GAL4 driver, or in SPR null mutant females. For the most restricted of positive GAL4 lines, ppk-GAL4, it was confirmed that these defects can indeed be attributed to a diminished response to SP. It was then determined that SPR is required in ppk+ sensory neurons in the female reproductive tract (Häsemeyer, 2009).
This study describes a set of internal ppk+ fru+ sensory neurons in the female reproductive tract and provides evidence that SPR functions in these neurons to trigger the behavioral changes induced by SP upon mating. This conclusion rests on two complementary sets of observations. First, SPR is required in both ppk+ and fru+ cells, because postmating responses are eliminated upon knockdown of SPR in either cell population. Second, SPR is sufficient in either ppk+ or fru+ cells alone, as expression in either restores the postmating response in SPR null mutant females. This forces the conclusion that SPR acts exclusively in cells that are both ppk+ and fru+. The sensory neurons innervating the uterus are the only cells that were identified that express both of these markers. There are typically four to six such cells, and it is not yet known if they are functionally equivalent, or if egg laying and receptivity are regulated by two distinct cell subtypes (Häsemeyer, 2009).
Silencing synaptic transmission of ppk+ fru+ neurons mimics the activity of SP, in that they both cause virgin females to become unreceptive and initiate egg laying. Thus, an attractive hypothesis is that activation of SPR by SP reduces the synaptic output of these neurons. Like other ppk+ neurons, the ppk+ fru+ uterus neurons are probably mechanosensory. They may therefore have an important function as uterus stretch receptors in the coordination of sperm transfer, fertilization, and egg release. They may have two distinct functional states, depending on the presence or absence of SP. Because receptivity can be genetically uncoupled from egg production and egg laying, it is inferred that SP can also act independently of any stretch signal in the uterus. Modulation of receptivity and egg laying might be mediated through either distinct ppk+ fru+ subtypes or distinct central synapses (Häsemeyer, 2009).
How might SP regulate these sensory neurons? Two possibilities are envisioned. First, the ppk+ fru+ neurons may detect SP in the reproductive tract and alter their firing rate accordingly. In this model, passage of SP into the hemolymph would not be required to induce the postmating response. A second possibility is that SP enters the circulatory system and acts presynaptically to modulate the release of these neurons at their central targets. The fact that SP can indeed be detected in the hemolymph of mated females does not in itself exclude the former possibility. At least some effects of SP, such as stimulating juvenile hormone synthesis in the corpus allatum, probably do require SP to enter the hemolymph. Similarly, the fact that SP triggers a postmating response even when injected directly into the hemolymph is also consistent with either model. The somata and some processes of the ppk+ fru+ neurons lie outside the uterus and would be readily accessible to factors in the hemolymph. A neural rather than a circulatory route has been proposed to mediate postmating responses in several species of moths. However, this conclusion is based upon the loss of this response upon nerve cord transection, a result predicted by both of these models. Thus, both models are consistent with currently available evidence from studies in Drosophila and other species, and distinguishing between them will require detailed studies of the physiological properties of the ppk+ fru+ neurons in response to SP (Häsemeyer, 2009).
The central targets of the ppk+ fru+ sensory neurons include the abdominal and/or subesophageal ganglia -- regions of the CNS likely to contain circuits that mediate behavioral responses to mating. The abdominal ganglion houses the octopaminergic neurons that are believed to regulate the release and passage of mature eggs from the ovary to the uterus. It is suspected that these neurons are direct or indirect targets of the ppk+ fru+ sensory neurons and that these circuits serve to ensure that ovulation and oviposition are coordinated with the presence of sperm (Häsemeyer, 2009).
Some ppk+ fibers project from the abdominal trunk nerve right through to the SOG, potentially forming a direct neural connection from the reproductive tract to the brain. It is suspected that these projections may feed into circuits that regulate female receptivity and other postmating behaviors. Virgin females are enticed to mate by the male's courtship song. Most auditory sensory neurons project to the mechanosensory neuropil in the lateral SOG, close to the terminal arborizations of the ppk+ neurons. The proximity of the auditory processing centers and the ascending ppk+ projections raises the attractive possibility that mating modulates an early step in song processing. The SOG also contains processes of the Ilp7 neurons, which function in egg-laying site selection after mating. Direct evidence for mating-induced changes in SOG circuit function is lacking in flies but has been obtained in other insects. In some species of moth, mating induces a long-term inhibition of the SOG neurosecretory cells that regulate female pheromone biosynthesis, making mated females less attractive to other males (Häsemeyer, 2009).
Having identified sensory neurons that detect SP in the reproductive tract, it will now be important to characterize the central pathways that process these signals to regulate female behavior. In the olfactory system, sensory neurons that detect pheromones are fru+, as are their postsynaptic partners in the brain. Given that the sensory neurons that detect SP are also fru+, and many fru+ neurons are also located in both the abdominal and subesophageal ganglia, it is enticing to think that a similar logic may apply in these pathways too. Elucidating the operation of these circuits should reveal how the female CNS integrates both external and internal information to switch between two very different behavioral patterns (Häsemeyer, 2009).
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date revised: 10 August 2010
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