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Mating pheromones of Nematoda: olfactory signaling with physiological consequences
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ScienceDirect Mating pheromones of Nematoda: olfactory signaling with physiological consequences Daniel HW Leighton and Paul W Sternberg Secreted pheromones have long been known to influence mating in the phylum Nematoda. The study of nematode sexual behavior has greatly benefited in the last decade from the genetic and neurobiological tools available for the model nematode Caenorhabditis elegans, as well as from the chemical identification of many pheromones secreted by this species. The discovery that nematodes can influence one another’s physiological development and stress responsiveness through the sharing of pheromones, in addition to simply triggering sexual attraction, is particularly striking. Here we review recent research on nematode mating pheromones, which has been conducted predominantly on C. elegans, but there are beginning to be parallel studies in other species. Address HHMI and Division of Biology and Biological Engineering, Caltech, Pasadena 91125, USA Corresponding author: Sternberg, Paul W ([email protected])
consisting of self-fertilizing hermaphrodites and rare males (reviewed in [2]). The hermaphrodites are incapable of mating with one another, and may only self-fertilize using a limited supply of self-sperm generated during larval development — reproduction after the exhaustion of self-sperm can only proceed by mating with a male. That mating is strictly unnecessary for survival of C. elegans makes this species an excellent model for the study of mating behavior [3], as the mating process can be disrupted at nearly every step without sacrificing the organism’s viability [4]. The discovery that many disparate species utilize ascarosides, a nematode-specific family of glycolipids, for communication, has greatly accelerated the rate of research in this field (reviewed in [5]). The research reviewed here largely took advantage of the ability to identify specific pheromones and challenge worms with pure synthetic compounds to uncover the precise neurological and physiological activities of these molecules.
Current Opinion in Neurobiology 2016, 38:119–124
Ascarosides
This review comes from a themed issue on Neurobiology of sex
The ascarosides are glycosides of the dideoxysugar ascarylose. These molecules always include a lipid tail, and may be conjugated to amino acid derivatives or other small molecules [6]. Ascarosides are now known to be produced by over twenty species of nematode from multiple clades, including free-living, vertebrate parasitic and insect parasitic worms [7]. Ascarosides have been demonstrated as mate-attracting pheromones both C. elegans [8] and Panagrellus redivivus [9], despite these species’ significant evolutionary distance, lying in different families. Ascarosides have also been shown to promote entrance into dauer, an alternative non-feeding and stress resistant larval stage, in C. elegans [10a], Heterorhabditis bacteriophora [11] and Pristionchus pacificus [12]. As would be expected of signaling molecules, ascaroside production varies with a worm’s age and environment [13]. In both C. elegans and P. redivivus, the different genders secrete specific ascarosides in vastly different quantities, with each gender producing the attractants for the other. In addition, the C. elegans hermaphrodite secreted pheromones are repulsive to other hermaphrodites [9,14]. Taken together, this evidence suggests that ascarosides are evolutionarily ancient signaling molecules that may serve as mating pheromones across much of Nematoda.
Edited by Barry Dickson and Catherine Dulac
http://dx.doi.org/10.1016/j.conb.2016.04.008 0959-4388/# 2016 Elsevier Ltd. All rights reserved.
Introduction Nematoda is a diverse phylum of worms that occupy a variety of terrestrial, aquatic and marine habitats. Most nematode species are gonochoristic, consisting of males and females that must locate one another and mate to reproduce. Research on nematode mating pheromones began in the 1960s with an eye toward understanding the mating habits of animal-parasitic and plant-parasitic nematodes. Since then, it has been found that dozens of nematode species, most of them vision-impaired and hearing-impaired by nature, locate one another by the reception of secreted molecules (reviewed in [1]). Most research on nematodes is conducted in the small, transparent Caenorhabditis elegans, which lives on rotting fruit and utilizes insect vectors as phoretic hosts. This nematode, unlike most, is androdiecious in nature, www.sciencedirect.com
In addition to being a highly diverse molecular family with over 150 members, the specific activity of each ascaroside is highly dependent on its chemical structure Current Opinion in Neurobiology 2016, 38:119–124
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Figure 1
O H3C HO
ascr#2
O
O OH
OH
O H3C HO
ascr#3
O
O OH O O
H3C HO
ascr#5
OH
O OH H N
O H3C HO
ascr#8
O
O
CO2H
OH
O
icas#9
H3C O
HN
O OH
O OH
O
OH
O
icas#3
H3C O
HN
OH
O
Dauer Formation
O
O
Male attraction
Aggregation Current Opinion in Neurobiology
The behavioral response of Caenorhabditis elegans to specific ascarosides is highly dependent on chemical structure.
[15] (Figure 1), and the same ascaroside can have extremely different effects on different species. For example, ascr#1 is a potent mate-attracting pheromone in P. redivivus, but promotes dauer formation in C. elegans [9,10a,16]. In fact, evolution of ascaroside signaling appears to be extremely rapid even within a species. The production and reception of dauer pheromones by both C. elegans and P. pacificus differs markedly amongst wild isolates of both species, possibly as a result of intense intraspecific competition over food resources [12,17]. Different strains of the same nematode are even known to mate at different rates, though this may have both pheromone-related and other reasons [18]. Ascarosides are also Current Opinion in Neurobiology 2016, 38:119–124
known to trigger feeding behavior in nematophagous fungi [19], and immune reactions in plant hosts [20], which likely apply additional pressure on the rapid evolution of nematode pheromones. Additionally, the ability of a nematode to locate a mate depends on more than simply chemotaxis to a pheromone cocktail. For example, Sammut et al. (2015) have recently shown that C. elegans males can be conditioned to associate a particular salt concentration with potential mates [21]. The essentially invariant anatomy of C. elegans, coupled with the ability to identify specific neurons under Nomarksi microscopy, has allowed researchers to identify www.sciencedirect.com
Pheromone signaling in nematodes Leighton and Sternberg 121
pheromone-responsive neurons through laser ablation experiments. The four male-specific CEM neurons are necessary for male attraction to two different ascarosides and the presence of multiple CEMs are argued to be necessary to generate the concentration tuning curve [10b]. Whole cell patch clamp electrophysiology indicates that the CEMs are primary sensors. In a given animal a particular CEM will respond positively or negatively to the ascaroside; this heterogeneous response might aid in the male nervous system calculating a derivative of pheromone concentration. The ADL, ASI, and ASK sensory neurons are also necessary for normal response to mating pheromones, and thus are expected to express mating pheromone receptors [22– 24]. One exciting finding from these studies is that the male neuronal response to mating pheromone is exhibited within the hermaphrodite nervous system as well. However, the stereotypical male behavioral response is simultaneously inhibited by neurons that only react to mating pheromone in the hermaphrodite [24]. Currently, six ascaroside receptors have been identified in C. elegans: srg-36, srg-37, srbc-64, srbc-66, daf-37 and daf38. Two of these receptors, srg-36 and srg-37, have homologs with conserved function in Caenorhabditis briggsae. All six receptors are G-protein-coupled-receptors (GPCRs), consistent with findings that GPCR function is important for sensation of ascarosides [25–28]. It has proven difficult, however, to determine precise relationships between pheromones and their receptors. It is not known whether ascaroside receptors function as single proteins, or as homo or heterodimers. Two yeast-based assays have recently been developed specifically for analysis of C. elegans GPCR activity [29,30]; this may allow for more rapid identification of ligand–receptor relationships than the mammalian and Xenopus based assays traditionally used for this purpose.
Hermaphrodite versus female behavior There has been a common conception in nematode literature that C. elegans hermaphrodites have lost certain female traits through evolution (e.g., [31–33]). Traits claimed to have been lost by hermaphrodites include the production of volatile mate-attracting pheromone and mating-induced torpor (which may aid the male in locating the vulva). In fact, evidence for both of these traits may be found in C. elegans, but mating behavior in general is repressed by the presence of sperm. As with a recently mated female, a young hermaphrodite exhibits no production of volatile pheromone, but resumes production upon sperm depletion, whether due to mutation or age [34]. This change in hermaphrodite pheromone production is also observed in the plant parasitic nematode Bursaphelenchus okinawaensis [35]. Garcia et al. [36] argued that hermaphrodites and females differ primarily because of age, but it is worth revisiting www.sciencedirect.com
the observations in light of the recent finding of spermregulated physiology in hermaphrodites. In particular, the ability of males to successfully insert their spicules into hermaphrodites or xenospecific females was assayed. There appears to be an affect of both age and sperm status. Specifically, the authors investigated the number of mating attempts made by a male before successful spicule insertion, depending on the strain of hermaphrodites he was presented with. The data for this experiment shows a dramatic reduction in the number of failed attempts if the hermaphrodite is spermless due to mutation, and a further reduction if the hermaphrodite is aged past the normal period of self-fertilization. These data indicate there are separate youth-correlated and spermcorrelated inhibitory effects on male copulation. Similar results were reported by another group in the same year [37]. Young C. elegans pseudofemales may be more difficult to mate with than young true females of another species, but the difference is far smaller than that between a female and a young hermaphrodite. C. elegans males do not induce torpor in females of other species upon mating, but this may simply be due to divergence of signals or receptors. Keeping in mind the significant differences in mating behavior between isolates of the same species, we believe a proper test of this torpor hypothesis would be to challenge males from multiple C. elegans isolates with aged and feminized hermaphrodites, also of various isolates. Although there are genuine differences between hermaphrodites and true females of closely related species, in many respects the hermaphrodites are simply behaving as sperm replete females.
Physiological effects of pheromone exposure A major shift in the discussion of nematode mating pheromones has occurred with the recognition that they are responsible for more than just chemoattraction. An early paper that attempted to address this topic, Timmermeyer et al. [38], showed that C. remanei females that had received copulatory plugs from their mates were more fecund than their unplugged sisters, despite receiving a similar number of matings. This observation suggests that male-deposited mating plugs, which may contain signaling compounds, might act to promote egglaying, oocyte development, or some other fertility promoting process in the receiving female. However, the possibility remains that plugged females had simply received or retained more sperm than their unplugged sisters in these experiments. Very recently, Aprison and Ruvinsky [39] showed that exposure to male pheromones has a substantial and beneficial effect on recovery from heat stress in both C. elegans and C. remanei [39]. Heat stress is known to reduce fertility in C. elegans hermaphrodites, to the point of sterility at sufficient extremes. Exposure to either males or male pheromones is sufficient to restore partial fertility. The authors focused on two ascaroside pheromones, ascr#3 and Current Opinion in Neurobiology 2016, 38:119–124
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ascr#10. These pheromones appear to promote clearance of gonadal blockages and improved sperm guidance, respectively. These two pheromones are in fact produced by both genders, but in different proportions. A hermaphroditelike synthetic cocktail does not promote recovery, though it is unknown how the hermaphrodite can exercise this level of discretion when the molecules are the same. Not all pheromone exposure serves to promote fertility. Dauer pheromones are believed to alert nematodes to conditions of overcrowding. In young larvae, these pheromones cause entry into the long-lived dauer state, but in adult hermaphrodites, these same pheromones cause reduced production of sperm guidance signals, leading to lower rates of fertilization [40]. Both this effect and the heat-stress amelioration of male pheromones are dependent on the gene daf-7, which encodes a TGF-b ligand secreted by sensory neurons [39,40]. It seems that C. elegans and other nematodes have evolved to reproduce at variable rates depending on environmental and social conditions.
Battle of the sexes It has been known for over 40 years that mating with a male reduces the lifespan of a nematode, first in Panagrellus [41], then in C. elegans [42] and more recently in C. remanei [43]. It has even been shown that in a mutant gonochoristic C. elegans strain raised for 100 generations, males have evolved to cause even more extreme reduction in lifespan, while the pseudofemales have evolved a resistance to this effect [44]. Although the act of mating with a male causes damage to a worm’s cuticle [45], this is not sufficient to reduce lifespan. Rather, the major impact on hermaphrodite or female lifespan appears to result from exposure to male pheromone and transfer of seminal fluid, and is mediated by hermaphrodite/female neuronal activity (especially insulin signaling). The shortening of the hermaphrodite lifespan is sometimes accompanied by substantial shrinking of the body and sensitization to osmotic stress before death [46,47]. It is frequently argued that the reduction in female/hermaphrodite lifespan is an example of the battle of the sexes — that males reduce the lifespan of their mate for selfish purposes. This argument typically appears in the absence of experiments to demonstrate that lifespan reduction is not coupled to hermaphrodite/ female benefit. Although the fact that lifespan reduction can be achieved without fertilization, or even mating, is strong evidence against a ‘death through reproductive exhaustion’ hypothesis, it does not rule out that the female is making physiological changes in anticipation of increased reproductive rate. This has been presented as a ‘life span versus reproduction’ hypothesis. Settling this question is complicated by the strong influence of experimental Current Opinion in Neurobiology 2016, 38:119–124
design on the relationship between longevity and hermaphrodite fecundity. Wu et al. (2012) found that perpetual mating with five males per hermaphrodite yields a positive correlation between lifespan and fecundity, mostly due to lifespans becoming so short, as hermaphrodites die before exit from the fertile phase of life. In a much more mild setup with one male per hermaphrodite, with mating only last for 24 h, there was a negative correlation between lifespan and fecundity [48]. Shi and Murphy [46] argue, among other reasons, that because daf-12 mutants can extend their fertile period through mating without suffering a reduction in lifespan, lifespan shortening is not linked to hermaphrodite benefit. However, the daf-12 mutants used in this study suffer from shortened lifespan to begin with. A convincing proof that lifespan reduction is to the hermaphrodite’s detriment would be a demonstration that pheromone-insensitive hermaphrodites can yield at least as many viable offspring after mating as wild-type hermaphrodites.
Conclusion More than ten years ago, virtually all research on nematode mating pheromones was limited to merely demonstrating that they existed in various species, and motivated chemoattraction. Few pheromones were chemically identified. In the last ten years, we have seen an accelerated pace of research on nematode pheromones thanks to the discovery of ascarosides, whose use as pheromones appears to be conserved across much of Nematoda. With the ability to now synthesize pure pheromones, we expect future research to uncover the neurological and biochemical activity of many pheromones that we may better understand how the worm sexes communicate in the wild. Although the vast majority of research in this field has been conducted in the model organism C. elegans, the knowledge of the conservation of ascaroside pheromones, as well as the increased ease with which even nonmodel organisms can be genetically modified, should make it easier for future research to uncover similar systems in other nematodes.
Conflict of interest statement Nothing declared.
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
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ScienceDirect Mating pheromones of Nematoda: olfactory signaling with physiological consequences Daniel HW Leighton and Paul W Sternberg Secreted pheromones have long been known to influence mating in the phylum Nematoda. The study of nematode sexual behavior has greatly benefited in the last decade from the genetic and neurobiological tools available for the model nematode Caenorhabditis elegans, as well as from the chemical identification of many pheromones secreted by this species. The discovery that nematodes can influence one another’s physiological development and stress responsiveness through the sharing of pheromones, in addition to simply triggering sexual attraction, is particularly striking. Here we review recent research on nematode mating pheromones, which has been conducted predominantly on C. elegans, but there are beginning to be parallel studies in other species. Address HHMI and Division of Biology and Biological Engineering, Caltech, Pasadena 91125, USA Corresponding author: Sternberg, Paul W ([email protected])
consisting of self-fertilizing hermaphrodites and rare males (reviewed in [2]). The hermaphrodites are incapable of mating with one another, and may only self-fertilize using a limited supply of self-sperm generated during larval development — reproduction after the exhaustion of self-sperm can only proceed by mating with a male. That mating is strictly unnecessary for survival of C. elegans makes this species an excellent model for the study of mating behavior [3], as the mating process can be disrupted at nearly every step without sacrificing the organism’s viability [4]. The discovery that many disparate species utilize ascarosides, a nematode-specific family of glycolipids, for communication, has greatly accelerated the rate of research in this field (reviewed in [5]). The research reviewed here largely took advantage of the ability to identify specific pheromones and challenge worms with pure synthetic compounds to uncover the precise neurological and physiological activities of these molecules.
Current Opinion in Neurobiology 2016, 38:119–124
Ascarosides
This review comes from a themed issue on Neurobiology of sex
The ascarosides are glycosides of the dideoxysugar ascarylose. These molecules always include a lipid tail, and may be conjugated to amino acid derivatives or other small molecules [6]. Ascarosides are now known to be produced by over twenty species of nematode from multiple clades, including free-living, vertebrate parasitic and insect parasitic worms [7]. Ascarosides have been demonstrated as mate-attracting pheromones both C. elegans [8] and Panagrellus redivivus [9], despite these species’ significant evolutionary distance, lying in different families. Ascarosides have also been shown to promote entrance into dauer, an alternative non-feeding and stress resistant larval stage, in C. elegans [10a], Heterorhabditis bacteriophora [11] and Pristionchus pacificus [12]. As would be expected of signaling molecules, ascaroside production varies with a worm’s age and environment [13]. In both C. elegans and P. redivivus, the different genders secrete specific ascarosides in vastly different quantities, with each gender producing the attractants for the other. In addition, the C. elegans hermaphrodite secreted pheromones are repulsive to other hermaphrodites [9,14]. Taken together, this evidence suggests that ascarosides are evolutionarily ancient signaling molecules that may serve as mating pheromones across much of Nematoda.
Edited by Barry Dickson and Catherine Dulac
http://dx.doi.org/10.1016/j.conb.2016.04.008 0959-4388/# 2016 Elsevier Ltd. All rights reserved.
Introduction Nematoda is a diverse phylum of worms that occupy a variety of terrestrial, aquatic and marine habitats. Most nematode species are gonochoristic, consisting of males and females that must locate one another and mate to reproduce. Research on nematode mating pheromones began in the 1960s with an eye toward understanding the mating habits of animal-parasitic and plant-parasitic nematodes. Since then, it has been found that dozens of nematode species, most of them vision-impaired and hearing-impaired by nature, locate one another by the reception of secreted molecules (reviewed in [1]). Most research on nematodes is conducted in the small, transparent Caenorhabditis elegans, which lives on rotting fruit and utilizes insect vectors as phoretic hosts. This nematode, unlike most, is androdiecious in nature, www.sciencedirect.com
In addition to being a highly diverse molecular family with over 150 members, the specific activity of each ascaroside is highly dependent on its chemical structure Current Opinion in Neurobiology 2016, 38:119–124
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Figure 1
O H3C HO
ascr#2
O
O OH
OH
O H3C HO
ascr#3
O
O OH O O
H3C HO
ascr#5
OH
O OH H N
O H3C HO
ascr#8
O
O
CO2H
OH
O
icas#9
H3C O
HN
O OH
O OH
O
OH
O
icas#3
H3C O
HN
OH
O
Dauer Formation
O
O
Male attraction
Aggregation Current Opinion in Neurobiology
The behavioral response of Caenorhabditis elegans to specific ascarosides is highly dependent on chemical structure.
[15] (Figure 1), and the same ascaroside can have extremely different effects on different species. For example, ascr#1 is a potent mate-attracting pheromone in P. redivivus, but promotes dauer formation in C. elegans [9,10a,16]. In fact, evolution of ascaroside signaling appears to be extremely rapid even within a species. The production and reception of dauer pheromones by both C. elegans and P. pacificus differs markedly amongst wild isolates of both species, possibly as a result of intense intraspecific competition over food resources [12,17]. Different strains of the same nematode are even known to mate at different rates, though this may have both pheromone-related and other reasons [18]. Ascarosides are also Current Opinion in Neurobiology 2016, 38:119–124
known to trigger feeding behavior in nematophagous fungi [19], and immune reactions in plant hosts [20], which likely apply additional pressure on the rapid evolution of nematode pheromones. Additionally, the ability of a nematode to locate a mate depends on more than simply chemotaxis to a pheromone cocktail. For example, Sammut et al. (2015) have recently shown that C. elegans males can be conditioned to associate a particular salt concentration with potential mates [21]. The essentially invariant anatomy of C. elegans, coupled with the ability to identify specific neurons under Nomarksi microscopy, has allowed researchers to identify www.sciencedirect.com
Pheromone signaling in nematodes Leighton and Sternberg 121
pheromone-responsive neurons through laser ablation experiments. The four male-specific CEM neurons are necessary for male attraction to two different ascarosides and the presence of multiple CEMs are argued to be necessary to generate the concentration tuning curve [10b]. Whole cell patch clamp electrophysiology indicates that the CEMs are primary sensors. In a given animal a particular CEM will respond positively or negatively to the ascaroside; this heterogeneous response might aid in the male nervous system calculating a derivative of pheromone concentration. The ADL, ASI, and ASK sensory neurons are also necessary for normal response to mating pheromones, and thus are expected to express mating pheromone receptors [22– 24]. One exciting finding from these studies is that the male neuronal response to mating pheromone is exhibited within the hermaphrodite nervous system as well. However, the stereotypical male behavioral response is simultaneously inhibited by neurons that only react to mating pheromone in the hermaphrodite [24]. Currently, six ascaroside receptors have been identified in C. elegans: srg-36, srg-37, srbc-64, srbc-66, daf-37 and daf38. Two of these receptors, srg-36 and srg-37, have homologs with conserved function in Caenorhabditis briggsae. All six receptors are G-protein-coupled-receptors (GPCRs), consistent with findings that GPCR function is important for sensation of ascarosides [25–28]. It has proven difficult, however, to determine precise relationships between pheromones and their receptors. It is not known whether ascaroside receptors function as single proteins, or as homo or heterodimers. Two yeast-based assays have recently been developed specifically for analysis of C. elegans GPCR activity [29,30]; this may allow for more rapid identification of ligand–receptor relationships than the mammalian and Xenopus based assays traditionally used for this purpose.
Hermaphrodite versus female behavior There has been a common conception in nematode literature that C. elegans hermaphrodites have lost certain female traits through evolution (e.g., [31–33]). Traits claimed to have been lost by hermaphrodites include the production of volatile mate-attracting pheromone and mating-induced torpor (which may aid the male in locating the vulva). In fact, evidence for both of these traits may be found in C. elegans, but mating behavior in general is repressed by the presence of sperm. As with a recently mated female, a young hermaphrodite exhibits no production of volatile pheromone, but resumes production upon sperm depletion, whether due to mutation or age [34]. This change in hermaphrodite pheromone production is also observed in the plant parasitic nematode Bursaphelenchus okinawaensis [35]. Garcia et al. [36] argued that hermaphrodites and females differ primarily because of age, but it is worth revisiting www.sciencedirect.com
the observations in light of the recent finding of spermregulated physiology in hermaphrodites. In particular, the ability of males to successfully insert their spicules into hermaphrodites or xenospecific females was assayed. There appears to be an affect of both age and sperm status. Specifically, the authors investigated the number of mating attempts made by a male before successful spicule insertion, depending on the strain of hermaphrodites he was presented with. The data for this experiment shows a dramatic reduction in the number of failed attempts if the hermaphrodite is spermless due to mutation, and a further reduction if the hermaphrodite is aged past the normal period of self-fertilization. These data indicate there are separate youth-correlated and spermcorrelated inhibitory effects on male copulation. Similar results were reported by another group in the same year [37]. Young C. elegans pseudofemales may be more difficult to mate with than young true females of another species, but the difference is far smaller than that between a female and a young hermaphrodite. C. elegans males do not induce torpor in females of other species upon mating, but this may simply be due to divergence of signals or receptors. Keeping in mind the significant differences in mating behavior between isolates of the same species, we believe a proper test of this torpor hypothesis would be to challenge males from multiple C. elegans isolates with aged and feminized hermaphrodites, also of various isolates. Although there are genuine differences between hermaphrodites and true females of closely related species, in many respects the hermaphrodites are simply behaving as sperm replete females.
Physiological effects of pheromone exposure A major shift in the discussion of nematode mating pheromones has occurred with the recognition that they are responsible for more than just chemoattraction. An early paper that attempted to address this topic, Timmermeyer et al. [38], showed that C. remanei females that had received copulatory plugs from their mates were more fecund than their unplugged sisters, despite receiving a similar number of matings. This observation suggests that male-deposited mating plugs, which may contain signaling compounds, might act to promote egglaying, oocyte development, or some other fertility promoting process in the receiving female. However, the possibility remains that plugged females had simply received or retained more sperm than their unplugged sisters in these experiments. Very recently, Aprison and Ruvinsky [39] showed that exposure to male pheromones has a substantial and beneficial effect on recovery from heat stress in both C. elegans and C. remanei [39]. Heat stress is known to reduce fertility in C. elegans hermaphrodites, to the point of sterility at sufficient extremes. Exposure to either males or male pheromones is sufficient to restore partial fertility. The authors focused on two ascaroside pheromones, ascr#3 and Current Opinion in Neurobiology 2016, 38:119–124
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ascr#10. These pheromones appear to promote clearance of gonadal blockages and improved sperm guidance, respectively. These two pheromones are in fact produced by both genders, but in different proportions. A hermaphroditelike synthetic cocktail does not promote recovery, though it is unknown how the hermaphrodite can exercise this level of discretion when the molecules are the same. Not all pheromone exposure serves to promote fertility. Dauer pheromones are believed to alert nematodes to conditions of overcrowding. In young larvae, these pheromones cause entry into the long-lived dauer state, but in adult hermaphrodites, these same pheromones cause reduced production of sperm guidance signals, leading to lower rates of fertilization [40]. Both this effect and the heat-stress amelioration of male pheromones are dependent on the gene daf-7, which encodes a TGF-b ligand secreted by sensory neurons [39,40]. It seems that C. elegans and other nematodes have evolved to reproduce at variable rates depending on environmental and social conditions.
Battle of the sexes It has been known for over 40 years that mating with a male reduces the lifespan of a nematode, first in Panagrellus [41], then in C. elegans [42] and more recently in C. remanei [43]. It has even been shown that in a mutant gonochoristic C. elegans strain raised for 100 generations, males have evolved to cause even more extreme reduction in lifespan, while the pseudofemales have evolved a resistance to this effect [44]. Although the act of mating with a male causes damage to a worm’s cuticle [45], this is not sufficient to reduce lifespan. Rather, the major impact on hermaphrodite or female lifespan appears to result from exposure to male pheromone and transfer of seminal fluid, and is mediated by hermaphrodite/female neuronal activity (especially insulin signaling). The shortening of the hermaphrodite lifespan is sometimes accompanied by substantial shrinking of the body and sensitization to osmotic stress before death [46,47]. It is frequently argued that the reduction in female/hermaphrodite lifespan is an example of the battle of the sexes — that males reduce the lifespan of their mate for selfish purposes. This argument typically appears in the absence of experiments to demonstrate that lifespan reduction is not coupled to hermaphrodite/ female benefit. Although the fact that lifespan reduction can be achieved without fertilization, or even mating, is strong evidence against a ‘death through reproductive exhaustion’ hypothesis, it does not rule out that the female is making physiological changes in anticipation of increased reproductive rate. This has been presented as a ‘life span versus reproduction’ hypothesis. Settling this question is complicated by the strong influence of experimental Current Opinion in Neurobiology 2016, 38:119–124
design on the relationship between longevity and hermaphrodite fecundity. Wu et al. (2012) found that perpetual mating with five males per hermaphrodite yields a positive correlation between lifespan and fecundity, mostly due to lifespans becoming so short, as hermaphrodites die before exit from the fertile phase of life. In a much more mild setup with one male per hermaphrodite, with mating only last for 24 h, there was a negative correlation between lifespan and fecundity [48]. Shi and Murphy [46] argue, among other reasons, that because daf-12 mutants can extend their fertile period through mating without suffering a reduction in lifespan, lifespan shortening is not linked to hermaphrodite benefit. However, the daf-12 mutants used in this study suffer from shortened lifespan to begin with. A convincing proof that lifespan reduction is to the hermaphrodite’s detriment would be a demonstration that pheromone-insensitive hermaphrodites can yield at least as many viable offspring after mating as wild-type hermaphrodites.
Conclusion More than ten years ago, virtually all research on nematode mating pheromones was limited to merely demonstrating that they existed in various species, and motivated chemoattraction. Few pheromones were chemically identified. In the last ten years, we have seen an accelerated pace of research on nematode pheromones thanks to the discovery of ascarosides, whose use as pheromones appears to be conserved across much of Nematoda. With the ability to now synthesize pure pheromones, we expect future research to uncover the neurological and biochemical activity of many pheromones that we may better understand how the worm sexes communicate in the wild. Although the vast majority of research in this field has been conducted in the model organism C. elegans, the knowledge of the conservation of ascaroside pheromones, as well as the increased ease with which even nonmodel organisms can be genetically modified, should make it easier for future research to uncover similar systems in other nematodes.
Conflict of interest statement Nothing declared.
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