Sperm status regulates sexual attraction in Caenorhabditis elegans

Mating behavior of animals is regulated by the sensory stimuli provided by the other sex. Sexually receptive females emit mating signals that can be inhibited by male ejaculate. The genetic mechanisms controlling the release of mating signals and encoding behavioral responses remain enigmatic. Here we present evidence of a Caenorhabditis elegans hermaphrodite-derived cue that stimulates male mating-response behavior and is dynamically regulated by her reproductive status. Wild-type males preferentially mated with older hermaphrodites. Increased sex appeal of older hermaphrodites was potent enough to stimulate robust response from mating-deficient pkd-2 and lov-1 polycystin mutant males. This enhanced response of pkd-2 males toward older hermaphrodites was independent of short-chain ascaroside pheromones, but was contingent on the absence of active sperm in the hermaphrodites. The improved pkd-2 male response toward spermless hermaphrodites was blocked by prior insemination or by genetic ablation of the ceh-18-dependent sperm-sensing pathway of the hermaphrodite somatic gonad. Our work suggests an interaction between sperm and the soma that has a negative but reversible effect on a hermaphrodite-derived mating cue that regulates male mating response, a phenomenon to date attributed to gonochoristic species only.     

Low potential for sexual selection in simultaneously hermaphroditic animals

Hermaphroditic animals are poorly represented in the sexual selection literature. This deficiency may reflect our inability to come to grips with hermaphroditism or, alternatively, it could be due to an inherent difference between hermaphrodites and gonochorists. Here we provide a number of reasons why sexual selection on traits related to mate acquisition can be expected to be intrinsically weaker in hermaphrodites. We show that the ''male'' fitness component, which can be increased by sexual selection in hermaphrodites, is only half that of pure males in a gonochorist population. This component can be reduced even further when hermaphrodites self-fertilize. As a result, the potential for sexual selection (ψ) on male characters in hermaphrodites is at most half that of gonochorists. Given a specific mate handling cost, sperm production cost and rate of encountering receptive mates, we calculate the optimal allocation to mate acquisition and sperm. Since both partners of a hermaphroditic pair invest in mate acquisition, hermaphrodites should optimally invest less in mate acquisition. This can further reduce ψ by up to one-half. A higher readiness to mate and high investment in sperm can lead to a further systematic reduction in P_s in hermaphrodites.     

Aspects of compound-conditioning to gustatory stimuli in the housefly Musca domestica L.

Houseflies (Musca domestica L.) were trained in a compound-conditioning paradigm where the conditioning stimuli were water and 1% sodium chloride solution. The unconditioned stimulus was 16% sucrose solution. A high degree of conditioning was produced. Control experiments for pseudoconditioning and sensitization revealed that the response of the flies to the procedure were due to an associative process. Experiments with double-water conditioned stimuli and with an interval between stimulus presentations indicate that the marked response to the first conditioned stimulus may be due to stimulus generalization in which the water, rather than the salt component of both stimuli served as the learning cue.     

Divergent male and female mate preferences do not explain incipient speciation between lizard lineages

Diversification in sexual signals is often taken as evidence for the importance of sexual selection in speciation. However, in order for sexual selection to generate reproductive isolation between populations, both signals and mate preferences must diverge together. Furthermore, assortative mating may result from multiple behavioral mechanisms, including female mate preferences, male mate preferences, and male–male competition; yet their relative contributions are rarely evaluated. Here, we explored the role of mate preferences and male competitive ability as potential barriers to gene flow between 2 divergent lineages of the tawny dragon lizard, Ctenophorus decresii, which differ in male throat coloration. We found stronger behavioral barriers to pairings between southern lineage males and northern lineage females than between northern males and southern females, indicating incomplete and asymmetric behavioral isolating barriers. These results were driven by both male and female mate preferences rather than lineage differences in male competitive ability. Intrasexual selection is therefore unlikely to drive the outcome of secondary contact in C. decresii, despite its widely acknowledged importance in lizards. Our results are consistent with the emerging view that although both male and female mate preferences can diverge alongside sexual signals, speciation is rarely driven by divergent sexual selection alone.     

Characterization of assortative mating in medaka: Mate discrimination cues and factors that bias sexual preference

Somatolactin alpha (SLα) is a peptide hormone that regulates skin color, and SLα-deficient and SLα-excess strains have been established in medaka (Oryzias latipes). Their skin colors differ conspicuously and males prefer to mate with females from the same strain. Pre-mating sexual isolation in this model vertebrate provides an ideal platform for investigating the molecular mechanisms of mate choice. Thus, we studied the sensory cues utilized by these fish to discriminate the same and different strains. When males were given a choice under monochromatic light, where the skin colors differed only in terms of brightness but not in hue, mating occurred but it was not assortative. This suggests that: (1) a visual cue is essential for mate discrimination rather than odor or acoustic cues; (2) the visual cue is color and not shape, size, or motion; and (3) the color cue needs to be perceived as the relative balance of brightness at multiple wavelengths rather than the brightness at a specific wavelength. In addition, we introduced another skin-color mutation into the SLα-excess strain and found that this new strain and the original SLα-excess strain, which also overexpressed SLα but exhibited distinct skin colors, preferred different colors. This demonstrates that SLα is not a primary determinant of sexual preference. The symmetrically biased sexual preferences of the SLα-deficient and SLα-excess strains may be acquired postnatally depending on their individual skin color or that of tank mates.     

Mate Guarding in the Androdioecious Clam Shrimp Eulimnadia texana: Male Assessment of Hermaphrodite Receptivity

Precopulatory mate guarding primarily occurs when males encounter receptive females at a low enough rate that such females become a valuable resource once encountered. Such circumstances are common in aquatic crustaceans wherein females are only receptive for a short period directly after molting. In these species, males commonly mate guard by physically attaching themselves to their prospective mates for hours to days at a time. To be effective in mate guarding, males must be able to assess the time to receptivity in their mates, which is commonly via chemical cues associated with molting. Clam shrimp in the genus Eulimnadia exhibit mate guarding, but with an important variation: these species are mixtures of males and hermaphrodites (androdioecy) rather than males and females. Nonetheless, the mate guarding behaviors of these shrimp are much the same as in other aquatic crustaceans. In this study, three projects were undertaken to determine the ability of Eulimnadia texana males to assess hermaphroditic receptivity. Males were found to be unable to assess receptivity without physically contacting hermaphrodites. However, after physical contact, males spent a significantly greater amount of time guarding receptive relative to non‐receptive hermaphrodites. Additionally, male interest in mate guarding was highest during the period between the dropping of one clutch of eggs and the extrusion of the following clutch. Because this period is also associated with hermaphroditic molting, it is consistent with the notion that males cue into chemicals associated with molting to determine hermaphroditic receptivity. These findings are consistent with previous studies of mating behavior in this species, and we discuss their importance to future tests of optimal mate guarding planned for these shrimp.     

Systematic comparison of bacterial feeding strains for increased yield of Caenorhabditis elegans males by RNA interference-induced non-disjunction

Rare Caenorhabditis elegans males arise when sex chromosome non-disjunction occurs during meiosis in self-fertilizing hermaphrodites. Non-disjunction is a relatively rare event, and males are typically observed at a frequency of less than one in five hundred wild-type animals. Males are required for genetic crosses and phenotypic analysis, yet current methods to generate large numbers of males can be cumbersome. Here, we identify RNAi reagents (dsRNA-expressing bacteria) with improved effectiveness for eliciting males. Specifically, we used RNAi to systematically reduce the expression of over two hundred genes with meiotic chromosome segregation functions, and we identified a set of RNAi reagents that robustly and reproducibly elicited male progeny.     

Developmental and reproductive consequences of prolonged non-aging dauer in Caenorhabditis elegans

When Caenorhabditis elegans encounters harsh environmental conditions, it enters a non-aging diapause (dauer), an alternative larval stage capable of long-term survival. This replaces the stage of normal development critical for development of the reproductive organs. Here, we report that increased duration of diapause causes a delay in post-dauer development, and also causes severe defects in the reproductive development of males and hermaphrodites. Thus, the dauer state, while allowing for survival under adverse conditions, has important developmental and reproductive consequences. This effect is more pronounced in males, possibly accounting for the increased survival of C. elegans hermaphrodites in challenging environmental conditions.     

Somatic damage to the X chromosome of the nematode Caenorhabditis elegans induced by gamma radiation

Summary Wild-type male embryos and young larvae of the nematode Caenorhabditis elegans were more sensitive than wild-type hermaphrodites to inactivation by gamma rays; wild-type males have one X chromosome per cell (XO), whereas wild-type hermaphrodites have two (XX). Furthermore, after transformation into fertile hermaphrodites by a her-1 mutation, XO animals were more radiosensitive than XX her-1 animals; and XX animals transformed into fertile males by a tra-1 mutation did not show increased radiosensitivity. It is concluded that wild-type males are more radiosensitive than wild-type hermaphrodites because they have one X chromosome rather than two, and the predominant mode of inactivation of XO animals involves damage to the single X chromosome. No sex-specific differences in survival were observed after UV irradiation.     

Mating Damages the Cuticle of C. elegans Hermaphrodites

Lifespan costs to reproduction are common across multiple species, and such costs could potentially arise through a number of mechanisms. In the nematode Caenorhabditis elegans, it has been suggested that part of the lifespan cost to hermaphrodites from mating results from physical damage owing to the act of copulation itself. Here, we examine whether mating damages the surface of the hermaphrodite cuticle via scanning electron microscopy. It is found that mated hermaphrodites suffered delamination of cuticle layers surrounding the vulva, and that the incidence of such damage depends on genetic background. Unmated hermaphrodites demonstrated almost no such damage, even when cultured in soil with potentially abrasive particles. Thus, a consequence of mating for C. elegans hermaphrodites is physical cuticle damage. These experiments did not assess the consequences of cuticle damage for lifespan, and the biological significance of this damage remains unclear. We further discuss our results within the context of recent studies linking the lifespan cost to mating in C. elegans hermaphrodites to male secretions.     

The evolution from females to hermaphrodites results in a sexual conflict over mating in androdioecious nematode worms and clam shrimp

The nematode worm Caenorhabditis elegans and the clam shrimp Eulimnadia texana are two well‐studied androdioecious species consisting mostly of self‐fertilizing hermaphrodites and few males. To understand how androdioecy can evolve, a simple two‐step mathematical model of the evolutionary pathway from a male–female species to a selfing‐hermaphrodite species is constructed. First, the frequency of mutant females capable of facultative self‐fertilization increases if the benefits of reproductive assurance exceed the cost. Second, hermaphrodites become obligate self‐fertilizers if the fitness of selfed offspring exceeds one‐half the fitness of outcrossed offspring. Genetic considerations specific to C. elegans and E. texana show that males may endure as descendants of the ancestral male–female species. These models combined with an extensive literature review suggest a sexual conflict over mating in these androdioecious species: selection favours hermaphrodites that self and males that outcross. The strength of selection on hermaphrodites and males differs, however. Males that fail to outcross suffer a genetic death. Hermaphrodites may never encounter a rare male, and those that do and outcross only bear less fecund offspring. This asymmetric sexual conflict results in an evolutionary stand‐off: rare, but persistent males occasionally fertilize common, but reluctant hermaphrodites. A consequence of this stand‐off may be an increase in the longevity of the androdioecious mating system.     

The Effect of Timing of Female Vibrational Reply on Male Signalling and Searching Behaviour in the Leafhopper Aphrodes makarovi

Sexual communication in animals often involves duetting characterized by a coordinated reciprocal exchange of acoustic signals. We used playback experiments to study the role of timing of a female reply in the species-specific duet structure in the leafhopper Aphrodes makarovi (Hemiptera: Cicadellidae). In leafhoppers, mate recognition and location is mediated exclusively by species- and sex-specific substrate-borne vibrational signals and a female signal emitted in reply to male advertisement calls is essential for recognition and successful location of the female. In A. makarovi, males have to initiate each exchange of vibrational signals between partners, and in a duet the beginning of a female reply overlaps the end of the male advertisement call. Results of playback treatments in which female replies were delayed and did not overlap with the male call revealed that in order to trigger an appropriate behavioural response of the male, female reply has to appear in a period less than 400 ms after the end of the initiating male call. Results also suggest that males are not able to detect a female reply while calling, since female reply that did not continue after the end of male call triggered male behaviour similar to behaviour observed in the absence of female reply. Together, our results show that vibrational duets are tightly coordinated and that the species-specific duet structure plays an important role in mate recognition in location processes.     

Investment in sensory structures,testis size,and wing coloration in males of a diurnal moth species: trade-offs or correlated growth?

For dioecious animals, reproductive success typically involves an exchange between the sexes of signals that provide information about mate location and quality. Typically, the elaborate, secondary sexual ornaments of males signal their quality, while females may signal their location and receptivity. In theory, the receptor structures that receive the latter signals may also become elaborate or enlarged in a way that ultimately functions to enhance mating success through improved mate location. The large, elaborate antennae of many male moths are one such sensory structure, and eye size may also be important in diurnal moths. Investment in these traits may be costly, resulting in trade‐offs among different traits associated with mate location. For polyandrous species, such trade‐offs may also include traits associated with paternity success, such as larger testes. Conversely, we would not expect this to be the case for monandrous species, where sperm competition is unlikely. We investigated these ideas by evaluating the relationship between investment in sensory structures (antennae, eye), testis, and a putative warning signal (orange hindwing patch) in field‐caught males of the monandrous diurnal painted apple moth Teia anartoides (Lepidoptera: Lymantriidae) in southeastern Australia. As predicted for a monandrous species, we found no evidence that male moths with larger sensory structures had reduced investment in testis size. However, contrary to expectation, investment in sensory structures was correlated: males with relatively larger antennae also had relatively larger eyes. Intriguingly, also, the size of male orange hindwing patches was positively correlated with testis size.     

The unexpected mating system of the androdioecious barnacle Chelonibia testudinaria (Linnaeus 1758)

Androdioecy was first described by Darwin in his seminal work on barnacle diversity; he identified males and hermaphrodites in the same reproductive population. Today, we realize that many androdioecious plants and animals share astonishing similarities, particularly with regard to their evolutionary history and mating system. Notably, these species were ancestrally dioecious, and their mating system has the following characteristics: hermaphrodites self‐fertilize frequently, males are more successful in large mating groups, and males have a mating advantage. A male mating advantage makes androdioecy more likely to persist over evolutionary times. Androdioecious barnacles, however, appear to persist as an outlier with a different evolutionary trajectory: they originate from hermaphroditic species. Although sexual systems of androdioecious barnacles are known, no information on the mating system of androdioecious barnacles is available. This study assessed the mating system of the androdioecious barnacle Chelonibia testudinaria. In contrast to other androdioecious species, C. testudinaria does not self‐fertilize, males do not have a mating advantage over hermaphrodites, and the average mating group is quite small, averaging only three individuals. Mating success is increased by proximity to the mate and penis length. Taken together, the mating system of C. testudinaria is unusual in comparison with other androdioecious plants and animals, and the lack of a male mating advantage suggests that the mating system alone does not provide an explanation for the maintenance of androdioecy in this species. Instead, we propose that sex‐specific life history equalizes male and hermaphroditic overall fitness.     

Mutations Causing Transformation of Sexual Phenotype in the Nematode CAENORHABDITIS ELEGANS

Ten mutations are described that transform genotypic hermaphrodites of the nematode Caenorhabditis elegans into phenotypic males. These fall into three autosomal complementation groups, termed tra-1, tra-2 , and tra-3. Two alleles of tra-1 produce almost complete transformation, to a fertile male phenotype; such transformed animals are useful for analyzing sex-linked genes. All alleles of tra-1 and tra-2 are recessive; the one known allele of tra-3 is both recessive and maternal in effect. Where tested, both XX and XXX hermaphrodites are transformed into males, but XO males (true males) are unaffected by these mutations. It is suggested that these genes are actually involved in hermaphrodite development and have no role in male development.