Gryllodes Sigillatus Research Articles

Competition, fluctuating asymmetry and sperm transfer in male gryllid crickets (Gryllus bimaculatus and Gryllodes sigillatus)

Sperm competition theory predicts that ejaculate expenditure by males should increase with the risk of competition from rivals. The tendency to modulate expenditure, however, may reflect various constraints acting on different individuals. We test this idea here by looking at the effects of competition and fluctuating asymmetry (FA) in two species of gryllid cricket. Male Gryllus bimaculatus and Gryllodes sigillatus modulated the sperm content of their spermatophores in relation to both apparent competition and FA in limb size. Males of both species increased the amount of sperm transferred when an apparent male competitor was present, sperm number increasing regardless of the species of apparent competitor in G. bimaculatus, but significantly more with a conspecific competitor in G. sigillatus. In addition, the relative size of the spermatophylax in G. sigillatus increased in the presence of apparent competition, while spermatophore transfer time decreased in G. bimaculatus. Sperm number showed opposite relationships with limb asymmetry in the two species, decreasing with increasing asymmetry in males in G. sigillatus, but increasing with asymmetry in both males and females in G. bimaculatus. G. bimaculatus, but not G. sigillatus, also transferred more sperm when paired with a larger female. Relationships between reproductive measures and limb asymmetry were significant only when competitors were present in G. bimaculatus, but were significant regardless of competition in G. sigillatus. The differences may reflect the additional burden of producing a spermatophylax in G. sigillatus.

Linear models for assessing mechanisms of sperm competition: the trouble with transformations.

Although sperm competition is a pervasive selective force shaping the reproductive tactics of males, the mechanisms underlying different patterns of sperm precedence remain obscure. Parker et al. (1990) developed a series of linear models designed to identify two of the more basic mechanisms: sperm lotteries and sperm displacement; the models can be tested experimentally by manipulating the relative numbers of sperm transferred by rival males and determining the paternity of offspring. Here we show that tests of the model derived for sperm lotteries can result in misleading inferences about the underlying mechanism of sperm precedence because the required inverse transformations may lead to a violation of fundamental assumptions of linear regression. We show that this problem can be remedied by reformulating the model using the actual numbers of offspring sired by each male, and log-transforming both sides of the resultant equation. Reassessment of data from a previous study (Sakaluk and Eggert 1996) using the corrected version of the model revealed that we should not have excluded a simple sperm lottery as a possible mechanism of sperm competition in decorated crickets, Gryllodes sigillatus.

LINEAR MODELS FOR ASSESSING MECHANISMS OF SPERM COMPETITION: THE TROUBLE WITH TRANSFORMATIONS

Although sperm competition is a pervasive selective force shaping the reproductive tactics of males, the mechanisms underlying different patterns of sperm precedence remain obscure. Parker et al. (1990) developed a series of linear models designed to identify two of the more basic mechanisms: sperm lotteries and sperm displacement; the models can be tested experimentally by manipulating the relative numbers of sperm transferred by rival males and determining the paternity of offspring. Here we show that tests of the model derived for sperm lotteries can result in misleading inferences about the underlying mechanism of sperm precedence because the required inverse transformations may lead to a violation of fundamental assumptions of linear regression. We show that this problem can be remedied by reformulating the model using the actual numbers of offspring sired by each male, and log-transforming both sides of the resultant equation. Reassessment of data from a previous study (Sakaluk and Eggert 1996) using the corrected version of the model revealed that we should not have excluded a simple sperm lottery as a possible mechanism of sperm competition in decorated crickets, Gryllodes sigillatus.

Polyandry and fitness of offspring reared under varying nutritional stress in decorated crickets.

Females, by mating with more than one male in their lifetime, may reduce their risk of receiving sperm from genetically incompatible sires or increase their prospects of obtaining sperm from genetically superior sires. Although there is evidence of both kinds of genetic benefits in crickets, their relative importance remains unclear, and the extent to which experimentally manipulated levels of polyandry in the laboratory correspond to those that occur in nature remain unknown. We measured lifetime polyandry of free-living female decorated crickets, Gryllodes sigillatus, and conducted an experiment to determine whether polyandry leads to an increase in offspring viability. We experimentally manipulated both the levels of polyandry and opportunities for females to select among males, randomly allocating the offspring of experimental females to high-food-stress or low-food-stress regimes to complete their development. Females exhibited a high degree of polyandry, mating on average with more than seven different males during their lifetime and up to as many as 15. Polyandry had no effect on either the developmental time or survival of offspring. However, polyandrous females produced significantly heavier sons than those of monandrous females, although there was no difference in the adult mass of daughters. There was no significant interaction between mating treatment and offspring nutritional regimen in their effects on offspring mass, suggesting that benefits accruing to female polyandry are independent of the environment in which offspring develop. The sex difference in the extent to which male and female offspring benefit via their mother's polyandry may reflect possible differences in the fitness returns from sons and daughters. The larger mass gain shown by sons of polyandrous females probably leads to their increased reproductive success, either because of their increased success in sperm competition or because of their increased life span.

The role of the male's cerci in copulation and mate guarding in decorated crickets (Gryllodes sigillatus)

AbstractCerci are paired, sensory appendages extending from the terminal abdominal segment of crickets. While the cerci are acutely sensitive to air currents and thereby function in the detection of potential predators, they are also known to play a role in co‐ordinating movements of males and females during copulation. The role of the male's cerci at four stages of the mating sequence (courtship, copulation, mate guarding and spermatophore removal) was examined by experimentally removing the cerci of male decorated crickets Gryllodes sigillatus and comparing their mating success with that of control males. The mating success of cercectomized males was significantly reduced relative to control males, primarily because of their greater inability to induce females to mount. Even when they succeeded in securing an initial mount, cercectomized males took significantly longer to transfer a spermatophore than did control males. Cercectomy had no influence on the efficacy of post‐copulatory mate guarding by males, nor did females discriminate against cercectomized males by removing their spermatophores sooner than those of control males. We conclude that the primary function of the male's cerci in sexual interactions in G. sigillatus is to provide tactile stimuli to females that either elicit or guide their mounting response.

Infection dynamics of a sexually transmitted nematode (Mehdinema alii) in the decorated cricket (Gryllodes sigillatus)

We investigated the transmission dynamics of a sexually transmitted nematode, Mehdinema alii, in the decorated cricket, Gryllodes sigillatus. Adult crickets were experimentally inoculated with infective stages of the nematode, called dauerlarvae, to determine the role of the female cricket in nematode transmission. We found that female crickets serve only as a means of mechanical transmission, so nematodes do not require passage through a female cricket to be infective. Dauerlarvae were experimentally placed on the genitalia of both sexes. In the male, the dauerlarvae migrated into the gut and proceeded to develop into adult nematodes, whereas those that were inoculated into the female genitalia failed to migrate into the gut. When dauerlarvae were inoculated directly into the female rectum, the nematodes failed to develop. Therefore, the female gut is not a suitable environment for nematode development. Dauerlarvae persisted in the female cricket for up to 12 days post inoculation and remained infective to male crickets.

Investment in Mate Guarding May Compensate for Constraints on Ejaculate Production in the Cricket Gryllodes sigillatus

Although there is a corpus of evidence that females of many taxa are choosy about males, there is less information on how males may react to females of different ‘quality’ (i.e. potential fecundity). The cricket Gryllodes sigillatus shows distinct mate guarding behaviour. We examined how long males mate guard females of different sizes (reflecting egg load and potential fecundity). We also examined the sperm number in ampullae donated to females of different sizes to see if males make a concomitant difference in investment in ejaculate. We also examined mate‐guarding behaviour and ejaculate size of males mated to virgin and nonvirgin females of the same size to see if males equate size with increased age and increased likelihood of mating (increased sperm competition). The results showed that males mate guard larger females for longer but make no difference in ejaculate investment between sizes of female. Males make no significant difference in mate guarding investment or ejaculate investment between virgins and nonvirgins of the same size. There is evidence that other species of crickets do alter their ejaculate depending on the female size and mating history, but have less distinct guarding behaviour. We suggest that mate‐guarding investment in G. sigillatus may serve a similar function to that of ejaculate investment in other crickets.

Ejaculate expenditures of male crickets in response to varying risk and intensity of sperm competition: not all species play games

Costs incurred in the manufacture of ejaculates may constrain the number of sperm that males can produce, so males should show some economy in their allocation of sperm across multiple matings. In species in which females mate with multiple males and are capable of storing sperm for extended periods, sperm allocation of males should be tailored to the risk of sperm competition. Recent game theory predicts that males should transfer the least sperm when there are no other rivals, and the most sperm when only one other rival is likely to inseminate the female. However, as the numbers of competitors increases beyond two, the models predict a corresponding decrease in ejaculate expenditure. We tested these predictions in three cricket species, Gryllodes sigillatus, Gryllus veletis, and Gryllus texensis, assessing the sperm allocation of males held under three levels of apparent interrival competition: no rivals, one rival and six rivals. Sperm allocation of G. veletis varied according to theory: males increased their sperm allocation with an increased risk of sperm competition (no rivals vs. one), but decreased their allocation with an increased intensity of sperm competition (one rival vs. six). Sperm allocation of male G. texensis showed no significant response to the density of rivals, and sperm allocation in G. sigillatus was influenced by an unexpected interaction between treatment density and the order in which males experienced the three treatments. The observed interspecific variation in facultative sperm allocation may be due to interspecific differences in population density, rearing environment, or female mating behavior. Key words: crickets, Gryllodes, nuptial food gifts, sexual selection, spermatophore, sperm competition. [Behav Ecol 12:740–745 (2001)]

Venereal worms: sexually transmitted nematodes in the decorated cricket.

The nematode, Mehdinema alii, occurs in the alimentary canal of the decorated cricket Gryllodes sigillatus. Adult nematodes occur primarily in the hindgut of mature male crickets, whereas juvenile nematodes are found in the genital chambers of mature male and female crickets. Here, we present experimental evidence for the venereal transmission of M. alii in G. sigillatus. Infectivity experiments were conducted to test for transmission via oral-fecal contamination, same-sex contact, and copulation. The infective dauers of the nematode are transferred from male to female crickets during copulation. Adult female crickets harboring infective dauers subsequently transfer the nematode to their next mates. Thus, M. alii is transmitted sexually during copulation.

Fluctuating asymmetry and sperm transfer in male decorated field crickets ( Gryllodes sigillatus )

Fluctuating asymmetry (FA) in limb size in female decorated field crickets (Gryllodes sigillatus) was associated with a reduction in the size of the spermatophore and the amount of sperm transferred by males and an increase in the time taken to transfer a spermatophore following introduction of a female. There was a weaker negative relationship between limb asymmetry in males and sperm number but no significant relationship between asymmetry in either sex and spermatophylax size. In line with a previous study, female size did not appear to influence spermatophore production or mating decisions by males. The results imply that developmental instability affects both gamete production and mating decisions among males, although the relationships between spermatophore size, sperm number and asymmetry in females are unlikely to be the result of males perceiving differences in female FA.

Morphological, Molecular, and Biological Characterization of Mehdinema alii (Nematoda: Diplogasterida) from the Decorated Cricket (Gryllodes sigillatus)

The nematode Mehdinema alii was recovered from the decorated cricket Gryllodes sigillatus (Walker). Morphometric comparisons are presented from 3 populations. The nematode is characterized by dense arrays of spines on the cuticle of the anterior half of the body and a highly elongate, tubular stoma with a dorsal denticle in the glottoid region. Females have a protruding vulva. Young females are amphidelphic, but the anterior ovary disappears in older females bearing multiple developing juveniles. The male is monorchic with asymmetrically placed genital papillae, distally fused spicules, and a highly complex gubernaculum bearing 2 cuticularized thorns that protrude through a separate, postcloacal opening. Adult nematodes are located primarily in the hindgut, whereas juveniles or dauers occur mainly in the genital chamber of both male and female crickets. Male crickets are significantly more likely to be infected than females. This male-biased infection may be linked to the venereal transmission mechanism of the dauers. Although morphologically unusual in many respects, placement of M. alii in Diplogasterida is supported by both the morphology of the anterior digestive tract as well as analysis of its 18S rDNA sequence. These sequence data suggest that M. alii groups most closely with members of the Cylindrocorporidae.

Mate Guarding in the Cricket Gryllodes sigillatus: Influence of Multiple Potential Partners

There are several hypotheses as to the function of postcopulatory mate guarding. Control over the mate‐guarding period by either sex could potentially influence relative reproductive success. Mate‐guarding behaviour in Gryllodes sigillatus was studied under several conditions: 1. undisturbed pairs; 2. pairs with a single male intruder; 3. pairs exposed acoustically, visually and olfactorily to several other males; 4. pairs exposed freely to several other males; and 5. pairs exposed freely to several other females. The results provide support for the spermatophore retention and rival defence hypotheses. The efficacy of mate guarding was not compromised by the pair being in acoustic, visual and olfactory contact with several other males. Once pairs were exposed to free contact with several other males, the spermatophore retention time by the female declined significantly, indicating that the mate guarder’s efficiency declines under competition from several rivals. In pairs exposed to contact with several females after mating, the mate‐guarding period and spermatophore retention time declined as the mate guarder abandoned the mated female and pursued the other females. Termination of the effective mate‐guarding period by either sex seems to be influenced by the number of other potential partners present.

Hydration benefits to courtship feeding in crickets

The spermatophore transferred by male decorated crickets (Gryllodes sigillatus) at mating includes a large gelatinous spermatophylax that the female consumes after copulation. Although previous studies have shown that G. sigillatus females gain no nutritional benefits from consuming food gifts, there may be other benefits to their consumption. We examined potential hydration benefits to females by experimentally manipulating both the availability of water and the number of food gifts that females consumed, and by measuring their effect on female fitness. Analysis of the number of nymphs produced by females revealed a significant interaction between the number of spermatophylaxes consumed and water availability. When spermatophylaxes were not provided, females given water ad libitum produced significantly more nymphs than females subjected to water stress. Female longevity was significantly affected by water availability, with an increase in the availability of water corresponding to a significant increase in female longevity. These data suggest that female G. sigillatus accrue fitness benefits by consuming spermatophylaxes when alternative sources of water are unavailable. In addition, females appear to allocate water contained in spermatophylaxes towards reproduction as opposed to survival.

Maturation of Oocytes and Vitellogenesis during the Development of Ovary in the House Cricket Gryllodes sigillatus (Walker) (Orthoptera:Gryllidae)

In G. sigillatus , the ovaries are panoistic type in which the ovarioles exhibit a progressive increase in dimension as the development proceeds. The yolk deposition was found to commence on the 4th day of adult emergence in the basal oocyte which attained a fully mature stage on day 8. During this process, considerable changes were observed in both the oocytes and the follicular cells from the second day of female adult emergence. In the earlier adult instars, yolk spheres appeared in the cortical region of the ooplasm and large yolk spheres in the central region during vitellogenic phase. At the time of vitellogenesis, the follicular epithelial cells of the basal oocytes were tall and columnar with intercellular spaces. As the vitellogenesis progressed, the cells became cuboidal and the oocytes developed vitelline membrane and chorion. The present study also reveals that vitellogenesis in Gryllodes sigillatus occurs both in the penultimate and basal oocytes simultaneously.

Paternity of offspring in multiply–mated, female crickets: the effect of nuptial food gifts and the advantage of mating first

The spermatophore transferred by male decorated crickets ( Gryllodes sigillatus ) includes a large gelatinous mass, the spermatophylax, that is consumed by the female after mating. This nuptial gift preoccupies the female while sperm are discharged from the remaining portion of the spermatophore, the sperm ampulla, into her reproductive tract. There is considerable variation in the mass of the spermatophylax, and about half of all males produce spermatophylaxes that are too small to ensure complete sperm transfer. We tested two hypotheses concerning the maintenance of this variation: (i) males trade–off investment in spermatophylaxes against copulation frequency; and (ii) males synthesize the largest spermatophylaxes of which they are physiologically capable. Males synthesizing large and small food gifts were permitted multiple mating opportunities with the same females, and allozyme markers were used to establish the paternity of offspring. There was a significant advantage to those males that mated first irrespective of gift size. This advantage probably arose, in part, because the sperm of first males would have had exclusive access to female9s eggs during the first 24 hours of oviposition, and underscores the benefits of matings with virgin females. The paternity of ‘small–gift’ males increased with gift mass, but there was no such increase in ‘large–gift’ males. This difference probably stems from the relationship between gift mass and sperm transfer: most of the gifts of the large–gift males would have been above the threshold needed to achieve complete inseminations, whereas those of small–gift males would have been below the threshold. Within mating–order positions, there was no significant difference in the paternity of large–gift and small–gift males, a result seemingly consistent with the ‘trade–off’ hypothesis. However, there was no correlation between spermatophylax mass and male mating frequency, so that the mechanism by which small–gift males offset their fertilization disadvantage remains unknown.

Snail sperm production characteristics vary with sperm competition risk

Sperm competition is widespread and influences both male investment in spermatogenic tissue and ejaculate characteristics. Sperm competition models assume trade-offs between sperm size and number, although such trade-offs may be difficult to detect. This study examines the effects of sperm competition risk on the sperm production characteristics of the freshwater snail Viviparus ater. In this prosobranch, females mate frequently and store sperm, generating sperm competition. Males produce two sperm morphs, fertile eupyrene sperm and non-fertilizing oligopyrene sperm. Non-fertilizing sperm may play a role in sperm competition and therefore, like fertilizing sperm, the number produced could vary relative to sperm competition risk. In addition, trade-offs between sperm number and sperm size may be expected. We manipulated the sex ratio of sexually mature snails and found the presence of rivals affected the ratio of oligopyrene/eupyrene sperm males produced. In experimental and natural populations, the number of oligopyrene sperm produced, but not the number of eupyrene sperm, was significantly higher when the sex ratio was male biased. Testis mass did not vary between experimental treatments. We also found a negative relationship between the number and size of oligopyrene sperm produced, which is consistent with evolutionary models of sperm competition, and is, to our knowledge, the first intraspecific demonstration of a trade-off between these traits.

Effects of the Consumption of Male Spermatophylax on the Oviposition Schedule of Females in the Decorated Cricket, Gryllodes sigillatus

The effects of the consumption of the spermatophylax produced by males on female fitness were studied in the decorated cricket, Gryllodes sigillatus. An increase in the number of spermatophylaces presented to females did not increase the total number of eggs made by females, the number of eggs laid, or the hatchability of eggs laid by females, but increased the number of eggs laid in the early stage of adult life of females. The duration of the egg stage decreased with the number of spermatophylaces presented to females. The implication of the results on the sham hypothesis that the spermatophylax does not have nutritional value is discussed.

Circadian locomotor rhythms in the cricket, Gryllodes sigillatus. II. Interactions between bilaterally paired circadian pacemakers.

The optic lobe is essential for circadian locomotor rhythms in the cricket, Gryllodes sigillatus. We examined potential interactions between the bilaterally paired optic lobes in circadian rhythm generation. When one optic lobe was removed, the free-running period of the locomotor rhythm slightly but significantly lengthened. When exposed to light-dark cycles (LD) with 26 hr period, intact and sham operated animals were clearly entrained to the light cycle, but a large number of animals receiving unilateral optic nerve severance showed rhythm dissociation. In the dissociation, two rhythmic components appeared; one was readily entrained to the given LD and the other free-ran with a period shorter than 24 hr, and activity was expressed only when they were inphase. The period of the free-running component was significantly longer than that of the animals with a single blinded pacemaker kept in LD13:13, suggesting that the pacemaker on the intact side had some influence on the blinded pacemaker even in the dissociated state. The ratio of animals with rhythm dissociation was greater with the lower light intensity of the LD. The results suggest that the bilaterally distributed pacemakers are only weakly coupled to one another but strongly suppress the activity driven by the partner pacemaker during their subjective day. The strong suppression of activity would be advantageous to keep a stable nocturnality for this cricket living indoors.

FEMALE CONTROL OF SPERM TRANSFER AND INTRASPECIFIC VARIATION IN SPERM PRECEDENCE: ANTECEDENTS TO THE EVOLUTION OF A COURTSHIP FOOD GIFT.

Manipulation of ejaculates is believed to be an important avenue of female choice throughout the animal kingdom, but evidence of its importance to sexual selection remains scarce. In crickets, such manipulation is manifest in the premature removal of the externally attached spermatophore, which may afford females an important means of postcopulatory mate choice. We tested the hypothesis that premature spermatophore removal contributes significantly to intraspecific variation in sperm precedence by (1) experimentally manipulating spermatophore attachment durations of competing male Gryllodes sigillatus and (2) employing protein electrophoresis to determine the paternity of doubly mated females. The relative spermatophore attachment durations of competing males had a significant influence on male paternity, but the pattern of sperm precedence deviated significantly from the predictions of an ideal lottery. Instead, paternity data and morphological evidence accorded best with a model of partial sperm displacement derived here. Our model is similar to a displacement model of Parker et al. in that sperm of the second male mixes instantaneously with that of the first throughout the displacement process, but the novel feature of our model is that the number of sperm displaced is only a fraction of the number of sperm transferred by the second male. Regardless of the underlying mechanism, female G. sigillatus can clearly alter the paternity of their offspring through their spermatophore-removal behavior, and employ such cryptic choice in favoring larger males and those providing larger courtship food gifts. We discuss how female control of sperm transfer and intraspecific variation in sperm precedence may be important precursors to the evolution of gift giving in insects.

Courtship feeding in decorated crickets: is the spermatophylax a sham?

Abstract. The spermatophore transferred by a male decorated cricket, Gryllodes sigillatus , at mating includes a large gelatinous spermatophylax, devoid of sperm, which the female removes and feeds on after copulation. Previous studies have shown that consumption of the spermatophylax keeps the female preoccupied while sperm from the remaining portion of the spermatophore is expelled into her reproductive tract; hence, the spermatophylax functions to ensure complete sperm transfer. To determine whether consumption of the spermatophylax also provides significant nutritional benefits to females and whether such benefits are contingent on food availability, female G. sigillatus were allowed to consume zero, one, or three spermatophylaxes per day and were either deprived of food, fed 30% of ad libitum food demands, or allowed unrestricted access to food (Purina® cricket chow). The number of spermatophylaxes consumed by females had no significant effect on the mass of eggs or number of nymphs produced, and there was no significant interaction between the number of spermatophylaxes consumed and food availability. Although there may be other benefits to its consumption, the present study suggests that as a food 'gift', the spermatophylax is a sham.