Males,but not females,contribute to sexual isolation between two sympatric species of <Emphasis Type="Italic">Gambusia</Emphasis>

Reproductive isolation restricts genetic exchange between species. Various pre- and post-mating barriers, such as behavior, physiology and gametic incompatibility, have been shown to evolve in sympatry. In certain scenarios, isolation can be asymmetrical, where species differentially prefer conspecifics. We examined sexual isolation via conspecific mate preference between Gambusia affinis and G. geiseri in both sexes. To investigate male contribution to sexual isolation, we compared the number of mating attempts (gonopodial thrusts) directed at either a conspecific or a heterospecific female, in both species. We also examined sperm priming and expenditure in males in the presence of conspecific or heterospecific females. We then measured female preference for either a conspecific or heterospecific male, in both species. We found that males of both species preferred to mate with conspecific females, but showed no difference in sperm production or expenditure between conspecific and heterospecific females. Females of both species did not prefer conspecific over heterospecific males. Our results suggest that sexual isolation might be mediated by male mate choice in this system and not female choice, suggesting that there is asymmetrical reproductive isolation between the sexes in G. affinis and G. geiseri, but symmetrical species isolation.     

Plasticity of the mate choice mind: courtship evokes choice‐like brain responses in females from a coercive mating system

Female mate choice is fundamental to sexual selection, and determining molecular underpinnings of female preference variation is important for understanding mating character evolution. Previously it was shown that whole‐brain expression of a synaptic plasticity marker, neuroserpin, positively correlates with mating bias in the female choice poeciliid, Xiphophorus nigrensis, when exposed to conspecific courting males, whereas this relationship is reversed in Gambusia affinis, a mate coercive poeciliid with no courting males. Here we explore whether species‐level differences in female behavioral and brain molecular responses represent ‘canalized’ or ‘plastic’ traits. We expose female G. affinis to conspecific males and females, as well as coercive and courting male Poecilia latipinna, for preference assays followed by whole‐brain gene expression analyses of neuroserpin, egr‐1 and early B. We find positive correlations between gene expression and female preference strength during exposure to courting heterospecific males, but a reversed pattern following exposure to coercive heterospecific males. This suggests that the neuromolecular processes associated with female preference behavior are plastic and responsive to different male phenotypes (courting or coercive) rather than a canalized response linked to mating system. Further, we propose that female behavioral plasticity may involve learning because female association patterns shifted with experience. Compared to younger females, we found larger, more experienced females spend less time near coercive males but associate more with males in the presence of courters. We thus suggest a conserved learning‐based neuromolecular process underlying the diversity of female mate preference across the mate choice and coercion‐driven mating systems.     

Habitat and life history differences between two species of Gambusia

Life history strategies reflect trade-offs that tend to maximize fitness, such as investment in a few large or many small offspring. We compared life histories of two temperate livebearing fishes Gambusia affinis and G. nobilis, an endangered species which is virtually unstudied. The two species persist in environments that differ widely in abiotic and biotic factors in the same local area. Gambusia affinis were typically found in habitats with high productivity and wide fluctuations in temperature, salinity and dissolved oxygen, whereas G. nobilis occurred in more stable spring-fed habitats. We collected data on life-history traits: embryo mass, brood size (number of embryos), total maternal reproductive effort, population sex ratios, and size (mass and length) distributions of adults and juveniles. There was no difference between species in reproductive effort per brood, but they differed in investment strategy. Gambusia affinis females produced large broods with small embryos, whereas G. nobilis females produced broods of fewer, larger embryos. These differences in life history strategies reflect a tradeoff between individual productivity and differential mortality rates in different environments. At our field site G. affinis persists as an annual species with relatively high growth rates and corresponding reproductive patterns, whereas G. nobilis females have a slower reproductive tempo and may live multiple years.     

Alternative reproductive tactics in male freshwater fish influence the accuracy of species recognition

Sexual conflict can result in coercive mating. Because males bear low costs of heterospecific mating, coercive males may engage in misdirected mating attempts toward heterospecific females. In contrast, sexual selection through consensual mate choice can cause mate recognition cues among species to diverge, leading to more accurate species recognition. Some species show both coercive mating and mate choice‐associated courtship behaviors as male alternative reproductive tactics. We hypothesized that if the selection pressures on each tactic differ, then the accuracy of species recognition would also change depending on the mating tactic adopted. We tested this hypothesis in the guppy (Poecilia reticulata) and mosquitofish (Gambusia affinis) by a series of choice experiments. Poecilia reticulata and G. affinis males both showed imperfect species recognition and directed all components of mating behavior toward heterospecific females. They tended to direct courtship displays more frequently toward conspecific than heterospecific females. With male P. reticulata, however, accurate species recognition disappeared when they attempted coercive copulation: they directed coercions more frequently toward heterospecific females. We also found that heterospecific sexual interaction had little effect on the fecundity of gravid females, which suggests that prepregnancy interactions likely underpin the exclusion of G. affinis by P. reticulata in our region.     

Interspecific interactions and learning variability jointly drive geographic differences in mate preferences

Co‐occurrence of closely related species can cause behavioral interference in mating and increase hybridization risk. Theoretically, this could lead to the evolution of more species‐specific mate preferences and sexual signaling traits. Alternatively, females can learn to reject heterospecific males, to avoid male sexual interference from closely related species. Such learned mate discrimination could also affect conspecific mate preferences if females generalize from between species differences to prefer more species‐specific mating signals. Female damselflies of the banded demoiselle (Calopteryx splendens) learn to reject heterospecific males of the beautiful demoiselle (C. virgo) through direct premating interactions. These two species co‐occur in a geographic mosaic of sympatric and microallopatric populations. Whereas C. virgo males have fully melanized wings, male C. splendens wings are partly melanized. We show that C. splendens females in sympatry with C. virgo prefer smaller male wing patches in conspecific males after learning to reject heterospecific males. In contrast, allopatric C. splendens females with experimentally induced experience with C. virgo males did not discriminate against larger male wing patches. Wing patch size might indicate conspecific male quality in allopatry. Co‐occurrence with C. virgo therefore causes females to prefer conspecific male traits that are more species specific, contributing to population divergence and geographic variation in female mate preferences.     

ACOUSTIC COMMUNICATION AND MATING BEHAVIOUR IN WATER BUGS OF THE GENUS MICRONECTA

ABSTRACT

The role of male sounds in attracting females and in mating was investigated in the three most common species of Micronecta found in ponds 60 km NE of Melbourne, Australia: M. concordia, M. tasmanica and M. robusta. In playback experiments using recorded male signals, females were attracted to signals of conspecific males, in preference to signals of heterospecific sympatric males. Studies of mating behaviour, using video recording, showed that signals were obligatory for mating. These findings strongly suggest that acoustic signalling is important in reproductive isolation in Micronecta. Comparisons between matings and rejected matings showed that, within each species, copulation only occurred following a certain minimum number of acoustic signals with more pulse-trains than the mean for the species. Low amplitude sounds (after signals) were also important in courtship, immediately preceding mating. No sounds occurred during copulation.     

Field bioassays show heterospecific mating preference asymmetry between hybridizing North American Papilio butterfly species (Lepidoptera: Papilionidae)

Few studies of interspecific mating preferences of naturally hybridizing species have been done in the field. Yet this is the only way potentially critical habitat-specific factors can be included in mating behavior evaluations. We conducted mate selection (male preference) studies using tethered pairs of heterospecific, size-matched virgin yellow females of Papilio glaucus and P. canadensis with free-flying male P. glaucus populations in south-central Florida, and with free-flying male P. canadensis populations in northern Michigan. Florida males clearly preferred the conspecific P. glaucus females of the pairs, as indicated by the touches/attempted matings and by the actual copulations observed. In 1997, 66.7% of the total attempts (n=168) and 94.2% of all copulations (n=69) were with the conspecific. In 1998 the same pattern of conspecific male preference was observed, with 67.3% of the touches/attempts (n=49) and 100% of the copulations (n=30) being with the P. glaucus females. In contrast, the natural populations of P. canadensis males did not show the expected conspecific preference. In fact, with wild P. canadensis males in 1997 a very strong preference was observed for the heterospecific P. glaucus females of the pairs, accounting for 75.8% of all touches/attempted matings (n=483) and 81.7% of all copulations (n=476). The strong asymmetry in interspecific mating preferences among these hybridizing Papilio species may reflect an ancestral trait in P. glaucus females that elicits strong preference in P. canadensis males, due perhaps to previously possessed male sensory bias or "good genes" indicators in the females. However, reinforcement of reproductive isolation in or near the hybrid zone of species overlap (sympatry) cannot be ruled out. Electronic Publication     

Mate Choice by Both Sexes Maintains Reproductive Isolation in a Species Flock of Pupfish (Cyprinodon spp) in the Bahamas

Female and male mate choices can reinforce reproductive isolation after sympatric speciation. Using a binary choice design, we examine the importance of visual cues in female mate choice in all three sympatric species of pupfish on San Salvador Island. We also examine the importance of olfactory cues in female choice of the hard‐shelled invertebrate specialist (Cyprinodon brontotheroides). We examine male mate choice in two of the three species, the scale eater (C. desquamator) and the detritivore (C. variegatus). Females of all three species use visual cues and prefer conspecific males. C. brontotheroides females do not use olfactory cues to discriminate between conspecific and heterospecific males. Males of C. desquamator and C. variegatus also preferentially court conspecific females. Thus, mutual mate choice, where both females and males exhibit mate choice, acts as a strong behavioral pre‐mating isolation mechanism in these sympatrically speciated pupfish.     

Mate discrimination in Littorina littorea (L.) and L. saxatilis (Olivi) (Mollusca:Prosobranchia)

The ability of males of Littorina littorea and L. saxatilis to discriminate between mates of different sex, species and size was examined. In partner choice experiments males of L. littorea had the possibility to initiate a copulation with either a female or a male. The males did not show a preference for either sex. There was therefore no evidence that they could determine the sex of a conspecific prior to copulation. The duration of intrasexual copulation was considerably shorter than for intersexual copulation, both in the field and in laboratory experiments. For the two species, intersexual copulations were far more frequent than intrasexual ones. This can partly be explained by the difference in copulation time.Few interspecific copulating pairs were found on the shore. This may reflect a low interspecific encounter rate rather than a mechanism of species recognition. On all of these occasions, however, the active male was of L. saxatilis. It is argued that selection against precopulatory species and sex recognition is a more likely explanation than an hypothesis that states that the required mutations for precopulatory mate identification has not yet occurred. L. littorea males copulated longer with large than with small females. Copulation time was short with parasitized females, which are sterile or of low fecundity. The allocation of mating effort by males is discussed.     

Multiple post‐mating barriers to hybridization in field crickets

Mechanisms that prevent different species from interbreeding are fundamental to the maintenance of biodiversity. Barriers to interspecific matings, such as failure to recognize a potential mate, are often relatively easy to identify. Those occurring after mating, such as differences in the how successful sperm are in competition for fertilisations, are cryptic and have the potential to create selection on females to mate multiply as a defence against maladaptive hybridization. Cryptic advantages to conspecific sperm may be very widespread and have been identified based on the observations of higher paternity of conspecifics in several species. However, a relationship between the fate of sperm from two species within the female and paternity has never been demonstrated. We use competitive microsatellite PCR to show that in two hybridising cricket species, Gryllus bimaculatus and G. campestris, sequential cryptic reproductive barriers are present. In competition with heterospecifics, more sperm from conspecific males is stored by females. Additionally, sperm from conspecific males has a higher fertilisation probability. This reveals that conspecific sperm precedence can occur through processes fundamentally under the control of females, providing avenues for females to evolve multiple mating as a defence against hybridization, with the counterintuitive outcome that promiscuity reinforces isolation and may promote speciation.     

Do Female Western Mosquitofish,Gambusia affinis,Prefer Ornaments That Males Lack?

Some species in the family Poeciliidae are known for extravagant male ornaments and courtship behavior (e.g., guppies), but the majority of poeciliids are characterized by coercive male copulation attempts that seem to circumvent female choice. In some lineages with male ornaments, female sensory bias may have preceded the evolution of corresponding male signals. We examined female preferences for colorful ornaments in Western mosquitofish, Gambusia affinis, in which males lack ornamentation and reproduce primarily through coercive mating attempts. We found that females exhibited a positional affinity for males that were artificially ornamented with blue coloration over males that had been treated with a transparent ornament. Females exhibited the opposite effect for males treated with red ornaments. In contrast, focal females did not exhibit behavioral discrimination between two live stimulus females or two models (silver fishing lures) with blue vs. transparent ornaments. This suggests a sexual context for female discrimination between males based on ornament color and whether an ornament was present. Because tribe Gambusiini is the basal branch of family Poeciliidae, the results of this study are consistent with the hypothesis that female responsiveness to male coloration is the ancestral poeciliid character state.     

Operational Sex Ratio and Individual Mating Frequencies in Two Bushcricket Species (Orthoptera,Tettigonioidea, Poecilimon)

During mating the male bushericket transfers a large spermatophore which is consumed by the female after copulation. Such a ‘nuptial gift' is likely to have strong effects on the mating frequency of both males and females and thus on the operational sex ratio (OSR): For the male a longer time period will be required in which he is prevented from further matings. For the female consumption may be advantageous increasing interest in matings. To quantify these effects field studies were carried out on two species of the genus Poecilimon. Both species differed widely in the mean number of matings observed per day (19% of all females in P. veluchianus compaied to 42% in P. affinis; Fig. 2) and thus in the distribution of female mating intervals (P. veluchianus 2 days, P. affinis 1 day (modes); Fig. 3). Separate experiments on the minimum male mating interval and the spermatophore weight also revealed distinct differences: the mean spermatophore weight was 26% of the male body weight in P. veluchianus and 15% in P. affinis (Fig. 6). Accordingly the refractory period was longer in P. veluchianus; 3 days after a previous mating nearh all males were ready to remate, compared to 2 days in P. affinis (Fig. 5). These differences, however, compensated one another in that the amount of spermatophylax material transferred per day was very similar in both species in relation to female body weight. Using data on female mating frequency per day and male remating ability, an upper estimate for the OSR was calculated. A lower limit for the OSR was given taking into account the most frequent female mating interval (big. 7). In both species the OSR remained relatively constant for large parts of the season. The medians (upper/lower estimate for the ratio males to females ready to mate: 2.95/1.19 in P. veluchianus; 2.22 0.88 in P. affinis) were very low, comparable to that in explosive breeding frog species. Thus in the bushcricket mating system, females may be able to demand large male mating efforts so that the operational sex ratio can come close to unity.     

Male age and sexual experience affect male mating behavior in the hawthorn spider mite,Amphitetranychus viennensis

As is reported, in species with first-male sperm precedence, male age and previous sexual experience play crucial roles in male mating behavior. In the hawthorn spider mite, Amphitetranychus viennensis Zacher, previous studies showed that only females that copulated for the first time could achieve fertilization. Based on this, the effects of male age and mating history on male mate choice and male mate competition were investigated. It was confirmed that males could distinguish virgins from fertilized females but they were unable to discriminate between virgins and unfertilized females. Interestingly, the copulation duration of males mated with fertilized females was much shorter than that of males mated with virgins or unfertilized females. Additionally, for male mating choice, males of all ages and more experienced males preferred 5-day-old virgin females, whereas only less experienced males preferred 1-day-old virgin females. In male-male competition, 3-day-old males were more competitive and obtained more copulations compared with others. Copula duration was closely related to male age. Though no significant differences were observed in mating competition between virgin and mated males, copula duration of males in first copulation was the longest and gradually shortened in subsequent copulations. In all, this investigation demonstrated that male age and sexual experience affected male mate choice and male-male competition, leading to further insight into the influences of male age and sexual experience on the reproductive fitness of both sexes.

     

Size‐Dependent Male Mate Preference and its Association with Size‐Assortative Mating in a Mangrove Snail,Littoraria ardouiniana

In many animals, body size plays a crucial role in mating success in the context of competition and preference for mates. Increasing evidence has shown that male mate preference can be size‐dependent and, therefore, an important driver of size‐assortative mating. To test this theory, mate choice experiments were performed during the three consecutive stages of mating behaviour, namely trail following, shell mounting and copulation, in the dioecious mangrove snail, Littoraria ardouiniana. These experiments identified two possible forms of size‐dependent male mate preference which could contribute to the formation of size‐assortative mating in these snails. Firstly, whereas small males were unselective, large males were selective and preferred to follow mucus trails laid by large females. Alternatively, the results can also be interpreted as all males were selective and adopted a mating strategy of selecting females similar to, or larger than, their own sizes. Both small and large males also copulated for longer with large than with small females, and this was more pronounced in large males. When two males encountered a female, they engaged in physical aggression, with the larger male excluding the smaller male from copulating with the female. This study, therefore, demonstrated that size‐dependent male mate preference may, along with male–male competition, play an important role in driving size‐assortative mating in these mangrove snails, and this may also be the case in other species that exhibit male mate choice.     

Sexual conflict and ecology: Species composition and male density interact to reduce male mating harassment and increase female survival

Sexual conflict is a pervasive evolutionary force that can reduce female fitness. Experimental evolution studies in the laboratory might overestimate the importance of sexual conflict because the ecological conditions in such settings typically include only a single species. Here, we experimentally manipulated conspecific male density (high or low) and species composition (sympatric or allopatric) to investigate how ecological conditions affect female survival in a sexually dimorphic insect, the banded demoiselle (Calopteryx splendens). Female survival was strongly influenced by an interaction between male density and species composition. Specifically, at low conspecific male density, female survival increased in the presence of heterospecific males (C. virgo). Behavioral mating experiments showed that interspecific interference competition reduced conspecific male mating success with large females. These findings suggest that reproductive interference competition between con‐ and heterospecific males might indirectly facilitate female survival by reducing mating harassment from conspecific males. Hence, interspecific competitors can show contrasting effects on the two sexes thereby influencing sexual conflict dynamics. Our results call for incorporation of more ecological realism in sexual conflict research, particularly how local community context and reproductive interference competition between heterospecific males can affect female fitness.     

Male mate choice in the gynogenetic–sexual complex of crucian carp,<Emphasis Type="Italic"> Carassius auratus</Emphasis>

The asexual all-female Japanese crucian carp, Carassius auratus langsdorfii (Teleostei: Cypriniformes), reproduces gynogenetically, relying on the sperm of males of the sexual "host," C. auratus subspp. Theoretically, frequency-dependent mating preference of males to conspecific females can lead to the coexistence of asexual and sexual fish, if all else is equal. Our specific questions are whether males prefer conspecific females over asexual females and whether individuals show dominance hierarchies that potentially cause frequency-dependent mating preference. In an individual choice experiment, a tank was partitioned into three compartments with the middle one for a single male and the two outer ones for a sexual and an asexual female. The males of C. auratus bürgeri demonstrated a significant preference for ovulated conspecific females over ovulated asexual females. In contrast, in a group mating experiment, a single experimental tank included two males, a sexual female, and an asexual female together, and males chased and mated with both asexual and sexual females equally. Male mate preference was weak in group mating, which is typical in natural populations. Males and females of crucian carp showed no apparent agonistic behavior to each other in the group mating experiment. This is different from other gynogenetic complexes with the dominance hierarchy of males showing strong frequency-dependent mating preference (e.g., Poeciliopsis). We conclude that male mate preference is unlikely to be a strong frequency-dependent force maintaining the coexistence of asexual–sexual complexes of Japanese crucian carp. Received in revised form: 5 February 2001 Electronic Publication     

The role of functional constraints in nonrandom mating patterns for a dance fly with female ornaments

Most hypotheses to explain nonrandom mating patterns invoke mate choice, particularly in species that display elaborate ornaments. However, conflicting selection pressures on traits can result in functional constraints that can also cause nonrandom mating patterns. We tested for functional load‐lifting constraints during aerial copulation in Rhamphomyia longicauda, a species of dance fly that displays multiple extravagant female‐specific ornaments that are unusual among sexual traits because they are under stabilizing selection. R. longicauda males provide females with a nuptial gift before engaging in aerial mating, and the male bears the entire weight of the female and nuptial gift for the duration of copulation. In theory, a male's ability to carry females and nuptial gifts could constrain pairing opportunities for the heaviest females, as reported for nonornamented dance flies. In concert with directional preferences for large females with mature eggs, such a load‐lifting constraint could produce the stabilizing selection on female size previously observed in this species. We therefore tested whether wild‐caught male R. longicauda collected during copulation were experiencing load‐lift limitations by comparing the mass carried by males during copulation with the male's wing loading traits. We also performed permutation tests to determine whether the loads carried by males during copulation were lighter than expected. We found that heavier males are more often found mating with heavier females suggesting that whereas R. longicauda males do not experience a load‐lift constraint, there is a strong relationship of assortative mating by mass. We suggest that active male mate choice for intermediately adorned females is more likely to be causing the nonrandom mating patterns observed in R. longicauda.     

Incomplete behavioural isolation and asymmetric female preference in darter sister species (Percidae: Etheostoma)

In this study, female preferences and behavioural isolation were estimated in a pair of allopatric sister species, Etheostoma duryi and Etheostoma flavum. Dichotomous mate preference trials were conducted to determine whether females prefer to associate with conspecific over heterospecific males and free‐spawning assays were conducted to determine whether those preferences translated into behavioural isolation. Dichotomous mate choice trials revealed asymmetric female preference, as female E. flavum preferred conspecific males, whereas female E. duryi showed no preference. Free‐spawning assays indicated that behavioural isolation remains incomplete between E. duryi and E. flavum (I_B = 0·19). In addition to female mating preferences, male behaviour also appeared to influence mating outcomes as male E. flavum consistently courted conspecific females more often in free‐spawning assays whereas male E. duryi did not. The data therefore suggest that despite marked divergence in male nuptial colour, divergence in female preferences between these species may not be sufficient to maintain species boundaries upon secondary contact. These results contrast with similar work in a sympatric darter species pair and may be explained by considering the contributions of reinforcement and differences in colour pattern as well as colour value.     

Predation Risk Reduces a Female Preference for Heterospecific Males in the Green Swordtail

The presence of a predator can result in the alteration, loss or reversal of a mating preference. Under predation risk, females often change their initial preference for conspicuous males, favouring less flashy males to reduce the risk of being detected by predators. Previous studies on predator‐induced plasticity in mate preferences have given females a choice between more and less conspicuous conspecific males. However, in species that naturally hybridize, it is also possible that females might choose an inconspicuous heterospecific male over a conspicuous conspecific male under predation risk. Our study addresses this question using the green swordtail (Xiphophorus helleri) and the southern platyfish (Xiphophorus maculatus), which are sympatric in the wild. We hypothesized that X. helleri females would prefer the sworded conspecific males in the absence of a predator but favour the less conspicuous, swordless, heterospecific males in the presence of a predator. Contrary to our expectation, females associated more with the heterospecific male than the conspecific male in the control (no predator) treatment, and they were non‐choosy in the predator treatment. This might reflect that females were attracted to the novel male phenotype when there was no risk of predation but became more neophobic after predator exposure. Regardless of the underlying mechanism, our results suggest that predation pressure may affect female preferences for conspecific versus heterospecific males. We also found striking within‐population, between‐individual variation in behavioural plasticity: females differed in the strength and direction of their preferences, as well as in the extent to which they altered their preferences in response to changes in perceived predation risk. Such variation in female preferences for heterospecific males could potentially lead to temporal and spatial variation in hybridization rates in the wild.     

Males and females contribute differently to the evolution of habitat segregation driven by hybridization

Costly heterospecific mating interactions, such as hybridization, select for prezygotic reproductive isolation. One of the potential traits responding to the selection arising from maladaptive hybridization is habitat preference, whose divergence results in interspecific habitat segregation. Theoretical studies have so far assumed that habitat preference is a sexually shared trait. However, male and female habitat preferences can experience different selection pressures. Here, by combining analytical and simulation approaches, we theoretically examine the evolution of sex-specific habitat preferences. Habitat segregation can have demographic consequences, potentially generating eco-evolutionary dynamics. We thus explicitly consider demography in the simulation model. We also vary the degrees of species discrimination to examine how mate choice influences the evolution of habitat preferences. Results show that both sexes can reduce hybridisation risk by settling in the habitats where abundant conspecific mates reside. However, when females can discriminate species, excess conspecific male aggregation intensifies male–male competition for mating opportunities, posing an obstacle to conspecific aggregation. Meanwhile, conspecific female aggregation attracts conspecific males, by offering the mating opportunity. Therefore, under effective species discrimination, females play a leading role in initiating habitat use divergence. Simulations typically result in either the coexistence with established habitat segregation or the extinction of one of the species. The former result is especially likely when the species differ to some extent in habitat preferences upon secondary contact. Our results disentangle the selection pressures acting on male and female habitat preferences, deepening our understanding of the evolutionary process of habitat segregation due to hybridization.