Asymmetric dominance and asymmetric mate choice oppose premating isolation after allopatric divergence

Assortative mating promotes reproductive isolation and allows allopatric speciation processes to continue in secondary contact. As mating patterns are determined by mate preferences and intrasexual competition, we investigated male–male competition and behavioral isolation in simulated secondary contact among allopatric populations. Three allopatric color morphs of the cichlid fish Tropheus were tested against each other. Dyadic male–male contests revealed dominance of red males over bluish and yellow‐blotch males. Reproductive isolation in the presence of male–male competition was assessed from genetic parentage in experimental ponds and was highly asymmetric among pairs of color morphs. Red females mated only with red males, whereas the other females performed variable degrees of heteromorphic mating. Discrepancies between mating patterns in ponds and female preferences in a competition‐free, two‐way choice paradigm suggested that the dominance of red males interfered with positive assortative mating of females of the subordinate morphs and provoked asymmetric hybridization. Between the nonred morphs, a significant excess of negative assortative mating by yellow‐blotch females with bluish males did not coincide with asymmetric dominance among males. Hence, both negative assortative mating preferences and interference of male–male competition with positive assortative preferences forestall premating isolation, the latter especially in environments unsupportive of competition‐driven spatial segregation.     

Altered mating behaviour in a Cry1Ac-resistant strain of Helicoverpa armigera (Lepidoptera: Noctuidae)

Randomness of mating between susceptible and resistant individuals is a major factor that closely relates to the refuge strategy of resistance management for Helicoverpa armigera (Hübner) to Bacillus thuringiensis cotton. The mating behaviour of Cry1Ac‐susceptible and Cry1Ac‐resistant strains of H. armigera was compared to investigate the randomness of their mating. The percentage of mating was lower for Cry1Ac‐resistant H. armigera compared with that of the susceptible strain under both no‐choice and multiple‐choice conditions. The low percentage of mating in the resistant strain indicates a reduced incidence of successful mating. The percentage of spermatophore‐containing mated female H. armigera in the crossing of susceptible females × resistant males was significantly lower than in the crossing of resistant females × susceptible males, but the observed mating frequencies of these two types of cross were similar to each other. This indicates that resistant males reduce the incidence of mating paternity more than they do their mating frequency. The percentages of heterogametic matings (susceptible females × resistant males, resistant females × susceptible males) in the multiple‐choice experiment were lower than those of homogametic matings (susceptible × susceptible, resistant × resistant) on peak mating nights. However, the difference between heterogametic and homogametic mating was not significant, indicating that there was a random mating between susceptible and resistant strains. The results presented here do not reflect reality in mating associated with Cry1Ac resistance but can provide insight into variable expression.     

The effects of host race, gender, and host plant distribution on alighting behavior, mating, and oviposition in Eurosta solidaginis

Eurosta solidaginis Fitch (Diptera: Tephritidae) has formed host races on Solidago altissima L. and Solidago gigantea Ait. (Asteraceae), and reproductive isolation between these host races is brought about in part by host‐associated assortative mating. Any non‐assortative mating creates the potential for gene flow between the populations, and we investigated the conditions that favored non‐assortative mating. We hypothesized that the frequency of non‐assortative mating would be influenced by differences in the behaviors of the host races and sexes and by the presence and pattern of distribution of the two host species. To test these hypotheses, we caged flies on four combinations of 32 potted host plants: all S. altissima, all S. gigantea, and cages with both host species arranged in either two pure species blocks or randomly dispersed. We recorded the number of flies of each host race that alighted on each host species and the frequency of mating within and between the host races. Males of both host races were observed on plants more frequently than females. Flies of the host race from S. gigantea (gig flies) were observed on plants in greater absolute numbers, and they mated more frequently than flies of the host race from S. altissima (alt flies). In all treatments, gig flies of both sexes were found on non‐natal host plants significantly more frequently than alt flies, and gig females showed a weaker preference for their host species than did gig males or alt flies of either gender for their respective natal hosts. Assortative mating predominated in all treatments, and flies from each host race mated more frequently in cages containing their own host plant. The frequency of non‐assortative mating varied among treatments, with the matings between alt ♀ × gig ♂ being more common in the pure S. altissima treatment and the gig ♀ × alt ♂ being more frequent in the pure S. gigantea and random treatments. Matings between gig ♂ × alt ♀ were more common overall than the reciprocal mating, because gig males were more active in pursuing matings and in alighting on the non‐natal host plant than alt flies. Non‐assortative matings were more frequent in the random than in the block treatments, but this difference was not significant. Because of strong selection against oviposition into the alternate host, we hypothesized that host plant distribution would not affect oviposition preference. We tested this hypothesis by examining the oviposition behavior of naïve, mated females in two treatments in which both host species were present: either arranged in blocks or randomly dispersed. Females oviposited only into their natal host, regardless of host plant distribution.     

Sampling scale can cause bias in positive assortative mating estimates: evidence from two intertidal snails

Assortative mating in the wild is commonly estimated by correlating between traits in mating pairs (e.g. the size of males and females). Unfortunately, such an approach may suffer from considerable sampling bias when the distribution of different expressions of a trait in the wild is nonrandom (e.g. when segregation of different size classes of individuals occurs in different microhabitats or areas). Consequently, any observed trait correlation in the wild can be an artefact of pooling heterogeneous samples of mating pairs from different microhabitats or areas rather than true nonrandom matings. This bias in estimating trait correlations as a result of sampling scale is termed the scale‐of‐choice effect (SCE). In the present study, we use two intertidal littorinid species from Hong Kong to show how the SCE can bias size‐assortative mating estimates from mating pairs captured in the wild, empirically demonstrating the influence of this effect on measures of positive assortative mating. This finding cautions that studies overlooking the SCE may have misinterpreted the magnitude and the cause of assortative mating, and we provide a new analytical approach for protecting against this potential bias in future studies.     

Remating, sperm transfer, and sperm displacement in the cactophilic species Drosophila buzzatii Patterson & Wheeler (Diptera: Drosophilidae)

The mating system of Drosophila buzzatii is characterized by short copulation duration, frequent remating in both males and females, and male ejaculate partitioning. Additional features of the system are strong sperm displacement and a high frequency of sterile matings. Remating frequencies and the effects of remating on various mating parameters were studied. In order to characterize variation, five isofemale lines from geographically distant localities in Australia (three localities), Brazil and the Canary Islands were used. Mating parameters studied were: premating time, copulation duration, interval between successive matings, and progeny number as a measure of sperm transfer. Variation for sperm displacement was studied in crosses between laboratory stocks and a number of isofemale lines from Australia. There were significant between‐line differences in female remating frequencies, premating time, copulation duration, interval between successive matings, and progeny numbers, indicating genetic variation for these traits. Females from the five lines mated on average 1.6 to 3.1 times in 4 h, with a maximum of eight matings for one female. The males were given a maximum of ten virgin females in sequence and more than one‐third of the males mated all ten females in the 2 h observation period. Copulation duration decreased and interval between matings increased with copulation number in multiply mated males. Mean copulation duration was c. 2 min. Sperm transfer, measured as the average number of progeny from a single mating, was low (c. 25) and multiply mated females gave more progeny than single mated females, although with much lower progeny numbers than observed in wild‐caught non‐virgin females. A surprisingly high proportion of observed matings gave no progeny, i.e. they were sterile matings. Sperm displacement was strong in most crosses and remained strong in multiply mated females. The results are discussed in relation to the evolution of mating patterns in Drosophila.     

Assortative Mating in Two Pheromone Strains of the Cabbage Looper Moth, Trichoplusia ni

The evolution of animal communication systems is an integral part of speciation. In moths, species specificity of the communication channel is largely a result of unique sex pheromone blends produced by females and corresponding specificity of male behavioral response. Insights into the process of speciation may result from studies of pheromone strains within a species in which reproductive isolation is not complete. Toward this end we investigated assortative mating based on female pheromone phenotypes and male response specificity between mutant and normal colonies of the cabbage looper moth, Trichoplusia ni. There was no evidence of assortative mating in small cages in which the density of moths was high. In larger cages with lower densities of moths, assortative mating was evident. In these larger cages, matings between normal males and normal females and mutant males and mutant females were more frequent than interstrain matings. Wind tunnel tests indicated that normal males responded preferentially to pheromone released by normal females, whereas mutant males did not discriminate between normal and mutant pheromone blends. In large field cages, pheromone traps baited with normal females caught equal numbers of mutant and normal males, while pheromone traps baited with mutant females caught primarily mutant males. The overall pattern of assortative mating could be explained primarily based on the normal males' preference for the pheromone blend released by normal females.     

MALE MATING PREFERENCES AND ASSORTATIVE MATING IN THE SOLDIER BEETLE

Mating preferences for a color characteristic were examined in three northern Georgia populations of the soldier beetle, Chauliognathus pennsylvanicus De Geer, by comparing observed and expected frequencies of matings to the same phenotype. The three populations are apparently in a zone of secondary contact between disparate color morphs. In the most northern population sampled, preferences were strong and were associated with positive assortative mating with respect to the color characteristic. In the southern population, neither assortative mating nor mating preference was strong, while in the middle population, preferences were expressed in the absence of assortative mating. Mating preferences cannot be attributed to host-plant choice, microhabitat choice, or simple conditioning on the phenotype of the last mate. However, they may represent part of a specific mate-recognition system which has been maintained in part of the zone of overlap but which has eroded in other areas.     

The effects of heterospecific mating frequency on the strength of cryptic reproductive barriers

Heterospecific mating frequency is critical to hybrid zone dynamics and can directly impact the strength of reproductive barriers and patterns of introgression. The effectiveness of post‐mating prezygotic (PMPZ) reproductive barriers, which include reduced fecundity via heterospecific matings and conspecific sperm precedence, may depend on the number, identity and order of mates. Studies of PMPZ barriers suggest that they may be important in many systems, but whether these barriers are effective at realistic heterospecific mating frequencies has not been tested. Here, we evaluate the strength of cryptic reproductive isolation in two leaf beetles (Chrysochus auratus and C. cobaltinus) in the context of a range of heterospecific mating frequencies observed in natural populations. We found both species benefited from multiple matings, but the benefits were greater in C. cobaltinus and extended to heterospecific matings. We found that PMPZ barriers greatly limited hybrid production by C. auratus females with moderate heterospecific mating frequencies, but that their effectiveness diminished at higher heterospecific mating frequencies. In contrast, there was no evidence for PMPZ barriers in C. cobaltinus females at any heterospecific mating frequency. We show that integrating realistic estimates of cryptic isolation with information on relative abundance and heterospecific mating frequency in the field substantially improves our understanding of the strong directional bias in F1 production previously documented in the Chrysochus hybrid zone. Our results demonstrate that heterospecific mating frequency is critical to understanding the impact of cryptic post‐copulatory barriers on hybrid zone structure and dynamics, and that future studies of such barriers should incorporate field‐relevant heterospecific mating frequencies.     

Studies on assortative mating. III. The effects of incomplete penetrance in assortative mating systems

Four models of positive and negative assortative mating systems, exclusive or partial, in relation to phenotypes determined by a pair of autosomal alleles with dominance but with incomplete penetrance of the dominant allele in heterozygous state, are presented. By introducing the parameter of incomplete penetrance in the models of assortative matings, matings between individuals with identical genotypes will occur within the negative systems, as well as matings between individuals with different genotypes within the positive assortative mating systems. Thus, incomplete penetrance has the effect of retarding equilibrium on an assortative mating system, making this equilibrium equivalent to a system with a lower degree of assortative mating, where the penetrance equals 1 or 0. Another conclusion of biological interest drawn from the mathematical analysis presented in this paper is that for any value of the penetrance, the frequency of 0.5 for recessive individuals is also typical of exclusive or partial negative assortative mating systems at equilibrium.     

Evolution of disassortative and assortative mating preferences based on imprinting

A two-locus haploid model of sexual selection is investigated to explore evolution of disassortative and assortative mating preferences based on imprinting. In this model, individuals imprint on a genetically transmitted trait during early ontogeny and choosy females later use those parental images as a criterion of mate choice. It is assumed that the presence or absence of the female preference is determined by a genetic locus. In order to incorporate such mechanisms as inbreeding depression and heterozygous advantage into our haploid framework, we assume that same-type matings are less fertile than different-type mating. The model suggests that: if all the females have a disassortative mating preference a viability-reducing trait may be maintained even without the fertility cost of same-type matings; a disassortative mating preference can be established even if it is initially rare, when there is a fertility cost of same-type matings. Further, an assortative mating preference is less likely to evolve than a disassortative mating preference. The model may be applicable to the evolution of MHC-disassortative mating preferences documented in house mice and humans.     

Comparison of sexual compatibility in crosses between the southern and northern populations of the cabbage beetle Colaphellus bowringi

It is widely accepted that the genetic divergence and reproductive incompat- ibility between closely related species and/or populations is often viewed as an important step toward speciation. In this study, sexual compatibility in crosses between the southern XS population and the northern TA population of the polyandrous cabbage beetle Co- laphellus bowringi was investigated by testing their mating preferences, mating latency, copulation duration, and reproductive performances of post-mating. In choice mating ex- periments, the percentages ofmatings were significantly higher in intra-population crosses than in inter-population crosses. Both isolation index （/） and index of pair sexual isolation （/PSi） indicated partial mating incompatibility or assortative mating in crosses between the two different geographical populations. In single pair mating experiments, XS females in inter-population crosses mated significantly later and copulated significantly shorter than those in intra-population crosses. However, TA females in inter-population crosses mated significantly earlier and copulated longer than those in intra-population crosses, suggesting that larger XS males may enhance heterotypic mating. The lifetime fecundity was highest in XS homotypic matings, lowest in TA homotypic matings, and intermedi- ate in heterotypic rnatings between their parents. The inter-population crosses resulted in significantly lower egg hatching rate and shorter female longevity than intra-population crosses. These results demonstrated that there exist some incompatibilities in premating, postmating-prezygotic, and postzygotic stages between the southern XS population and northern TA population of the cabbage beetle Colaphellus bowringi.     

Assortative mating in fallow deer reduces the strength of sexual selection

Background

Assortative mating can help explain how genetic variation for male quality is maintained even in highly polygynous species. Here, we present a longitudinal study examining how female and male ages, as well as male social dominance, affect assortative mating in fallow deer (Dama dama) over 10 years. Assortative mating could help explain the substantial proportion of females that do not mate with prime-aged, high ranking males, despite very high mating skew. We investigated the temporal pattern of female and male matings, and the relationship between female age and the age and dominance of their mates.

Results

The peak of yearling female matings was four days later than the peak for older females. Younger females, and especially yearlings, mated with younger and lower-ranking males than older females. Similarly, young males and lower-ranking males mated with younger females than older males and higher-ranking males. Furthermore, the timing of matings by young males coincided with the peak of yearling female matings, whereas the timing of older male matings (irrespective of rank) coincided with the peak of older female matings.

Conclusions

Assortative mating, through a combination of indirect and/or direct female mate choice, can help explain the persistence of genetic variation for male traits associated with reproductive success.     

Effects of multiple matings on reproductive fitness of male and female Diaeretiella rapae

Mating frequency and the amount of sperm transferred during mating have important consequences on progeny sex ratio and fitness of haplodiploid insects. Production of female offspring may be limited by the availability of sperm for fertilizing eggs. This study examined multiple mating and its effect on fitness of the cabbage aphid parasitoid Diaeretiella rapae McIntosh (Hymenoptera: Aphidiidae). Female D. rapae mated once, whereas males mated with on average more than three females in a single day. The minimum time lag between two consecutive matings by a male was 3 min, and the maximum number of matings a male achieved in a day was eight. Sperm depletion occurred as a consequence of multiple mating in D. rapae. The number of daughters produced by females that mated with multiple‐mated males was negatively correlated with the number of matings achieved by these males. Similarly, the proportion of female progeny decreased in females that mated with males that had already mated three times. Although the proportion of female progeny resulting from multiple mating decreased, the decrease was quicker when the mating occurred on the same day than when the matings occurred once per day over several days. Mating success of males initially increased after the first mating, but then males became ‘exhausted’ in later matings; their mating success decreased with the number of prior matings. The fertility of females was affected by mating with multiple‐mated males. The study suggests that male mating history affects the fitness of male and female D. rapae.     

Role of Contests in the Scramble Competition Mating System of a Leaf Beetle

Male competition for mates can occur through contests or a scramble to locate females. We examined the significance of contests for mates in the leaf beetle Chrysomela aeneicollis, which experiences a short breeding season. During peak mating season, 18–52% of beetles are found in male-female pairs, and nearly half of these are copulating. Sex ratios do not differ from parity, females are larger than males, and positive size-assortative mating occurs. Males fight (2–4% of beetles) over access to females, and disruption of mating usually follows these contests. In the laboratory, we compared mating and fighting frequencies for males found in mating pairs (field-paired) and single males placed into an arena with a field-paired female. Mating pairs were switched in half of arenas (new male-female pairs) and maintained in the other half. For 2 days, each male was free to move about and fight; thereafter males were tethered to prevent contests. Mating frequencies were significantly greater for field-paired than single males in both situations. Male size was not related to mating frequency; however, large females received more matings than small ones. These data suggest that males fight for high quality females, but otherwise search for as many matings as possible.     

Frequency of interspecific matings and of hybrid offspring in sympatric populations of Vtviparus ater and V. contectus (Mollusca: Prosobranchia)

In species pairs able to produce fertile hybrids, the amount of introgression can be limited by the frequency of interspecific copulations. We investigated the frequency of interspecific copulations between Kviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assortative mating, the frequency of interspecific copulations was substantial. At sites where V. contectus was rare, 5.9% of all matings, and up to 73% of matings in which V. contectus was involved, were interspecific. At sites where both species were abundant 14.8% of all matings were interspecific. Male-male matings occurred in approximately 3% of copulations. In 1993, 12 out of 325 V. ater females collected from 10 sympatric populations and held in isolation for two to five months gave birth to 25 hybrid offspring, out of a total of 2548 offspring produced by all females. The proportion of hybrid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since interspecific copulations are frequent and hybrid offspring are produced in nature, we conclude that assortative mating is not an effective isolating mechanism between V. ater and V. contectus.     

细纹豆芫菁交配与繁殖力的关系

将采自野外的细纹豆芫菁EpicautamannerhimiMkl的雌雄成虫各50头在室内进行人工随机配对,共发生75次交配,平均交配1.5次。雄虫1生可交配0～4次,雌虫0～2次。交配持续时间为(188±55)min,交配持续时间与交配次数之间、交配持续时间与繁殖力之间均无相关性。交配次数与两性的繁殖力呈负相关。交配后有36头雌虫43次产卵,其中有35次产卵发生在本次交配后,有8次产卵发生在连续2次交配后。作者认为雌虫在性感受性上的差异,与不育雄虫参与雌虫的前次交配有关。雄虫能否产生足够数量的交配因子来抑制雌虫的性感受性,是决定雌虫在产卵前交配次数的重要因素。     

The Effects of Assortative Mating and Migration on Cytonuclear Associations in Hybrid Zones

We examine the influence of nonrandom mating and immigration on the evolutionary dynamics of cytonuclear associations in hybrid zones. Recursion equations for allelic and genotypic cytonuclear disequilibria were generated under models of (1) migration alone, assuming hybrid zone matings are random with respect to cytonuclear genotype; and (2) migration in conjunction with refined epistatic mating, in which females of the pure parental species preferentially mate with conspecific males. Major results are as follows: (a) even the slightest migration removes the dependency of the final outcome on initial conditions, producing a unique equilibrium in which both pure parental genotypes are maintained in the hybrid zone; (b) in contrast to nuclear genes, the dynamics of cytoplasmic allele frequencies appear robust to changes in the assumed mating system, yet are particularly sensitive to gene flow; (c) continued immigration can generate permanent cytonuclear disequilibria, whether mating is random or assortative; and (d) the order of population censusing (before versus after reproduction by immigrants) can have a dramatic effect on the magnitude but not the pattern of cytonuclear disequilibria. Using the maximum likelihood method, the parameter space of migration rates and assortative mating rates was examined for best fit to observed cytonuclear disequilibria data in a hybrid population of Hyla tree frogs. An epistatic mating model with a total immigration rate of about 32% per generation produces equilibrium gene frequencies and cytonuclear disequilibria consistent with the empirical observations.     

Seasonal changes in humidity level in the tropics impact body color polymorphism and desiccation resistance in Drosophila jambulina—Evidence for melanism-desiccation hypothesis

Drosophila jambulina exhibits color dimorphism controlled by a single locus but its ecological significance is not clear. Dark and light morphs differ significantly in body melanisation, desiccation resistance, rate of water loss, mating activity and fecundity. Interestingly, this species lacks clinal variation for body size, desiccation resistance and life history traits. For body melanisation, lack of geographical variation as well as plastic effects is not in agreement with a thermal melanism hypothesis. However, based on field data, there are seasonal changes in phenotypic frequencies of dark and light body color morphs which correlate significantly with variation in humidity levels. Under short-term (8 h) desiccation stress, we observed higher number of assortative matings, longer copulation period and increased fecundity for dark strains as compared with light strains. By contrast, both the morphs when exposed to high humid conditions exhibited higher assortative matings and fecundity for light strains as compared with dark strains. In tropical populations of D. jambulina, body color polymorphism seems to be maintained through humidity changes as opposed to thermal melanism. Thus, seasonal changes in the frequency of body color morphs in this tropical species supports melanism-desiccation hypothesis.     

Mate Choice and Spawning Success in the Fighting Fish Betta splendens: the Importance of Body Size, Display Behavior and Nest Size

Courtship displays should be exaggerated enough to attract mates and yet tempered so as not to deter them. We tested this hypothesis in the fighting fish Betta splendens by studying courtship displays and body size and their relationships with male parental quality and female fecundity, as well as the effects of display behavior and body size on mate choice decisions and spawning success. Because of their high degree of parental investment, males are expected to be discriminating in their choice of mates. Males who displayed more frequently built larger nests, a measure of parental quality, but larger males did not. When females were paired with males with high display rates, however, the pair had fewer eggs in their nest, even when accounting for female body mass. In a mate choice test using computer‐generated male stimuli that differed only in display behavior, females showed no preferences for displaying males vs. non‐displaying males, or for males with higher display rates vs. lower display rates. In similar tests in which the computer‐generated males differed only in size, females preferred larger males, but also preferred males that differed with respect to body size (negative assortative mating). Males preferred computer‐generated females that performed courtship displays over non‐displaying females, but showed no preferences for female body size. Neither a female's body size nor her display behavior was a significant predictor of her fecundity as estimated by the number of eggs released during spawning. Thus, our results suggest that female B. splendens must balance male parental quality (nest size) with the risk of potentially disruptive or dangerous behavior during spawning, and that females may minimize these risks through negative size‐assortative mating. Female display behavior, while unrelated to fecundity in our study, may attract males because it indicates reproductive readiness or serves a species‐recognition function.     

Mating system in a population of the wasp <Emphasis Type="Italic">Polistes dominulus</Emphasis> (Christ) (Hymenoptera,Vespidae)

The specific features of mating females and males of different morphs collected from a population of the wasp P. dominulus in Southern Ukraine were studied. Data analysis of mating individuals in experiments indicated a random mate choice by size and assortative mating by coloration. The most actively copulating males in the experiments had light-colored mesopleuron, sternite, and fore and middle coxae. They were similar in the color pattern and size to resident males, which defend their territories in the habitat. Most P. dominulus females mated only once. In their coloration, those females represented the majority of the individuals in the population. The color variability of the foundresses was shown to change in a cyclic pattern. The possibility that such variability may result from selective or assortative mating is discussed. The color and pattern traits of P. dominulus females and males could serve as markers of their reproductive strategies and their reproductive success on the whole.