Interaction of rearing environment and reproductive tactic on gene expression profiles in Atlantic salmon

Organisms that share the same genotype can develop into divergent phenotypes, depending on environmental conditions. In Atlantic salmon, young males of the same age can be found either as sneakers or immature males that are future anadromous fish. Just as the organism-level phenotype varies between divergent male developmental trajectories, brain gene expression is expected to vary as well. We hypothesized that rearing environment can also have an important effect on gene expression in the brain and possibly interact with the reproductive tactic adopted. We tested this hypothesis by comparing brain gene expression profiles of the two male tactics in fish from the same population that were reared in either a natural stream or under laboratory conditions. We found that expression of certain genes was affected by rearing environment only, while others varied between male reproductive tactics independent of rearing environment. Finally, more than half of all genes that showed variable expression varied between the two male tactics only in one environment. Thus, in these fish, very different molecular pathways can give rise to similar macro-phenotypes depending on rearing environment. This result gives important insights into the molecular underpinnings of developmental plasticity in relationship to the environment.     

Phenotypic variation between dimorphic males in corkwing wrasse

The age-frequency distributions for sneaker males (individuals with female secondary sexual characters) and territorial males (individuals with typical male sexual characters) of corkwing wrasse Symphodus melops in a Swedish population did not differ. Otolith analysis revealed that sneaker males were smaller than their territorial male counterparts as juveniles, but there was no evidence of a clear-cut threshold to determine the subsequent reproductive patterns. Thus, the expression of male morph in corkwing wrasse most likely is related to early growth pattern and the different morphs could be fixed for life. Adult sneaker males in general were smaller than territorial males of the same age. The relative gonad size and the abdomen in sneaker males were substantially enlarged compared with territorial males, but there was no difference in gonad water content between the two types of males. Sneaker males weighed less in relation to their length than territorial males. These results support theoretical sperm competition models which predict that the male morph with reduced attractiveness to females (i.e. sneakers) should invest more into spermatogenesis in order to achieve fertilization than the attractive male morph (i.e. territorial males).     

Male sexual polymorphism, alternative reproductive tactics, and androgens in combtooth blennies (pisces: blenniidae)

In species in which intense intermale competition for the access to females is present males of lower competitive ability may adopt alternative reproductive tactics (ART) to get access to mates. These ART translate in many cases into male sexual polymorphism, with individuals following distinctly different tactics. Usually two alternative male morphs can be recognized in species with ART: (1) bourgeois males that compete for access to mates invest in typically male behaviors, such as building elaborated nests or displaying ornaments; and (2) parasitic males that take advantage of the success of the bourgeois males in attracting females and attempt "sneaker" fertilizations (e.g., sneaker and satellite males). In combtooth blennies (Blenniidae) the co-occurrence of ART and male sexual polymorphism has been described for two temperate species: the peacock blenny, Salaria pavo, and the Azorean rock-pool blenny, Parablennius sanguinolentus parvicornis. Interestingly, while in the peacock blenny the alternative male morph adopts a sneaker tactic, in the rock-pool blenny parasitic males act as satellites to nest-holder males. Thus, this variation in the ART expressed in these two closely related species allows for a comparative study of the proximate and ultimate factors affecting the expression of the two ART. In this article we summarize the available information on androgen levels in bourgeois and parasitic males of natural populations of the two species and of recent studies on the effect of exogenous administration of androgens on tactic switching in parasitic males of the two species. The information is discussed within the frame of the relative plasticity hypothesis, which predicts that plastic alternative morphs should show differences in hormone levels and that the administration of sex steroids should be effective in promoting the switch from the parasitic to bourgeois tactic. The evidence is only partly consistent with this hypothesis. Alternatively, a social transduction hypothesis that better fits the available data on androgens and ART in teleost is proposed. It states that the observed differences in androgen levels between alternative morphs should not be interpreted as an organization vs activation effect of steroids, but rather as the limited vs lifelong responsiveness of the neuroendocrine axis to social regulation.     

Age‐dependent male mating tactics in a spider mite—A life‐history perspective

Males often fight with rival males for access to females. However, some males display nonfighting tactics such as sneaking, satellite behavior, or female mimicking. When these mating tactics comprise a conditional strategy, they are often thought to be explained by resource holding potential (RHP), that is, nonfighting tactics are displayed by less competitive males who are more likely to lose a fight. The alternative mating tactics, however, can also be explained by life‐history theory, which predicts that young males avoid fighting, regardless of their RHP, if it pays off to wait for future reproduction. Here, we test whether the sneaking tactic displayed by young males of the two‐spotted spider mite can be explained by life‐history theory. We tested whether young sneaker males survive longer than young fighter males after a bout of mild or strong competition with old fighter males. We also investigated whether old males have a more protective outer skin—a possible proxy for RHP—by measuring cuticle hardness and elasticity using nanoindentation. We found that young sneaker males survived longer than young fighter males after mild male competition. This difference was not found after strong male competition, which suggests that induction of sneaking tactic is affected by male density. Hardness and elasticity of the skin did not vary with male age. Given that earlier work could also not detect morphometric differences between fighter and sneaker males, we conclude that there is no apparent increase in RHP with age in the mite and age‐dependent male mating tactics in the mite can be explained only by life‐history theory. Because it is likely that fighting incurs a survival cost, age‐dependent alternative mating tactics may be explained by life‐history theory in many species when reproduction of old males is a significant factor in fitness.     

Alternative reproductive tactics and the propensity of hybridization

One explanation for hybridization between species is the fitness benefits it occasionally confers to the hybridizing individuals. This explanation is possible in species that have evolved alternative male reproductive tactics: individuals with inferior tactics might be more prone to hybridization provided it increases their reproductive success and fitness. Here we experimentally tested whether the propensity of hybridization in the wild depends on male reproductive tactic in Calopteryx splendens damselflies. Counter to our expectation, it was males adopting the superior reproductive tactic (territoriality) that had greatest propensity to hybridize than males adopting the inferior tactics (sneakers and floaters). Moreover, among the territorial males, the most ornamented males had greatest propensity to hybridize whereas the pattern was reversed in the sneaker males. Our results suggest that there is fluctuating selection on male mate discrimination against heterospecific females depending on both ornament size and the male’s reproductive tactic.     

Tactic changes in dusky frillgoby Bathygobius fuscus sneaker males: effects of body size and nest availability

Field and laboratory studies were conducted to examine the effects of nest availability and body size on changes in male mating tactics from sneaking to nest‐holding in the dusky frillgoby Bathygobius fuscus. In the field, the body size of nest‐holding males decreased from early to mid‐breeding season, suggesting the possibility of a change in the tactics of sneaker males to nest‐holding. Many sneaker males did not use vacant spawning nests even when size‐matched nests were available, but they continued to reproduce as sneakers. Similarly, in aquarium experiments with available vacant nests, some sneaker males became nest‐holders irrespective of their body size, but some did not. These results showed that nest availability is not a limiting factor for changes in tactics by sneaker males in this species. Because tactic‐unchanged sneaker males were co‐housed with larger nest‐holding males in the tanks, the body size of nearby nest‐holding males may have affected the decision to change tactics for sneaker males. Moreover, smaller individuals among tactic‐changed males tended to spend more time until spawning, probably because they had relatively larger costs and smaller benefits of reproduction as nest‐holding males compared to larger males.     

Size Dependent Male Reproductive Tactic in the Two-Spotted Goby (Gobiusculus flavescens)

Male investment in testes and sperm duct gland in the polygamous nest breeding two-spotted goby Gobiusculus flavescens (Fabricius) was investigated in relation to time in reproductive season and individual physical parameters. This small teleost fish is most likely the most abundant species found along the rocky shores of the North East Atlantic. The two-spotted goby has a single reproductive season, during which nest-caring males can raise several clutches of offspring. According to the literature the males are on average larger than the females. Here we report for the first time a population showing a reversal of this trend, with males on average being smaller than females, a difference likely caused by a large proportion of small males. Early in the breeding season these small males have typical sneaker characters, with relatively large testes and small seminal duct glands compared to the larger dominant territorial males. The presence of these two alternative male reproductive tactics is confirmed by histological studies, which shows the presence of sperm in the sperm duct glands (SDG) of smaller males, but not in the SDG of intermediate and larger males. To our knowledge, males with typical sneaker characters have not been reported in earlier studied populations of two-spotted goby. Interestingly we found that testes investment declined significantly over the course of the breeding season, and that this reduction was significantly more pronounced in small compared to the large males. Further, a significant increase in seminal duct gland (SDG) mass was observed for the smaller males over the breeding season. We propose that this indicates a possible shift in mating tactic by smaller males from a parasitic to a nest-holding tactic over the course of the breeding season. Thus, the observed size dependent plasticity in investment in SDG over time suggests that the reproductive tactic of G. flavescens is conditional, and possibly influenced by mate availability and male—male competition.     

Alternative mate-finding tactics in a non-territorial damselfly (Odonata: Coenagrionidae)

Males of the non-territorial damselfly Enallagma hageni have two alternative tactics for finding mates: (1) they search the banks of the pond for unmated females (searching tactic), or (2) wait at oviposition sites for females that resurface prematurely from underwater oviposition (waiting tactic). Although the searching tactic yielded more fertilizations than the waiting tactic, for time invested, the waiting tactic became increasingly successful later in the reproductive season due to changes in female oviposition behaviour. The two tactics can be maintained in the population because males can mate by the waiting tactic during the afternoon when few females are available to searchers. Among males visiting the breeding site an equal number of times, males mating by a mixture of tactics were as successful as males mating only by the main tactic. Because marked males were found to use both tactics, these behaviours are interpreted as evidence of behavioural plasticity within individuals, representing one conditional evolutionary strategy.     

The mate choice brain: comparing gene profiles between female choice and male coercive poeciliids

Genes that mediate mate preferences potentially play a key role in promoting and maintaining biological diversity. In this study, we compare mate preference behavior in two related poeciliid fishes with contrasting behavioral phenotypes and relate these behavioral differences to gene profiles in the brain. Results reveal that one poeciliid fish, the Northern swordtail, exhibits robust mate preference as compared to the Western mosquitofish, which utilizes a coercive mating system. Female swordtails display no significant difference in association time between male- and female-exposure trials, whereas female mosquitofish spend significantly less time associating with males relative to females. Furthermore, the preference strength for large males is significantly lower in female mosquitofish relative to swordtails. We then examine expression of three candidate genes previously shown to be associated with mate preference behavior in female swordtails and linked to neural plasticity in other vertebrates: neuroserpin (NS), neuroligin-3 (NLG-3) and N-methyl-d-aspartate receptor (NMDA-R). Whole brain gene expression patterns reveal that two genes (NS and NLG-3) are positively associated with mate preference behavior in female swordtails, a pattern opposing that of the mosquitofish. In mosquitofish females, these genes are downregulated when females express biases toward males yet are elevated in association with total motor activity patterns under asocial conditions, suggesting that the presence of males in mosquitofish species may inhibit expression of these genes. Both gene expression and female behavioral responses to males exhibit opposing patterns between these species, suggesting that this genetic pathway may potentially act as a substrate for the evolution of mate preference behavior.     

Pre-oviposition Ejaculation in Externally Fertilizing Fish: How Sneaker Male Rose Bitterlings Contrive to Mate

Pre-oviposition ejaculation as a mating tactic of sneaker males in the rose bitterling, Rhodeus ocellatus, was studied under natural and artificial conditions. In a small pond in Yao city, Osaka, Japan, the operational sex ratio of males and females was found to be approximately 3.5:1 and the proportion of males to the mussels, which serve as spawning beds for the rose bitterling, was approximately 2:1. Competitively subordinate rose bitterling males which spawned into the mussels participated in mating by sneaking, because not all males could occupy territories around the mussels. The sneaker males often released sperm not only after but also before egg-laying (this ejaculation movement by the male before egg-laying is termed ‘pre-oviposition ejaculation’). In pair spawning with sneaker, the sneakers frequently performed pre-oviposition ejaculations, which territorial males never performed. In the field, pre-oviposition ejaculations by sneakers coincided with the leading of females by territorial males. Under artificial conditions, I demonstrated by using electrophoretic paternal analyses that the pre-oviposition ejaculations by the sneakers were more effective than the post-oviposition ejaculations by the territorial males. In addition, there were negative size-dependences in ejaculation achievement rate and fertilization success of the sneaker males.     

Behaviour and success of sneaker males in the sand goby, Pomatoschistus minutus

In this study we investigated in laboratory conditions the presence of alternative reproductive tactics in the sand goby and describe proximate factors affecting their expression and success. We describe the reciprocal interactions of resident males, females, and sneaking males. The pre-spawning phase proved to be important for successful nest intrusions by sneakers. The number of sneakers had no effect on the frequency of successful intrusions. When small males had exclusive access to nest sites, they built a nest and courted females, showing a full behavioural repertoire. The intensity of courtship was, however, strongly positively correlated with body size. Using microsatellite DNA markers we assessed paternity shares of territorial and sneaker males in a subset of all replicates. Following successful nest intrusion sneaker males fertilised 5–10% of the eggs. Our interpretation of the results is that sneaking in the sand goby is a conditional tactic, one that is less successful than the normal nest guarding behaviour, at least for one spawning event. Received in revised form: 9 March 2001 Electronic Publication     

Description of alternative male reproductive tactics in a shell-brooding cichlid, Telmatochromis vittatus, in Lake Tanganyika

Telmatochromis vittatus (Cichlidae) is a Tanganyikan substrate brooder which spawns in the gastropod-shell nests of a cichlid, Lamprologus callipterus. We describe male reproductive tactics of T. vittatus in and around the shell nests, where males of various sizes were found. Based on utilization patterns of the shell nests, interactions among males, and spawning behaviors, males could be categorized into four types based on reproductive tactics and in order of body size: sneaker males, satellite males, territorial males and piracy males. Size range of males in tactic groups rarely overlapped. Territorial males defended shell nests harboring multiple females, but during pair-spawning they were occasionally taken over by large piracy males that visited several nests repeatedly. Small sneaker males darted to pair-spawning territorial males and might ejaculate sperm. Satellite males did not perform parasitic spawning but pair-spawned in a single shell outside the nests. Spawning of satellite males was infrequently parasitized. The largest gonado-somatic index (GSI) was found in sneaker males followed by piracy males, territorial males and satellite males, suggesting that gonadal investment of males using the four tactics may be consistent with intensity or risk of sperm competition.     

Effects of ploidy and sex-locus genotype on gene expression patterns in the fire ant Solenopsis invicta

Males in many animal species differ greatly from females in morphology, physiology and behaviour. Ants, bees and wasps have a haplodiploid mechanism of sex determination whereby unfertilized eggs become males while fertilized eggs become females. However, many species also have a low frequency of diploid males, which are thought to develop from diploid eggs when individuals are homozygous at one or more sex determination loci. Diploid males are morphologically similar to haploids, though often larger and typically sterile. To determine how ploidy level and sex-locus genotype affect gene expression during development, we compared expression patterns between diploid males, haploid males and females (queens) at three developmental timepoints in Solenopsis invicta. In pupae, gene expression profiles of diploid males were very different from those of haploid males but nearly identical to those of queens. An unexpected shift in expression patterns emerged soon after adult eclosion, with diploid male patterns diverging from those of queens to resemble those of haploid males, a pattern retained in older adults. The finding that ploidy level effects on early gene expression override sex effects (including genes implicated in sperm production and pheromone production/perception) may explain diploid male sterility and lack of worker discrimination against them during development.     

Interspecific profiling of gene expression informed by comparative genomic hybridization: A review and a novel approach in African cichlid fishes

Modern genomic approaches have facilitated great progress in our understanding of the molecular and genetic underpinnings of ecological and evolutionary processes. Analysis of gene expression through heterologous hybridization in particular has enabled genome-scale studies in many ecologically and evolutionarily interesting species. However, these studies have been hampered by the difficulty of comparing-on a common array platform-gene-expression profiles across species due to sequence divergence altering the dynamics of hybridization. All too often, comparisons of expression profiles across species were limited to contrasting lists of gene or even of just functional categories. Here we review these issues and propose a novel solution. Exploiting the diverse cichlid lineages of East Africa as our model-system, we then present results from an experimental case study that compares the neural gene-expression profiles of males and females of two species that differ in mating system. Using a single microarray platform that contains genes from one species, Astatotilapia burtoni, we conducted a total of 16 direct comparisons for neural gene-expression level between individual males and females from a pair of sister species, the polygynous Enantiopus melanogenys and the monogamous Xenotilapia flavipinnis. Next, we conducted a meta-analysis with previously published data from two different intra-specific expression studies to determine whether sex-specific neural gene expression is more closely associated with behavioral phenotype than it is with gonadal sex. Our results indicate that the gene expression profiles are species-specific to a large extent, as relatively few genes show conserved expression patterns associated with either sex. Finally, we describe how competitive genomic DNA hybridizations between the two focal species allow us to assess the degree to which divergence of sequences biases the results. We propose a masking technique that correlates interspecific expression ratios obtained with cDNA with hybridization ratios obtained with genomic DNA for the same set of species and determines threshold sequence divergence to reduce false positives. Our approach should be applicable to a wide range of interesting questions related to the evolution and ecology of gene expression.     

Major molecular differences between mammalian sexes are involved in drug metabolism and renal function

Many anatomical differences exist between males and females; these are manifested on a molecular level by different hormonal environments. Although several molecular differences in adult tissues have been identified, a comprehensive investigation of the gene expression differences between males and females has not been performed. We surveyed the expression patterns of 13,977 mouse genes in male and female hypothalamus, kidney, liver, and reproductive tissues. Extensive differential gene expression was observed not only in the reproductive tissues, but also in the kidney and liver. The differentially expressed genes are involved in drug and steroid metabolism, osmotic regulation, or as yet unresolved cellular roles. In contrast, very few molecular differences were observed between the male and female hypothalamus in both mice and humans. We conclude that there are persistent differences in gene expression between adult males and females. These molecular differences have important implications for the physiological differences between males and females.     

Sexual and social stimuli elicit rapid and contrasting genomic responses

Sensory physiology has been shown to influence female mate choice, yet little is known about the mechanisms within the brain that regulate this critical behaviour. Here we examine preference behaviour of 58 female swordtails, Xiphophorus nigrensis, in four different social environments (attractive and unattractive males, females only, non-attractive males only and asocial conditions) followed by neural gene expression profiling. We used a brain-specific cDNA microarray to identify patterns of genomic response and candidate genes, followed by quantitative PCR (qPCR) examination of gene expression with variation in behaviour. Our microarray results revealed patterns of genomic response differing more between classes of social stimuli than between presence versus absence of stimuli. We identified suites of genes showing diametrically opposed patterns of expression: genes that are turned 'on' while females interact with attractive males are turned 'off' when interacting with other females, and vice versa. Our qPCR results identified significant predictive relationships between five candidate genes and specific mate choice behaviours (preference and receptivity) across females exposed to males, with no significant patterns identified in female or asocial conditions or with overall locomotor activity. The identification of stimulus- and behaviour-specific responses opens an exciting window into the molecular pathways associated with social behaviour and mechanisms that underlie sexual selection.