Male mate choice relies on major histocompatibility complex class I in a sex-role-reversed pipefish

Occurrence of the pugnose pipefish Bryx dunckeri in the Sargasso Sea

Juvenile specimens of the pugnose pipefish, Bryx dunckeri, were collected during a multipurpose research survey conducted within the Sargasso Sea Subtropical Convergence Zone, extending the known distribution range of this species to include open ocean areas of the Western North Atlantic. Novel spatial data are of scientific interest as information on the distribution, population structure, and population size of this species is limited. Additionally, we present detailed photographs and morphological data on the collected specimens. The results are discussed in relation to the dispersal abilities and population structure in syngnathids.

Response to MHC-based olfactory cues in a mate choice context in two species of darter (Percidae: Etheostoma)

Mate choice is hypothesized to play an important role in maintaining high diversity at major histocompatibility complex (MHC) genes in vertebrates. Many studies have revealed that females across taxa prefer the scent of males with MHC genotypes different to their own. In this study we tested the "opposites-attract" hypothesis in two species of darter with known differences in female criteria used in mate choice: in the fantail darters (a paternal-care species), females prefer males with visual traits related to nest guarding and egg tending, while in rainbow darters (not a paternal-care species) female mate choice criteria are unknown. In dichotomous mate-choice trials, we presented females of both species with the scents of conspecific males with MHC class IIb genotypes that were either similar or dissimilar to that of the focal female. We evaluated the proportion of time each female spent with each male and calculated the average strength of female preference for both species. Female fantail darters demonstrated a preference for the scent of males with similar (rather than dissimilar) MHC genotypes, but this result was not statistically significant. Rainbow darter females showed no preference for the scent of males with similar or dissimilar MHC genotypes. Our results do not support the "opposites-attract" hypothesis in darters.

Mate Discrimination Using Chemical Cues by Male Guppies

Males often perform mate choice with the aim of maximizing reproductive success. To identify profitable mates, the males of some animals are known to use visual and chemical cues derived from females. In this study, we aimed to examine mate discrimination by male guppies (Poecilia reticulata) using chemical cues received from females under different reproductive statuses, i.e., virgin females, pregnant females, females after copulation with another male, and post-partum females. We conducted a dichotomous choice experiment for each combination of chemical stimuli from females under different reproductive statuses. In experiment 1, in which females were removed from water that was subsequently used as a chemical stimulus, male preferences did not differ significantly in all combinations of chemical stimuli from females under different reproductive statuses. However, in experiment 2, in which females remained within bottles containing the water used as a chemical stimulus, with the exception of one combination of chemical stimuli, significant differences in male preferences were detected for chemical stimuli derived from females under different reproductive statuses. Overall, males generally showed a preference for chemical stimulus received from females after copulation with other males. The findings of this study indicate that male guppies can discriminate the reproductive status of females based solely on chemical cues that may disappear or degenerate within a short period of time, thereby facilitating profitable mate choice.

Does the female seahorse still prefer her mating partner after a period of separation?

For species showing sexual monogamy, once one male and one female form a mating pair bond, they will be faithful to each other in subsequent breeding events. However, if their pair bond is broken for some reason, do they continue to prefer their partner when they come together again for mating? In other words, can the broken pair bond of sexually monogamous species be repaired? This is an interesting question but not yet well answered. To address this question, in the present study we used the lined seahorse (Hippocampus erectus), a typical sexually monogamous species, to study the partner preference of a female individual who experienced a complete separation followed by a reunion with her partner. Our main findings are as follows: (i) The female seahorse no longer prefers her partner after a separation, whether it is a former partner or a recent partner. No preference for partner-males may indicate that the broken pair bond cannot be repaired. (ii) The female seahorse maintains sexual fidelity to her partner in the absence of separation. However, once the health of her partner decreases, the female will switch mate, and her courtship with the new partner can take place during the pregnancy of her original partner. The first finding may provide insight into whether monogamous species still have an opportunity to reselect a new partner in the future to correct their poor choice once they have mated with a low-quality partner. The answer is that they can still gain an opportunity as long as the pair bonds with their current partners are broken. The second finding may reveal the conditions and timing at which a female seahorse switches her mate. These findings help us better understand the mating system of the seahorse H. erectus.

Songs versus colours versus horns: what explains the diversity of sexually selected traits?

Papers on sexual selection often highlight the incredible diversity of sexually selected traits across animals. Yet, few studies have tried to explain why this diversity evolved. Animals use many different types of traits to attract mates and outcompete rivals, including colours, songs, and horns, but it remains unclear why, for example, some taxa have songs, others have colours, and others horns. Here, we first conduct a systematic survey of the basic diversity and distribution of different types of sexually selected signals and weapons across the animal Tree of Life. Based on this survey, we describe seven major patterns in trait diversity and distributions. We then discuss 10 unanswered questions raised by these patterns, and how they might be addressed. One major pattern is that most types of sexually selected signals and weapons are apparently absent from most animal phyla (88%), in contrast to the conventional wisdom that a diversity of sexually selected traits is present across animals. Furthermore, most trait diversity is clustered in Arthropoda and Chordata, but only within certain clades. Within these clades, many different types of traits have evolved, and many types appear to have evolved repeatedly. By contrast, other major arthropod and chordate clades appear to lack all or most trait types, and similar patterns are repeated at smaller phylogenetic scales (e.g. within insects). Although most research on sexual selection focuses on female choice, we find similar numbers of traits (among sampled species) are involved in male contests (44%) and female choice (55%). Overall, these patterns are largely unexplained and unexplored, as are many other fundamental questions about the evolution of these traits. We suggest that understanding the diversity of sexually selected traits may require a shift towards macroevolutionary studies at relatively deep timescales (e.g. tens to hundreds of millions of years ago).

Within-generation and transgenerational plasticity of mate choice in oceanic stickleback under climate change

Plasticity, both within and across generations, can shape sexual traits involved in mate choice and reproductive success, and thus direct measures of fitness. Especially, transgenerational plasticity (TGP), where parental environment influences offspring plasticity in future environments, could compensate for otherwise negative effects of environmental change on offspring sexual traits. We conducted a mate choice experiment using stickleback ( Gasterosteus aculeatus) with different thermal histories (ambient 17°C or elevated 21°C) within and across generations under simulated ocean warming using outdoor mesocosms. Parentage analysis of egg clutches revealed that maternal developmental temperature and reproductive (mesocosm) environment affected egg size, with females that developed at 17°C laying smaller eggs in 21°C mesocosms, likely owing to metabolic costs at elevated temperature. Paternal developmental temperature interacted with the reproductive environment to influence mating success, particularly under simulated ocean warming, with males that developed at 21°C showing lower overall mating success compared with 17°C males, but higher mating success in 21°C mesocosms. Furthermore, mating success of males was influenced by the interaction between F1 developmental temperature and F0 parent acclimation temperature, demonstrating the potential role of both TGP and within-generation plasticity in shaping traits involved in sexual selection and mate choice, potentially facilitating rapid responses to environmental change. This article is part of the theme issue 'The role of plasticity in phenotypic adaptation to rapid environmental change'.

Reduced sexual size dimorphism in a pipefish population where males do not prefer larger females

Within a species' distribution, populations are often exposed to diverse environments and may thus experience different sources of both natural and sexual selection. These differences are likely to impact the balance between costs and benefits to individuals seeking reproduction, thus entailing evolutionary repercussions. Here, we look into an unusual population (Baltic Sea) of the broadnosed pipefish, Syngnathus typhle, where males do not seem to select females based on size and hypothesize that this pattern may derive from a reduction in direct benefits to the male. We further hypothesize that if larger females do not persistently secure a higher reproductive success, either through pre- or postcopulatory sexual selection, a decrease in sexual size dimorphism in the Baltic population should be apparent, especially when contrasted with a well-studied population, inhabiting similar latitudes (Swedish west coast), where males prefer larger females. We found that, in the Baltic population, variation in female quality is low. We were unable to find differences in abortion rates or protein concentration in oocytes produced by females of contrasting sizes. Direct benefits from mating with large partners seem, thus, reduced in the Baltic population. We also found no evidence of any postcopulatory mechanism that could favor larger mothers as embryo development was unrelated to female size. While female size can still be selected through intrasexual competition or fecundity selection, the pressure for large female body size seems to be lower in the Baltic. Accordingly, we found a noticeable decrease in sexual size dimorphism in the Baltic population. We conclude that, although far from negating the significance of other selective processes, sexual selection seems to have a decisive role in supporting pipefish sexual size asymmetries.

Disentangling the mechanisms of mate choice in a captive koala population

Successful captive breeding programs are crucial to the long-term survival of many threatened species. However, pair incompatibility (breeding failure) limits sustainability of many captive populations. Understanding whether the drivers of this incompatibility are behavioral, genetic, or a combination of both, is crucial to improving breeding programs. We used 28 years of pairing data from the San Diego Zoo koala colony, plus genetic analyses using both major histocompatibility complex (MHC)-linked and non-MHC-linked microsatellite markers, to show that both genetic and non-genetic factors can influence mating success. Male age was reconfirmed to be a contributing factor to the likelihood of a koala pair copulating. This trend could also be related to a pair's age difference, which was highly correlated with male age in our dataset. Familiarity was reconfirmed to increase the probability of a successful copulation. Our data provided evidence that females select mates based on MHC and genome-wide similarity. Male heterozygosity at MHC class II loci was associated with both pre- and post-copulatory female choice. Genome-wide similarity, and similarity at the MHC class II DAB locus, were also associated with female choice at the post-copulatory level. Finally, certain MHC-linked alleles were associated with either increased or decreased mating success. We predict that utilizing a variety of behavioral and MHC-dependent mate choice mechanisms improves female fitness through increased reproductive success. This study highlights the complexity of mate choice mechanisms in a species, and the importance of ascertaining mate choice mechanisms to improve the success of captive breeding programs.

Sex in murky waters: algal-induced turbidity increases sexual selection in pipefish

ABSTRACT

Algal-induced turbidity has been shown to alter several important aspects of reproduction and sexual selection. However, while turbidity has been shown to negatively affect reproduction and sexually selected traits in some species, it may instead enhance reproductive success in others, implying that the impact of eutrophication is far more complex than originally believed. In this study, we aimed to provide more insight into these inconsistent findings. We used molecular tools to investigate the impact of algal turbidity on reproductive success and sexual selection on males in controlled laboratory experiments, allowing mate choice, mating competition, and mate encounter rates to affect reproduction. As study species, we used the broad-nosed pipefish, Syngnathus typhle, a species practicing male pregnancy and where we have previously shown that male mate choice is impaired by turbidity. Here, turbidity instead enhanced sexual selection on male size and mating success as well as reproductive success. Effects from mating competition and mate encounter rates may thus override effects from mate choice based on visual cues, producing an overall stronger sexual selection in turbid waters. Hence, seemingly inconsistent effects of turbidity on sexual selection may depend on which mechanisms of sexual selection that have been under study.

SIGNIFICANCE STATEMENT

Algal blooms are becoming increasingly more common due to eutrophication of freshwater and marine environments. The high density of algae lowers water transparency and reduces the possibility for fish and other aquatic animals to perform behaviors dependent on vision. We have previously shown that pipefish are unable to select the best partner in mate choice trials when water transparency was reduced. However, fish might use other senses than vision to compensate for the reduction in water transparency. In this study, we found that when fish were allowed to freely interact, thereby allowing competition between partners and direct contact between the fish, the best partner was indeed chosen. Hence, the negative effects of reduced water visibility due to algal blooms may be counteracted by the use of other senses in fish.

Can MHC-assortative partner choice promote offspring diversity? A new combination of MHC-dependent behaviours among sexes in a highly successful invasive mammal

Sexual selection involving genetically disassortative mate choice is one of several evolutionary processes that can maintain or enhance population genetic variability. Examples of reproductive systems in which choosers (generally females) select mates depending on their major histocompatibility complex (MHC) genes have been reported for several vertebrate species. Notably, the role of MHC-dependent choice not in mating contexts, but in other kinds of social interactions such as in the establishment of complex social systems, has not yet drawn significant scientific interest and is virtually absent from the literature. We have investigated male and female MHC-dependent choice in an invasive population of North American raccoons (Procyon lotor) in Germany. Both male and female raccoons rely on olfaction for individual recognition. Males have an unusually complex social system in which older individuals choose unrelated younger ones to form stable male coalitions that defend territories and a monopoly over females. We have confirmed that females perform MHC-disassortative mate choice and that this behaviour fosters genetic diversity of offspring. We have also observed that males build coalitions by choosing male partners depending on their MHC, but in an assortative manner. This is the first observation of antagonistic MHC-dependent behaviours among sexes. We show that this is the only combination of MHC-dependent partner choice that leads to outbreeding. In the case of introduced raccoons, such behaviours can act together to promote the invasive potential of the species by increasing its adaptive genetic divergence.

MHC-dependent mate choice is linked to a trace-amine-associated receptor gene in a mammal

Major histocompatibility complex (MHC) genes play a pivotal role in vertebrate self/nonself recognition, parasite resistance and life history decisions. In evolutionary terms, the MHC’s exceptional diversity is likely maintained by sexual and pathogen-driven selection. Even though MHC-dependent mating preferences have been confirmed for many species, the sensory and genetic mechanisms underlying mate recognition remain cryptic. Since olfaction is crucial for social communication in vertebrates, variation in chemosensory receptor genes could explain MHC-dependent mating patterns. Here, we investigated whether female mate choice is based on MHC alleles and linked to variation in chemosensory trace amine-associated receptors (TAARs) in the greater sac-winged bat (Saccopteryx bilineata). We sequenced several MHC and TAAR genes and related their variation to mating and paternity data. We found strong evidence for MHC class I-dependent female choice for genetically diverse and dissimilar males. We also detected a significant interaction between mate choice and the female TAAR3 genotype, with TAAR3-heterozygous females being more likely to choose MHC-diverse males. These results suggest that TAARs and olfactory cues may be key mediators in mammalian MHC-dependent mate choice. Our study may help identify the ligands involved in the chemical communication between potential mates.

Sexual Selection and the differences between the sexes in Mandrills (Mandrillus sphinx)

Sexual selection has become a major focus in evolutionary and behavioral ecology. It is also a popular research topic in primatology. I use studies of mandrills (Mandrillus sphinx), a classic example of extravagant armaments and ornaments in animals, to exemplify how a long-term, multidisciplinary approach that integrates field observations with laboratory methods can contribute to on-going theoretical debates in the field of sexual selection. I begin with a brief summary of the main concepts of sexual selection theory and the differences between the sexes. I then introduce mandrills and the study population and review mandrill life history, the ontogeny of sex differences, and maternal effects. Next, I focus on male-male competition and female choice, followed by the less well-studied questions of female-female competition and male choice. This review shows how different reproductive priorities lead to very different life histories and divergent adaptations in males and females. It demonstrates how broadening traditional perspectives on sexual selection beyond the ostentatious results of intense sexual selection on males leads to an understanding of more subtle and cryptic forms of competition and choice in both sexes and opens many productive avenues in the study of primate reproductive strategies. These include the potential for studies of postcopulatory selection, female intrasexual competition, and male choice. These studies of mandrills provide comparison and, I hope, inspiration for studies of both other polygynandrous species and species with mating systems less traditionally associated with sexual selection.

Next-generation genotyping of hypervariable loci in many individuals of a non-model species: technical and theoretical implications

BACKGROUND

Across species, diversity at the Major Histocompatibility Complex (MHC) is critical to disease resistance and population health; however, use of MHC diversity to quantify the genetic health of populations has been hampered by the extreme variation found in MHC genes. Next generation sequencing (NGS) technology generates sufficient data to genotype even the most diverse species, but workflows for distinguishing artifacts from alleles are still under development. We used NGS to evaluate the MHC diversity of over 300 captive and wild ring-tailed lemurs (Lemur catta: Primates: Mammalia). We modified a published workflow to address errors that arise from deep sequencing individuals and tested for evidence of selection at the most diverse MHC genes.

RESULTS

In addition to evaluating the accuracy of 454 Titanium and Ion Torrent PGM for genotyping large populations at hypervariable genes, we suggested modifications to improve current methods of allele calling. Using these modifications, we genotyped 302 out of 319 individuals, obtaining an average sequencing depth of over 1000 reads per amplicon. We identified 55 MHC-DRB alleles, 51 of which were previously undescribed, and provide the first sequences of five additional MHC genes: DOA, DOB, DPA, DQA, and DRA. The additional five MHC genes had one or two alleles each with little sequence variation; however, the 55 MHC-DRB alleles showed a high dN/dS ratio and trans-species polymorphism, indicating a history of positive selection. Because each individual possessed 1-7 MHC-DRB alleles, we suggest that ring-tailed lemurs have four, putatively functional, MHC-DRB copies.

CONCLUSIONS

In the future, accurate genotyping methods for NGS data will be critical to assessing genetic variation in non-model species. We recommend that future NGS studies increase the proportion of replicated samples, both within and across platforms, particularly for hypervariable genes like the MHC. Quantifying MHC diversity within non-model species is the first step to assessing the relationship of genetic diversity at functional loci to individual fitness and population viability. Owing to MHC-DRB diversity and copy number, ring-tailed lemurs may serve as an ideal model for estimating the interaction between genetic diversity, fitness, and environment, especially regarding endangered species.