Sexual selection as a side-effect of sexual conflict in the seaweed fly, Coelopa ursina (Diptera: Coelopidae)

Microbiome Dynamics and Functional Composition in Coelopa frigida (Diptera, Coelopidae): Insights into Trophic Specialization of Kelp Flies

Coelopidae (Diptera), known as kelp flies, exhibit an ecological association with beached kelp and other rotting seaweeds. This unique trophic specialization necessitates significant adaptations to overcome the limitations of an algal diet. We aimed to investigate whether the flies' microbiome could be one of these adaptive mechanisms. Our analysis focused on assessing composition and diversity of adult and larval microbiota of the kelp fly Coelopa frigida. Feeding habits of the larvae of this species have been subject of numerous studies, with debates whether they directly consume kelp or primarily feed on associated bacteria. By using a 16S rRNA metabarcoding approach, we found that the larval microbiota displayed considerably less diversity than adults, heavily dominated by only four operational taxonomic units (OTUs). Phylogenetic placement recovered the most dominant OTU of the larval microbiome, which is the source of more than half of all metabarcoding sequence reads, as an undescribed genus of Orbaceae (Gammaproteobacteria). Interestingly, this OTU is barely found among the 15 most abundant taxa of the adult microbiome, where it is responsible for less than 2% of the metabarcoding sequence reads. The other three OTUs dominating the larval microbiome have been assigned as Psychrobacter (Gammaproteobacteria), Wohlfahrtiimonas (Gammaproteobacteria), and Cetobacterium (Fusobacteriota). Moreover, we also uncovered a distinct shift in the functional composition between the larval and adult stages, where our taxonomic profiling suggests a significant decrease in functional diversity in larval samples. Our study offers insights into the microbiome dynamics and functional composition of Coelopa frigida.

Size variation, allometry and mating success in Aotearoa|New Zealand kelp flies (Coelopidae)

The need to respond quickly to the presence of an ephemeral resource required for breeding is often a feature of scramble competition mating systems. Scramble competition mating systems can feature extreme levels of sexual conflict and coercive mating by males. As a result, sexual selection can act on various traits used by males to overcome female resistance behaviours. Selection on these traits may result in significant intra and intersexual size variation and sexual dimorphism. Additionally, traits that influence mating success in males often show positive static allometry. Kelp flies (Coelopidae) are a small family of Diptera which specialise on wrack (beach cast marine macroalgae), a highly ephemeral resource. The mating system of these flies involves high levels of sexual conflict, with females rejecting all male mating attempts. In this study we describe intra and intersexual size variation and static allometry of traits in two of Aotearoa|New Zealand’s species, Coelopella curvipes and Chaetocoelopa littoralis. In addition, we investigate the mating behaviour of C. littoralis under ecologically relevant mating conditions. We found high levels of variation in both species with significant evidence of sexual dimorphism across all traits measured in C. littoralis, and in mid tibia length in C. curvipes. Furthermore, mid tibia length in both species exhibits positive static allometry and is disproportionally larger in larger males, suggesting that this trait in particular may be under strong sexual selection. We found that larger male C. littoralis which attempt to mate are significantly more likely to mate successfully demonstrating a large-size advantage in this species similar to findings across the Coelopidae. However, we only found a non-significant trend towards a mating advantage for males with longer mid-tibiae. We discuss these findings with reference to the population dynamics and ecology of these species.

A large chromosomal inversion shapes gene expression in seaweed flies (Coelopa frigida)

Inversions often underlie complex adaptive traits, but the genic targets inside them are largely unknown. Gene expression profiling provides a powerful way to link inversions with their phenotypic consequences. We examined the effects of the Cf-Inv(1) inversion in the seaweed fly Coelopa frigida on gene expression variation across sexes and life stages. Our analyses revealed that Cf-Inv(1) shapes global expression patterns, most likely via linked variation, but the extent of this effect is variable, with much stronger effects in adults than larvae. Furthermore, within adults, both common as well as sex-specific patterns were found. The vast majority of these differentially expressed genes mapped to Cf-Inv(1). However, genes that were differentially expressed in a single context (i.e., in males, females, or larvae) were more likely to be located outside of Cf-Inv(1). By combining our findings with genomic scans for environmentally associated SNPs, we were able to pinpoint candidate variants in the inversion that may underlie mechanistic pathways that determine phenotypes. Together the results of this study, combined with previous findings, support the notion that the polymorphic Cf-Inv(1) inversion in this species is a major factor shaping both coding and regulatory variation resulting in highly complex adaptive effects.

Mating behaviour, mate choice and female resistance in the bean flower thrips (Megalurothrips sjostedti)

Many species of thrips (Thysanoptera) in the family Thripidae form mating aggregations, but the adaptive significance of these aggregations and the extent of male and female mate choice is poorly understood. We studied the mating behaviour of the bean flower thrips Megalurothrips sjostedti (Trybom) (Thysanoptera: Thripidae), which forms male aggregations and occurs across sub-Saharan Africa. We tested whether males choose mates by female age or mating status. No-choice mating bioassays with one male and one female were used to simulate the way males usually encounter only one female at a time in aggregations in the field. Virgin females violently resisted mating attempts by males, but we found no compelling evidence to establish whether this was indiscriminate or was screening suitable males. Younger males (1–2 days old) did not discriminate females by age (1–2 or 7–10 days old), but older males (7–10 days old) avoided mating with older females. Any male choice by female mating status (virgin or mated) was weak or absent. The mating behaviour of M. sjostedti shows broad similarities with that of other thrips species that form aggregations, but also shows some distinct and novel differences, which can help our understanding of the adaptive significance of aggregations.

Females of a solitary bee reject males to collect food for offspring

The time dedicated to courtship and copulation is the most general cost of mating for females. However, quantitative estimates of this cost and the consequences for female mating behavior have been investigated for only a few model organisms and mostly under laboratory conditions. We determined the costs of copulations and persistent courtship by males in terms of time for females of the solitary bee Anthrenoides micans. We estimated the rate and duration of male mating behaviors and the consequences for sexual interactions for females with respect to the loss of foraging opportunity in the wild. Males invested most of their time searching for mates and intercepted foraging females every 3 min. Copulas lasted, on average, 10 times longer than the time females took to resist male mating attempts. Despite the high frequency of these rejections (82%), females spent 3-fold more time copulating than rejecting males. Considering the rate of encounters with males and the mean duration of flower visits by females, we estimated that females would perform 64% fewer flower visits per hour if they accepted all copulation attempts. The loss of time is especially significant in the natural habitat of the species, where host cacti blossom for extraordinary short periods of time and females compete with other cacti-specialized bees and conspecifics. Because the offspring production of a female solitary bee depends on its pollen collection capacity, reduced foraging performance directly influences female reproductive success.

Artificial selection on sexual aggression: Correlated traits and possible trade-offs

Forced copulation is an extreme form of sexual aggression that can affect the evolution of sex-specific anatomy, morphology, and behavior. To characterize mechanistic and evolutionary aspects of forced copulation, we artificially selected male fruit flies based on their ability to succeed in the naturally prevalent behavior of forced matings with newly eclosed (teneral) females. The low and high forced copulation lineages showed rapid divergence, with the high lineages ultimately showing twice the rates of forced copulation as the low lineages. While males from the high lineages spent more time aggressively pursuing and mounting teneral females, their behavior toward non-teneral and heterospecific females was similar to that of males from the low lineages. Males from the low and high lineages also showed similar levels of male-male aggression. This suggests little or no genetic correlations between sexual aggression and non-aggressive pursuit of females, and between male aggression toward females and males. Surprisingly however, males from the high lineages had twice as high mating success than males from the low lineages when allowed to compete for consensual mating with mature females. In further experiments, we found no evidence for trade-offs associated with high forced mating rates: males from the high lineages did not have lower longevity than males from the low lineages when housed with females, and four generations of relaxed selection did not lead to convergence in forced mating rates. Our data indicate complex interactions among forced copulation success and consensual mating behavior, which we hope to clarify in future genomic work.

Exaggerated displays do not improve mounting success in male seaweed flies Fucellia tergina (Diptera: Anthomyiidae)

Signals of individual quality are assumed to be difficult to exaggerate, either because they are directly linked to underlying traits (indices) or because they are costly to perform (handicaps). In practise advertisement displays may consist of conventional and costly components, for instance where a morphological structure related to body size is used in visual displays. In this case, there is the potential for dishonest displays, due to the population level variance around the relationship between body size and display structures. We examine the use of wing flicking displays that we observed in situ in a strandline dwelling seaweed fly Fucellia tergina, using overall body size and the size of their eyes as underlying indicators of condition. Males displayed far more frequently than females, and were also observed to frequently mount other flies, a behaviour that was rare in females. The rate of display was greater for males that had positive residual values from relationships between wing length and body length. In other words those males with larger than expected wings for their underlying quality displayed more frequently, indicating that these displays are open to exaggeration. Males with larger than expected wings (for the size of their body or eyes), however, mounted less frequently. We suggest that small bodied males are less successful in terms of mounting, but that those small males with relatively large wings may attempt to compensate for this through increased display effort.

Male-specific genotype by environment interactions influence viability selection acting on a sexually selected inversion system in the seaweed fly, Coelopa frigida

In the seaweed fly, Coelopa frigida, a large chromosomal inversion system is affected by sexual selection and viability selection. However, our understanding of the interaction between these two selective forces is currently limited as research has focused upon a limited range of environments. We allowed C. frigida larvae to develop in two different algae, Fucus and Laminaria, and then measured viability and body size for each inversion genotype. Significant male-specific genotype-by-environment interactions influenced viability and body size. For males developing in Laminaria, the direction of viability selection acts similarly on the inversion system as the direction of sexual selection. In contrast, for males developing in Fucus, viability selection opposes sexual selection. These results demonstrate that through considering viability selection in different environments, the costs and benefits associated with sexual selection can be found to vary.

The role of sexual selection and conflict in mediating among-population variation in mating strategies and sexually dimorphic traits in Sepsis punctum

The black scavenger fly Sepsis punctum exhibits striking among-population variation in the direction and magnitude of sexual size dimorphism, modification to the male forelimb and pre-copulatory behaviour. In some populations, male-biased sexual size dimorphism is observed; in other, less dimorphic, populations males court prior to mating. Such variation in reproductive traits is of interest to evolutionary biologists because it has the potential to limit gene flow among populations, contributing to speciation. Here, we investigate whether large male body size and modified forefemur are associated with higher male mating success within populations, whether these traits are associated with higher mating success among populations, and if these traits carry viability costs that could constrain their response to sexual selection. Flies from five distinct populations were reared at high or low food, generating high and low quality males. The expression of body size, forelimb morphology and courtship rate were each greater at high food, but high food males experienced higher mating success or reduced latency to first copulation in only one of the populations. Among populations, overall mating success increased with the degree of male-bias in overall body size and forelimb modification, suggesting that these traits have evolved as a means of increasing male mating rate. The increased mating success observed in large-male populations raises the question of why variation in magnitude of dimorphism persists among populations. One reason may be that costs of producing a large size constrain the evolution of ever-larger males. We found no evidence that juvenile mortality under food stress was greater for large-male populations, but development time was considerably longer and may represent an important constraint in an ephemeral and competitive growth environment.

Control of copula duration and sperm storage by female Queensland fruit flies

Copula duration and sperm storage patterns can directly or indirectly affect fitness of male and female insects. Although both sexes have an interest in the outcome, research has tended to focus on males. To investigate female influences, we compared copula duration and sperm storage of Queensland fruit fly females that were intact, or had been incapacitated through decapitation or abdomen isolation. We found that copulations were far longer when females had been incapacitated, indicating that constraints imposed on copula duration by intact females had been relaxed. Repeatability of copula duration for males was very low regardless of female treatment, and this is also consistent with strong female influence. Number of sperm in the spermathecae was not influenced by female treatment, suggesting that female abdominal ganglia control the transport of sperm to these long-term storage organs. However, more sperm were found in the ventral receptacles of incapacitated females compared to intact females. Overall, results implicate cephalic ganglia in regulation of copula duration and short-term sperm storage in the ventral receptacle and abdominal ganglia in regulation of long-term sperm storage in the spermathecae.

Evolution of genitalia: theories, evidence, and new directions

Many hypotheses have been proposed to explain why male intromittent genitalia consistently tend to diverge more rapidly than other body traits of the same individuals in a wide range of animal taxa. Currently the two most popular involve sexual selection: sexually antagonistic coevolution (SAC) and cryptic female choice (CFC). A review of the most extensive attempts to discriminate between these two hypotheses indicates that SAC is not likely to have played a major role in explaining this pattern of genital evolution. Promising lines for future, more direct tests of CFC include experimental modification of male genital form and female sensory abilities, analysis of possible male-female dialogues during copulation, and direct observations of genital behavior.

The effect of an inversion system and the time interval between matings on postcopulatory sexual selection in the seaweed fly, Coelopa frigida

The seaweed fly, Coelopa frigida, exhibits LMSP. A large chromosomal inversion system affects many traits including egg-to-adult viability via heterosis. Consequently, there is also considerable potential for cryptic female mate choice to operate on the basis of sperm karyotype. Here, we investigated the effect of time interval and chromosomal inversion karyotype on postcopulatory sexual selection. Homokaryotypic females were mated with a male of the same and a male of the opposite homokaryotype. The order of the matings was varied so cryptic female mate choice could operate either in concert or antagonistically with LMSP. LMSP was found when there was a 24 h time interval between matings, irrespective of the order in which the males were mated. However, when the males were mated in quick succession the order of mating was important. When LMSP and cryptic female mate choice work in concert a high level of LMSP was found. However, when the male of opposite homokaryotype mated first, then first male sperm precedence was observed. This suggests that polyandrous females might be able to bias paternity but only when matings occur in quick succession. Consequently, population density is likely to affect the operation of postcopulatory sexual selection.

Sexually antagonistic coevolution in a mating system: combining experimental and comparative approaches to address evolutionary processes

We combined experimental and comparative techniques to study the evolution of mating behaviors within in a clade of 15 water striders (Gerris spp.). Superfluous multiple mating is costly to females in this group, and consequently there is overt conflict between the sexes over mating. Two alternative hypotheses that could generate interspecific variation in mating behaviors are tested: interspecific variation in optimal female mating rate versus sexually antagonistic coevolution of persistence and resistance traits. These potentially coevolving traits include male grasping and female antigrasping structures that further the interests of one sex over the other during premating struggles. Both processes are known to play a role in observed behavioral variation within species. We used two large sets of experiments to quantify behavioral differences among species, as well as their response to an environmentally (sex-ratio) induced change in optimal female mating rate. Our analysis revealed a large degree of continuous interspecific variation in all 20 quantified behavioral variables. Nevertheless, species shared the same set of behaviors, and each responded in a qualitatively similar fashion to sex-ratio alterations. A remarkably large proportion (> 50%) of all interspecific variation in the magnitude of behaviors, including their response to sex ratio, could be captured by a single multivariate axis. These data suggest tight coevolution of behaviors within a shared mating system. The pattern of correlated evolution was best accounted for by antagonistic coevolution in the relative abilities of each sex to control the outcome of premating struggles. In species where males have a relative advantage, mating activity is high, and the opposite is found in species where females have gained a relative advantage. Our analyses also suggested that evolution has been unconstrained by history, with no consistent evolutionary tendency toward or away from male or female relative advantage.

Male mating preference for female survivorship in the seaweed fly Gluma musgravei (Diptera: Coelopidae)

The seaweed fly mating system is characterized by pre-mating struggles during which females exhibit a mate rejection response involving kicking, shaking and abdominal curling. Males must resist rejection until females become passive and allow copulation to take place. However, despite the vigorous nature of the struggle males frequently dismount passive females without attempting copulation. Here we show that rejected females suffered higher post-encounter mortality rates than those accepted by males in the seaweed fly Gluma musgravei. Furthermore, we show that males also preferentially mounted females with higher future longevity. We propose that this male mate choice for female survivorship has evolved as a result of females often having to survive for long periods after mating until suitable oviposition sites become available. Such male preferences for female survivorship may be common in species in which oviposition must sometimes be substantially delayed after mating.

Female mate choice for large males in several species of seaweed fly (Diptera: Coelopidae)

The mating system of the seaweed fly Coelopa frigida involves a vigorous premating struggle during which females attempt to dislodge mounted males by kicking and shaking from side to side. Additionally females can prevent engagement of genitalia by curling their abdomens downwards. Large males gain a mating advantage. Male size is partially determined by a chromosomal inversion polymorphism which is maintained by strong heterosis. Thus female mate choice on the basis of size will affect offspring fitness. We report the occurrence of premating struggles and mate choice for large males in five additional species of seaweed fly, namely, C. nebularum, C. vanduzeei, C. pilipes, Gluma musgravei and G. nitida. Four of these species appear to lack the inversion system, suggesting that mate choice for large males can be maintained in its absence and also evolved before its establishment. Gluma females had stronger preferences than Coelopa females and showed an additional response to mounting, namely, curling their abdomens upwards into the male. This may allow assessment of male size. Copyright 2000 The Association for the Study of Animal Behaviour.

Female mating behaviour, sexual selection and chromosome I inversion karyotype in the seaweed fly, coelopa frigida

Previous studies of the seaweed fly, Coelopa frigida, have revealed the operation of several different forces of sexual selection. The overall pattern of mate choice seen in natural populations is not consistent with the predictions of indirect sexual selection as females do not express preferences that maximize the fitness of their offspring, even though the benefits from such choice are relatively large in this species. Thus, the maintenance of female mate choice for large male size must instead either be a result of a direct benefit to the female of mating with large males, or a side-effect of the evolution of another character, in other words pleiotropy. In order to separate these two alternatives the genetical basis of female mating behaviour needs to be studied. Previous studies have revealed associations between chromosomal inversion karyotype and both general female willingness to mate and mate choice for large male size, however these associations were lost after several generations of laboratory culture. Here several isokaryotypic lines from wild collections of flies were derived. The willingness to mate and mate choice of females from each line were determined. Pairs of lines of opposite inversion karyotype that significantly differed in either or both willingness to mate and mate choice were crossed. The mating behaviour and inversion karyotype of the F2 progeny (all F1 progeny are heterokaryotypes) were determined. Clear differences in the general levels of female willingness to mate were found between the two inversion homokaryotypes in several families, whereas variation in the strength of female choice for large male size were not revealed. It is suggested that mate choice in this species occurs as a pleiotropic effect of selection acting on female willingness to mate.