Isolated Random Forest Assisted Spatio-Temporal Ant Colony Evolutionary Algorithm for Cell Tracking in Time-Lapse Sequences.

Multi-Object tracking in real world environments is a tough problem, especially for cell morphogenesis with division. Most cell tracking methods are hard to achieve reliable mitosis detection, efficient inter-frame matching, and accurate state estimation simultaneously within a unified tracking framework. In this paper, we propose a novel unified framework that leverages a spatio-temporal ant colony evolutionary algorithm to track cells amidst mitosis under measurement uncertainty. Each Bernoulli ant colony representing a migrating cell is able to capture the occurrence of mitosis through the proposed Isolation Random Forest (IRF)-assisted temporal mitosis detection algorithm with the assumption that mitotic cells exhibit unique spatio-temporal features different from non-mitotic ones. Guided by prediction of a division event, multiple ant colonies evolve between consecutive frames according to an augmented assignment matrix solved by the extended Hungarian method. To handle dense cell populations, an efficient group partition between cells and measurements is exploited, which enables multiple assignment tasks to be executed in parallel with a reduction in matrix dimension. After inter-frame traversing, the ant colony transitions to a foraging stage in which it begins approximating the Bernoulli parameter to estimate cell state by iteratively updating its pheromone field. Experiments on multi-cell tracking in the presence of cell mitosis and morphological changes are conducted, and the results demonstrate that the proposed method outperforms state-of-the-art approaches, striking a balance between accuracy and computational efficiency.

Obligate chimerism in male yellow crazy ants.

Multicellular organisms typically develop from a single fertilized egg and therefore consist of clonal cells. We report an extraordinary reproductive system in the yellow crazy ant. Males are chimeras of haploid cells from two divergent lineages: R and W. R cells are overrepresented in the males' somatic tissues, whereas W cells are overrepresented in their sperm. Chimerism occurs when parental nuclei bypass syngamy and divide separately within the same egg. When syngamy takes place, the diploid offspring either develops into a queen when the oocyte is fertilized by an R sperm or into a worker when fertilized by a W sperm. This study reveals a mode of reproduction that may be associated with a conflict between lineages to preferentially enter the germ line.

First proteomic analysis of the role of lysine acetylation in extensive functions in Solenopsis invicta.

Lysine acetylation (Kac) plays a critical role in the regulation of many important cellular processes. However, little is known about Kac in Solenopsis invicta, which is among the 100 most dangerous invasive species in the world. Kac in S. invicta was evaluated for the first time in this study. Altogether, 2387 Kac sites were tested in 992 proteins. The prediction of subcellular localization indicated that most identified proteins were located in the cytoplasm, mitochondria, and nucleus. Venom allergen Sol i 2, Sol i 3, and Sol i 4 were found to be located in the extracellular. The enriched Kac site motifs included Kac H, Kac Y, Kac G, Kac F, Kac T, and Kac W. H, Y, F, and W frequently occurred at the +1 position, whereas G, Y, and T frequently occurred at the -1 position. In the cellular component, acetylated proteins were enriched in the cytoplasmic part, mitochondrial matrix, and cytosolic ribosome. Furthermore, 25 pathways were detected to have significant enrichment. Interestingly, arginine and proline metabolism, as well as phagosome, which are related to immunity, involved several Kac proteins. Sequence alignment analyses demonstrated that V-type proton ATPase subunit G, tubulin alpha chain, and arginine kinase, the acetylated lysine residues, were evolutionarily conserved among different ant species. In the investigation of the interaction network, diverse interactions were adjusted by Kac. The results indicated that Kac may play an important role in the sensitization, cellular energy metabolism, immune response, nerve signal transduction, and response to biotic and abiotic stress of S. invicta. It may be useful to confirm the functions of Kac target proteins for the design of specific and effective drugs to prevent and control this dangerous invasive species.

Origin and elaboration of a major evolutionary transition in individuality.

Obligate endosymbiosis, in which distantly related species integrate to form a single replicating individual, represents a major evolutionary transition in individuality1-3. Although such transitions are thought to increase biological complexity1,2,4-6, the evolutionary and developmental steps that lead to integration remain poorly understood. Here we show that obligate endosymbiosis between the bacteria Blochmannia and the hyperdiverse ant tribe Camponotini7-11 originated and also elaborated through radical alterations in embryonic development, as compared to other insects. The Hox genes Abdominal A (abdA) and Ultrabithorax (Ubx)-which, in arthropods, normally function to differentiate abdominal and thoracic segments after they form-were rewired to also regulate germline genes early in development. Consequently, the mRNAs and proteins of these Hox genes are expressed maternally and colocalize at a subcellular level with those of germline genes in the germplasm and three novel locations in the freshly laid egg. Blochmannia bacteria then selectively regulate these mRNAs and proteins to make each of these four locations functionally distinct, creating a system of coordinates in the embryo in which each location performs a different function to integrate Blochmannia into the Camponotini. Finally, we show that the capacity to localize mRNAs and proteins to new locations in the embryo evolved before obligate endosymbiosis and was subsequently co-opted by Blochmannia and Camponotini. This pre-existing molecular capacity converged with a pre-existing ecological mutualism12,13 to facilitate both the horizontal transfer10 and developmental integration of Blochmannia into Camponotini. Therefore, the convergence of pre-existing molecular capacities and ecological interactions-as well as the rewiring of highly conserved gene networks-may be a general feature that facilitates the origin and elaboration of major transitions in individuality.

Apoptosis as a primary defense mechanism in response to viral infection in invasive fire ant Solenopsis invicta.

Apoptosis is considered an innate defense mechanism of insect hosts at the early stage of pathogen infection. The present study attempts to determine whether apoptosis is involved in defending the fire ant, Solenopsis invicta from a natural viral pathogen Solenopsis invicta virus 1 (SINV-1). Results of TEM examination and TUNEL assay both revealed the signature of apoptosis in the midgut epithelium of SINV-1-infected fire ant larvae. A time-course study was conducted to monitor changes in the dynamics of SINV-1 viral titers and apoptosis levels in the midgut epithelium of SINV-1-infected larvae. We found that the viral titer significantly decreases as apoptosis level increases, suggesting that the apoptotic epithelium constitutes a barrier against dissemination of SINV-1. These findings serve as the very first empirical evidence for the virus-induced apoptosis in ants and also help explain some previously observed mortality patterns and behavioral alterations associated with SINV-1 in fire ants.

Morphology of the mandibular gland of the ant Paraponera clavata (Hymenoptera: Paraponerinae).

The ant Paraponera clavata (Fabricius, 1775) is the only extant species of Paraponerinae and is widely distributed in Brazilian forests. Aspects of its biology are documented extensively in the literature; however, knowledge of P. clavata internal morphology, specifically of exocrine glands, is restricted to the venom apparatus. The objective of this study was to describe the mandibular gland morphology of P. clavata workers. The mandibular gland is composed of a reservoir connected to a cluster of Type III secretory cells with cytoplasm rich in mitochondria and lipid droplets, similar to that of other ants. Notably, the glandular secretion is rich in protein and has a solid aspect. This is the first morphological description of the mandibular gland of P. clavata. RESEARCH HIGHLIGHTS: This study presents the morphological description of the mandibular gland of Paraponera clavata (Hymenoptera: Paraponerinae). Singular characteristics of the gland are described: the glandular secretion is rich in protein and has a solid aspect.

Interspecific comparison of mushroom body synaptic complexes of dimorphic workers in the ant genus Pheidole.

Social insects may have morphologically and behaviorally specialized workers that vary in requirements for sensory information processing, making them excellent systems to examine the relationship between brain structure and behavior. The density and size of synaptic complexes (microglomeruli, MG) in the mushroom bodies (MB) have served as proxies for processing ability and synaptic plasticity, and have been shown to vary among insect species that differ in behavioral complexity. To understand the relationship between behavioral specialization and synaptic structure, we examined age-related changes in MG density and size between minor worker and soldier subcastes in two species of Pheidole ants, P. dentata and P. morrisi, that differ in behavior. We hypothesized that task-diverse minor workers would have more densely packed MG than soldiers, and that species-specific differences in soldier repertories would be reflected in MG structure. We also examined MG variation in young and mature minor workers and soldiers, predicting that as workers age and develop behaviorally, MG would decrease in density in both subcastes due to synaptic pruning. Results support the hypothesis that MG density in the lip (olfactory) and collar (visual) regions of the MBs decrease with age in association with increases in bouton size in the lip. However, minors had significantly lower densities of MG in the lip than soldiers, suggesting MG may not show structural variation according to subcaste-related differences in cognitive demands in either species.

Three-dimensional visualization and a deep-learning model reveal complex fungal parasite networks in behaviorally manipulated ants.

Some microbes possess the ability to adaptively manipulate host behavior. To better understand how such microbial parasites control animal behavior, we examine the cell-level interactions between the species-specific fungal parasite Ophiocordyceps unilateralis sensu lato and its carpenter ant host (Camponotus castaneus) at a crucial moment in the parasite's lifecycle: when the manipulated host fixes itself permanently to a substrate by its mandibles. The fungus is known to secrete tissue-specific metabolites and cause changes in host gene expression as well as atrophy in the mandible muscles of its ant host, but it is unknown how the fungus coordinates these effects to manipulate its host's behavior. In this study, we combine techniques in serial block-face scanning-electron microscopy and deep-learning-based image segmentation algorithms to visualize the distribution, abundance, and interactions of this fungus inside the body of its manipulated host. Fungal cells were found throughout the host body but not in the brain, implying that behavioral control of the animal body by this microbe occurs peripherally. Additionally, fungal cells invaded host muscle fibers and joined together to form networks that encircled the muscles. These networks may represent a collective foraging behavior of this parasite, which may in turn facilitate host manipulation.

Cytogenetic studies on populations of Camponotus rufipes (Fabricius, 1775) and Camponotus renggeri Emery, 1894 (Formicidae: Formicinae).

Two valid ant species, Camponotus rufipes and Camponotus renggeri, have recently been the subject of a broad discussion with reference to taxa synonymization. Both species are quite common among the Neotropical myrmecofauna and share some unique traits, such as the shape of the scape and the pilosity patterns of the tibiae and scapes. A single morphological trait can help distinguish these species; however, only a combination of different approaches can enlighten our view of the complex phylogenetic relationships prevailing in the different populations of these two taxa. Therefore, focusing on the taxonomic issues concerning these two species, a cytogenetic survey including 10 populations of C. rufipes and two populations of C. renggeri was performed. In order to better understand the extent of the relationship between C. rufipes and C. renggeri, two common Neotropical Camponotus species, C. atriceps and C. cingulatus were taken as outgroups. All four species of Camponotus that were studied had 2n = 40 chromosomes (4sm+34st+2t); however, the abundance of chromosome rearrangements observed, combined with several chromosome markers, suggest that C. rufipes and C. renggeri are two good distinct species although closely related. The already reported chromosome translocation 2n = 39 (1m+4sm+32st+2t) for C. rufipes has been found in different populations as in the unprecedented chromosome inversions found both in C. rufipes and in C. renggeri populations. Within the C. renggeri chromosome inversions, both the heterozygous state 2n = 40 (1m+3sm+34st+2t) and the homozygous state, 2n = 40 (2m+2sm+34st+2t) were identified. However, only heterozygous specimens for chromosome inversions were found among C. rufipes, with karyotype configurations distinct from those found in C. renggeri, with 2n = 40 (1m+4sm+34st+2t). None of the populations studied showed signs of mosaic individuals. With respect to rDNA clusters, the 18S rDNA seemed to be more restricted inside the genome, as C. renggeri showed four 18S rDNA clusters, whereas, C. rufipes, C. atriceps, and C. cingulatus showed only two clusters. The chromosome locations of the 5S rDNA clusters were pointed for the first time in Formicidae, and showed itself to be more widely spread over the genome. By combining different chromosome banding approaches it was possible to demonstrate the crucial importance that chromosome inversions played on the karyotype evolution within these ants. The results also showed that chromosome translocations might be a consequence of the chromatin dynamic condition observed among Camponotus species. The homozygosis condition found in a C. renggeri from a Brazilian savanna population for chromosome inversions and the contrasting heterozygous condition for a different kind of chromosome inversion in C. rufipes from the Brazilian coastal rainforest, opens the window for a chromosome race hypothesis within the group C. renggeri and C. rufipes. The wide distribution, rich ecological interactions, genetic diversity, and morphological variability among C. renggeri and C. rufipes justify questioning of the actual taxonomic status of these species. The answer of this puzzle is clear when observing the number of 18S rDNA clusters of these ants, as C. rufipes has only two clusters whereas C. renggeri has four.

Compound eye and ocellar structure for walking and flying modes of locomotion in the Australian ant, Camponotus consobrinus.

Ants are unusual among insects in that individuals of the same species within a single colony have different modes of locomotion and tasks. We know from walking ants that vision plays a significant role in guiding this behaviour, but we know surprisingly little about the potential contribution of visual sensory structures for a flying mode of locomotion. Here we investigate the structure of the compound eye and ocelli in pedestrian workers, alate females and alate males of an Australian ant, Camponotus consobrinus, and discuss the trade-offs involved in optical sensitivity and spatial resolution. Male ants have more but smaller ommatidia and the smallest interommatidial angles, which is most likely an adaptation to visually track individual flying females. Both walking and flying forms of ants have a similar proportion of specialized receptors sensitive to polarized skylight, but the absolute number of these receptors varies, being greatest in males. Ocelli are present only in the flying forms. Each ocellus consists of a bipartite retina with a horizon-facing dorsal retina, which contains retinula cells with long rhabdoms, and a sky-facing ventral retina with shorter rhabdoms. We discuss the implications of these and their potential for sensing the pattern of polarized skylight.

Sperm traits differ between winged and wingless males of the ant Cardiocondyla obscurior.

Size and shape of sperm cells vary tremendously throughout the animal kingdom. The adaptive significance of this variation is not fully understood. In addition to sperm-female interactions and the environmental conditions, the risk of sperm competition might affect number, morphology and other "quality" traits of sperm. In the male-diphenic ant Cardiocondyla obscurior, winged sneaker males have limited sperm number, because their testes degenerate shortly after adult emergence, as is typical for males of social Hymenoptera. In contrast, wingless fighter males continuously replenish their sperm supply due to their exceptional lifelong spermatogenesis. While winged males usually have to compete with several other winged males for virgin queens, wingless males are able to monopolize queens by killing all other rivals. Hence, this presents a unique system to investigate how alternative reproductive tactics and associated physiology affect sperm morphology and viability. We found that sperm-limited males invest into sperm number instead of sperm size. Variance in sperm length is smaller in winged males, probably reflecting that they have to compete with several other males. Finally, sperm viability is equally high in both male phenotypes.

A bacterial filter protects and structures the gut microbiome of an insect.

Associations with symbionts within the gut lumen of hosts are particularly prone to disruption due to the constant influx of ingested food and non-symbiotic microbes, yet we know little about how partner fidelity is maintained. Here we describe for the first time the existence of a gut morphological filter capable of protecting an animal gut microbiome from disruption. The proventriculus, a valve located between the crop and midgut of insects, functions as a micro-pore filter in the Sonoran Desert turtle ant (Cephalotes rohweri), blocking the entry of bacteria and particles ⩾0.2 µm into the midgut and hindgut while allowing passage of dissolved nutrients. Initial establishment of symbiotic gut bacteria occurs within the first few hours after pupation via oral-rectal trophallaxis, before the proventricular filter develops. Cephalotes ants are remarkable for having maintained a consistent core gut microbiome over evolutionary time and this partner fidelity is likely enabled by the proventricular filtering mechanism. In addition, the structure and function of the cephalotine proventriculus offers a new perspective on organismal resistance to pathogenic microbes, structuring of gut microbial communities, and development and maintenance of host-microbe fidelity both during the animal life cycle and over evolutionary time.

The first cytogenetic data on Strumigenys louisianae Roger, 1863 (Formicidae: Myrmicinae: Dacetini): the lowest chromosome number in the Hymenoptera of the neotropical region.

In the present study, the first cytogenetic data was obtained for the ant species Strumigenys louisianae, from a genus possessing no previous cytogenetic data for the Neotropical region. The chromosome number observed was 2n = 4, all possessing metacentric morphology. Blocks rich in GC base pairs were observed in the interstitial region of the short arm of the largest chromosome pair, which may indicate that this region corresponds to the NORs. The referred species presented the lowest chromosome number observed for the subfamily Myrmicinae and for the Hymenoptera found in the Neotropical region. Observation of a low chromosome number karyotype has been described in Myrmecia croslandi, in which the occurrence of tandem fusions accounts for the most probable rearrangement for its formation. The accumulation of cytogenetic data may carry crucial information to ensure deeper understanding of the systematics of the tribe Dacetini.

The antennal sensory array of the nocturnal bull ant Myrmecia pyriformis.

Insects use antennal sensilla to not only detect chemical and mechanical cues but also to sense changes in temperature, humidity and CO(2) levels. Very little is known about the variation in numbers, size and structure of sensilla in ants. Here we describe in detail the array of sensilla on the apical segment of the antennae of the nocturnal Australian bull ant Myrmecia pyriformis. Using scanning electron microscopy techniques we identified eight types of sensilla: trichodea curvata, basiconica, trichodea, coelocapitular, chaetica, trichoid II, ampullacea and coeloconica. Mapping the spatial location of each sensillum revealed distinct distribution patterns for different types of sensilla which were consistent across different individuals. We found, in most cases, the number of sensilla increases with the size of the apical antennomere, which in turn increases with body size. Conversely, the size of sensilla did not appreciably increase with the size of the apical antennomere. We discuss the size, numbers and distribution of sensilla of M. pyriformis compared to other ant species. Lastly, given the inconsistent use of sensillum nomenclature and difficulties associated in reliable identification we have attempted to consolidate the ant sensilla literature to make possible interspecific comparisons.

Influences of pyriproxyfen on fecundity and reproduction of the Pharaoh ant (Hymenoptera: Formicidae).

The effects of the juvenile hormone analog pyriproxyfen (at concentrations of 0.1, 0.5, and 1%) on egg production, number of ovarioles, and length of oocytes were examined in queens of the Pharaoh ant Monomorium pharaonis (L.). Pyriproxyfen significantly reduced egg production in queens from week 3 onwards. Queens that were exposed to 1% pyriproxyfen stopped producing eggs at week 8. After 8 wk, ovaries were dissected from all queens, and the number of ovarioles and the length of the largest oocytes were recorded. The ovaries of queens in treated colonies were smaller than those in untreated queens, and the number of ovarioles in the ovaries was significantly lower in all pyriproxyfen-treated queens. Queens treated with the highest concentrations of pyriproxyfen tended to have significantly shorter oocytes than untreated queens. Histological studies of the ovaries revealed that pyriproxyfen caused vacuolation in the ovarioles, thickening of the tunica propria, development of small eggs, and underdevelopment of nurse cells and the follicular epithelium. Exposure to pyriproxyfen reduced egg production and induced severe morphological changes in the ovaries of queens, and the effects increased with increased concentration of pyriproxyfen.

Sperm bundles in the seminal vesicles of sexually mature Lasius ant males.

In many insects, sperm cells are produced in bundles with their heads being held together by a glycoprotein matrix secreted by a cyst cell. Mature sperm cells in the seminal vesicles are usually free, but in sawflies and several other insects, such structures (spermatodesmata) remain intact and sperm cells may be ejaculated as bundles. Here we report the occurrence of spermatodesmata in mature males of the ant Lasius pallitarsis. Microscopic investigations of the abdominal contents of males immediately prior to their nuptial flights showed that the anterior ends of numerous sperm cells were embedded in an oval-shaped 20 by 30 micrometer extracellular fibrous cap. Individual sperm ranged in length from 55 to 75 micrometers with an average overall length of 65 micrometers. The bulb-shaped heads of the sperm were relatively small, only about 1.5 micrometers in length and about 1.1 micrometers in diameter. The diameter of the sperm tails was approximately 1 micrometer. Observations of live preparations of the spermatodesmata showed increasingly active undulating wave-like movement of the sperm tails as the slide preparations aged. This appears to be the first case of sperm bundles being present in the seminal vesicles of mature ant males--males that are immediately poised to complete their nuptial mating flight.

Division of labor and structural plasticity in an extrinsic serotonergic mushroom body neuron in the ant Pheidole dentata.

Worker polyphenisms in ants enable insightful analyses of neuronal underpinnings of division of labor, a crucial aspect of animal social organization. In the ant Pheidole dentata, which has a dimorphic worker caste, serotonin titer increases in the brain with age, modulating pheromonal recruitment communication and foraging, behaviors characteristic of mature individuals. Serotonin-immunoreactive (5HT-IR) neurons are found in the mushroom bodies (MB) and may modulate multi-sensory information processing associated with cues and social signals guiding task performance. The volume of this neuropil correlates with worker subcaste and age in P. dentata, but the role of structural variation in individual extrinsic MB neurons in division of labor in ants is poorly understood. We tested the hypothesis that branching complexity in a 5HT-IR calyx input neuron (CIN) in the MBs increases with age in minor workers of P. dentata in association with task repertoire expansion. We further predicted that major workers, which are defense specialists, have less elaborate CIN axonal arbors at any age in comparison to minor workers, which are task generalists. Contrary to our predictions, immunohistochemical and morphometric analyses revealed significantly greater CIN branching in both newly eclosed and mature major workers, and identified an effect of worker age on branching complexity only in majors. Our results indicate a modulatory role of the CIN in subcaste-specific behaviors and suggest behavioral specialization may be associated with the elaboration of specific MB neurons.

Morphophysiological differences between the metapleural glands of fungus-growing and non-fungus-growing ants (Hymenoptera, Formicidae).

The metapleural gland is an organ exclusive to ants. Its main role is to produce secretions that inhibit the proliferation of different types of pathogens. The aim of the present study was to examine the morphophysiological differences between the metapleural gland of 3 non-fungus-growing ants of the tribes Ectatommini, Myrmicini, and Blepharidattini and that of 5 fungus-growing ants from 2 basal and 3 derived attine genera. The metapleural gland of the non-fungus-growing ants and the basal attine ants has fewer secretory cells than that of the derived attine ants (leaf-cutting ants). In addition, the metapleural gland of the latter had more clusters of secretory cells and sieve plates, indicating a greater storage capacity and demand for secretion in these more advanced farming ants. The glands of the derived attine ants also produced higher levels of polysaccharides and acidic lipids than those of Myrmicini, Blepharidattini, and basal attines. Our results confirm morphophysiological differences between the metapleural glands of the derived attines and those of the basal attines and non-fungus-growing ants, suggesting that the metapleural glands of the derived attines (leaf-cutting ants) are more developed in morphology and physiology, with enhanced secretion production (acidic lipids and protein) to protect against the proliferation of unwanted fungi and bacteria in the fungal garden, it is possible that leaf-cutting ants may have evolved more developed metapleural glands in response to stronger pressure from parasites.

Visual experience affects both behavioral and neuronal aspects in the individual life history of the desert ant Cataglyphis fortis.

The individual life history of the desert ant Cataglyphis fortis is characterized by a fast transition from interior tasks to mainly visually guided foraging. Previous studies revealed a remarkable structural synaptic plasticity in visual and olfactory input regions within the mushroom bodies of the ants' brain centers involved in learning and memory. Reorganization of synaptic complexes (microglomeruli) was shown to be triggered by sensory exposure rather than an internal program. Using video analyses at the natural nest site and activity recordings after artificial light treatments we investigated whether the ants get exposed to light before onset of foraging and whether this changes the ants' activity levels. We asked whether synaptic reorganization occurs in a similar time window by immunolabeling and quantification of pre- and postsynaptic compartments of visual and olfactory microglomeruli after periods of light-exposure. Ants reverted back to dark nest conditions were used to investigate whether synaptic reorganization is reversible. The behavior analyses revealed that late-interior ants (diggers) are exposed to light and perform exploration runs up to 2 days before they start foraging. This corresponds well with the result that artificial light treatment over more than 2-3 days significantly increased the ants' locomotor activities. At the neuronal level, visual exposure of more than 1 day was necessary to trigger reorganization of microglomeruli, and light-induced changes were only partly reversible in the dark. We conclude that visual preexposure is an important and flexible means to prepare the ants' visual pathway for orientation capabilities essential during foraging.

Polyandry reduces sperm length variation in social insects.

Postcopulatory sexual selection, either in the form of sperm competition or cryptic female choice, is an important selective force that is thought to have generated the enormous variation in sperm morphology observed interspecifically. However, the evolutionary significance of intraspecific variation in sperm morphology, and the role that postcopulatory sexual selection plays in influencing this variation, remains poorly investigated in invertebrates. Here, we tested the hypothesis that postcopulatory sexual selection reduces variation in sperm morphology, both between and within males, in 27 species of eusocial ants and bees. These eusocial species offer an unusual opportunity to assess how selection acts on variance in sperm morphology, as haploid males produce clonal, haploid sperm that does not experience haploid-diploid conflict. We provide solid evidence that males of polyandrous ant and bee species indeed produce less-variable sperm, indicating that sperm competition selected for sperm of superior quality. Our results offer a mechanistic explanation for the evolution of high-quality sperm and provide comprehensive evidence that sperm morphology of social insects is influenced by sexual selection.