Inbreeding and the evolution of sociality in arthropods

Mechanisms of dispersal and colonisation in a wind-borne cereal pest, the haplodiploid wheat curl mite

Dispersal and colonisation determine the survival and success of organisms, and influence the structure and dynamics of communities and ecosystems in space and time. Both affect the gene flow between populations, ensuring sufficient level of genetic variation and improving adaptation abilities. In haplodiploids, such as Aceria tosichella (wheat curl mite, WCM), a population may be founded even by a single unfertilised female, so there is a risk of heterozygosity loss (i.e. founder effect). It may lead to adverse outcomes, such as inbreeding depression. Yet, the strength of the founder effect partly depends on the genetic variation of the parental population. WCM is an economically important pest with a great invasive potential, but its dispersal and colonisation mechanisms were poorly studied before. Therefore, here we assessed WCM dispersal and colonisation potential in relation to the genetic variation of the parental population. We checked whether this potential may be linked to specific pre-dispersal actions (e.g. mating before dispersal and collective behaviour). Our study confirms that dispersal strategies of WCM are not dependent on heterozygosity in the parental population, and the efficient dispersal of this species depends on collective movement of fertilised females.

Nest-departure behaviour of gynes and drones in the invasive yellowjacket Vespula germanica (Hymenoptera: Vespidae)

Inbreeding costs can be high in haplodiploid hymenopterans due to their particular mechanism of sex determination (i.e., single-locus complementary sex-determination system, sl-CSD), as it can lead to the production of sterile males. Therefore, mechanisms contributing to reduced inbred matings can be beneficial. In this sense, asynchronous nest departure of sibling drones and gynes could reduce kin encounters in social hymenopterans. Using six observation colonies, we determined under field conditions the nest departure behaviour of sibling reproductives of the social wasp Vespula germanica (Hymenoptera: Vespidae). We determined that sexuals leave the nests definitively and detected asynchronous departure not fixed to a particular caste at a seasonal scale in some colonies, as gynes or drones delayed their departure as a function of the departure of the opposite sex, depending on the colony. At a higher temporal resolution (i.e., within a day), we discovered that drones consistently began to leave nests 1 h before gynes and this difference was driven by those individuals that left on the same day as did the opposite-sex kin. Even though other mechanisms such as polyandry and differential dispersal could also be important at reducing inbred matings in the species, the observed departure patterns (i.e., in some colonies actually leave together with the opposite caste, while in others temporal segregation seems to occur) from nests could be complementary to the former and be important at reducing the negative effects of inbreeding in this invasive species.

Drone aggregation behavior in the social wasp Vespula germanica (Hymenoptera: Vespidae): Effect of kinship and density

Inbreeding can have negative consequences on population viability because of the reduced fitness of the progeny. In general, most species have developed mechanisms to minimize inbreeding such as dispersal and kin avoidance behavior. In the eusocial Hymenoptera, related individuals typically share a common nest and have relatively short mating periods, this could lead to inbreeding, and because of their single-locus complementary sex determination system, it may generate diploid males that could result in infertile triploid progeny representing a cost for the colony. Vespula germanica, is an eusocial wasp that has invaded many parts of the world, despite likely facing a reduced genetic pool during the arrival phases. We ask whether male wasp display specific aggregation behavior that favors genetic diversity, key to reduce inbreeding. Through a set of laboratory experiments, we investigated the effects of drone nestmateship and density on the aggregation behavior of V. germanica drones. We show that drones avoid aggregating with their nestmates at all densities while non-nestmates are avoided only at high densities. This suggests that lek genetic diversity and density could be regulated through drone behavior and in the long run minimize inbreeding favoring invasion success.

Dispersal, endosymbiont abundance and fitness-related consequences of inbreeding and outbreeding in a social beetle

Most social species outbreed. However, some have persistent inbreeding with occasional outbreeding, and the decision of the individual regarding whether to stay in the natal group and inbreed or to disperse, with the potential to outbreed, is flexible and may depend on social, genetic and ecological benefits and costs. Few of these factors have been investigated experimentally in these systems. The beetle Coccotrypes dactyliperda Fabricius, 1801 (Scolytidae: Xyloborinae) lives in extended family colonies inside date seeds. The beetles inbreed, but some individuals disperse away from the natal seed and may outbreed. We investigated dispersal behaviour and assessed fitness-related measures in inbred and outbred offspring, in addition to the relative abundance of two endosymbionts. We predicted inbred offspring to have higher fitness-related measures and a reduced tendency to disperse than outbred offspring, owing to fitness benefits of cooperation within the colony, whereas increased endosymbiont abundance will promote dispersal of their hosts, thus enhancing their own spread in the population. Dispersing beetles were more active than ones that remained in the natal seed. As predicted, fewer inbred offspring dispersed than outbred offspring, but they matured and dispersed earlier. Fitness-related measures of inbred mothers were either lower (number of offspring) or not different (body mass) from those of outbred mothers. Inbred dispersers had greater amounts of Wolbachia, suggesting a role in dispersal. The results support the hypothesis that inbred females reduce dispersal and that early maturation and dispersal are likely to be benefits of increased cooperation in brood care.

Subsocial Cockroaches Nauphoeta cinerea Mate Indiscriminately with Kin Despite High Costs of Inbreeding

Many animals have evolved strategies to reduce risks of inbreeding and its deleterious effects on the progeny. In social arthropods, such as the eusocial ants and bees, inbreeding avoidance is typically achieved by the dispersal of breeders from their native colony. However studies in presocial insects suggest that kin discrimination during mate choice may be a more common mechanism in socially simpler species with no reproductive division of labour. Here we examined this possibility in the subsocial cockroach Nauphoeta cinerea, a model species for research in sexual selection, where males establish dominance hierarchies to access females and control breeding territories. When given a binary choice between a sibling male and a non-sibling male that had the opportunity to establish a hierarchy prior to the tests, females mated preferentially with the dominant male, irrespective of kinship or body size. Despite the lack of kin discrimination during mate choice, inbred-mated females incurred significant fitness costs, producing 20% less offspring than outbred-mated females. We discuss how the social mating system of this territorial cockroach may naturally limit the probability of siblings to encounter and reproduce, without the need for evolving active inbreeding avoidance mechanisms, such as kin recognition.

The Elaborate Postural Display of Courting Drosophila persimilis Flies Produces Substrate-Borne Vibratory Signals

Sexual selection has led to the evolution of extraordinary and elaborate male courtship behaviors across taxa, including mammals and birds, as well as some species of flies. Drosophila persimilis flies perform complex courtship behaviors found in most Drosophila species, which consist of visual, air-borne, gustatory and olfactory cues. In addition, Drosophila persimilis courting males also perform an elaborate postural display that is not found in most other Drosophila species. This postural display includes an upwards contortion of their abdomen, specialized movements of the head and forelegs, raising both wings into a "wing-posture" and, most remarkably, the males proffer the female a regurgitated droplet. Here, we use high-resolution imaging, laser vibrometry and air-borne acoustic recordings to analyse this postural display to ask which signals may promote copulation. Surprisingly, we find that no air-borne signals are generated during the display. We show, however, that the abdomen tremulates to generate substrate-borne vibratory signals, which correlate with the female's immobility before she feeds onto the droplet and accepts copulation.

Male meiosis in Crustacea: synapsis, recombination, epigenetics and fertility in Daphnia magna

We present the first detailed cytological study of male meiosis in Daphnia (Crustacea: Branchiopoda: Cladocera)—an aquatic microcrustacean with a cyclical parthenogenetic life cycle. Using immunostaining of the testes in Daphnia magna for baseline knowledge, we characterized the different stages of meiotic division and spermiogenesis in relation to the distribution of proteins involved in synapsis, early recombination events and sister chromatid cohesion. We also studied post-translational histone modifications in male spermatocytes, in relation to the dynamic chromatin progression of meiosis. Finally, we applied a DNA fragmentation test to measure sperm quality of D. magna, with respect to levels of inbreeding. As a proxy for fertility, this technique may be used to assess the reproductive health of a sentinel species of aquatic ecosystems. Daphnia proves to be a model species for comparative studies of meiosis that is poised to improve our understanding of the cytological basis of sexual and asexual reproduction.

Evidence for inbreeding depression and pre-copulatory, but not post copulatory inbreeding avoidance in the cabbage beetle Colaphellus bowringi

Inbreeding is known to have adverse effects on fitness-related traits in a range of insect species. A series of theoretical and experimental studies have suggested that polyandrous insects could avoid the cost of inbreeding via pre-copulatory mate choice and/or post-copulatory mechanisms. We looked for evidence of pre-copulatory inbreeding avoidance using female mate preference trials, in which females were given the choice of mating with either of two males, a sibling and a non-sibling. We also tested for evidence of post-copulatory inbreeding avoidance by conducting double mating experiments, in which four sibling females were mated with two males sequentially, either two siblings, two non-siblings or a sibling and a non-sibling in either order. We identified substantial inbreeding depression: offspring of females mated to full siblings had lower hatching success, slower development time from egg to adult, lower survival of larval and pupal stages, and lower adult body mass than the offspring of females mated to non-sibling males. We also found evidence of pre-copulatory inbreeding avoidance, as females preferred to mate with non-sibling males. However, we did not find any evidence of post-copulatory inbreeding avoidance: egg hatching success of females mating to both sibling and non-sibling males were consistent with sperm being used without bias in relation to mate relatedness. Our results suggest that this cabbage beetle has evolved a pre-copulatory mechanism to avoid matings between close relative, but that polyandry is apparently not an inbreeding avoidance mechanism in C. bowringi.