Cytoplasmic Incompatibility and Bacterial Density in Nasonia Vitripennis

Cytoplasmically (maternally) inherited bacteria that cause reproductive incompatibility between strains are widespread among insects. In the parasitoid wasp Nasonia, incompatibility results in improper condensation and fragmentation of the paternal chromosomes in fertilized eggs. Some form of genome imprinting may be involved. Because of haplodiploidy, incompatibility results in conversion of (diploid) female eggs into (haploid) males. Experiments show that bacterial density is correlated with compatibility differences between male and female Nasonia. Males from strains with high bacterial numbers are incompatible with females from strains with lower numbers. Temporal changes in compatibility of females after tetracycline treatment are generally correlated with decreases in bacterial levels in eggs. However, complete loss of bacteria in mature eggs precedes conversion of eggs to the ``asymbiont' compatibility type by 3-4 days. This result is consistent with a critical ``imprinting' period during egg maturation, when cytoplasmic bacteria determine compatibility. Consequent inheritance of reduced bacterial numbers in F(1) progeny has different effects on compatibility type of subsequent male vs. female progeny. In some cases, partial incompatibility occurs which results in reduced offspring numbers, apparently due to incomplete paternal chromosome elimination resulting in aneuploidy.     

Can maternally transmitted endosymbionts facilitate the evolution of haplodiploidy?

Whilst many invertebrate taxa are haplodiploid, the factors underlying the evolution of haplodiploidy remain unresolved. We investigate theoretically whether haplodiploidy might evolve as an outcome of the co-evolution between maternally inherited endosymbionts and their hosts. First, we substantially extend a recently developed model that involves maternally inherited endosymbionts that kill male offspring by eliminating the paternal genome. We also put forward a new hypothesis and develop a model that involves bacteria that induce cytoplasmic incompatibility (CI). Based on these models, we explore the co-evolutionary events that might occur between hosts and symbionts. We find that both with male-killers and CI-inducing endosymbionts, the hosts are likely to develop increased viability of haploid males, which can be considered a preadaptation to haplodiploidy. In addition, populations with haploidizing male-killers can in some cases evolve directly towards a genetic system of paternal genome elimination, a special form of haplodiploidy. These results are combined with consideration of mechanism and ecology to appraise the likelihood of male-killers and CI inducing bacteria being involved in the evolution of haplodiploidy.     

The role of endosymbionts in the evolution of haploid-male genetic systems in scale insects (Coccoidea)

There is an extraordinary diversity in genetic systems across species, but this variation remains poorly understood. In part, this is because the mechanisms responsible for transitions between systems are often unknown. A recent hypothesis has suggested that conflict between hosts and endosymbiotic microorganisms over transmission could drive the transition from diplodiploidy to systems with male haploidy (haplodiploidy, including arrhenotoky and paternal genome elimination [PGE]). Here, we present the first formal test of this idea with a comparative analysis across scale insects (Hemiptera: Coccoidea). Scale insects are renowned for their large variation in genetic systems, and multiple transitions between diplodiploidy and haplodiploidy have taken place within this group. Additionally, most species rely on endosymbiotic microorganisms to provide them with essential nutrients lacking in their diet. We show that species harboring endosymbionts are indeed more likely to have a genetic system with male haploidy, which supports the hypothesis that endosymbionts might have played a role in the transition to haplodiploidy. We also extend our analysis to consider the relationship between endosymbiont presence and transitions to parthenogenesis. Although in scale insects there is no such overall association, species harboring eukaryote endosymbionts were more likely to be parthenogenetic than those with bacterial symbionts. These results support the idea that intergenomic conflict can drive the evolution of novel genetic systems and affect host reproduction.     

Perspective: maternal kin groups and the origins of asymmetric genetic systems-genomic imprinting, haplodiploidy, and parthenogenesis

The genetic systems of animals and plants are typically eumendelian. That is, an equal complement of autosomes is inherited from each of two parents, and at each locus, each parent's allele is equally likely to be expressed and equally likely to be transmitted. Genetic systems that violate any of these eumendelian symmetries are termed asymmetric and include parent-specific gene expression (PSGE), haplodiploidy, thelytoky, and related systems. Asymmetric genetic systems typically arise in lineages with close associations between kin (gregarious siblings, brooding, or viviparity). To date, different explanatory frameworks have been proposed to account for each of the different asymmetric genetic systems. Haig's kinship theory of genomic imprinting argues that PSGE arises when kinship asymmetries between interacting kin create conflicts between maternally and paternally derived alleles. Greater maternal than paternal relatedness within groups selects for more "abstemious" expression of maternally derived alleles and more "greedy" expression of paternally derived alleles. Here, I argue that this process may also underlie origins of haplodiploidy and many origins of thelytoky. The tendency for paternal alleles to be more "greedy" in maternal kin groups means that maternal-paternal conflict is not a zero-sum game: the maternal optimum will more closely correspond to the optimum for family groups and demes and for associated entities such as symbionts. Often in these circumstances, partial or complete suppression of paternal gene expression will evolve (haplodiploidy, thelytoky), or other features of the life cycle will evolve to minimize the conflict (monogamy, inbreeding). Maternally transmitted cytoplasmic elements and maternally imprinted nuclear alleles have a shared interest in minimizing agonistic interactions between female siblings and may cooperate to exclude the paternal genome. Eusociality is the most dramatic expression of the conflict-reducing effects of haplodiploidy, but its original and more widespread function may be suppression of intrafamilial cannibalism. In rare circumstances in which paternal gene products gain access to maternal physiology via a placenta, PSGE with greedy paternal gene expression can persist (e.g., in mammals).     

Biparental inheritance of RAPD markers in males of the pseudo-arrhenotokous mite Typhlodromus pyri.

In pseudo-arrhenotokous mites, haploid males develop from fertilized eggs that undergo paternal genome loss (PGL) during early embryogenesis. We present evidence that some of the paternal genome may be retained in males of the predatory mite Typhlodromus pyri Scheuten (Acari: Phytoseiidae). Two reproductively compatible populations were differentiated by two random amplified polymorphic DNA markers and the inheritance pattern in the offspring was analysed. Maternal transmission rates are variable and independent of the sex of the offspring and of the marker. These data suggest a nuclear origin and independent segregation of the markers. One marker (330 base pairs (bp)) was paternally transmitted to male as well as female offspring, the other (990 bp) was paternally transmitted to all females and some of the male offspring. We propose that the paternal set of inactivated chromosomes may be partially retained in some tissues of the haploid males or, alternatively, that a B chromosome does not follow the process of PGL in male embryos, thereby segregating with the maternal set. The possible mechanisms controlling the condensation and the segregation of the chromosome(s) retained are discussed on the basis of current hypotheses on chromosome inactivation in insects.     

Phylogenetic congruence of armored scale insects (Hemiptera: Diaspididae) and their primary endosymbionts from the phylum Bacteroidetes

Insects in the sap-sucking hemipteran suborder Sternorrhyncha typically harbor maternally transmitted bacteria housed in a specialized organ, the bacteriome. In three of the four superfamilies of Sternorrhyncha (Aphidoidea, Aleyrodoidea, Psylloidea), the bacteriome-associated (primary) bacterial lineage is from the class Gammaproteobacteria (phylum Proteobacteria). The fourth superfamily, Coccoidea (scale insects), has a diverse array of bacterial endosymbionts whose affinities are largely unexplored. We have amplified fragments of two bacterial ribosomal genes from each of 68 species of armored scale insects (Diaspididae). In spite of initially using primers designed for Gammaproteobacteria, we consistently amplified sequences from a different bacterial phylum: Bacteroidetes. We use these sequences (16S and 23S, 2105 total base pairs), along with previously published sequences from the armored scale hosts (elongation factor 1alpha and 28S rDNA) to investigate phylogenetic congruence between the two clades. The Bayesian tree for the bacteria is roughly congruent with that of the hosts, with 67% of nodes identical. Partition homogeneity tests found no significant difference between the host and bacterial data sets. Of thirteen Shimodaira-Hasegawa tests, comparing the original Bayesian bacterial tree to bacterial trees with incongruent clades forced to match the host tree, 12 found no significant difference. A significant difference in topology was found only when the entire host tree was compared with the entire bacterial tree. For the bacterial data set, the treelengths of the most parsimonious host trees are only 1.8-2.4% longer than that of the most parsimonious bacterial trees. The high level of congruence between the topologies indicates that these Bacteroidetes are the primary endosymbionts of armored scale insects. To investigate the phylogenetic affinities of these endosymbionts, we aligned some of their 16S rDNA sequences with other known Bacteroidetes endosymbionts and with other similar sequences identified by BLAST searches. Although the endosymbionts of armored scales are only distantly related to the endosymbionts of the other sternorrhynchan insects, they are closely related to bacteria associated with eriococcid and margarodid scale insects, to cockroach and auchenorrynchan endosymbionts (Blattabacterium and Sulcia), and to male-killing endosymbionts of ladybird beetles. We propose the name "Candidatus Uzinura diaspidicola" for the primary endosymbionts of armored scale insects.     

The evolution of haplodiploidy by male‐killing endosymbionts: importance of population structure and endosymbiont mutualisms

Haplodiploid inheritance systems, characterized by male transmission of only their maternally inherited genomic elements, have evolved more than 20 times within the animal kingdom. A number of theoretical studies have argued that infection with certain male‐killing endosymbionts can potentially lead to the evolution of haplodiploidy. By explicitly investigating the coevolutionary dynamics between host and endosymbiont, we show that the assumptions of current models cannot explain the evolution of haplodiploidy very well, as the endosymbiont will often go extinct in the long term. Here, we provide two additional mechanisms that can explain the stable evolution of haplodiploidy by male‐killing endosymbionts. First of all, a spatially structured population can facilitate the long‐term persistence of haplodiploidy, but this applies only when levels of inbreeding are very high. By contrast, endosymbionts that are mutualistic with their hosts provide a much more general and promising route to the stable evolution of haplodiploidy. This model is the first to provide a formal explanation of the supposed association between the evolution of haplodiploidy and the highly inbred lifestyles of some ancestors, while it also provides a hypothesis for the evolution of haplodiploidy in more outbred ancestors.     

Factors Affecting the Strength of Cardinium-Induced Cytoplasmic Incompatibility in the Parasitic Wasp Encarsia pergandiella (Hymenoptera: Aphelinidae)

Bacteria that cause cytoplasmic incompatibility (CI) are among the most common maternally transmitted parasites of insects. In CI, uninfected females produce few or no offspring when they mate with infected males and, as a result, are often at a reproductive disadvantage relative to infected females. Two different bacteria are known to cause CI, Wolbachia and Cardinium. CI Cardinium was discovered more recently and has been little studied. Here, factors that could influence the reduction in reproductive output in a CI cross, or CI “strength,” were explored in the parasitic wasp Encarsia pergandiella. Cardinium in this wasp exhibits variable CI strength. Experiments tested the effect of male age, male size, male host species, Cardinium density, and male development time on CI strength. We found a striking effect of male development time, with males that took longer to develop exhibiting stronger CI when mated to uninfected females. Male age had little effect; although in one experiment, the oldest males exhibited stronger CI. Male size, host species, and bacterial density had no effect on the strength of CI. Identifying the factors that control CI are crucial for understanding the dynamics of infection, as well as the success of strategies that aim to use CI microbes to control insect pests and disease vectors.     

Sex-Limited Mitochondrial DNA Transmission in the Marine Mussel Mytilus Edulis

Mitochondrial DNA (mtDNA) was thought to be inherited maternally in animals, although paternal leakage has been reported in mice and Drosophila. Recently, direct evidence of extensive paternal inheritance of mtDNA has been found in the marine mussel Mytilus. We give evidence that whereas female mussels are homoplasmic for a genome that is transmitted to eggs, male mussels are heteroplasmic for this genome and for a second genome that is transmitted preferentially to sperm. The results provide support for the existence of separate male and female routes of mtDNA inheritance in mussels. The two genomes show a base sequence divergence exceeding 20% at three protein coding genes, consistent with long term maintenance of the heteroplasmic state. We propose that the two genomes differ in fitness in males and females, possibly as a result of interaction with nuclear genes.     

Female‐dependent transmission of paternal mtDNA is a shared feature of bivalve species with doubly uniparental inheritance (DUI) of mitochondrial DNA

Several species from a number of bivalve molluscan families are known to have a paternally transmitted mitochondrial genome, along with the standard maternally transmitted one. The main characteristic of the phenomenon, known as doubly uniparental inheritance (DUI), is the coupling of sex and mtDNA inheritance: males receive both genomes but transmit only the paternal to their progeny; females either do not have the paternal genome or, if they do, they do not transmit it to their progeny. In the families Mytilidae and Veneridae, both of which have DUI, a female individual is either female‐biased (it produces only, or nearly so, female progeny), male‐biased (it produces mainly male progeny) or non‐biased (it produces both genders in intermediate frequencies). Here we present evidence for a same pattern in the freshwater mussel, Unio delphinus (Unionidae). These results suggest that the maternal control of whether a fertilized egg will develop into a male or a female individual (and the associated feature of whether it will inherited or not inherit the paternal mtDNA) is a general characteristic of species with DUI.     

The life cycle of Micromalthus debilisLeConte (1878) (Coleoptera: Archostemata: Micromalthidae): historical review and evolutionary perspective

Micromalthus debilis 33 ), has one of the most bizarre life cycles of any metazoan. Reproduction is typically by thelytokous, viviparous, larviform females, but there is also a rare arrhenotokous phase. The active first‐instar (triungulin) larva develops into a legless, feeding (cerambycoid) larva. This form either pupates, leading to a diploid adult female, or develops into any of three subsequent types of reproductive paedogenetic forms: (1) a thelytokous female that produces triungulins via viviparity; (2) an arrhenotokous female that produces a single egg that develops into the short‐legged (curculionoid) larva, eventually devouring its mother and becoming a haploid adult male; or (3) an amphitokous female that can follow either of the two above reproductive pathways. We speculate that Micromalthus is dependent on maternally transmitted bacteria for the ability to digest rotting wood, and that these bacteria are senescent in males, causing males to be obligately cannibalistic. Obligate male cannibalism, in turn, would have dramatically increased the cost of males, and have created a strong selective advantage for cyclic thelytoky and the other features of the Micromalthus life cycle that minimize the role of the male.     

The evolutionary dynamics of haplodiploidy: Genome architecture and haploid viability

Haplodiploid reproduction, in which males are haploid and females are diploid, is widespread among animals, yet we understand little about the forces responsible for its evolution. The current theory is that haplodiploidy has evolved through genetic conflicts, as it provides a transmission advantage to mothers. Male viability is thought to be a major limiting factor; diploid individuals tend to harbor many recessive lethal mutations. This theory predicts that the evolution of haplodiploidy is more likely in male heterogametic lineages with few chromosomes, as genes on the X chromosome are often expressed in a haploid environment, and the fewer the chromosome number, the greater the proportion of the total genome that is X‐linked. We test this prediction with comparative phylogenetic analyses of mites, among which haplodiploidy has evolved repeatedly. We recover a negative correlation between chromosome number and haplodiploidy, find evidence that low chromosome number evolved prior to haplodiploidy, and that it is unlikely that diplodiploidy has reevolved from haplodiploid lineages of mites. These results are consistent with the predicted importance of haploid male viability.     

The evolution of unusual chromosomal systems in coccoids: extraordinary sex ratios revisited

Coccoids (scale insects) exhibit a wide variety of chromosomal systems. In many species, paternal chromosomes are eliminated from the male germline such that all of a male's sperm transmit an identical set of maternal chromosomes. In such species, an offspring's sex is determined by whether or not paternal chromosomes are inactivated in the egg's cytoplasm after fertilization. This paper presents a model of the evolution of paternal genome loss in coccoids from an ancestral system of XX-XO sex determination. The model is based on Hamilton's (1967) theory that different genetic elements within the genome have different unbeatable sex ratios. In this model (1) meiotic drive by the X chromosome in XO males causes female-biased sex ratios; (2) the maternal set of autosomes in males evolves effective sex linkage to exploit X-drive; and (3) genes expressed in mothers are selected to convert some of their XX daughters into sons. A similar model may explain the evolution of haplodiploidy.     

Male Killing Spiroplasma Preferentially Disrupts Neural Development in the Drosophila melanogaster Embryo

Male killing bacteria such as Spiroplasma are widespread pathogens of numerous arthropods including Drosophila melanogaster. These maternally transmitted bacteria can bias host sex ratios toward the female sex in order to ‘selfishly’ enhance bacterial transmission. However, little is known about the specific means by which these pathogens disrupt host development in order to kill males. Here we show that a male-killing Spiroplasma strain severely disrupts nervous tissue development in male but not female D. melanogaster embryos. The neuroblasts, or neuron progenitors, form properly and their daughter cells differentiate into neurons of the ventral nerve chord. However, the neurons fail to pack together properly and they produce highly abnormal axons. In contrast, non-neural tissue, such as mesoderm, and body segmentation appear normal during this time, although the entire male embryo becomes highly abnormal during later stages. Finally, we found that Spiroplasma is altogether absent from the neural tissue but localizes within the gut and the epithelium immediately surrounding the neural tissue, suggesting that the bacterium secretes a toxin that affects neural tissue development across tissue boundaries. Together these findings demonstrate the unique ability of this insect pathogen to preferentially affect development of a specific embryonic tissue to induce male killing.     

Biochemical evidence of haplodiploidy in the whiteflyBemisia tabaci

Polymorphic esterase and acetylcholinesterase alleles in the whiteflyBemisia tabaci were studied using electrophoretic and colorimetric assays. The segregation of these alleles between parental and F₁ generations provided unequivocal evidence of haplodiploidy in this pest species. Unmated females, heterozygous at a polymorphic locus, produced a 1:1 ratio of haploid males expressing either of the maternal alleles. Although male offspring were produced by both virgin and mated females, the segregation of alleles showed they were always haploid (hemizygous) for the marker enzymes. Females only arose from fertilized eggs and invariably expressed paternal and maternal alleles.     

Gender-specific bacterial composition of black flies (Diptera: Simuliidae)

Although hematophagous black flies are well-known socioeconomic pests and vectors of disease agents, their associated bacteria are poorly known. A systematic analysis of the bacterial community associated with freshly emerged adult black flies of four North American species, using cultivation-independent molecular techniques, revealed 75 nonsingleton bacterial phylotypes. Although 17 cosmopolitan phylotypes were shared among host species, each fly species had a distinct bacterial profile. The bacterial composition, however, did not correlate strongly with the host phylogeny but differed between male and female flies of the same species from the same habitat, demonstrating that a group of insects have a gender-dependent bacterial community. In general, female flies harbor a less diverse bacterial community than do males. The anatomical locations of selected bacteria were revealed using fluorescence in situ hybridization. Understanding the physiological function of the associated bacterial community could provide clues for developing novel pest-management strategies.     

Bacterial communities of the reproductive organs of virgin and mated common bedbugs,Cimex lectularius

1. Microbes associated with reproductive organs of animals are either sexually transmitted or opportunistic. Both can affect host defence, immunity, and future colonisation with other microbes. There are only few studies on the microbiota of reproductive organs in insects and how they are affected by copulation. 2. This study examines the bacterial communities associated with reproductive organs in the common bedbug Cimex lectularius, a well-established insect model for the effects of microbes on male and female reproduction. Combining a metagenomic approach with a controlled mating scheme, we found 31 sequence variants (SVs) across 55 organ samples, with on average three SVs in each sample. Male and female reproductive organs harboured distinct bacterial communities in terms of present SVs. 3. Using a community ecology approach, we found three potential indications of sexual transmission of bacteria in the common bedbug: (i) copulation increased the similarity of the communities of male and female organs; (ii) mated individuals harboured bacteria that were found in non-mated individuals of the opposite sex but not in non-mated individuals of the same sex; and (iii) bacterial communities showed a high SV turnover between non-mated and mated individuals, suggesting a mating-induced replacement of bacteria. 4. Our findings show that the community ecology approach is useful to examine the bacterial dynamics on reproductive organs, especially when combined with studies that quantify the frequency of transmission and/or estimate the effect of the transmitted microbes on the host immune system and the host endosymbionts.     

Wright-Fisher model of social insects with haploid males and diploid females

An inhomogeneous discrete Markov model is formulated for sexual random mating in finite populations of haploid male and diploid female individuals. This is a Wright-Fisher type of model for social insects. The generations are non-overlapping and of given finite sizes. Bottlenecks are included, allowing different sizes to change from generation to generation. Mutations and selection are included in this exact model for the stochastic process. Computations of the exact Markov model are presented, focussing on the sexually asymmetric genetic drift caused by haplodiploidy.     

Decoupling of host-symbiont-phage coadaptations following transfer between insect species

Transferring endosymbiotic bacteria between different host species can perturb the coordinated regulation of the host and bacterial genomes. Here we use the most common maternally transmitted bacteria, Wolbachia pipientis, to test the consequences of host genetic background on infection densities and the processes underlying those changes in the parasitoid wasp genus Nasonia. Introgressing the genome of Nasonia giraulti into the infected cytoplasm of N. vitripennis causes a two-order-of-magnitude increase in bacterial loads in adults and a proliferation of the infection to somatic tissues. The host effect on W. pipientis distribution and densities is associated with a twofold decrease in densities of the temperate phage WO-B. Returning the bacteria from the new host species back to the resident host species restores the bacteria and phage to their native densities. To our knowledge, this is the first study to report a host-microbe genetic interaction that affects the densities of both W. pipientis and bacteriophage WO-B. The consequences of the increased bacterial density include a reduction in fecundity, an increase in levels of cytoplasmic incompatibility (CI), and unexpectedly, male-to-female transfer of the bacteria to uninfected females and an increased acceptance of densely infected females to interspecific mates. While paternal inheritance of the W. pipientis was not observed, the high incidence of male-to-female transfer in the introgressed background raises the possibility that paternal transmission could be more likely in hybrids where paternal leakage of other cytoplasmic elements is also known to occur. Taken together, these results establish a major change in W. pipientis densities and tissue tropism between closely related species and support a model in which phage WO, Wolbachia, and arthropods form a tripartite symbiotic association in which all three are integral to understanding the biology of this widespread endosymbiosis.