Gregory D. D. Hurst

Mitochondrial DNA as a marker of molecular diversity: a reappraisal

Over the last three decades, mitochondrial DNA has been the most popular marker of molecular diversity, for a combination of technical ease‐of‐use considerations, and supposed biological and evolutionary properties of clonality, near‐neutrality and clock‐like nature of its substitution rate. Reviewing recent literature on the subject, we argue that mitochondrial DNA is not always clonal, far from neutrally evolving and certainly not clock‐like, questioning its relevance as a witness of recent species and population history. We critically evaluate the usage of mitochondrial DNA for species delineation and identification. Finally, we note the great potential of accumulating mtDNA data for evolutionary and functional analysis of the mitochondrial genome.

Problems with mitochondrial DNA as a marker in population, phylogeographic and phylogenetic studies: the effects of inherited symbionts

Mitochondrial DNA (mtDNA) has been a marker of choice for reconstructing historical patterns of population demography, admixture, biogeography and speciation. However, it has recently been suggested that the pervasive nature of direct and indirect selection on this molecule renders any conclusion derived from it ambiguous. We review here the evidence for indirect selection on mtDNA in arthropods arising from linkage disequilibrium with maternally inherited symbionts. We note first that these symbionts are very common in arthropods and then review studies that reveal the extent to which they shape mtDNA evolution. mtDNA diversity patterns are compatible with neutral expectations for an uninfected population in only 2 of 19 cases. The remaining 17 studies revealed cases of symbiont-driven reduction in mtDNA diversity, symbiont-driven increases in diversity, symbiont-driven changes in mtDNA variation over space and symbiont-associated paraphyly of mtDNA. We therefore conclude that these elements often confound the inference of an organisms evolutionary history from mtDNA data and that mtDNA on its own is an unsuitable marker for the study of recent historical events in arthropods. We also discuss the impact of these studies on the current programme of taxonomy based on DNA bar-coding.

Male-killing Wolbachia in two species of insect

The inherited bacterium Wolbachia spreads through the manipulation of host reproduction, and has been suggested to be an important factor in arthropod evolution, from host speciation to the evolution of sex–determination systems. Past work has shown that members of this group may produce cytoplasmic incompatibility, feminize genetically male hosts, and induce host parthenogenesis. Here, we report an expansion of the range of reproductive manipulations produced by members of this clade, recording Wolbachia strains that kill male hosts during embryogenesis in two host species, the ladybird Adalia bipunctata, and the butterfly Acraea encedon. Both male–killing bacteria belong to the B group of Wolbachia. However, phylogenetic analyses were unable to resolve whether the bacteria in the two species are monophyletic, or represent independent origins of male–killing among the B–group Wolbachia. We also found significant divergence within the wsp gene of Wolbachia strains found in different A. bipunctata individuals, suggesting this host species contains two Wolbachia strains, diverged in wsp sequence but monophyletic. Our observations reinforce the notion that Wolbachia may be an important agent driving arthropod evolution, and corroborates previous suggestions that male–killing behaviour is easily evolved by invertebrate symbionts.

Sex-ratio-distorting Wolbachia causes sex-role reversal in its butterfly host.

Sex‐role‐reversed mating systems in which females compete for males and males may be choosy are usually associated with males investing more than females in offspring. We report that sex‐role reversal may also be caused by selfish genetic elements which distort the sex ratio towards females. Some populations of the butterflies Acraea encedon and Acraea encedana are extremely female biased because over 90% of females are infected with a Wolbachia bacterium that is maternally inherited and kills male embryos. Many females in these populations are virgins suggesting that their reproductive success may be limited by access to males. These females form lekking swarms at landmarks in which females exhibit behaviours which we interpret as functioning to solicit matings from males. The hypothesis that female A. encedon swarm in order to mate is supported by the finding that, in release‐recapture experiments, mated females tend to leave the swarm while unmated females remained. This behaviour is a sex‐role‐reversed form of a common mating system in insects in which males form lekking swarms at landmarks and compete for females. Female lekking swarms are absent from less female‐biased populations and here the butterflies are instead associated with resources in the form of the larval food plant.

Evolutionary consequences of Wolbachia infections

The past decade has revealed the bacterium Wolbachia as the most widespread symbiont of arthropods and nematodes. Behind this evolutionary success is an remarkable variety of effects on host biology, ranging from manipulation of reproduction in favor of females to more classical mutualistic interactions. Here we discuss the potential of Wolbachia for promoting evolutionary changes in its hosts.

Why do maternally inherited microorganisms kill males

Maternally-inherited male killing microorganisms are known in a number of insect species. We here discuss the evolutionary reasons for such behaviour through examining the ongoing dynamics of these elements. In cases where death occurs late in the life cycle, horizontal transmission associated with male death occurs but in others no such direct advantage to killing the individual appears to exist. The evidence that early male killing is analogous to primary sex ratio distortion through increasing the fitness with respect to producing females from increases in resources to surviving (female) offspring and through the prevention of inbreeding is discussed. The early life history of many of these species suggests an early resource advantage may accrue from male killing. Inbreeding avoidance appears to be an important factor in at least two cases. The potential for horizontal transmission to maintain these elements is also discussed. It is concluded that male killing appears to be an adaptive strategy for the microorganism in most cases.

Inherited microorganisms, sex-specific virulence and reproductive parasitism

Parasites show an amazing repertoire of adaptations, highlighted by complex life cycles that allow both survival in the host and transmission among hosts. However, there is one heterogeneous group of microorganisms whose adaptations are perhaps even more surprising: parthenogenesis induction, feminization of genetic males, killing of male hosts and sperm-mediated sterilization of uninfected eggs. The common feature of these microorganisms is their mode of transmission: inheritance from mother to offspring. Here, we present an introduction to hereditary symbiosis, focusing on microsporidia and bacteria that manipulate host reproduction in arthropods (reproductive parasites). We also discuss the implications of one of these microorganisms, Wolbachia, for the control of arthropod pests and vectors and for the therapy of filarial diseases. Finally, we discuss whether some parasites of vertebrates might show sex-specific virulence.

The butterfly Danaus chrysippus is infected by a male-killing Spiroplasma bacterium

Many insects carry maternally inherited bacteria which kill male offspring. Such bacteria will spread if male death benefits the female siblings who transmit the bacterium, and they are therefore expected in insects with antagonistic sibling interactions. We report that the butterfly Danaus chrysippus is host to a maternally inherited male-killing bacterium. Using diagnostic PCR and rDNA sequence, the bacterium was identified as a Spiroplasma closely related to 2 ladybird beetle male-killers and the tick symbiont Spiroplasma ixodetis. The male-killer was found to have a geographically restricted distribution, with up to 40% of females being infected in East Africa, but no detectable infection in small samples from other populations. Danaus chrysippus is a surprising host for a male-killer as its eggs are laid singly. This suggests that the ecological conditions permitting male-killers to invade may be more widespread than previously realized.

Selfish genetic elements and speciation

This review concerns the importance of selfish genetic elements (SGEs) in speciation. We assess the importance of medea genes, meiotic drive elements, transposable elements and the bacterium Wolbachia in the creation of postzygotic isolation. Although all of these elements can contribute to postzygotic isolation, their contribution will often disappear if there is gene flow between the populations. Further, there is the possibility that incompatibilities produced by SGEs may lessen over time. We conclude that although some of the case studies are tantalizing, particularly those associated with Wolbachia, the role of selfish genetic elements in speciation remains unproven.

Wolbachia infection associated with all-female broods in Hypolimnas bolina (Lepidoptera : Nymphalidae): evidence for horizontal transmission of a butterfly male killer

Inherited bacteria that kill male hosts during embryogenesis infect a wide range of insect species. In order to ascertain if there are patterns to host infection, with particular male killing bacteria specialising on particular taxa, we investigated the male killing trait in the butterfly Hypolimnas bolina. All-female broods were first reported in this species in the 1920s. Investigation of this system in the Fiji Islands revealed the causal agent of sex ratio distortion in H. bolina to be a male killing Wolbachia bacterium. This bacterium is identical in wsp and ftsZ sequence to a male killer in the butterfly Acraea encedon in Tanzania, suggesting it has moved between host species, yet retained its phenotype. The prevalence of the Wolbachia was calculated for three different island groups of Fiji, and found to vary significantly across the country. Antibiotics failed to cure either the male killing trait or the Wolbachia infection. The implications of these results are discussed.