Phenotypic plasticity in chemical defence of butterflies allows usage of diverse host plants

Host plant specialization is a major force driving ecological niche partitioning and diversification in insect herbivores. The cyanogenic defences of Passiflora plants keep most herbivores at bay, but not the larvae of Heliconius butterflies, which can both sequester and biosynthesize cyanogenic compounds. Here, we demonstrate that both Heliconius cydno chioneus and H. melpomene rosina have remarkable plasticity in their chemical defences. When feeding on Passiflora species with cyanogenic compounds that they can readily sequester, both species downregulate the biosynthesis of these compounds. By contrast, when fed on Passiflora plants that do not contain cyanogenic glucosides that can be sequestered, both species increase biosynthesis. This biochemical plasticity comes at a fitness cost for the more specialist H. m. rosina, as adult size and weight for this species negatively correlate with biosynthesis levels, but not for the more generalist H. c. chioneus. By contrast, H. m rosina has increased performance when sequestration is possible on its specialized host plant. In summary, phenotypic plasticity in biochemical responses to different host plants offers these butterflies the ability to widen their range of potential hosts within the Passiflora genus, while maintaining their chemical defences.     

Hybridization and speciation

Hybridization has many and varied impacts on the process of speciation. Hybridization may slow or reverse differentiation by allowing gene flow and recombination. It may accelerate speciation via adaptive introgression or cause near‐instantaneous speciation by allopolyploidization. It may have multiple effects at different stages and in different spatial contexts within a single speciation event. We offer a perspective on the context and evolutionary significance of hybridization during speciation, highlighting issues of current interest and debate. In secondary contact zones, it is uncertain if barriers to gene flow will be strengthened or broken down due to recombination and gene flow. Theory and empirical evidence suggest the latter is more likely, except within and around strongly selected genomic regions. Hybridization may contribute to speciation through the formation of new hybrid taxa, whereas introgression of a few loci may promote adaptive divergence and so facilitate speciation. Gene regulatory networks, epigenetic effects and the evolution of selfish genetic material in the genome suggest that the Dobzhansky–Muller model of hybrid incompatibilities requires a broader interpretation. Finally, although the incidence of reinforcement remains uncertain, this and other interactions in areas of sympatry may have knock‐on effects on speciation both within and outside regions of hybridization.     

A major locus controls a biologically active pheromone component in Heliconius melpomene

Understanding the production, response, and genetics of signals used in mate choice can inform our understanding of the evolution of both intraspecific mate choice and reproductive isolation. Sex pheromones are important for courtship and mate choice in many insects, but we know relatively little of their role in butterflies. The butterfly Heliconius melpomene uses a complex blend of wing androconial compounds during courtship. Electroantennography in H. melpomene and its close relative Heliconius cydno showed that responses to androconial extracts were not species specific. Females of both species responded equally strongly to extracts of both species, suggesting conservation of peripheral nervous system elements across the two species. Individual blend components provoked little to no response, with the exception of octadecanal, a major component of the H. melpomene blend. Supplementing octadecanal on the wings of octadecanal-rich H. melpomene males led to an increase in the time until mating, demonstrating the bioactivity of octadecanal in Heliconius. Using quantitative trait locus (QTL) mapping, we identified a single locus on chromosome 20 responsible for 41% of the parental species’ difference in octadecanal production. This QTL does not overlap with any of the major wing color or mate choice loci, nor does it overlap with known regions of elevated or reduced F_ST. A set of 16 candidate fatty acid biosynthesis genes lies underneath the QTL. Pheromones in Heliconius carry information relevant for mate choice and are under simple genetic control, suggesting they could be important during speciation.     

THE DYNAMICS OF RECIPROCAL SELECTIVE SWEEPS OF HOST RESISTANCE AND A PARASITE COUNTER‐ADAPTATION IN DROSOPHILA

Host–parasite coevolution can result in consecutive selective sweeps of host resistance alleles and parasite counter‐adaptations. To illustrate the dynamics of this important but little studied form of coevolution, we have modeled an ongoing arms race between Drosophila melanogaster and the vertically transmitted sigma virus, using parameters we estimated in the field. We integrate these results with previous work showing that the spread of a resistance allele of the ref(2)P gene in the host was followed by the spread of a virus genotype, which overcomes this resistance. In line with these observations, our model predicts that there can be rapid selective sweeps in both the host and parasite, which can drive large changes in the prevalence of infection. The virus will tend to be ahead in the arms race, as incomplete dominance slows down host adaptation and selection for host resistance is weaker than selection for parasites to overcome resistance—the “life‐dinner” principle. This asymmetry in the adaptation rates results in a partial sweep of the host resistance allele, as it loses its advantage part way through the selective sweep. This well‐understood natural system illustrates how the outcome of host–parasite coevolution is determined by different population genetic parameters in the field.     

Immunohistochemical localization of irisin in skin,eye, and thyroid and pineal glands of the crested porcupine (Hystrix cristata)

Irisin was first identified in muscle cells. We detected irisin immunoreactivity in various organs of the crested porcupine (Hystrix cristata). In the epidermis, irisin immunoreactivity was localized mainly in stratum basale, stratum spinosum and stratum granulosum layers; immunoreactivity was not observed in the stratum corneum. In the dermis, irisin was found in the external and internal root sheath, cortex and medulla of hair follicles, and in sebaceous glands. Irisin immunoreactivity was found in the neural retina and skeletal muscle fibers associated with the eye. The pineal and thyroid glands also exhibited irisin immunoreactivity.     

Are we underestimating the diversity and incidence of insect bacterial symbionts? A case study in ladybird beetles

Vertically transmitted bacterial symbionts are common in arthropods. However, estimates of their incidence and diversity are based on studies that test for a single bacterial genus and often only include small samples of each host species. Focussing on ladybird beetles, we collected large samples from 21 species and tested them for four different bacterial symbionts. Over half the species were infected, and there were often multiple symbionts in the same population. In most cases, more females than males were infected, suggesting that the symbionts may be sex ratio distorters. Many of these infections would have been missed in previous studies as they only infect a small proportion of the population. Furthermore, 11 out of the 17 symbionts discovered by us were either in the genus Rickettsia or Spiroplasma, which are rarely sampled. Our results suggest that the true incidence and diversity of bacterial symbionts in insects may be far greater than previously thought.     

How many species are infected with Wolbachia? Cryptic sex ratio distorters revealed to be common by intensive sampling.

Inherited bacterial symbionts from the genus Wolbachia have attracted much attention by virtue of their ability to manipulate the reproduction of their arthropod hosts. The potential importance of these bacteria has been underlined by surveys, which have estimated that 17% of insect species are infected. We examined whether these surveys have systematically underestimated the proportion of infected species through failing to detect the low-prevalence infections that are expected when Wolbachia distorts the sex ratio. We estimated the proportion of species infected with Wolbachia within Acraea butterflies by testing large collections of each species for infection. Seven out of 24 species of Acraea were infected with Wolbachia. Four of these were infected with Wolbachia at high prevalence, a figure compatible with previous broad-scale surveys, whilst three carried low-prevalence infections that would have had a very low likelihood of being detected by previous sampling methods. This led us to conclude that sex-ratio-distorting Wolbachia may be common in insects that have an ecology and/or genetics that permit the invasion of these parasites and that previous surveys may have seriously underestimated the proportion of species that are infected.     

Phylogenetic discordance at the species boundary: comparative gene genealogies among rapidly radiating Heliconius butterflies

Recent adaptive radiations provide excellent model systems for understanding speciation, but rapid diversification can cause problems for phylogenetic inference. Here we use gene genealogies to investigate the phylogeny of recent speciation in the heliconiine butterflies. We sequenced three gene regions, intron 3 ( approximately 550 bp) of sex-linked triose-phosphate isomerase (Tpi), intron 3 ( approximately 450 bp) of autosomal mannose-phosphate isomerase (Mpi), and 1,603 bp of mitochondrial cytochrome oxidase subunits I and II (COI and COII), for 37 individuals from 25 species of Heliconius and related genera. The nuclear intron sequences evolved at rates similar to those of mitochondrial coding sequences, but the phylogenetic utility of introns was restricted to closely related geographic populations and species due to high levels of indel variation. For two sister species pairs, Heliconius erato-Heliconius himera and Heliconius melpomene-Heliconius cydno, there was highly significant discordance between the three genes. At mtDNA and Tpi, the hypotheses of reciprocal monophyly and paraphyly of at least one species with respect to its sister could not be distinguished. In contrast alleles sampled from the third locus, Mpi, showed polyphyletic relationships between both species pairs. In all cases, recent coalescence of mtDNA lineages within species suggests that polyphyly of nuclear genes is not unexpected. In addition, very similar alleles were shared between melpomene and cydno, implying recent gene flow. Our finding of discordant genealogies between genes is consistent with models of adaptive speciation with ongoing gene flow and highlights the need for multiple locus comparisons to resolve phylogeny among closely related species.