No abstract

The swallowtail butterfly, Papilio dardanus , is an iconic example of a polymorphic Batesian mimic. The expression of various female-limited colour forms is thought to be controlled by a single autosomal locus, termed H , whose function in determining the wing pattern remains elusive. As a step towards the physical mapping of H , we established a set of 272 polymorphic amplified fragment length polymorphism (AFLP) markers ( Eco RI- Mse I). Segregation patterns in a ‘female-informative’ brood (exploiting the absence of crossing over in female Lepidoptera) mapped these AFLPs to 30 linkage groups (putative chromosomes). The difference between the hippocoon and cenea female forms segregating in this family resides on a single one of these linkage groups, defined by 14 AFLPs. In a ‘male-informative’ cross (markers segregating within a linkage group), a pair of AFLPs co-segregated closely with the two female forms, except in four recombinants out of 19 female offspring. Linkage with these AFLP markers using four further female-informative families demonstrated that the genetic factor determining other morphs ( poultoni , lamborni and trimeni ) also maps to this same linkage group. The candidate gene invected , obtained in a screen for co-segregation of developmental genes with the colour forms, resides in a 13.9 cM interval flanked by the two AFLP markers. In the male-informative family invected co-segregated perfectly with the hippocoon / cenea factor, despite the four crossovers with the AFLPs. These findings make invected , and possibly its closely linked paralogue engrailed , strong candidates for H . This is supported by their known role in eyespot specification in nymphalid butterfly wings.

Background: Skippers (Family: Hesperiidae) are a large group of butterflies with ca. 4000 species under 567 genera. The lack of a time-calibrated higher-level phylogeny of the group has precluded understanding of its evolutionary past. We here use a 10-gene dataset to reconstruct the most comprehensive time-calibrated phylogeny of the group, and explore factors that affected the diversification of these butterflies.

The African Mocker Swallowtail, Papilio dardanus, is a textbook example in evolutionary genetics. Classical breeding experiments have shown that wing pattern variation in this polymorphic Batesian mimic is determined by the polyallelic H locus that controls a set of distinct mimetic phenotypes. Using bacterial artificial chromosome (BAC) sequencing, recombination analyses and comparative genomics, we show that H co-segregates with an interval of less than 500 kb that is collinear with two other Lepidoptera genomes and contains 24 genes, including the transcription factor genes engrailed (en) and invected (inv). H is located in a region of conserved gene order, which argues against any role for genomic translocations in the evolution of a hypothesized multi-gene mimicry locus. Natural populations of P. dardanus show significant associations of specific morphs with single nucleotide polymorphisms (SNPs), centred on en. In addition, SNP variation in the H region reveals evidence of non-neutral molecular evolution in the en gene alone. We find evidence for a duplication potentially driving physical constraints on recombination in the lamborni morph. Absence of perfect linkage disequilibrium between different genes in the other morphs suggests that H is limited to nucleotide positions in the regulatory and coding regions of en. Our results therefore support the hypothesis that a single gene underlies wing pattern variation in P. dardanus.

Sexually antagonistic selection can drive both the evolution of sex chromosomes and speciation itself. The tropical butterfly the African Queen, Danaus chrysippus, shows two such sexually antagonistic phenotypes, the first being sex-linked colour pattern, the second, susceptibility to a male-killing, maternally inherited mollicute, Spiroplasma ixodeti, which causes approximately 100% mortality in male eggs and first instar larvae. Importantly, this mortality is not affected by the infection status of the male parent and the horizontal transmission of Spiroplasma is unknown. In East Africa, male-killing of the Queen is prevalent in a narrow hybrid zone centred on Nairobi. This hybrid zone separates otherwise allopatric subspecies with different colour patterns. Here we show that a neo-W chromosome, a fusion between the W (female) chromosome and an autosome that controls both colour pattern and male-killing, links the two phenotypes thereby driving speciation across the hybrid zone. Studies of the population genetics of the neo-W around Nairobi show that the interaction between colour pattern and male-killer susceptibility restricts gene flow between two subspecies of D. chrysippus. Our results demonstrate how a complex interplay between sex, colour pattern, male-killing, and a neo-W chromosome, has set up a genetic ‘sink' that keeps the two subspecies apart. The association between the neo-W and male-killing thus provides a ‘smoking gun' for an ongoing speciation process.

Abstract. When a butterfly species has a polymorphic female, with one of the forms closely resembling the male, it is customary to suppose that this form is ancestral, and that the ‘odd’ forms have arisen later. R. I. Vane‐Wright, on the other hand, has suggested that in some species the male‐like form may be a ‘transvestite’ female, the ancestral form of the female having been strikingly unlike the male. As later‐derived forms are usually, but not always, genetically dominant to ancestral forms, we can make some choice between these hypotheses by discovering the dominance relations of the male‐like and the ‘odd’ forms of the female. In the mimetic Papilio aegeus the male‐like form is shown to be recessive to the ‘odd’ (mimetic) form, as has essentially been the case in all other butterflies so far investigated. Papilio phorcas is now shown to be the exception: the ‘odd’ (non‐mimetic) form is recessive to the male‐like form. We conclude that usually the male‐like form is ancestral, but that P.phorcas may be an authentic example of ‘transvestism’, or the ‘transfer’ of male epigamic colour to the female of the species. The yellow, male‐like pattern of the mimetic Papilio dardanus may be dominant or recessive to the mimetic forms according to the genetic background: largely recessive in Madagascar, and southern and western Africa, dominant to most forms in Ethiopia, and probably dominant to one mimetic form but recessive to the others in Kenya. All female dardanus patterns, both mimetic and yellow, are strongly dominant to both female phorcas patterns in P. dardanus × P. phorcas hybrids (P. ‘nandina’). The simplest explanation of this situation is that the male‐like pattern of dardanus is ancestral, and that dominance has become locally reversed in Ethiopia. The dominance relations, and the sex‐ or autosomal‐linkage of two forms can be determined without pedigree‐breeding, simply by observing a few offspring each from a large number of wild‐caught females.

Butterflies are a diverse and charismatic insect group that are thought to have evolved with plants and dispersed throughout the world in response to key geological events. However, these hypotheses have not been extensively tested because a comprehensive phylogenetic framework and datasets for butterfly larval hosts and global distributions are lacking. We sequenced 391 genes from nearly 2,300 butterfly species, sampled from 90 countries and 28 specimen collections, to reconstruct a new phylogenomic tree of butterflies representing 92% of all genera. Our phylogeny has strong support for nearly all nodes and demonstrates that at least 36 butterfly tribes require reclassification. Divergence time analyses imply an origin ~100 million years ago for butterflies and indicate that all but one family were present before the K/Pg extinction event. We aggregated larval host datasets and global distribution records and found that butterflies are likely to have first fed on Fabaceae and originated in what is now the Americas. Soon after the Cretaceous Thermal Maximum, butterflies crossed Beringia and diversified in the Palaeotropics. Our results also reveal that most butterfly species are specialists that feed on only one larval host plant family. However, generalist butterflies that consume two or more plant families usually feed on closely related plants.

Many tropical environments experience cyclical seasonal changes, frequently with pronounced wet and dry seasons, leading to a highly uneven temporal distribution of resources. Short‐lived animals inhabiting such environments often show season‐specific adaptations to cope with alternating selection pressures. African Bicyclus butterflies show strong seasonal polyphenism in a suite of phenotypic and life‐history traits, and their adults are thought to undergo reproductive diapause associated with the lack of available larval host plants during the dry season. Using 3 years of longitudinal field data for three species in Malawi, dissections demonstrated that one forest species reproduces continuously, whereas two savannah species undergo reproductive diapause in the dry season, either with or without pre‐diapause mating. Using additional data from field‐collected and museum samples, we then documented the same three mating strategies for a further 37 species. Phylogenetic analyses indicated that the ancestral state was a non‐diapausing forest species, and that habitat preference and mating strategy evolved in a correlated fashion. Bicyclus butterflies underwent rapid diversification during the Late Miocene, coinciding with expansions into more open savannah habitat. We conclude that the ability to undergo reproductive diapause was a key trait that facilitated colonization and eventual radiation into savannahs in the Late Miocene.