title: A neo-W chromosome in a tropical butterfly links colour pattern, male-killing, and speciation.
abstract: 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.
authors.affiliation: Natural History Museum, Eton College, Windsor SL4 6DW, UK.
authors.forename: David A S
authors.initials: DA
authors.lastname: Smith
authors.affiliation: BirdLife International, Africa Partnership Secretariat, Box 3502-00100, Nairobi, Kenya Department of Zoology, National Museums of Kenya, Box 4068-00100, Nairobi, Kenya.
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Gordon
authors.affiliation: Institut für Biologie, Zentrum für medizinische Struktur- und Zellbiologie, Universität Lübeck, Ratzeburger Allee 160, 23538 Lübeck, Germany.
authors.forename: Walther
authors.initials: W
authors.lastname: Traut
authors.affiliation: Emerging Infectious Diseases Lab, ICIPE, Box 30772-00506, Nairobi, Kenya.
authors.forename: Jeremy
authors.initials: J
authors.lastname: Herren
authors.affiliation: African Butterfly Research Institute (ABRI), Box 14308-0800, Nairobi, Kenya.
authors.forename: Steve
authors.initials: S
authors.lastname: Collins
authors.affiliation: Insect Committee of Nature Kenya, Box 24467-00100, Nairobi, Kenya.
authors.forename: Dino J
authors.initials: DJ
authors.lastname: Martins
authors.affiliation: Department of Zoology, National Museums of Kenya, Box 4068-00100, Nairobi, Kenya.
authors.forename: Kennedy
authors.initials: K
authors.lastname: Saitoti
authors.affiliation: Department of Zoological Sciences, Kenyatta University, Box 43844-00100, Nairobi, Kenya.
authors.forename: Piera
authors.initials: P
authors.lastname: Ireri
authors.affiliation: Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Penryn, Cornwall TR10 9EZ, UK r.ffrench.constant@exeter.ac.uk.
authors.forename: Richard
authors.identifier: 0000-0001-5385-9888
authors.initials: R
authors.lastname: Ffrench-Constant
mesh_terms: D000818:Animals; D002080:Butterflies / Q000235:genetics*; D003116:Color; D005260:Female; D051456:Gene Flow; D049810:Genetic Speciation; D005828:Genetics, Population; D007630:Kenya; D008297:Male; D010641:Phenotype; D010858:Pigmentation / Q000235:genetics*; D012730:Sex Chromosomes / Q000235:genetics*
keywords: Danaus chrysippus; Spiroplasma; colour pattern; male-killing; neo-W chromosome; speciation