Neo Sex Chromosomes, Colour Polymorphism and Male-Killing in the African Queen Butterfly, <i>Danaus chrysippus</i> (L.)

• Main Authors: David A.S. Smith, Walther Traut, Simon H. Martin, Piera Ireri, Kennedy S. Omufwoko, Richard ffrench-Constant, Ian J. Gordon
• Format: Article
• Language: English
• Published: MDPI AG 2019-09-01
• Series: Insects
• Subjects: colour polymorphism; <i>Danaus chrysippus</i>; defence; ‘magic trait’; male-killing; mimicry; neo sex chromosomes; resource competition; speciation
• Online Access: https://www.mdpi.com/2075-4450/10/9/291

<i>Danaus chrysippus</i> (L.), one of the world&#8217;s commonest butterflies, has an extensive range throughout the Old-World tropics. In Africa it is divided into four geographical subspecies which overlap and hybridise freely in the East African Rift: Here alone a male-killing (MK) endosymbiont, <i>Spiroplasma ixodetis</i>, has invaded, causing female-biased populations to predominate. In ssp. <i>chrysippus</i>, inside the Rift only, an autosome carrying a colour locus has fused with the W chromosome to create a neo-W chromosome. A total of 40&#8722;100% of Rift females are neo-W and carry <i>Spiroplasma</i>, thus transmitting a linked, matrilineal neo-W, MK complex. As neo-W females have no sons, half the mother&#8217;s genes are lost in each generation. Paradoxically, although neo-W females have no close male relatives and are thereby forced to outbreed, MK restricts gene flow between subspecies and may thus promote speciation. The neo-W chromosome originated in the Nairobi region around 2.2 k years ago and subsequently spread throughout the Rift contact zone in some 26 k generations, possibly assisted by not having any competing brothers. Our work on the neo-W chromosome, the spread of <i>Spiroplasma</i> and possible speciation is ongoing.