Thanks to a fused chromosome.
In a narrow zone around Nairobi, in Kenya, females from two subspecies of African Queen butterflies abound, but the males are dying before they even hatch.
Like African Queens across the continent, these butterflies are infected with a microbe called Spiroplasma. But unlike the others, when the butterflies living in this particular zone are infected, their sons pay the price.
According to a study published in Proceedings of the Royal Society B, the likely culprit is a chromosomal anomaly specific to the butterflies in Nairobi.
In contrast to humans, for which sex is determined by our X and Y chromosome, butterflies go by the ZW system: ZZ codes for a males, while ZW confers a female. But somehow in the butterflies living in the male-free zone, a non-sex chromosome wound up fusing with the W to form what researchers refer to as a “neo-W.” It is this fused chromosome that they believe makes these African Queens susceptible to the male-killing effects of Spiroplasma.
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"The neo-W effectively acts as a genetic sink for all males, and butterfly populations around Nairobi are nearly all female,” said lead author David Smith, formerly from the Natural History Museum at Eton College, in a press release.
With the populations cleaned of males, the only chance for procreation within the zone comes when an immigrant male finds his way over. It also means that members of the two subspecies occupying the zone cannot mate with each other, being all female. By keeping their genes from co-mingling, the researchers expect that over time, this barrier to mating will cause the two subspecies to diverge into completely separate species.
"Whilst we don't understand the precise molecular mechanisms behind this chromosomal merger, this means that no males are made in the hybrid zone, and that mating success in the zone is effectively zero, thereby creating a barrier with a new species on either side," said study co-author Richard ffrench-Constant, from the Centre for Ecology and Conservation at the University of Exeter.
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