Feb. 28 (UPI) -- African monarch butterflies aren't like other monarch species. Though their coloration is intended to advertise toxicity and ward off would-be predators, their pattern is highly variable.
According to a new study, published this week in the journal PLOS Biology, a male-killing bacteria explains the African monarch's diversity of patterns.
Usually, warning patterns like the kind displayed by toxic insects are consistent across individuals. Consistency helps predators learn not to mess with toxic species.
The answer to why the African monarch, Danaus chrysippus, breaks the rule of uniformity was found in their genes.
Previous studies showed females from an eastern population of African monarch butterflies have a unique chromosomal arrangement, with the chromosome containing genes response for color patterning fused to one of their sex chromosomes, the W chromosome. Scientists dubbed this fused chromosome the neo-W chromosome.
Earlier research also showed the same females were infected with a bacteria called Spiroplasma. The infection kills all of a female butterfly's sons.
Until now, scientists weren't sure if there was a connection between the bacteria, the female's novel chromosomal makeup and the species' highly variable color patterns.
When scientists took a closer look at the bacteria's DNA and the female butterflies' unusual chromosomes, they found the infection has enabled the rapid spread of the neo-W chromosome throughout the African monarch butterfly population. They also found that because the bacteria only allows for female offspring, it encourages the inheritance of a specific color pattern gene.
The promotion of a particular color pattern gene should produce a consistent color pattern across the butterfly population, but tests showed the color pattern genes from fathers override the female color traits.
Fathers with different color patterns produce daughters with different color patterns.
Previous studies have demonstrated that pattern variability observed among African monarchs appears seasonal. The latest research suggests seasonal wind patterns cause male migrants from different regions to end up in East Africa at certain times throughout the year.
Because the prominent bacteria limits the production of male offspring, the patterns passed on by visiting fathers never proliferate. They hit a genetic dead end. This dynamic ensures no dominant pattern emerges.
"The relatively fast emergence and spread of a new chromosome, combined with the short life cycle of the butterfly, allows us to study how the microbe is altering the evolution of the butterfly, almost in real-time," study author Simon Martin, an evolutionary biologist at the University of Edinburgh in Scotland, said in a news release. "We are continually discovering new ways in which microbes manipulate their hosts, and male-killing is just one example of this. It makes you wonder to what extent the evolution of other organisms -- even humans -- is affected by such unseen forces."