A female apple maggot. Photo: Joseph Berger/Bugwood.org, CC BY 3.0
- Railroad worms are beginning to offer evolutionary biologists a front-row seat to an accelerated form of evolution once thought to be impossible.
- After apples were brought to North America around 1620, railroad worms – which until then laid their eggs only in fruits of the downy hawthorn tree – began to lay their eggs in apples as well.
- A recent study by researchers at NCBS Bengaluru found that the worms were able to take advantage of the opportunity because their brains developed very quickly.
Minneapolis: Over millennia, farming has transformed landscapes and ecosystems. As we transported plants across the oceans, we also unwittingly enabled the flourishing and evolution of other species. Some, like the cabbage white butterfly, spread with their host plants to every continent except South America and Antarctica. Others evolved to take advantage of the new resources on offer.
Railroad worms are one such pest. They have vexed small-scale apple farmers in North America for 180 years. As it happens, they are also beginning to offer evolutionary biologists a front-row seat to an accelerated form of evolution once thought to be impossible.
European settlers brought apples to North America around 1620. As the invaders colonised the continent, so did the apple tree. And as there were more and more apples, the railroad worms – which until then laid their eggs only in fruits of the downy hawthorn (Crataegus mollis) tree – began to lay their eggs in these fruits as well.
A recent study by researchers at the National Center of Biological Sciences (NCBS), Bengaluru, found that the worms were able to take advantage of the opportunity because their brains developed very quickly.
Benjamin Walsh, the state entomologist of Illinois, first proposed this explanation in 1864, only five years after Charles Darwin’s On the Origin of Species was published. Walsh speculated that railroad worms shifted to apples from haws – as the fruits of the hawthorn are called – by splitting into distinct host-races. A host-race is a subpopulation of a fruit-eating insect that prefers a specific host.
For example, the railroad worms that laid their eggs in apples came to be called apple flies, a new host-race. The rest of the population was called the hawthorn flies.
Switching hosts is more than a matter of taste. The host fruits are where these flies mate and lay eggs. If one subpopulation of a species prefers a different host, it and other subpopulations won’t mate and, over time, may just become separate species.
“For a long time, most biologists held the view that the evolution of new species required populations to be completely physically isolated,” Thomas Powell, an assistant professor at Binghamton University, New York, told The Wire Science.
Walsh’s idea challenged this notion. “Shifting hosts is one of the most common ways [in which] new insects emerge,” Shannon Olsson, a principal investigator at NCBS and who has been working on the flies since 2005, said. (Her team also studies the coffee white stem borer, a coffee plant pest in India.)
The two host-races, apple flies and hawthorn flies, don’t meet the criteria to be considered separate species. “You can’t tell an apple fly from a hawthorn fly genetically or morphologically,” Olsson said. “They can mate with each other and produce fertile offspring.”
Instead, the two host-races are considered to be undergoing speciation.
While host-races have many similarities, they differ in one crucial aspect: behaviour. They’re more attracted to chemical compounds released by their respective host fruits than they are to the emissions of other fruits.
The most pressing question in the field, according to Powel, is pinning down the genes that drive this difference. Understanding their behaviour better could unravel this mystery.
Apple flies and hawthorn flies mate at different times of the year, when their respective hosts fruit: haws in August, apples in September.
To find out how ancestral hawthorn flies changed their mating timing, Hinal Kharva, a PhD student at NCBS and an author of the new study, and her collaborators travelled to the US in the summer of 2018, where they collected apple and hawthorn fly larvae, or maggots.
Normally, after an adult fly lays its eggs in the fruit, their maggots consume it. Once the fruit falls off the tree the maggots crawl out and go underground, where hormonal changes cause them to transform into a pupa (equivalent to cocoons in butterflies). They spend the winter as pupae, in dormancy.
The maggots that Kharva collected became pupae in a lab and ended up being transported to India in an icebox. Once back in Bengaluru, she put them in a refrigerator to match their native winter conditions.
In April, when the cold in North America recedes, the pupae exit dormancy and begin their journey to adulthood. At this stage, the apple flies are ahead relative to hawthorn flies because their parents emerged in August, giving them a head start.
Among insects, a lot of the chemicals that affect behaviour also affect development. They include neurotransmitters like dopamine and serotonin.
When Kharva took the pupae out of the fridge in April, she tracked their transition to adulthood and measured the levels of dopamine and serotonin and some other neurochemicals in their bodies. To her surprise, she found that apple flies had shorter development periods and lower serotonin and dopamine levels.
“There’s no reason for them to develop their brains faster. Apple flies come out a month earlier than hawthorn flies, but both their life cycles are a year long,” Olsson said.
For apple flies, this began in August and for hawthorn flies in September. After exiting dormancy, apple flies seemed to mysteriously accelerate their development.
Kharva said she believes some ancestors of today’s apple flies may have been developing quicker even before apples turned up. Natural selection acts on a diversity of traits. And with lower levels of neurochemicals, Kharva said, “maybe these early emerging flies had different brains.”
That is, their ‘altered’ brains and the absence of hawthorns could have allowed them to switch quickly to the newly introduced apples.
Amitabh Joshi, professor at Jawaharlal Nehru Center for Advanced Scientific Research said their results are convincing – but that their paper doesn’t prove that faster development or lower levels of neurochemicals actually affect behaviour.
Olsson agreed: they can artificially lower or raise the neurochemicals in a developing pupa to check whether that changes their fruit preference as adults – but they haven’t performed those experiments yet.
If they do and those findings hold up, it could mean “some of the same genes that control life-cycle timing also influence brain chemistry to affect behaviour,” Powell said.
Minor genetic differences could alter seemingly independent things about an animal’s biology. This, Powell added, means constraints on the evolution of new species in the absence of a geographic separation may not be as strong as previously thought.
That in turn means evolution – even in complex multicellular organisms – could be faster and that organisms could respond more reflexively to changing environments than previously thought.
Siddhant Pusdekar is a candidate at the University of Minnesota’s Ecology, Evolution and Behaviour PhD programme.