How a farmer pest is prompting scientists to rethink the speed of evolution

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 a form of accelerated evolution once considered impossible.
  • After the introduction of apples to North America around 1620, railroad worms – which until then had only laid their eggs in the fruit of downy hawthorn – also began to lay their eggs in apples.
  • A recent study by researchers at NCBS Bengaluru found that the worms were able to take advantage of this opportunity because their brains grew very quickly.

Minneapolis: Over the millennia, agriculture has transformed landscapes and ecosystems. By transporting plants across the oceans, we have also unwittingly allowed other species to flourish and evolve. Some, like the cabbage white butterfly, have spread with their host plants to all continents except South America and Antarctica. Others have evolved to take advantage of the new resources available.

Railroad worms are one such pest. They vexed small apple growers in North America for 180 years. They also happen to be beginning to offer evolutionary biologists a front-row seat to a form of accelerated evolution once considered impossible.

European settlers brought apples to North America around 1620. As invaders colonized the continent, so did the apple tree. And as there were more and more apples, the railroad worms – which until then had only laid their eggs in the fruits of the downy hawthorn (Crataegus mollis) tree – also began to lay their eggs in these fruits.

A recent study by researchers from the National Center for Biological Sciences (NCBS) in Bangalore, found that the worms were able to take advantage of the opportunity because their brains grew very rapidly.

Benjamin Walsh, Illinois state entomologist, first proposed this explanation in 1864, only five years after the intervention of Charles Darwin About the origin of species has been published. Walsh speculated that the railroad worms moved to the apples of the hawthorns – as the fruits of the hawthorn tree are called – by splitting into two race-hosts. 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 were called apple maggot, a new host race. The rest of the population was called the hawthorn flies.

Changing hosts is more than a matter of taste. Host fruits are where these flies mate and lay their eggs. If a subpopulation of a species prefers a different host, it and other subpopulations will not mate and, over time, might just become separate species.

“For a long time, most biologists believed that the evolution of new species required populations to be completely physically isolated,” said Thomas Powell, an assistant professor at Binghamton University in New York. The science of yarn.

Walsh’s idea challenged that notion. “Host switching is one of the most common ways [in which] new insects are emerging,” said Shannon Olsson, a senior researcher at NCBS who has worked on flies since 2005. (Her team is also studying the coffee white stem borera coffee pest in India.)

The two host races, apple maggots and hawthorn maggots, do not meet the criteria to be considered separate species. “You can’t genetically or morphologically tell an apple maggot from a hawthorn maggot,” Olsson said. “They can mate and produce fertile offspring.”

Instead, the two host races are considered to be undergoing speciation.

Although the host races have many similarities, they differ in one crucial aspect: behaviour. They are more attracted to chemical compounds released by their respective host fruits than to emissions from other fruits.

The most pressing question in the field, according to Powel, is to pin down the genes that drive this difference. A better understanding of their behavior could unravel this mystery.

Coupling

Apple maggots and hawthorn maggots mate at different times of the year when their respective hosts produce fruit: hawthorns in August, apples in September.

To find out how ancestral hawthorn flies changed their mating timing, Hinal Kharva, an NCBS doctoral student and author of the new study, and her collaborators traveled to the United States 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, its maggots consume it. Once the fruit falls from the tree, the maggots crawl up and go underground, where hormonal changes cause them to pupate (the equivalent of cocoons in butterflies). They overwinter as pupae, dormant.

The maggots collected by Kharva grew into pupae in a lab and ended up being transported to India in a cooler. Once back in Bangalore, she put them in a fridge to match their native winter conditions.

In April, when the cold weather in North America recedes, the pupae come out of their dormancy and begin their journey to adulthood. At this stage, the apple maggots are ahead of the hawthorn maggots because their parents emerged in August, giving them a head start.

Among insects, many of the chemicals that affect behavior 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 levels of dopamine, serotonin and other neurochemicals in their bodies. To her surprise, she found that apple flies had shorter developmental periods and lower levels of serotonin and dopamine.

“There’s no reason for them to develop their brains any faster. Apple maggots emerge a month earlier than hawthorn maggots, but both of their life cycles last a year,” Olsson said.

For apple maggots it started in August and for hawthorn maggots in September. After emerging from dormancy, apple maggots seemed to mysteriously accelerate their development.

Kharva said she thinks some ancestors of today’s apple maggots may have developed faster before apples even appeared. Natural selection acts on a variety of traits. And with lower levels of neurochemicals, Kharva said, “maybe those early emergent flies had different brains.”

That is, their “altered” brains and lack of hawthorns might have allowed them to quickly switch to newly introduced apples.

Amitabh Joshi, a professor at the Jawaharlal Nehru Center for Advanced Scientific Research, said their findings are compelling, but their paper does not prove that faster development or lower levels of neurochemicals actually affect behavior.

Olsson agreed: They can artificially lower or raise neurochemicals in a developing pupa to test whether this changes their preference for fruit in adulthood — but they haven’t performed those experiments yet.

If they do, and these findings hold, it could mean that “some of the same genes that control life cycle timing also influence brain chemistry to affect behavior,” Powell said.

Minor genetic differences could alter seemingly unrelated elements of an animal’s biology. This, Powell added, means the constraints on the evolution of new species in the absence of geographic separation may not be as strong as previously thought.

This in turn means that evolution – even in complex multicellular organisms – could be faster, and organisms could respond more reflexively to changing environments than previously thought.

Siddhant Pusdekar is a candidate in the doctoral program in Ecology, Evolution, and Behavior at the University of Minnesota.