In evolutionary terms, every invasive species is extraordinary. Take an organism out of its native habitat that it’s specifically evolved to survive in over millions and millions of years, plop it somewhere new, and–by chance–it flourishes. Adding to the miraculousness: Invasive species often arrive in their new environments in initially small numbers, carrying far less genetic diversity than they have in their home range. A drop in genetic diversity, called a “genetic bottleneck,” means less potential for adaptability and flexibility in the face of challenges.
This is the genetic paradox of invasion; small, genetically homogenous groups of organisms, taken far from home, can still become pervasive pests. For example, the famous case of cane toads, brought to Australia to try to control insects munching on sugarcane crops, quickly became an-ever-expanding menace in their own right.
Clearly, many invasive species manage to thrive despite the evolutionary roadblocks, and cane toads aren’t the only interlopers disrupting the down-under ecosystem. New research of a stinging, buzzing environmental threat offers some insight into how and why.
Surprising selection
In a study published February 29 in the journal Current Biology, scientists cataloged the spread of Asian honey bees (Apis cerana) in Australia, and analyzed the invasive populations’ genetic journey. They found that the tens of thousands of hives now buzzing across northeastern Australia likely originated from a single bee colony (one breeding queen and her workers), introduced to a Queensland port around 2007. Despite that extreme initial genetic bottleneck, over the course of just 10 years, the insects started re-diversifying and adapting to their foreign habitat via natural selection, according to the research. One queen bee held enough genetic diversity to kickstart an entire, viable population. “Our data support the view that genetic bottlenecks may have little impact on adaptive potential,” write the study authors.
“We weren’t expecting to find selection,” says lead researcher Kathleen Dogantzis, a biologist at York University in Toronto. Usually, patterns of natural selection take a long time to emerge. “The assumption is that it takes populations a lot longer to get acclimatized and adapted to a new environment. But we were able to show that, in a very short period of time–within this 10-year period–certain regions of the genome are contributing,” to the bees’ population growth, she explains.
“Despite having very little genetic diversity, these insects managed to use what they brought with them as the basis for adaptation,” says co-senior study author Amro Zayed, a biology professor at York University.
The study is “a fantastic story and incredibly insightful,” says Andrew Suarez, an entomology professor at the University of Illinois who studies biological invasions, but was uninvolved in the new work. Not only were Dogantzis, Amro, and their colleagues able to suss out the surprising, probable origins of the invasive bees, but they also zeroed in on what, specific, genetic changes the insects are undergoing in Australia. Selection appears to be acting on bee genes associated with social structure, reproduction, and foraging, per the study. The findings illustrate evolution in real time, showing what sorts of pressures the bees are subject to in their adopted range and how they’ve managed to respond. “I was excited to see that,” adds Suarez.
An invasive species’ origins
Asian honey bees are native to a wide swath of Asia, from Afghanistan to Japan. In their home range they’re critical pollinators and an important part of the ecological web. But in Australia, where the honey bees aren’t native, they may compete with native insects, birds, and mammals for flower resources, and nest in tree cavities that would otherwise offer important habitat for native species. The Asian honey bees also threaten the human-managed hives of European honey bees that are used to boost agricultural production.
The closest native population to Australia is in Indonesia, but people brought the insects to New Guinea in the 1970s for their honey and farming purposes. And this is where the colony that made it to Queensland shores came from, according to the new study. The researchers compared genome sequences from the native Indonesian population, the introduced New Guinean colonies, and the invasive Australian hives and found that the Australian and New Guinean bees were mostly closely related.
Bee bonanza
From there, Amro, Dogantzis, and their co-researchers looked at how the Australian bees’ whole genomes were changing each year from 2008 to 2018, as the insects’ numbers exploded to an estimated 10,000-50,000 colonies. Through a multi-step analysis, they pinpointed 481 tiny genetic variants (known as single nucleotide polymorphisms or SNPs) that could be having an outsized impact on bee survival, and appear to be undergoing positive selection. In other words, these 481 gene alterations spread through the population in a non-random pattern that suggests they’re beneficial to the bees. 471 of these variants could be traced back to the Indonesian or New Guinean bee population–indicating that almost all of these adaptations were carried by that first colony in Australia, while just a few could be the product of new mutations.
Using the well-studied European honey bee genome as a reference, the scientists were able to predict what these SNPs do. They found that several of the genes are related to reproduction, honey bee caste development, and foraging behavior–all traits likely to be important for survival and managing a new environment. “A species can adapt very rapidly, even when most genetic diversity is lost,” says co-senior study author Ros Gloag, an evolutionary biologist at The University of Sydney. “It’s because natural selection finds something to work with, even when diversity is low,” she adds.
Limits and possibilities
There are some limitations to how widely these findings, alone, can or should be extrapolated. For one, “we can only observe invasions if they’re successful,” says Zayed–which means there’s an inherent data bias. Nobody knows how many Asian honey bee colonies reached Australia before the one that led to a successful invasion. So, though it’s possible for one colony to multiply and diversify, it certainly shouldn’t be an expectation in conservation efforts, he adds. Plus, honey bees and other social insects have certain advantages when it comes to populating new habitats. Queens can mate with multiple males and store their sperm, continually laying eggs that reflect the diversity of more than one paternal line. Bees and ants reproduce quickly–a queen can lay up to thousands of eggs in a day. And the colonial structure means that gathering resources and defending against danger is easier.
Then, there are inherent limits to the researchers’ genetic analyses, says Natalie Hofmeister, an assistant professor of evolutionary biology at the University of Michigan who wasn’t involved in the new study. The approach can detect patterns and suggest likely correlations, but not prove causes of genetic change, she explains. Hofmeister further notes that the methods the researchers used weren’t developed to pick up on rapid evolutionary changes and it’s possible (though unlikely) that the 481 SNPs weren’t the result of selection.
Nevertheless, “it’s an elegant study,” she says, that adds to a growing body of research on the evolution of biological introductions and sets up lots of future hypotheses worth testing. To better protect against future species invasions and understand what allows some animals to succeed over others, we need more research like this across taxa, Hofmeister says.
Suarez agrees. The strategy of looking at whole genomes at multiple time points is “really exciting,” he says. Though species invasions carry many environmental downsides, the silver lining is that they present unique opportunities to study evolution in action. In his view, the research sheds light on how one particular insect invasion has unfolded, but also boosts our broader understanding of what’s biologically possible and how to gauge the risk of future species introductions–just a single colony can spur a whole population. The knowledge, Suarez says, could potentially help both those trying to control the spread of harmful animals and conservationists seeking to save endangered animals with dwindling populations. “There’s so many lessons that we can learn across biology from this sort of approach,” he says.