A recent study finds evidence that the “apple maggot”, a major agricultural pest in North America, may have diverged from its native fruit-tree host 150 years ago in just a single generation.
The usual host of the native fly Rhagoletis pomonella is the fruit of the North American hawthorn tree. However, in the mid-1800s a fragmented group of R. pomonella began to infest the fruit of a closely related tree in the eastern United States – the apple tree. Today the fly is often called the “apple maggot” and is a prominent agricultural pest of apple production in North America.
Apple trees are native to central Asia and were introduced to the new world in the 17th century by Europeans. Interestingly, apple trees begin to fruit about a month before native North American hawthorns, which made it necessary for the divergent apple-loving R. pomonella group to suddenly adapt the timing of egg-laying in its one-year life cycle to coincide with the fruiting time of its new host.
This divergent group thrived in their new niche and, as a result, there are now two races of R. pomonella in the United States: one with a life cycle timed for hawthorns (native) and one adapted for apples (divergent). The process that created these two races is called divergent ecological selection because the organisms have adopted different lifestyles based on their ecological niches; in this case, their fruit of choice for egg-laying.
Importantly, these two groups are still considered the same species since they interbreed and share genetic information, but they may be on their way to “speciating”: the process by which one species splits into two or more distinct species.
A recent study published in the journal Ecology Letters used experimental evolution and genetic analysis tools to answer a few questions about this interesting scenario. First, can this recent natural divergence be simulated in the lab? Second, how did R. pomonella diverge into two races? Third, are the two races likely to fully speciate?
To answer the first question, researchers exposed flies from the native hawthorn-loving race to environmental conditions experienced by the recently diverged apple-loving race (i.e. warmer temperatures during egg-laying, different fruit host, timing of mating). After a single generation under these conditions they compared the genome of the environmentally-manipulated hawthorn-loving flies to the genome of true apple-loving flies found in nature.
Their results showed a striking similarity between the genomes after just one generation. There are over 32,000 documented single nucleotide polymorphisms (differences in one base pair of DNA) and many structural chromosome regions that differ between the genomes of native and divergent races, and many of these arose in just the one generation of the experimental evolution conditions. These results show that many of the traits associated with the apple pest might have arisen in just a single generation.
So how did such a major shift occur so quickly? The researchers argue this is partly because there is a high level of “linkage disequilibrium” within R. pomonella, which means that many regions of the genome are directly linked to, or dependent on, other regions. In other words, one small change in the DNA may change other parts indirectly.
The major shift may also be partly due to the high level of natural genetic variation within the R. pomonella genome. This means that ecological selection had a larger diversity of traits to “choose” from to promote the divergence. A major contributor of this natural diversity is the structure of the genome itself, which contains many areas where genes can be cut out, flipped around, and replaced in various orders.
The question of speciation cannot have a definite answer (science is an educated prediction of the future, not a guarantee), but based on the evidence in the current study it is likely that these two fly races will eventually no longer be capable of interbreeding and will become distinct species. However, currently about 4% of the native and divergent flies interbreed per generation, which should be enough to maintain the species. This may change as time goes on and the forces of divergent ecological selection pull the species apart.
This study shows that major divergent ecological selection can occur over time scales as short as one generation. It also explains possible reasons why this happened in R. pomonella, mainly based on the characteristics of its genome such as linkage and high natural diversity. This process of selective divergence has likely occurred many times in the past and may be occurring in other species now, slowly driving evolution by natural selection on Earth.