Hawthorns to apples, flies to parasitoids – how new species create more species

Scene from Goodwill Hunting where Matt Damon boasts of the awesomeness of sequential speciation.

What determines species diversity?

This is one of the most important questions facing biological sciences. It turns out that species themselves may create new biodiversity. The idea is rather simple: as new species colonize and adapt to new environments, they create new opportunities for other organisms to colonize and adapt, potentially setting off a chain reaction of adaptation events within communities and across ecosystems. The process, referred to as “sequential” or “cascading” divergence, has been hypothesized to help explain a number of biological patterns including periods where there has been an explosion of new species following mass extinctions and the inordinate amount of species diversity found in tropical environments.

Do we observe sequential speciation in nature?

Well one of the most likely places sequential speciation might occur is within plant-feeding insects. Plant-feeding insects are the most biodiverse group of animals on the planet with as many as 30 million species. There are also many insects, some known as parasitoids, which use plant-feeding insects as their food, often by inserting their young within them so that they may eat their insect hosts from the inside out. Thus, it seems reasonable to think that, given the extraordinary diversity of plant-feeding insects and the many parasitoids that can potentially feed on them, that there would be a wealth of opportunity for sequential speciation to occur.  
 

The apple maggot fly, Rhagoletis pomonella, on its native host hawthorn. Photo credit: Hannes Schuler.

If we knew of a plant-feeding insect that recently diverged – split into new species – then we could look to see if their parasitoids have followed, also diverging into new species. Well…about 400 years ago, domesticated apples were introduced from Europe to the United States. These new apples did not go unnoticed by some of the locals. A species of fly that had long been living in the US that fed on the fruits of hawthorn plants (Rhagoletis) started laying eggs in apple fruits, and before long, the species had begun to split into two diverging lineages – one that still fed on the hawthorn plants and another that fed on the newly introduced apples.

What kept these diverging flies from merging back into one species? Well, the hawthorn and apple fruits produce different chemical signals (they basically smell different) and the flies evolved to use these differences to find their “favorite” host. Because the flies mate on their host plant, apple-loving flies only mated with apple-loving flies while hawthorn-loving flies only mated with hawthorn-loving flies. Over time, the two forms mated less and less and began to become genetically different. Also, the fruits develop at different times of the year, so the flies had to adjust their internal clocks to match when their food was available. That is they evolved (adapted) to match the timing of their host. Those best synchronized to their host, whether it be apple or hawthorn, had the highest fitness – passing on more descendants with similar traits – until they now not only differ in what smells they like, but when they are flying around looking for mates. Because they began to start mating at different times of the year, those who chose apple became less and less likely to mate with those that stuck with hawthorn. Crazy enough, all of this has actually occurred multiple times in other closely related flies, adapting to other plants, such as blueberries, snowberries and dogwoods.

So, did the parasitoids split into new species, like their fly hosts?

The parasitoid wasp, Utetes canaliculatus, searching for a Rhagoletis host on a snowberry shrub. Photo credit:  Hannes Schuler.

If this was happening, we would expect to see some of the parasitoids develop preferences for different fruits, just like the flies they were following. In research conducted in 2009, Andrew Forbes and his colleagues we were able to show that indeed this is exactly what happened. Not only that, the parasitoids adjusted their internal clocks to match their new host, just as the flies did. Both of these ecological changes helped the parasitoids split into two species (well at least on their way to being different species), those that attack flies on apple and those on hawthorn.

Now here’s where our new research comes in. Was it only a single parasitoid that followed the new species of fly that came to love apple, or were there other parasitoids that followed as well? That is, did a whole community of parasitoids adapt as they followed the Rhagoletis fly as it diverged into two species, all because apple trees were introduced into the US? Yes, that is exactly what happened. Three parasitoid wasps (Diachasma alloeum, Utetes canaliculatus and Diachasmimorpha mellea) basically split into new species of parasitoid wasps as they followed their fly hosts into different host plant environments. As you might have guessed, the new species of wasps also showed the same host plant related ecological differences as their fly hosts – they had a preference for odors of the different plants and adjusted their timing to match that of their host. Thus, the creation of new species can rapidly amplify biodiversity of entire communities. The question now is, just how widespread is sequential speciation and how often does it really contribute to the formation of biodiversity?

The study "Sequential Divergence and the Multiplicative Origin of Community Diversity" published in PNAS can be found at: www.pnas.org/content/112/44/E5980.abstract

About the author:
Glen Hood (left) is a PhD candidate in Jeffrey Feder’s (right) lab at the University of Notre Dame. He studies ecological speciation in Rhagoletis fruit flies and their community of parasitoid wasps. The only thing that parallels his love for biology is his passion for music. Glen is also looking for a postdoc, so be sure to contact him if you want him or his beard in your lab.