Do we need to redefine the term species in the wake of avian malaria research?

Blog on the SBS seminar (11^th March 2021) from Prof Javier Pérez-Tris written by Nancy Stewart, student from the “Meet the scientists” course

The naming and characterization of individual species is fundamental to biology in representing the principle taxonomic unit in grouping organisms. This is one of the first things we learn about the natural world and may seem simple in some cases, such as spotting the stripes on a tiger, but sometimes it is far less straight forward. A species is usually defined as individuals capable of sexual reproduction to produce fertile offspring. However, when we are talking about single celled organisms this becomes much more difficult to define. Such species do not interbreed like macro-cellular species, so the traditional definition of species does not explain differences between groups of individuals. So how do we group species in the case of single-celled organisms?

This is an issue faced by scientists studying avian malaria parasites such as Professor Javier Pérez-Tris, at Computense University of Madrid. I was lucky enough to hear him speak on this topic at a recent SBS seminar. Pérez-Tris studies avian malaria to find how this disease spreads across the world in migratory birds and how malaria causing parasites impact on populations of bird species that are particularly vulnerable to this disease. Pérez-Tris showed how the discovery of new species can completely change our outlook on avian malaria. These parasites are single-celled organisms with very few physical traits which can be used to distinguish individual species. They have been studied for almost a century and for a long time only a few hundred species were thought to exist.

Many scientists accepted this as fact. It wasn’t that they were missing species but that there just weren’t many species to observe using classical approaches, such as microscopic observation. Usually for new species to evolve, the environment needs to be variable with different niches for the organisms to take advantage of. However, avian malaria parasites live in blood, which is a uniform environment devoid of such variation.  Additionally, the geographical barriers which create most species were thought to be absent for avian malaria parasites. Birds migrate huge distances and so parasites infecting birds in Africa could infect bird species in Europe for example.

Recently, a new generation of researchers armed with technology which allows them to delve into the DNA of these organisms have rejected this long-held hypothesis. By sequencing the DNA of single-celled organisms, they have discovered many more avian malaria parasites and found new ways that the parasites can adapt and evolve into new species. Because only a few species were thought to exist, they were considered to be generalist species, infecting many bird species. However, by looking at the DNA of avian malaria causing parasite groups, Pérez-Tris has split them into multiple species with individual genetic lineage (or probable species) infecting a different bird species. He also discovered that many parasite species do not infect birds at migration sites with most parasites being inactive and unable to infect new individuals during migration. The research also helped to identify that there are, indeed, some generalist parasite species present infecting many different bird species with parasites remaining active during migration and, thus, capable of infecting birds in different geographical locations. It, therefore, seems that generalist parasite species are extremely successful relative to specialist parasites.

This leaves many of us asking why specialists would ever evolve? If being a generalist and infection during migration is so successful, then why would parasites evolve to do the opposite? Well, Pérez-Tris proposes that although generalists are successful in the short term, these species tend to go extinct or evolve into specialist species in the long term, which is not a suitable or stable strategy in the long term.

Having changed the landscape of avian malaria research, researchers are now grappling with the much more complex issue of what exactly distinguishes one species form another. Although this seems a simple question, it rarely produces a simple answer, as exemplified by avian malaria research. If we look at Pérez-Tris’ research on the avian malaria parasite of two warbler species in Europe, we see how complex this issue can be. He found two distinct genetic lineages of parasite, each found almost exclusively in one bird species. Despite the overlapping ranges of the bird species, it was very rare to find a single parasite infecting the other bird species, or for coinfections to occur with parasite lineages live very separate lives. This would be enough for most of us to assume they are separate species. However, when Pérez-Tris looked at the DNA of the two parasites he found very little difference between the groups, which challenges the definition of such parasites as being separate species.

This work leaves many of us in awe of new genetic and genomic approaches in addressing the fundamental question: How do you define distinct parasite species? What was previously defined by physical traits and life strategies is now being redefined using DNA which changes our understanding of the concept dramatically. The work highlights questions, such as: How related do two individuals need to be before they are considered as one species? How does this relate to the physiological make up of such organisms?

Hopefully, we’ll see Pérez-Tris produce more answers for us in the not-too-distant future.