Photo by Adam Baker licensed under CC BY-NC 2.0

The mass blooming of bluebonnets (Lupinus texensis) is truly one of southern North America’s most stunning natural spectacles. Celebrated across its native range, the bluebonnet has greatly benefited from supplemental planting by humans. Indeed, in states like Texas, hundreds of miles of roadsides are seeded with bluebonnets every year and the end result can be spectacular. The popularity of mass seeding of this wonderful species has led some to ask how the practice may be affecting the genetic diversity of the species throughout its range.

Before we get into population genetics, it is worth getting to know this plant a bit better. Bluebonnets are a type of winter annual lupine endemic to southern Texas and northern Mexico. Their highly camouflaged seeds usually begin to germinate late in the fall after enough weathering has weakened the hard seed coat the protects the embryo. Seedlings remain small throughout fall and winter, rarely growing more than a few tiny, palmate leaves. Once spring arrives, growth accelerates.

Within a few short weeks, most individuals will have already pushed up a spike chock full of their characteristic blue and white flowers. Their main pollinators are bumblebees such as the American bumblebee (Bombus pensylvanicus). Once pollinated, plants don’t waste any time producing seeds. Bluebonnets utilize an explosive seed dispersal mechanism, which can be pretty fun to witness in person. As the pods mature, they gradually dry out, creating a lot of tension. Eventually, the tension within the pod becomes so great that the whole structure gives in and explodes, launching seeds as far as 13 feet (4 m) away from the parent plant where they will wait until fall returns.

Photo by Danny Barron licensed under CC BY-NC-ND 2.0

Although 13 feet may sound like a decent distance for a plant the size of a bluebonnet to launch its seeds, it pales in comparison to many other forms of seed dispersal. As such, one would expect bluebonnets within any given population to be more closely related to one another than they would be to bluebonnets growing in other, more distant populations. It is this assumption that led scientists to ask how intentional seeding of bluebonnets may be affecting the genetics of these plants. Before we jump into their findings, I first want to make one thing very clear.

I am in no way disparaging intentional seeding of native plants, especially not by municipalities! I think the practice of seeding with native plants is vital to any environmental management practice we humans undertake. That being said, it is important that we try to understand how any of our actions may be impacting any aspect of biodiversity. Now, onto the research.

By sampling the DNA of both natural and intentionally planted populations across a wide swath of bluebonnet’s endemic range, scientists revealed an intriguing picture of their genetic structure. Simply put, there is surprisingly little. Where they expected to find genetic differences among populations, they instead found a lot of uniformity. It is almost as if populations were mixing their genetic material across the range of the species.

There are a few possible explanations that could explain this pattern. For one, it is possible that estimates of seed dispersal in this species are vastly underestimated. Perhaps seed dispersal events regularly exceed previous estimates of around 13 feet. Along a similar line of reasoning, it is also possible that bluebonnets don’t rely solely on ballistics to get their seeds out into the environment. If birds or mammals occasionally move seeds long distances, this could eventually lead to genetic mixing among different populations. However, such possibilities are unlikely given the nature of bluebonnet seeds and the fact that animals are far more likely to act as seed predators for bluebonnets than seed dispersers.

Scientists have also put forth the possibility that bluebonnets in both natural and cultivate populations simply haven’t been isolated long enough for genetic differences to emerge among populations. However, this does not explain why there is so few genetic differences among widely separated natural populations.

The most likely reason why bluebonnets are so alike genetically is intentional planting. Though plenty of effort is put into ensuring that bluebonnet plantings are done using seeds sourced within 124 miles (200 km) from the planting site, we simply can’t rule out the idea that genes from individuals sourced from cultivation are not completely swamping the gene pools of wild populations as they are sowed along roadsides and into other planting projects.

To be fair, though these findings are compelling, we can’t necessarily jump to any conclusions as to whether such genetic swamping is a net negative or net positive for bluebonnets across their range. The scientists involved with the study do mention that swamping of fractured wild bluebonnet populations with genes of cultivated individuals could prove beneficial for the species, especially as the impact of human development continues to increase. It is possible that cultivated individuals that are selected because they perform well in human-dominated environments are introducing genes into wild populations that may allow them to cope with the increased human disturbances.

The alternative argument to that point is that we are swamping wild populations with potentially deleterious alleles at a faster rate than natural selection can purge them from the population. If this is the case, we may see a gradual decline in some populations that grow more and more out of sync with their local environment.

Though it is far too early to draw any hard fast conclusions about the impacts of genetic swamping, the genetic patterns that have been uncovered among bluebonnets are important to document. Now that we know that genetic diversity is low across populations, we can begin to dive deeper into both the mechanisms that created said patterns and their impacts on various populations. Once again, this is not an argument against intentional seeding and planting of native plants. Instead, it is a nice reminder that even the best intentions can have vast and unintended consequences that we need to study in more detail.