The great garden escape: the transition from ornamental to naturalized species

This summer I’ve had the pleasure to travel and explore the region I live in. Visiting beautiful arboretums and botanical gardens, driving to parks both new and familiar and hiking the trails, and picking the leaves off invasive flora… Okay, that last one might seem a little strange, let me explain.

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The view from the trail. I love being a field ecologist!

The last several weeks I have spent collecting leaves of my two favorite invasive plants, Viburnum dilatatum and Viburnum sieboldii as part of my dissertation research investigating the genetic diversity of these charismatic plants. My question is how did these plants escape from the comfort of their gardens and go off into the wilds of the forest? So what does genetic diversity tell us? Simply, how genetically homogenous or heterogeneous individuals are within a population. By comprehending the genetic diversity of these viburnum populations compared to that of other viburnum populations I can hopefully understand how they did just that.

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The two invasive perpetrators in question. Left: Viburnum dilatatum and Right: Viburnum sieboldii

Currently, these two species are densely distributed along the I-95 corridor from the greater New York metropolitan area down through the greater Philadelphia area. So I have been traveling to parks where there are naturalized populations of these viburnum species, as well as arboretums where these plants reside in their living collection. I sample the leaves from individuals within these populations and living collections and bring them back to the lab to isolate their DNA. Once their DNA is isolated I can amplify certain regions of the DNA (AKA genetic marker) and send them out for sequencing. Once the region of interest is sequenced I can compare nucleotide sequence to understand how closely related individuals are within a population (i.e. population genetics) and how closely related populations are to one another (i.e. landscape genetics).

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Density of populations per county.

So in real world terms what does that mean? Well, one would expect high genetic diversity if plants reproduce sexually because each offspring is their own unique individual. The more unique individuals there are, the more diverse (or heterogeneous) the population is. However, if plants reproduce vegetatively (AKA clonally) or with individuals that are closely related than genetic diversity would be low and the population would be more homogenous.

So how do I compare populations? Using a haplotype network map (click here for a more detailed explanation). What the heck is a haplotype? Well let’s go back to biology 101. A haploid is one set of chromosomes; the haplotype focuses on certain gene region inherited from one parent. It allows us to figure out the structure of a population. A haplotype network looks like this.

Haplotype Network

Some preliminary data from my research used to create this haplotype network.

Each circle is an individual within a population and each population is represented by a different color. So in the above picture there are four populations and roughly two individuals per population. As you can see each circle is connected by lines with hatch marks. The number of hatch marks represents the number of differences in the nucleotide sequence between the gene region of interest in each individual; in this case it is rbcLa gene, AKA ribulose biphosphate carboxylase large chain (the gene responsible for making RuBisCo, remember, the stuff used in photosynthesis). As you can see most of the individuals are closely related except for 7A. Are you bored yet? Well, back to the bigger picture.

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An example of an aligned nucleotide sequence from the rbcLa gene for those seven individuals.

Once you have these nucleotide sequences from the region of interest you can compare the populations to understand the genetic diversity. You can overlay these haplotype networks on a map to get a practical idea of how the populations are structured to compare between populations. For instance by comparing individuals from naturalized populations around the region to that of individuals from arboretums I can deduce if plants are escaping from multiple arboretums where they reside in the living collection or if there is one source of spread.

Haplotype Map

Haplotypes from Philadelphia, central New Jersey, and New York City. Everyone is a little bit different.

This methodology will help me answer my original question, which was just how did these species escape cultivation. Species newly established away from their original location, whether native or invasive, have to overcome a founder effect, essentially reestablishing a colony with only a few original members of the species. With such a small number of individuals in a new location it usually means that the founder population has low genetic diversity. Ornamental plants also face the additional challenge of being clonal because they are often vegetatively propagated from cuttings of the original plant. Low genetic diversity can be a serious problem for species survival (explained here), were as high genetic diversity usually ensures a healthy population. So how did they do it; hybridization with other viburnum species or crossing with cultivars of the same species?

This fall as I sit in the lab and isolate DNA and amplify microsatellites I will find the answer to this question. This insight will hopefully help to develop strategies to help control the spread of these and potentially other invasive species. Whether it is advocating to remove them from the living collections or control and eradicate known naturalized populations. Whatever the solution, my research hopes to aid in this endeavor.

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