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Book Review

Wave the Yellow Flag

Original Paper: Gaskin, John F., Monica L. Pokorny, and Jane M. Mangold. “An unusual case of seed dispersal in an invasive aquatic; yellow flag iris (Iris pseudacorus).” Biological Invasions (2016): 1-9. doi:10.​1007/​s10530-016-1151-0 

Figure 1: A native blue flag iris growing on the banks of Blackbird Creek, a wetland that is part of the Delaware Bay estuary.

Figure 1: A native blue flag iris growing on the banks of Blackbird Creek, a wetland that is part of the Delaware Bay estuary.

Blue flag irises: a wetland flower

Irises are a beautiful flower commonly found in full bloom this time of year. But irises are not just for home gardens, they are also a common wetland plant!

Anyone with a careful eye who has been to the banks of freshwater pond, river, or even upper reach of an estuary may have spotted a wild iris. Blue flag irises are lovely purple-blue flowers that are native to the wetland habitats of the United States and offer a beautiful splash of color along wetland shorelines (Figure 1).

So if you saw a yellow variant of this blue iris, you’d probably think it was an adaptation or a byproduct of cross-pollination. Unfortunately, you would be wrong. The yellow flag iris (Iris pseudacorus) is an invasive wetland plant native to Europe that is slowly starting to invade our wetlands (Figure 2). These irises are used as ornamental wetland flowers, but are now being found in the wild along freshwater coasts throughout the United States.

A lovely invader: The yellow flag iris

Yellow flag irises grow in saturated (very wet) soils and can even survive living partially submerged in water, which is why they are most commonly found within a meter of a water bank. In their native European range, yellow flag irises mostly reproduce asexually by forming clones underground when pieces of their rhizome (root system) break off. In Ireland, clumps of genetically identical yellow flag irises have been found up to 20 meters wide!

Figure 2: A clump of invasive yellow flag irises growing along the banks of the Blackbird Creek River in Delaware.

Figure 2: A clump of invasive yellow flag irises growing along the banks of the Blackbird Creek River in Delaware.

Understanding how an invasive plant reproduces is critical to understand how to manage, reduce, or even eliminate it from an ecosystem. Generally speaking, the best invasive species are those that can reproduce quickly. Yellow flag irises can reproduce asexually by cloning itself or sexually by dispersing seeds. Wetland plants, including irises, are unique in that most seeds are dispersed by floating on water, allowing them to travel pretty far distances!

So, Gaskin et al. set out to investigate: what type of reproduction strategy are invasive yellow flag irises using?

The Study Approach

The investigators collected tissue samples from twenty different populations of yellow flag irises throughout the Pacific Northwest. These populations were different clumps of irises at least 5 meters apart since previous research has found 5 meters is usually enough distance for the clumps to be genetically unique. In total, 171 plant samples were collected!

DNA was extracted from each plant sample and amplified using technique called polymerase chain reaction (PCR). In this study, that amplified DNA was on primer pairs that would help determine polymorphisms, loosely speaking, how genetically diverse one individual plant was from another. This will help determine whether different populations of yellow flag irises were clones (asexual reproduction likely from pieces of their rhizomes breaking off) or unique individuals (seeds).

Figure 3: Conceptual image adapted from Gaskin et al. (2016) showing the Simple Matching Value. Two plants that score a one are genetically clones while the further away from one implies a greater degree of genetically diversity. 98% of the individuals tested were found to be unique.

Figure 3: Conceptual image adapted from Gaskin et al. (2016) showing the Simple Matching Value test. Two plants that score a one are genetically clones while the further away from one implies a greater degree of genetic diversity.

Gaskin et al. did a few different tests, including a Simple Matching Values test which is a ratio of matching loci (the location of a gene on a chromosome that can be determined with the extracted DNA) over the total number of loci between two plants. The closer that number is to 1, the closer they are genetically (Figure 3).

The researchers also tested seed viability to help better understand how well the yellow flag irises’ seeds could establish new populations. This was done using dyes. Seeds that absorb water will respire if they have the ability to grow. Seed embryos turn red from the dye if they respired.

The Findings

Genetically identical clones were only found in five plant pairs, even taking into account method error. This means 98% of the 171 plant samples were genetically unique (Figure 3). Additionally, one third of the yellow flag iris populations with >8 plants had a low single genetic cluster value, indicating that the cluster had more than 1 genetic source.

Figure 4: Blue and yellow flag irises growing side by side in a tributary of Delaware Bay.

Figure 4: Blue and yellow flag irises growing side by side in a tributary of Delaware Bay.

All these factors point to sexual reproduction, not the expected vegetative reproduction approach, to be the mechanism for the yellow flag iris invasion. This was an interesting finding since asexual reproduction is the most common spreading tactic of yellow flag irises in their native European habitats.

Additionally, 99.1% of the seeds tested were viable, aka ready to become a new plant. Each iris had 5-6 seed pods which each contained an average of 120 seeds. Now that’s a lot of potential new plants! Since seeds can float, new populations can be found and established over great distances from the original parents!

The Significance

The yellow flag iris is quite beautiful, but is currently invading wetlands in the United States. They produce a large number of viable seeds each year which can float down rivers and upper parts of estuaries, allowing for new populations to be created over a large distance. While it is unclear what ecological effects this invasion will cause, it is possible they may out-compete the native blue flag irises since they grow in the same areas (Figure 4). Land managers hoping to reduce or control its spread will likely have to find a way to limit seed dispersal or viability, which is a hard thing to do. Until then, we are only just beginning to understand how pervasive this invasion will get!

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