Population genetics to uncover life history and inform conservation of a federally threatened tiny plant

Article: 

Edwards, C. E., Albrecht, M. A., Bassüner, B. & Yatskievych, G. A. Population genetic analysis reveals a predominantly selfing mating system and strong genetic structuring in a naturally fragmented, threatened plant. Conserv. Genet.20, 1437–1448 (2019). https://doi.org/10.1007/s10592-019-01226-9

 Featured Image: the study species, Geocarpon, also known as Tiny Tim or earth fruit. Source: David Seidensticker, flickr.com

The forces shaping biodiversity

The world is facing an unprecedented loss of biodiversity. Scientists, policymakers, and citizens are working together to monitor the world’s natural diversity and its loss, and to outline conservation strategies.

There are three generally recognized levels of biodiversity: diversity of ecosystems, diversity of species, and diversity on the genetic level. Each level is important to conserving the world’s biodiversity as a whole.

A higher level of genetic diversity is associated with improved fitness (the ability of an individual to survive and reproduce), greater capacity of a species to adapt to environmental change, and healthier ecosystem functioning and resilience.

Properly measuring genetic diversity in the lab requires an understanding of the natural processes occurring in nature that shape genetic diversity.

Life history is defined by the Oxford Dictionary as an organism’s pattern of survival and reproduction. Characteristics such as a species’ mating system (sexual or asexual or both, does everyone mate, and who is mating with who?), methods of seed dispersal, pollinators, and the connectivity of habitats directly influence genetic diversity levels and distribution, the knowledge of which directly influences how we choose to conserve species.

Genetic diversity is typically measured at the population level, where a population is defined as a group of organisms in the same species, sharing the same geographical area and interbreeding.It can be difficult to observe the above plethora of life history characteristics out in the field, but population genetics, a field that studies the differences within and among populations of the same species, can give us clues to help unravel these questions.

In their recent study, published in Conservation Genetics, Edwards and coworkers aimed to use population genetics to demonstrate the influence of life history traits on the population structure of a rare species, with the aim of assessing and informing conservation strategies.

A tiny and timid subject

Their focus species goes by several names: Mononeuria minima, Geocarpon, Tiny Tim, earth fruit. If you haven’t guessed from one of its aliases, this plant is extremely small. It is also extremely rare. Although it is found in many states, its distribution is patchy- the plant is picky about where it chooses to grow. Geocarpon has a short lifespan, so can only be seen for a few weeks out of the year, and is listed as federally threatened under the Endangered Species Act. Needless to say, these characteristics make Geocarpon an elusive study subject!

Geocarpon plants and their habitat: a) a flowering plant next to a dime for scale, b) an individual Geocarpon, c) a landscape view of Geocarpon’s sandstone glade habitat in Missouri, d) a Geocarpon next to a quarter for scale. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature. Conservation Genetics, Edwards et al (2019).

The goal

Edwards and coworkers aimed to use population genetics to understand the basic biology of the species, including its mating system and whether separate populations mate with one another. They also wanted to use this information to assess and inform current conservation strategies for this species.

Methods

 Plants were collected across a range of states: Texas, Louisiana, Arkansas, and Missouri. 15-30 individual plants were collected from each site. This range in sample numbers exists because of the rarity of the species: the scientists did not want to remove too many plants from populations of variable sizes since Geocarpon is federally threatened.

They extracted DNA and performed analyses to determine the variation within populations, and how much separate populations differed from one another.

Uniformity within populations and high variation among populations

After genetic analyses, the team of scientists found that there was very little variation within populations. This finding suggests that the plants reproduce largely asexually.

There are also high levels of inbreeding, meaning that sexually reproducing individuals within populations are closely related to one another.

Separate populations were very different in terms of their genetic makeup, suggesting that there is little mixing of mates from different populations. Similarities only occurred when populations were geographically close to one another, suggesting that Geocarpon is a homebody: its seeds don’t travel far. Based on these data, Edwards and coworkers defined a Geocarpon population as a group of individuals separated by no more than 0.5 km.

The distribution of sample sites. Colors indicate genetic clusters. Reprinted by permission from Springer Nature Customer Service Centre GmbH: Springer Nature. Conservation Genetics, Edwards et al (2019).

So what is the fate of Tiny Tim?

The study confirmed that life history characteristics influence population structure. Low seed dispersal and high rates of selfing led to high population structuring (ie, populations are genetically separate and can be easily distinguished from one another).

Based on this finding, Edwards and colleagues assessed the current conservation strategy in place for Geocarpon. This strategy states that in order to be delisted from the Endangered Species Act, 15 viable populations of Geocarpon must be protected. These populations must span a range of environments and represent the full spectrum of the species’ genetic variability. The viability of populations must also be confirmed over 15 years of monitoring (USFWS 1993).

Our ability to meet these requirements was hindered by the lack of understanding of the influence of life history characteristics on the population structure of Geocarpon. Now that we have a working definition of a Geocarpon population, these criteria, and how well we are meeting them, should be reassessed.

The study’s findings show us that conserving this species will be challenging. Because each population is unique, in order to fully conserve the range of diversity, we must protect many populations. However, the resources and political will for conservation, especially of non-charismatic species, are limited. The authors suggested prioritizing populations which harbor alleles (versions of genes) that are not present in any other population. Additionally, they suggested ex situ (outside of its natural habitat; ie, in a botanical garden) conservation efforts when in situ (on-site) methods are not feasible.

This study goes further than many in not only assessing the genetic diversity of a species, but also discussing the life history factors that shape this diversity, and how conservation strategies can be developed in light of this information.

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Sarah Shainker

Sarah Shainker

Sarah is a first year Phd student at the University of Alabama in Birmingham with interests in evolutionary ecology, conservation genetics, citizen science, and macroalgae. Before beginning grad school, she worked as an outdoor educator in the north Georgia mountains and as a coastal resource management volunteer for Peace Corps Philippines. Twitter: @SarahShainker

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