Experiments in the Woods

Let’s say you wanted to have more genetic insight about a group of people in a region.  You would expect that individuals within families would be more similar than individuals who are unrelated.  But what might not be apparent without analysis is how much genetic diversity occurs over the entire region.  Genetic diversity is one component that can have a large impact on traits within a population. Take height in humans for example, a highly quantitative trait with a large range of phenotypes, or observed values.  Tall parents generally have tall children, short parents generally have short children.  But environmental effects such as malnourishment can also impact how short or tall a person is.  Say individuals within a family are generally short and exhibit low genetic diversity for regions of DNA, or loci that are partly responsible for height when looking at the family on a whole. In comparison, a family of mixed height people possesses high genetic diversity in these areas, again on a family basis. We could postulate that greater genetic diversity at these loci leads to a greater range of observed height within families.  We would say that the family of short people had loci that were fixed, or not diverse at the locus level. It is then a major function of population studies to estimate genetic diversity in order to understand how traits are inherited, how individuals are related, and how inheritance and relatedness affect the phenotypes we observe. 

In my case, I know of groups of plants but have no idea of how they are related at the local, regional, or national level.  Knowing this information can help refine the analysis of the iron acquisition traits we’re interested in (see previous post).  As mentioned in the example above, tall parents generally have tall children.  Plants adapted to a stressful soil type might have offspring that are equally adapted, however I can’t determine that yet.  But if the plants in a population are all closely related and intermating, this relatedness would be useful information to know in case these traits are conserved (common) in family structures. 

So how can you analyze a family of plants in the wild? First, you need to create one!  Within each population I observe, I identify individual plants that have flower buds and are capable of producing seed.  These plants are given an identification tag and leaves are sampled to determine genetic features (such as relatedness and diversity) among individuals in a population.  Each plant represents a family of half-siblings housed on a maternal plant:  pollination was at random with the paternal parents being unknown.  We can then estimate how each maternal parent performs individually or as a mean of all maternal parents in a population based on the performance of the offspring plants.  I’ll explain more later, but this is known as a half-sib mating design.

Mature R. viscosum individuals to be used as maternal parents.  Each flower will be pollinated randomly through natural pollinators.  Orange tags are placed on plants containing a labeling system and unique number.  Leaves from the parents are sampled and their location saved as GPS coordinates so that seed can be recovered when it is ripe in the fall.

I let pollination occur naturally, which for this species mostly occurs via butterflies and other insects.  By using this scheme, I can not only determine genetic diversity within each population, but compare it to other populations sampled.  This will help us get a better picture of how diverse this species is across its range.  In designating certain plants in a population as parents and by genotyping them (estimating the amount of genetic diversity present at the DNA level), we can also estimate the effect that relatedness has on traits of interest in the progeny.  The progeny will be grown from the seeds collected off these parental plants this coming fall and evaluated for the traits of interest (see previous post).  Relatedness will likely vary depending on location as some populations were smaller and more isolated than others. 

R. viscosum populations sampled throughout eastern Texas and western Louisiana.  Populations contained between 3 and 50+ mature individuals.  

A main reason for using parents in both half-sibling family and population analysis is that it is logistically simple.  A single plant can serve two functions, both as a parent in a mating design and as an individual for population analysis.  Because this species is never common and populations are isolated, other sampling strategies such sampling a plant every 5 miles are not realistic.  This is known as a transect, and is more appropriate for estimating genetic diversity in species which are common and continuous across a large area.  In my case populations are clearly defined and can be tested, through the genetic diversity we identify, to determine how unique the adaptations within and among each population are. 

I’ve been blogging from a McDonald’s as it is the only place in town with wifi, and will keep updating whenever I’m hungry.