Download Genetic basis of flowering time variation in Arabidopsis thaliana

Genetic basis of flowering time variation in Arabidopsis thaliana
Reiko Akiyama
Because plants are sessile organisms, adaptation to the local environment is important for the persistence
of plant populations. More generally, the genetic basis of adaptation is of fundamental interest in
evolutionary biology. Adaptation to the local environment is often reflected as a correlation between the
trait under natural selection and the selective factors in the environment. Flowering time is a key lifehistory trait in the annual herb Arabidopsis thaliana. Previous studies demonstrated that northern
populations of A. thaliana flower late compared to southern populations when grown in common
environments. FLOWERING LOCUS C (FLC) is one of the candidate genes that control flowering time in
A. thaliana. When functional, FLC induces late flowering, but many mutations that disturb normal gene
activity have also been observed. One of them is the presence of a transposable element (transposon) and
another one is the substitution of a nucleotide by another called single nucleotide polymorphisms (SNPs).
If FLC is involved in the control of flowering time variation in natural populations, we expect to find an
association between those mutations and flowering time. If selection acts on flowering time varies with
latitude, there should also be a correlation between mutations at FLC and latitude.
To understand the genetic basis of variation in flowering time in natural populations of A. thaliana, I first
investigated the genetic variation in the candidate flowering time gene FLC in 17 populations from
Scandinavia (15 populations) and Italy (2 populations) and then examined the relationship between
polymorphism at FLC and latitude. I chose to study a region at FLC whose importance for normal gene
activity was known and for which both transposon and SNPs had been characterized. After scoring all
individuals for these mutations, I analyzed the data to determine whether there was a correlation between
them and latitude. I also carried out a population genetics analyses to characterise the observed variation at
FLC. There was a weak correlation between genetic variation at FLC and latitude, however, the pattern
was not pronounced. The population genetics analyses showed that the genetic variation at FLC did not
depart significantly from other genes in A. thaliana, and was consistent with the standard neutral model of
evolution, which indicated that the genetic variation at FLC was not likely to have been subject to
selection.
Since most recent studies show that A. thaliana displayed a strong population structure which could have
an effect similar to that of selection also at neutral markers, we cannot directly associate flowering time
and genetic variation in an individual gene. Thus, before carrying out an association study, I excluded
population structure by creating unstructured populations. If the correlation between the genetic variation
and latitude still exists even in an unstructured population, then we can conclude that there is a significant
association between flowering time and the genetic variation. I used some known molecular markers that
can be assumed to be unaffected by selection in order to create unstructured population. There was
extensive population structure and I got approximately as few as ten individuals for each unstructured
population. Since a higher number of individuals is needed to carry out a meaningful association study, I
did not proceed further.
In summary, both the weak correlation between the genetic variation at FLC and latitude, and the result
of the statistical analyses suggest that FLC is not the only gene that controls flowering time in A. thaliana
natural populations. Strong population structure in the studied populations shows that we cannot ignore
population structure in studies of A. thaliana, which has long been considered to have weak population
structure.
Degree project in Biology20p, spring semester 2005
Department of Plant Ecology
Biology Education Centre
Supervisor: Jon Ågren.
Examensarbete i biologi, 20p, VT 2005
Avdelningen för växtekologi
Institionen för biologisk grundutbildning
Handledare: Jon Ågren