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 life-history 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