Background

The research in my lab is focused on understanding the molecular and population genetic basis of adaptation. In particular we are interested in how population genetic processes, such as genetic drift, recombination and natural selection, are affected by population structure. Most plant species are geographically structured and it is important to determine how and to what degree this structure influences the evolutionary process.

 

Nucleotide polymorphism, linkage disequilibrium and haplotype structure in Populus tremula
Natural selection can act to differentiate populations for adaptive traits even in the face of substantial migration. A classic example in forest trees is adaptations to the steep latitudinal gradient in the length of growing season that characterizes northern environments. Trees often show latitudinal clines in important traits such as the timing of bud set, the onset of flowering and frost hardening even through population differentiation at neutral molecular markers is generally low. I am interested in the genetic basis and evolution of phenotypic traits that are responsible for the adaptation of plants to such environments and how this process in influenced by population structure. Methods for analyzing the genetic basis of adaptive traits (such as QTL analyses and association mapping studies) can be confounded by the presence of population structure since it results in spurious correlations between phenotypic traits and marker alleles, thus making it more difficult to locate genes responsible for phenotypic variation.

We are currently investigating levels of nucleotide polymorphism and linkage disequilibrium in the deciduous tree Populus tremula to infer patterns of haplotype structure over a latitudinal gradient across Sweden.  The aim is to evaluate the possible utility of association mapping methods for investigating the genetic basis of adaptive traits. Knowledge of population structure is essential in association studies and it will be estimated in P. tremula using SSR and SNP markers. Once population structure is estimated it can be incorporated into analyses of the pattern of linkage disequilibrium and haplotype structure of the P. tremula genome.

The evolution of dioecy (separate sexes) and sex determining systems in plants
Separate sexes and chromosomal sex determination is the rule among most higher animals. Among plants dioecy is far less common, c. 5% of all angiosperm species are thought to be dioecious and chromosomal sex determination is even less common.

Plants have evolved a wide variety of sex determining mechanisms, including genic (Mercurialis and Cucumis) and chromosmal (Silene and Rumex) sex determination. In several plant species sex is also environmentally determined.

One of the most striking features of the separation of the sexes is that is that selection may act differently on genes expressed in males and in females. The extreme example of this is the degeneration of the non-recombining part of chromosome that is specific to the heterogametic sex in chromosomal sex determination systems. There are several possible explanations for the degeneration of the Y-chromosome including Müller's ratchet, selective sweeps, background selection, sexual selection and the accumulation of repetitive sequences. These forces are expected to leave different signatures in the patterns of variation at homologous genes located on the sex chromosomes.

We are currently tracing the evolutionary history of sex chromosome evolution in the genus Silene using sex-linked microsatellites. We are also investigating the mechanisms behind sex determination in Populus and Salix.

 

Ingvarsson, P.K. () Restoring genetic variation lost - the genetic rescue hypothesis. Trends in Ecology and Evolution 16: 62-63.

 

Ingvarsson, P.K. and Taylor, D.R. () Genealogical evidence for epidemics of selfish genes. Proceedings of the National Academy of Sciences of the United States of America 99: .

 

Ingvarsson, P.K. () A metapopulation perspective on genetic diversity and differentiation in partially self-fertilizing plants. Evolution, 56: .

 

Ingvarsson, P.K., Ribstein, S. and Taylor, D.R. () Molecular evolution of insertions and deletion in the chloroplast genome of Silene. Molecular Biology and Evolution, in press.