What are Polyploids and why are they so great?

Many species, including humans, have two sets of chromosomes (diploids), one from the mother and another set from the father. Polyploids have more than two complete sets, which may come from the same (autopolyploid) or even different (allopolyploid) parental species. The more sets of chromosomes, the more sophisticated patterns of behaviour they may show when cells divide, creating a wider range of genetic possibilities for the offspring.

 




We can see below that many of our most important crop species are polyploids. For example, potato, the world’s third most important food crop, is an autotetraploid species with four complete sets of chromosomes (4x). To feed the growing human population, we face the challenge of breeding new improved varieties of these polyploid crops that are more robust, higher yielding and able to face the challenges caused by climate change. To meet this challenge we aim to understand how polyploidy affects the genetic makeup of plants in terms of both structure and function.

A Showcase of Polyploids...

Arabidopsis as a model autotetraploid

One of the approaches we are taking involves the model plant Arabidopsis thaliana, a simple weed in the mustard family that grows happily in the glasshouse. By artificially creating tetraploids(4x) from diploid (2x) plants, we can study the effects of genome duplication at many different levels. Many polyploids have bigger organs than the corresponding diploids. See here the increase in flower size in the plant on the left (autotetraploid) compared to its diploid parent on the right.





Using Arabidopsis as our model, we are interested in the consequences of polyploidy for fundamental cellular processes, including meiosis, a central process in the life cycle of all sexually reproducing organisms. During meiosis (see below) there is a reciprocal exchange of genetic material between sets of chromosome. This process of chromosome reshuffling or recombination generates new genetic variation that is the raw material for both natural selection and artificial selection used in plant breeding. We seek to understand how polyploidisation affects this process, and in doing so, our work is important for developing breeding strategies for the world’s most important diploid and polyploid food crops.

Meiosis in a diploid cell.

Genetic Analysis of Complex Traits in Plants

As quantitative geneticists, we are interested in the genetic basis of complex traits that are controlled by many different genes and also influenced by environmental factors. Most of the traits that are important to plant (or animal) breeders are complex in nature, including yield, resistance to stresses such as drought, and disease resistance. Our work involves developing novel methods to discover the genes underlying complex traits in diploid and polyploidy species.

 

Why is Genetic Analysis with Polyploid Species a Challenge?

Developing methods for mapping complex traits in polyploids is challenging yet essential for any breeding program that aims to improve the performance of polyploid species. Polyploids display more complicated patterns of gene segregation (separation) during meiosis compared with diploids. In a diploid meiosis there is only one way that the two copies of chromosome can pair. Compare this with an autotetraploid meiosis, where the four copies of each chromosome can pair up in three different ways, or even form a group of four chromosomes (a quadrivalent). Such added complexity creates a huge number of possible outcomes during meiosis. We are developing experimental and statistical methods and tools for genetic analysis in polyploids that take full account of the complex nature of inheritance in these species, which can be used to map complex traits important in agriculture.