The evolution of chromosome structure meiotic pairing and silencing of the heterologous sex chromosomes in the plant genus Silene

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Human individuals normally develop as either a male or a female and the genetic mechanism that determines the sex of an organism is based on sex chromosomes: Females have two X chromosomes, while males have one X and one Y chromosome, which is male-specific. During embryonic development the individual genes on the Y chromosome trigger a chain of events leading to the development of a male. Apart from the sex chromosomes, cells contain other chromosomes, called autosomes. Each chromosome is normally represented twice, for example, in the human there are 46 chromosomes, consisting of a pair of sex chromosomes and 22 pairs of autosomes. In the egg and sperm the chromosomes are represented once. In order to achieve this the meiotic pathway occurs, the chromosomes form pairs, which is relatively straightforward for the autosomes and XX in females. Things become more complicated when two different sex chromosomes, X and Y, have to pair in males. The cell has to ensure that a correct pairing is formed between the X and Y chromosomes and depends on pairing in a small region that is similar in both chromosomes, known as the pairing region. The mechanisms involved in these processes are studied to some extent in humans and the mouse, but no information is available for independently evolved plant sex chromosomes. Understanding how sex chromosomes have evolved independently in evolution and how they function in different organisms is of great biological interest. In this project we will study how plant sex chromosomes make sure the pairing occurs correctly and how the structure and function of plant sex chromosomes evolved. Plants often have male and female organs on the same individual. Species with separate male and female individuals are rare in plants and some of these species are known to contain sex chromosomes. The white campion, Silene latifolia has separate sexes (male and female plants) and has a sex chromosome system similar to mammals; females have two X chromosomes, while males contain X and Y chromosomes. The sex determination system in this species is relatively young and likely to have evolved 10-15 million years ago (MYA) compared to ~320 MYA in mammals. Sex chromosomes have evolved only in a few species of the genus Silene, allowing us to compare the structure and behaviour of chromosomes in species with and without sex chromosomes. This provides a unique opportunity to study evolutionary changes that have led to sex chromosome evolution. The two questions we want to understand are how have the sex chromosomes evolved in the campions, and how they negotiate meiosis successfully, given the X and Y chromosomes are largely different to each other and therefore have specific problems in pairing, synapsis and recombination. For this purpose we will search for DNA probes that hybridise specifically with S. latifolia sex chromosomes. The location of these probes on the sex chromosomes (and chromosomes homologous to sex chromosomes) in different species will be studied and compared using fluorescent in situ hybridisation, the method which allows to localise fluorescent-labelled DNA probes hybridised to chromosomes fixed to a glass slide. The pairing of sex chromosomes during cell devision (meiosis) will be studied using fluorescent-labelled antibodies, the proteins that specifically bind other proteins, in our case the proteins involved in chromosome pairing and recombination. Using these tools we will be able to observe the paired and unpaired regions of the sex chromosomes as fluorescing foci on chromosome spreads. The comparison of structure and meiotic behaviour of sex chromosomes in Silene on the one hand and mammals on the other will help to understand whether and to what extent meiotic processes and controls are conserved across all eukaryotes. A plant model such as the one provided by the Silene genus is very advantageous and likely to be useful for general principles in meiosis and therefore reproduction in humans.