Why did the chicken cross the road? Because there was a crossover hotspot.

Gene mutations often get a bad name. But they are essential for populations to become more diverse. By acquiring new traits – ideally superhero powers or at least vaguely useful ones – a population can move from lower to higher fitness levels over time. ‘Fitness ‘ in this context does not mean completing the London Marathon but to survive and produce offspring, and sexual reproduction speeds up the whole thing. Sexual reproduction requires two haploid gametes which cells produce through meiotic cell division. In the first stage of meiosis matching chromosomes cross over and swap chunks of DNA with each other. This exchange of genetic material creates new gene combinations and possibly new traits.

http://commons.wikimedia.org/wiki/File:MajorEventsInMeiosis.jpg#file
http://commons.wikimedia.org/wiki/File:MajorEventsInMeiosis.jpg#file

Dr Ian Henderson, University of Cambridge, received the 2013 SEB President’s Medal in the plant section, in recognition for his achievements. Dr Henderson and his co-workers study crossover hotspots in meiotic recombination in the model plant Arabidopsis thaliana. Chromosome crossovers during meiosis does not happen randomly, but rather within certain chromosome regions. Crossover events occur more frequently in regions with many genes, but less often around the centromeres. Centromeres are anchor points on chromosomes to which the microtubule spindle attaches before lining them up and pulling them apart. This makes them very important structures and crossovers close to or within them can have serious consequences such as incorrect chromosome separation or breakages.

From left: President's Medallist 2013 Dr Ian Henderson and Young Scientist Finalists Vidya Pawar, Niaz Ali, Plant section chair Prof Christine Raines.
From left: President’s Medallist 2013 Dr Ian Henderson and Young Scientist Finalists Vidya Pawar, Niaz Ali, Plant section chair Prof Christine Raines.

Centromeres are ‘protected’ by epigenetic regulation, which marks the areas on chromosomes where crossovers are allowed to happen. This is possible because DNA is wrapped around proteins, so-called histones, similar to spools with thread wrapped around them. A range of enzymes can attach chemical tags such as methyl groups to DNA or to histones and these modifications influence gene expression.

Yelina, Henderson and colleagues mapped chromosome crossover events in Arabidopsis thaliana wild type plants containing the fully functional DNA methyltransferase METHYLTRANSFERASE1 (MET1) and compared them to met1 mutant plants. As MET1 is not working properly in met1 mutant lines, DNA methylation is massively reduced in these plants. Chromosome crossovers in met1 lines occurred more frequently close to centromeres compared to wild type plants (Yelina et al., PLOS Genetics 2012).

This post is a continuation of my coverage from the The Society for Experimental Biolgy (SEB) annual meeting, which took place in Valencia at the beginning of July 2013. You can read my other posts here.