Laboratory of Chromosome Dynamics and Evolution

National Heart, Lung, And Blood Institute

Investigators

Linked publications, trials & patents

Abstract

Genetic elements compete for transmission to the next generation through meiosis, when haploid gametes are created from a diploid parent. Although Mendel's Law of Segregation states that each genetic element has an equal chance to transmit to the gametes, it is increasingly clear that this law is often violated by selfish elements. This non-Mendelian transmission of selfish elements in meiosis is called meiotic drive with significant impacts on chromosome evolution and sexual reproduction. To reveal the molecular basis underlying non-Mendelian segregation, we currently focus on selfish R2d2, a non-centromeric locus on mouse chromosome 2. This selfish element shows over 90% transmission ratio distortion with mild embryonic lethality. The underlying cell biological basis for both biased segregation in female meiosis and the embryonic lethality is unknown. To study the dynamics of the R2d2 locus during female meiosis, we developed two strategies to visualize this locus in mouse oocytes. First system is based on the recently developed FISH technique called Oligo-paint. In collaboration with Elissa Lais lab in NIDDK, we implemented the Oligo-paint technique to mouse oocyte cells. Oligo-paint of the R2d2 locus allows us to analyze if the R2d2 locus has different chromatin structure compared to the neighboring regions and perform IF-FISH to screen what proteins are recruited to the R2d2 locus to cheat the segregation process. In the past year, we analyzed the recombination landscape around the R2d2 locus using this Oligo-paint technique and found that selfish R2d2 is cheating more often in meiosis II division than in meiosis I. To understand how selfish R2d2 is cheating in meiosis II, we developed another method to live-image this locus based on the CRISPR/Cas9 technique. In this method, we microinject mouse oocytes with deactivated version of Cas9 protein fused to GFP (dCas9-GFP) in complex with sgRNA that recognize the R2d2 DNA sequence. dCas9-GFP specifically localized to the R2d2 locus, which allowed us to perform live-cell imaging to capture the cheating behaviors in action. The live-imaging reveled that selfish R2d2 is cheating in a very different way from selfish centromeres, which bias their orientation towards the egg side of the spindle in metaphase. Collectively, the Oligo-paint and dCas9 systems offer excellent opportunity to reveal the cell biological basis underlying non-Mendelian segregation of selfish R2d2 and how this cheating affects fertility. Meiotic drive is fundamental to sexual reproduction and has been recognized as a powerful force in genetics and evolutionary biology since first described in maize in 1942. The underlying mechanisms have long been mysterious to cell biologists. This project tackles this exciting problem, all the way from developing experimental systems to revealing how selfish elements challenge Mendel and affect fitness. Moreover, our work will lead to a deeper understanding of the interactions between chromosomes and spindle microtubules. These interactions are highly error-prone in humans and a major cause of infertility, which could be caused by selfish behaviors of meiotic drivers.