Project Summary/Abstract Chromosome abnormalities due to meiotic errors are a leading cause of birth defects and spontaneous abortions in humans. Our overarching goal is to understand how the organization of chromosomes in the nucleus contributes to the correct pairing, synapsis, and recombination of homologous chromosomes during meiosis I prophase– and how infidelity in these processes lead to chromosomal abnormalities. The basic mechanisms leading to homolog pairing, synapsis and recombination are well conserved. The study of a wide range of organisms has ultimately led to insights into human gamete aneuploidy and infertility. We use two model organisms, budding yeast and zebrafish, each providing a unique lens to address how the 3D configuration of chromosomes is governed to accommodate the changes in the nuclear landscape throughout meiotic prophase. Our work addresses three key questions in the field of chromosome biology: 1) How do chromosomes balance the contributions of diffusive versus active motion, 2) How does movement promote molecular transactions between chromosomes? And 3) how do cells sense and respond to unpaired meiotic chromosomes to ensure reproductive fidelity? 1) To understand how chromosomes move, we will examine the contributions of diffusion, constrained diffusion, and active motor-driven movement on chromosomal loci in yeast by comparing data sets of XYZ coordinates of tagged loci over time using our newly developed imaging pipeline. We will compare these outcomes with newly developed models of chromosome behavior based on simple biophysical properties of polymers. We will test if the nuclear pore complex also contributes to chromosome motion, building on our discovery of a role of the NPC in meiotic chromosome dynamics. 2) To understand how the organization of chromosomes in the much larger vertebrate nucleus contributes to effective homolog pairing we will build on our recent work in zebrafish showing the initial events of pairing and synapsis all take place at the telomere bouquet, suggesting that pairing in the larger nucleus is accommodated by temporally and spatially sequestering pairing factors in time and space. We will test if telomere attachment or positioning at the nuclear membrane is important for pairing, and we will identify the epigenetic markers that define pairing-competent features of chromosomes. 3) To understand how cells respond to unpaired chromosomes and how does this response differ between species, and even between sexes of the same species, we will take advantage of our recent findings that synaptic errors cause arrest in spermatogenesis in zebrafish males. Furthermore, synaptic errors in females are tolerated, thus raising the tantalizing possibility that the surveillance and silencing of asynapsed chromosomes checkpoints does not operate in zebrafish.