Chromosome-Specific Bias in Double Crossover Events During Meiosis in the Holocentric Pantry Moth 

LS Lee¹, RK Runyan¹, A Mitra², L Raab^1,3, C Heryanto¹, M Richmond⁴, L Benner¹, R Dale², and LF Rosin¹.

1. Unit on Chromosome Dynamics, Division of Developmental Biology, Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD.

2. Bioinformatics and Scientific Programming Core, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD.

3. Laboratory of Biochemistry and Genetics, National Institute of Diabetes and Kidney and Digestive Diseases, National Institutes of Health, Bethesda, MD.

4. Stowers Institute for Medical Research, Kansas City, MO. 

Abstract Text:

Meiosis is the specialized cell division that produces sperm and egg cells. During meiosis I, homologous chromosomes pair for genetic recombination. Recombination not only facilitates the exchange of genetic information but more importantly, it creates physical links between the homologs (chiasma or crossovers; COs). COs are essential for holding homologs together and ensuring their equal segregation at anaphase I. COs occur at sites of programmed DNA double-stranded breaks (DSBs), but ten times more DSBs form than COs, which are limited to one per chromosome in most species. In humans, the majority of meiotic errors result from defects in the number and placements of COs, leading to infertility or aneuploidy in progeny. Yet, how DSBs are designated to become COs is unclear, particularly in humans where it is difficult to study meiosis. This necessitates the use of model organisms for studying CO designation. Moths, in particular, are excellent model systems to study meiosis due to their mammalian-like number of chromosomes and the fact that prophase occurs throughout development in both males and females, making pairing highly accessible. Our lab utilizes the pantry moth Plodia interpunctella, due to its small size and Drosophila-like rearing qualities. Our previous cytological analyses of Plodia chromosomes during meiosis revealed the presence of two different metaphase I structures: rod-shaped bivalents and ring-shaped bivalents. We hypothesized that these two structures form as the result of either single or double distal crossovers, respectively, but crossover formation has never been studied in Plodia or any other Lepidopteran system. To uncover how many COs form per chromosome in Plodia, how this is regulated, and how this influences the bivalent structure at metaphase I, we are employing a combination of genomics, cytology, and proteomics. We generated haplotype maps for two different lab strains of Plodia using a combination of PacBio long-read sequencing and Illumina short-read sequencing. By sequencing the genomes of more than 50 inter-strain hybrids, we were able to precisely map COs in this species. We found extremely high crossover assurance and interference in Plodia, with most chromosomes harboring only a single, distal CO. Curiously, and in agreement with our cytology, chromosomes 1, 3, 5, and 22 were found to have an extremely high frequency of double crossovers (between 30-50%). Genome-wide, we saw no evidence of a “centromere” or “telomere” effect impacting the placement of COs. Instead, COs most often form near telomeres in regions that are devoid of active chromatin marks. Future studies will include the cytological mapping of COs, as well as proteomics to identify CO regulators in Plodia. With this information, we will be better able to uncover conserved and unique aspects of the CO designation pathways in different species, allowing for the development of new therapeutic targets or approaches for human infertility.