Two mitochondrial DNA polymorphisms modulate cardiolipin binding and lead to synthetic lethality

Genetic screens have been used extensively to probe interactions between nuclear genes and their impact on phenotypes. Probing interactions between mitochondrial genes and their phenotypic outcome, however, has not been possible due to a lack of tools to map the responsible polymorphisms. Here, using a toolkit we previously established in Drosophila, we isolate over 300 recombinant mitochondrial genomes and map a naturally occurring polymorphism at the cytochrome c oxidase III residue 109 (CoIII109) that fully rescues the lethality and other defects associated with a point mutation in cytochrome c oxidase I (CoIT300I). Through lipidomics profiling, biochemical assays and phenotypic analyses, we show that the CoIII109 polymorphism modulates cardiolipin binding to prevent complex IV instability caused by the CoIT300I mutation. This study demonstrates the feasibility of genetic interaction screens in animal mitochondrial DNA. It unwraps the complex intra-genomic interplays underlying disorders linked to mitochondrial DNA and how they influence disease expression.

The chromodomain protein MRG-1 facilitates SC-independent homologous pairing during meiosis in Caenorhabditis elegans

Homologous chromosome pairing is a prerequisite to establish physical linkage between homologs, which is critical for faithful chromosome segregation during meiosis I. The establishment of pairing is genetically separable from subsequent synapsis, defined as stabilization of pairing by the synaptonemal complex (SC). The underlying mechanism of presynaptic pairing is poorly understood. In the nematode Caenorhabditis elegans, a unique cis-acting element, the pairing center (PC), is essential for presynaptic pairing; however, it is not known whether and how the remainder of the chromosome contributes to presynaptic pairing. Here we report direct evidence for presynaptic pairing activity intrinsic to non-PC regions, which is facilitated by a conserved chromodomain protein, MRG-1. In mrg-1 loss-of-function mutants, pairing is compromised specifically in non-PC regions, leading to nonhomologous SC assembly. Our data support a model in which presynaptic alignment in non-PC regions collaborates with initial PC pairing to ensure correct homologous synapsis.