Molecular Characterization of the Highly Penetrant Paternal Transmission of Mitochondrial DNA Observed in a Hybrid Line between Nematodes C. briggsae and C. nigoni

It is well known that both parents contribute to a half of genetic materials to their offspring in organisms with sexual mode of reproduction. One exception to this is the cytoplasm genetic materials, including those from plastids and mitochondria, which are transmitted exclusively from a mother. Wide conservation of maternally exclusive inheritance of cytoplasm genetic materials indicates its functional significance in safeguarding an organism’s fitness during its reproduction and development. “Leakage” of a tiny amount of paternal mitochondrial DNA (mtDNA) is found associated with diabetes and cancer in humans. However, why only maternal mtDNAs are specifically transmitted during reproduction and development remains largely unknown.

Here we focus on the mechanism of targeted removal of mtDNA in nematode species. This is because our accidental observation of “mixed” mitochondrial DNAs in a hybrid between nematodes Caenorhabditis briggsae and C. nigoni, both of which are closely related to model organism, C. elegans. The two species can mate with each other but most of their hybrid progeny are dead as an embryo. Only a few of their progeny can manage to survive up to adulthood, making the nematode species pair an ideal model for studing speciation genetics. To this end, we generated a large cohort of C. briggsae transgenic strains expressing GFP in pharynx. Each strain carries an independent GFP insertion in different part of C. briggsae genome. By backcrossing the GFP into C. nigoni for at least 15 generations, we were able to produce a total of 110 hybrid strains that carry a GFP-linked genomic fragment in an otherwise C. nigoni background. To our surprise, we observed a fully penetrant transmission of C. briggsae paternal mtDNA in one of the hybrid strains, providing a golden opportunity to identify the factors that are responsible for specific removal of paternal mitochondria in the sexually reproduced progeny.

Given the tight linkage between the GFP insertion and the paternal inheritance of mtDNA, we hypothesize that the GFP insertion could disrupt some DNA elements that are responsible for the paternal inheritance of mtDNA. We propose to map the GFP insertion site by Next-Generation Sequencing and/or long-read PacBio sequencing, generate multiple null alleles targeting the elements and perform complementation test to functionally determine the identity of the elements. We will also perform functional characterization of the elements to understand how they contribute to the specific clearance of paternal mitochondria. The proposed work will provide mechanistic insights into exclusive transmission of maternal mtDNA in most eukaryotic species.