Molecular characterization of hybrid male sterility and inviability produced by X-linked introgression between C. briggsae and C. sp.9

It is well known that closely related species are able to mate with each other but their hybrid progeny frequently suffer sterility or developmental defects or death, a phenomenon called post-zygotic hybrid incompatibility (HI). HI is a fundamental biological process for maintaining genetic stability of a living species. Mechanisms of HI have been intensively studied between genetic model organisms such as Drosophila species and their sister species with which the model species can mate and produce hybrid viable progeny, but remain largely unknown in other species. Nematode C. elegans as a well-established model organism is prevented from HI study mainly owing to the lack of a sister species. Recent discovery of C. briggsae sister species, C. sp.9 opens up the possibility of studying genetic and molecular mechanisms of HI in nematode species. To empower the nematode species as a model for HI study, we randomly labeled C. briggsae chromosome with GFP markers which were introduced into C. sp.9 by repeated backcrossing, resulting in a short GFP-linked C. briggsae genomic fragments in an otherwise C. sp.9 background. By defining the boundaries of the introduced C. briggsae fragments in C. sp.9, we found that the presence C. briggsae genomic fragments in C. sp.9 background produced pervasive HIs and were able to assign the hybrid male sterile or inviable loci predominately into the center and the right arms of the X chromosome. The work provides a framework for dissecting the molecular mechanisms of the interspecific hybrid male sterility or lethality in nematode species for the first time. In the proposed study, we will first produce an improved genome assembly for both C. briggsae and C. sp.9 for comparative genomics using next generation of sequencing. We will next examine whether there is any abnormal gene expression in the gonads of hybrid sterile males as opposed to that of fertile males by genomic sequencing of RNA transcripts. We will then investigate whether the introduced C. briggsae-specific transposable elements become mobile in a naïve C. sp.9 background by Southern blot and next generation of sequencing. Finally, we will attempt to determine the molecular identity of a genetic locus within a 490-Kb interval on the X chromosome because substitution of C. sp.9 sequence with C. briggsae’ counterpart within the interval produces dead males. The proposed work will shed light on how hybrid male progeny between closely related nematode species becomes sterile or dead at the molecular level.