Molecular Insights into Female Hybrid Sterility in Interspecific Crosses between and .

Species of the genus have served as favorite models in speciation studies; however, genetic factors of interspecific reproductive incompatibility are under-investigated. Here, we performed an analysis of hybrid female sterility by crossing females and males. Using transcriptomic data analysis and molecular, cellular, and genetic approaches, we analyzed differential gene expression, transposable element (TE) activity, piRNA biogenesis, and functional defects of oogenesis in hybrids. Premature germline stem cell loss was the most prominent defect of oogenesis in hybrid ovaries. Because of the differential expression of genes encoding piRNA pathway components, and , the functional RDC complex in hybrid ovaries was not assembled. However, the activity of the RDC complex was maintained in hybrids independent of the genomic origin of piRNA clusters. Despite the identification of a cohort of overexpressed TEs in hybrid ovaries, we found no evidence that their activity can be considered the main cause of hybrid sterility. We revealed a complicated pattern of Vasa protein expression in the hybrid germline, including partial piRNA targeting of the allele and a significant zygotic delay in expression. We arrived at the conclusion that the hybrid sterility phenotype was caused by intricate multi-locus differences between the species.