Interspecific hybridization is considered a genomic stress resulting in new gene expression patterns and transposable elements (TE) deregulation. Understanding expression changes in hybrids vs parental species is essential to unravel their potential role in speciation processes. The hybridization between Drosophila buzzatii and D. koepferae species produces sterile males and fertile females that can be backcrossed (BC) with parental D. buzzatii males, gradually restoring male fertility in the offspring from the third BC generation onwards. The main goal of this project is to identify the key genes involved in hybrid male sterility and the TE activity over different generations of backcrossing until male fertility recovering. We studied the testicular transcriptome of parental species, F1 and four generations of backcross hybrids. We observed that 26% of genes were deregulated in F1 hybrids vs parental species and all those located on the Y chromosome showed underexpression vs D. buzzatii in F1 and BC1 hybrids.

4.8 and 0.7 % of TE copies were differentially expressed in F1 hybrids vs D. buzzatii and D. koepferae respectively, with a general trend towards underexpression. The total number of copies from parental species differentially expressed in backcrosses decreased, but most of the D. buzzatii ones were overexpressed, except in BC1. Some piRNA pathway genes, involved in TE silencing, showed a deregulation in F1 vs both parental species and is maintained until BC1 in some of them, with Fs(1)Yb being the only gene exhibiting deregulation in all crosses. Finally, 21% of the genes differentially expressed in parental species have specific TE insertions within or their vicinity.

This work provides the first comprehensive transcriptomic study of Drosophila male interspecific hybrids, including the Y chromosome. It gives us important insights into how deregulation of gene and TE expression, mediated by genomic stress, can affect hybrid sterility.