Nuclear-Cytoplasmic Conflict in Hybrids of Roach Rutilus rutilus and Bream Abramis brama as a Consequence of the Species Divergence in Body and Genome Sizes

Abstract—The species divergence in body size is often associated with changes in the genome size and the rate of mitochondrial DNA (mtDNA) evolution. In the course of hybridization of species with different body sizes, these differences can lead to the problems of nuclear-cytoplasmic compatibility and to the activation of the embryonic genome that are manifested in hybrids as developmental anomalies and a decrease in adaptability. The bream Abramis brama (L.) is larger than the roach Rutilus rutilus (L.) in body and genome sizes. Previously, a change in the donor genome of male to the level of maternal species was established in the first generation of hybrids by the ITS1 region of ribosomal DNA, which can presumably affect the inheritance of morphological traits from the paternal species and, particularly, body size. The body length and height, as well as a number of diagnostic morphological traits, are measured, and genotyping (rDNA ITS1 and mtDNA cyt b) of underyearlings and mature individuals of the bream, roach, F1 hybrids and underyearlings of backcrosses is carried out. Sexually mature hybrids of both directions of crossing were similar in body length to R. rutilus, which indicates a violation of the paternal effect when inheriting the body length of a larger species (bream). A violation of the inheritance of A. brama body length in the hybrids of R. rutilus × A. brama (♀R < ♂A) crossing is considered a deviation in the development and, obviously, can affect the adaptation of hybrids, which causes the rarity of this variant in nature. At the same time, alloplasmic _ARR backcrosses restore the bream body length even in the presence of the roach nuclear genome, which indicates the effect of mitochondrial genes on the inheritance of this trait. We attribute the observed deviation in the development of RA hybrids with roach mtDNA to an insufficient level of transcription of ribosomal genes due to a decrease in the number and variability of the donor bream rDNA copies. In addition, a high level of roach mtDNA polymorphism indicates a suboptimal mitonuclear match of respiratory complexes in RA hybrids, which negatively affects key physiological processes, including the growth and development of a large body size. Presumably, the development of a large body size as a complex trait with high aerobic demands is blocked in RA hybrids for energetic reasons. It was demonstrated that differences between the families of rDNA and mtDNA repeated sequences in the number and variability of copies in the R. rutilus and A. brama genomes can lead to regulatory nuclear-cytoplasmic genome incompatibility in F1 hybrids.