Unique nucleolar dominance patterns in different ploidy hybrid lineages derived from Cyprinus carpio (♀) × Megalobrama amblycephala (♂)

Abstract

Nucleolar dominance is one of the epigenetic phenomena that refers to the dominance effect exhibited by the suppression of ribosomal RNA gene loci from one parent in heterozygous organisms. In this paper, the genetic and expression changes of 45S rRNA genes in different generations of the crucian carp-like homodiploid fish lineage (2nNCRC, F1, F2, F3, F5) and the autotetraploid carp lineage (4nNC, F1-F3) formed by distant hybridization of common carp (Cyprinus carpio, COC) (♀) × blunt snout bream (Megalobrama amblycephala, BSB) (♂) were mainly studied. The results showed that the nucleolar dominance patterns were different in different generations of the 2nNCRC lineage and the 4nNC lineage. In the different generations of the 2nNCRC lineage, 2nNCRC-F1 (F3, F5) exhibited the phenomenon of nucleolar dominance, while 2nNCRC-F2 did not and expressed the mutant 45S rRNA. The establishment of the nucleolar dominance pattern presented a more complicated situation, which may be related to the inconsistent coping mechanism among different generations after suffering the effect of “genome shock” at the early stage of the formation of the 2nNCRC lineage. Among the different generations of the 4nNC lineage, 4nNC-F2 existed the phenomenon of nucleolar dominance and expressed mutant 45S rRNA. However, 4nNC-F1 (F3) did not exist in the phenomenon of nucleolar dominance. The establishment of the nucleolar dominance pattern showed a process of instability-stability-instability, revealing the epigenetic instability of this lineage. This result may be related to the comprehensive effect caused by the continuous “genome shock” at the early stage of the formation of the 4nNC lineage, which can provide clues for further revealing the possible mechanism of occurrence and survival of polyploidization vertebrates. Moreover, this study found that the nucleolar dominance was biased towards BSB in the 2nNCRC lineage, while it was biased towards COC in the 4nNC lineage. The inconsistent results appeared in the different ploidy lineages formed by the same distant hybridization combination, which further verified that the dominance of transcription advantages species did not necessarily determine the biased characteristics of nucleolar dominance. Two lineages derived a mutant ITS1 sequence, which the mechanism may be related to some mechanisms triggered by the “genome shock” effect. This study will further fill a gap in studying nucleolar dominance in the early stage of the formation of homodiploid and polyploid vertebrates and provide important experimental data for studying fish epigenetics, which is of great significance in fish genetic breeding and biological evolution.