Route of NCLDV evolution: the genomic accordion

•General presentation of the NCLDV family.•Description of the main evolutionary forces acting on their genomes (gene duplications and deletions, gene transfers, movement of mobile elements).•NCLDV genomes evolve as genomic accordions with successive steps of expansion and shrinkage.

Nucleo cytoplasmic large DNA virus (NCLDV) superfamily forms a diverse group of viruses that infects a wide range of eukaryotic hosts (e.g. vertebrates, insects, protests, etc.). These viruses are characterized by a huge range in genome size (between 100 kb and 1.2 Mb), coupled with an extraordinary diverse genomic repertoire. Here I will review some recent results that shed light on the origin and genome evolution of these viruses, introducing the idea that these viruses evolved using a complex process of genomic accordion that imply successive steps of genome expansions (duplication and gene transfers) and genome reduction, in addition to movement and amplification of diverse mobile genetic elements.