Plasmids as secondary chromosomes

Large replicons secondary to the main chromosome have been termed both 'second chromosomes' if they carry essential genes and are indispensable for cell viability, as well as 'megaplasmids' if they do not use chromosome-type but plasmid-type replication systems. Recently, the term "chromid" was introduced to distinguish this replicon as it is neither a chromosome nor a plasmid. Three criteria were defined: (i) chromids have plasmid-type maintenance and replication systems, (ii) chromids have a nucleotide composition close to that of the chromosome and (iii) chromids carry core genes that are found on the chromosome in other species (Harrison et al., 2010). Although this adds to the complexity of the nomenclature found in the literature, it reflects the necessity to clearly differentiate these types of replicons.