Figure 3.

What makes a megaplasmid. Grey arrows; megaplasmids arise and persist when plasmids are selected to carry more accessory gene content: big genes or operons, multiple different environmentally correlated traits, and/or genes that reduce physiological burdens. At the same time, ‘selfish’ traits that promote plasmid vertical and horizontal transmission can also drive up plasmid size. Blue arrows; new genes can be acquired by the activities of other mobile genetic elements, including transposons, plasmids, ICEs and phage (not shown). Red arrows; chromids are thought to develop from megaplasmids by acquiring core genes, potentially through an intermediate step of carrying a redundant copy. Chromids also tend to lose the ability for horizontal transmission. Consequently, ‘locked in’ to a particular genome, the nascent chromid acquires the compositional signatures of the host chromosome.