Parsimony reconstruction of ancestral genome sizes. Genome sizes are indicated (in Mbp) at specific nodes and in the right margin for the extant genomes. A single event of genome reduction appears to have occurred in the ancestor of the unicellular and mostly marine cyanobacteria (Prochlorococcus/Synechococcus). In contrast, the history of Clade 1 cyanobacteria involves a mix of genome expansions and reductions. The most notable expansion events are evident in the genome of Acaryochloris marina (Acam), Trichodesmium erythraeum (Trie), Nostoc punctiforme PCC 73102 (Nosp) and Microcoleus chthonoplastes (Micc). Reductions in this clade include Cylindrospermopsis raciborskii (Cylr), Raphidiopsis brookii (Rapb), Synechococcus sp. PCC 7002 (Syn7002), Synechocystis sp. PCC 6803 (Scys6803) and cyanobacterium UCYN-A (Ucyn). Note that the genome size of the obligate symbiont 'Nostoc azollae' 0708 (NoAz) has not changed considerably since the ancestor shared with Cylr and Rapb although the pseudogenization in NoAz is considerable (Figure 3).