Fig. 2. Evolution of TnI isoforms.

A, The evolutionary lineage demonstrates that fast skeletal muscle TnI (fTnI) was the first vertebrate TnI from an ancestor that also gave rise to invertebrate TnI. Slow skeletal muscle TnI and then cardiac TnI emerged subsequently from gene duplications. B, The phylogenetic tree of TnI isoforms of representative vertebrate and invertebrate species reveals that each muscle type-specific isoform is conserved among species whereas the three isoforms are significantly diverged. An exception is the fish slow skeletal muscle and cardiac TnI, indicating their early states after divergence.