Figure 6. Reconstructing the past.

Ancestral stickleback populations contain pelvic fins (A,C). Certain freshwater populations have reduced fins (B; arrowhead). The fins are restored in these populations upon expression of a transgene containing 2.5 kb of the 5′ flanking region of the Pitx1 locus from a population with pelvic fins. This regulatory sequence was attached to the coding region of Pitx1 derived from a population lacking pelvic fins. Expression of this transgene restores the pelvic fins in populations normally lacking them — compare (C) with transgene to (D) lacking the transgene. Reproduced with permission from [122].