Models of chromosome evolution during 2R WGD. (A) Vertebrate groups A, B, and E in Fig. 2F. Pairs of light blue boxes with few ohnologs may be the remains of genome rearrangements between the two WGD events. (B) Two distinct ancestor chromosomes (A and B) were duplicated by the first WGD, and duplicated chromosomes A1 and B1 underwent a chromosome fusion. (C) One ancestor chromosome consisting of two parts (A and B) was duplicated by the first WGD, and a copy of the duplicated chromosomes was split into two chromosomes (A1 and B1) by a chromosome fission event. (D) After the second round of WGD, two of the four sister chromosomes underwent two independent chromosome fissions. Because the two fissions split two chromosomes (A11 and A12) at different chromosome positions, blocks A21b and A22a have some ohnologs in common. (E) Both of the fusion and fission models in B and C produce the same distribution of ohnologs, especially pairs of light blue boxes without ohnologs. (F) The distribution originating from independent fission events in D is distinguishable from that in E as indicated by one light blue box.
