Figure 5.

Nuclear architectures of avian chromosomes. (A) Microchromosomes of birds, possibly those of vertebrate ancestors, are clustered around the nucleolus in the nuclear center, which might be associated with specific centromeric repeats. Such a radial chromosome conformation can promote trans-contacts between microchromosomes. In contrast, macrochromosomes are distributed in nuclear peripheries with few trans-contacts. (B) 3D evolution of emu sex chromosomes. About 150 MYA, S0 formed in the ancestor of birds, and WS0 started to degenerate. During the heterochromatinization process, WS0 became anchored to the nuclear lamina like any other heterochromatic regions. About 23 MYA, one W-linked inversion has produced S1. Possibly due to the spreading of heterochromatin of S0 (red), the S1 region (WS1B, orange) adjacent to the WS0 underwent heterochromatinization earlier than the other region (WS1A), and evolved larger inactive/B compartments. This increased the cis-contacts within S1 but decreased cis-contacts between the S1 and PAR. The further spreading of heterochromatin into WS1A may be also halted by the selection on the female-related genes (red dots) located in the WS1A or the natural selection acting to preserve the TAD boundary between the WS1A and WS1B.