Figure 4.

Facultative Chromocenters, Translocation Breakpoints, and Their Consequences.

(A) A given facultative chromocenter enters prophase as already condensed centromere-containing chromosomal segment (early condensing chromatin). Euchromatin in prophase is late condensing (late-condensing chromatin) and decreasingly diffuse until middle/late metaphase. It is the change in condensation state of euchromatin that is critical for a total chromosome length during progression of the prophase stage. For simplicity, the illustrated cycling nucleus possesses only seven facultative chromocenters. Only two prophase chromosomes are shown (in boxes). C-banding-positive terminal heterochromatin, C-banding-positive NOR-heterochromatin, and NORs are also shown.

(B) As a consequence of the organization shown in (A), prophase chromosomes can be drawn as a simplified model for a general discussion on translocations in Oenothera. For this illustrative purpose, the term “early-condensing chromatin” and “facultative heterochromatin/facultative chromocenter,” although reflecting different stages (prophase versus interphase), designate the same region, i.e., middle chromosome region. 1 to 3, Schematically depicted three different reciprocal translocations. Breakpoints are indicated by triangles, and translocating segments are within dashed ovals. Translocation 1 does not change chromosome morphology since it involves breakpoints occurring at the junction between middle chromosome regions and euchromatin (yellow triangles). Translocation 2 involves breakpoints within the decondensed (active) 45S rDNA (NOR) and leads to multiplication and diversification of 45S rDNA sites. Translocation 3, involving breakpoints occurring within middle chromosome regions (red triangles), causes significant alterations in chromosome morphology.

(C) Cycling nucleus with the idealized Rabl arrangement. The breakpoint regions (junctions between facultative heterochromatin and euchromatin) are located at the same distance from the centromere pole. Thus, such an organization should facilitate physical interactions between breakpoint sites, allowing regular exchanging of euchromatic segments in Oenothera. For simplicity, only three interphase chromosomes were depicted.