Fig. 5

Hypothesis on the R. bergii neo-XY sex chromosome evolution based on satDNAs distribution patterns. a First the de novo origin of the neo-sex chromosomes involved an X-A centric fusion. After the neo-XY arisen, the neo-X and neo-Y presents conserved homology (indicated in blue) and recombining events are still able to occur between the XR arm and the neo-Y. A pericentric inversion has become established on the neo-Y involving more than 90% of its length followed by heterochromatinization (indicated in black) and recombination suppression between neo-XY. These rearrangements and recombination suppression could favored the accumulation of the satDNAs on the neo-Y and generate the different neo-Y variants as indicated in (b), as follows: (i) three large and independent paracentric inversions involving Rber299 displacing this satDNAs towards interstitial or distal regions originating the variants II, III and V; (ii) a paracentric inversion involving both Rber248 and Rber299 and (iii) amplifications of Rber1 and Rber59 (not observed in the variant I, probably by low copy number and FISH resolution) generating the pattern IV, in which the neo-Y is enlarged. Each satDNAs mapped on the neo-Y chromosome are indicated by colors directly on the images