“…In a diversity of organisms, heterochromatinization, accompanied by amplification of tandem repeats, represents an important step in the morphological differentiation of simple sex chromosome systems, especially in the ZW ones (Nanda et al, 2000;Kondo et al, 2004;Marchal et al, 2004;Peichel et al, 2004;Charlesworth et al, 2005;Ezaz et al, 2009;Kejnovský et al, 2009). Same type of studies provided evidence that rDNA cistrons can also occur on the sex chromosomes of distinct organisms (see, for example, Goodpasture and Bloom, 1975;Schmid et al, 1983;Yonenaga-Yassuda et al, 1983;Morielle and Varella-Garcia, 1988;Cioffi et al, 2010).…”