Nucleoli Numbers in <i>Danthonia</i> Polypliods

Abstract

It has been indicated that the species in the genus Danthonia constitute a polyploid series. Species which are 2n, 4n, 6n, 8n, and 12n have been demonstrated. Nucleoli numbers were counted in these species at mitotic telophase. It was observed that the diploids produce either 2 or 4 nucleoli, the tetraploids 2 or 4, the hexaploids 2 (except for the 42-chromosome hexaploid with four nucleoli) and all the higher polyploids four nucleoli.The fact that D. disticha possesses both a chromosome pair having secondary constrictions and a satellited pair, whereas D. curva (both diploids) lack a satellited chromosome pair was assumed to indicate that the first mentioned species retained the original diploid condition. D. curva evidently therefore got rid of one nucleolus-organizer pair. Tetraploids with four nucleoli could have originated out of diploids with two nucleoli. Similarly the octaploids could have been derived from tetraploids with two nucleoli, and the 72-chromosome species out of hexaploids with only a single nucleolus pair.It was assumed that the extra nucleolus-organizers could have got lost through mutations. The fact that the genus is very old seems to support this assumption. Taking all the polyploids into consideration it would appear as if they tend to revert back to the diploid condition as far as nucleoli are concerned.