Basic Chromosome Numbers in the Genus Cerastium

Abstract

AN investigation of mitotic chromosomes of Cerastium semidecandrum L. (British material) has revealed that, of the 36 metaphase chromosomes, four are nucleolar-organizing chromosomes. These each have a satellite. The nucleoli fuse early in telophase since the resting nuclei have only one, two or, more rarely, three nucleoli, though once a cell with four nucleoli was seen. It would thus seem that these 2n = 36 plants are tetraploid, although no plant with 2n = 18 has, as yet, been reported, and that nine is finally established as the basic number. From chromosome counts most species in the genus have nine as the basic number; however, there is a group of species, all annuals, which have the chromosome number 2n = 38. In these plants the nucleolar-organizing chromosomes cannot be distinguished in somatic tissues; but in the pollen-mother cells there are two nucleolar-organizing chromosomes clearly visible during prophase. These two chromosomes each have a distinct deeply staining region in contact with the nucleolus, such as McClintock¹ described in Zea mays. It would thus seem that in this section of the genus the plants are truly diploid with 19 as the basic number. A list of species for which chromosome counts are available is given below, with the species grouped according to their basic numbers. Unless otherwise stated the material is continental; undated counts are published here for the first time.