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.