CYTOGENETIC STUDIES IN AMARANTHUS: I. CYTOLOGICAL ASPECTS OF SEX DETERMINATION IN DIOECIOUS SPECIES

Cytological observations are contained in this report on four dioecious species of Amaranthns, two of which were formerly included in the genus Acnida. The species studied are A. arenicola, A. palmeri, A. tamariscinus, and A. tuberculatus. Three of these species have a diploid chromosome number of 32, whereas A. palmeri has a somatic chromosome number of 34. A single spontaneous triploid (2n = 48) female plant was found in collections of A. tamariscinus and a tetraploid (2n = 64) male plant in collections of A. tuberculatus. A fifth species, A. australis, has previously been reported as having 32 somatic chromosomes. The chromosome numbers for half the dioecious species of Amaranthus have now been determined. Detailed observations on the dividing chromosomes of these species in mitosis, in meiosis, and in the first division of the nucleus in the pollen grain have been made and have failed to distinguish heteromorphic chromosomes which might be associated with sex determination. The small size of the chromosomes has made detailed morphological studies impractical and there is no marked difference in absolute size of the chromosomes between species. Since haploid numbers of 16 and 17 are found in both monoecious and dioecious species, it would seem that the aneuploid condition in Amaranthus arose early and hybridization within the genus has resulted in promoting the genie condition which has been necessary for the expression of the dioecious condition.