The origin and meiotic behaviour of hexaploid northern wheatgrass (Agropyron dasystachyum)

The occurrence of hexaploid (2n = 6x = 42) forms in some otherwise natural tetraploid populations of Agropyron dasystachyum (2n = 4x = 28) was cytologically detected and studied. The hexaploid plants are morphologically similar to the tetraploids except for a small reduction in the anther size. The general survey of chromosome numbers of natural Northern Wheatgrass (A. dasystachyum 2n = 4x = 28) populations derived from eight different regions of Alberta indicated that the occurrence of hexaploid variants was not restricted to a single locality. A comparative study of chromosome pairing in the natural and the synthetic hexaploids revealed that the naturally occurring 42-chromosomed plants of A. dasystachyum originated as a result of fertilization between unreduced (SSHH) and the natural (SH) gametes, both coming from the tetraploid form of A. dasystachyum. Based on chromosome pairing, the genomes of the natural hexaploid A. dasystachyum have been designated as SSSHHH. The natural hexaploids appear to intercross among themselves and also with tetraploids producing euploid and aneuploid hybrids. The possible evolutionary significance of these findings is briefly discussed.