A cytotaxonomic study of the genus Sorghum made clear the need for a taxonomic revision. Six subgenera have been established on the basis of cytology, morphology, and distribution. The subgenera Parasorghum and Stiposorghum have a basic chromosome number of five. The pachytene chromosomes of species in the subgenus Stiposorghum compare favorably with those of Zea mays. The other four subgenera, Eusorghum (S. vulgare), Chaetosorghum, Heterosorghum, and Sorghastrum, have "basic" chromosome numbers that are multiples of ten. These four groups may have had their origin in tetraploid progenitors. The closely related genus Cleistachne has a basic chromosome number of nine. This advanced genus my have originated from a tetraploid Sorghum ancestor after a chromosome had been eliminated. The following diagram illustrates the probable course of evolution:
Chaetosorghum (x=10) Cleistachne (x=9)
Eusorghum (x=10) Sorghastrum (x=10)
Other genera of Â
Â ¨Parasorghum (x=5)¨Stiposorghum (x=5)
The probable center of origin of the genus and probably of the tribe Andropogoneae may be found in southeastern Asia. This hypothesis successfully meets the greatest number of objections.
It is interesting to speculate whether the tribe Tripsaceae which is closely related to the Andropogoneae also may have had its origin in southeastern Asia. Genera are found in the Tripsaceae with the basic chromosome number of 5, 9, and 10. The genus Coix with the basic chromosome number of five is restricted to southeastern Asia; Tripsacum, an advanced genus, with a basic chromosome number of nine is found in the New World with a genus (Zea) having a basic chromosome number of ten. Whereas Zea mays may have originated in the New World, its ancestral form may have had its origin in southeastern Asia.
E. D. Garber