DNA Comparisons among Barley, Oats, Rye, and Wheat.

F all groups of plants, none can match the cereal grains in importance to O m a n . Their origins in cultivation have been traced back some 9,000 years to the beginnings of agriculture ( HELBAEK 1960). Extensive cytogenetic studies have been undertaken with this group of plants from which their taxonomic and supposed evolutionary relationships may be deduced (BELL 1965; RILEY 1965). Some of the relationships in the family to which the cultivated cereals belong are diagrammed in Figure 1 which includes only the relevant genera. The grass family, Gramineae, contains a very large number of polyploid forms. STEBBINS (1956b) has calculated that nearly 70% of grass species are polyploid, a value which is twice the average of flowering plants as a whole. In addition there has been a striking development of allopolyploid groups including the three major crop plants: barley, oats and wheat. STEBBIN'S (1956a,b) suggestions for a revision of the scheme presented in Figure 1 include the proposition that the tribe Triticeae be considered as a single genus. This change was proposed on the basis of cytological evidence and the uniquely high number of bigeneric hybrids. BOWDEN (1959) proposed on legal criteria that Triticum and Aegilops be considered as members of a single genus instead of separate genera in the subtribe Triticinae. They were so incorporated by MORRIS and SEARS (1967). The present investigation was initiated to compare DNA base sequences within the Gramineae. DNA comparisons can yield quantitative data on relatedness and divergence among organisms (SCHILDKRAUT et al. 1962; MCCARTHY and BOLTON 1963; HOYER, MCCARTHY and BOLTON 1964; LAIRD and MCCARTHY 1968). BENDICH and BOLTON (1967) have recently demonstrated the applicability of these same methods to plant material. The basis of this approach depends upon the accumulation of heritable changes in the genes of organisms manifested in the base sequence of DNA. Such changes occur in the sequential order of bases in the polynucleotide structure of DNA and in the number of similar regions of DNA created by gene duplication. By separating the two DNA strands of one organism and recombining them with separated DNA strands of another organism, a heteroduplex or hybrid DNA is produced. The quantity of hybrid DNA is easily measurable when one of the two DNAs is radiolabeled. It is generally recognized that the thermal stability of a DNA/DNA or RNA/