Preferential Selection for Wheat‐rye Substitutions in 42‐Chromosome Triticale1

A survey of available data indicates that many 42‐chromosome triticale varieties (X Triticosecale Wittmack) have one or more D(R) genome substitutions (wheat‐rye, respectively) which are the result of natural and/or artificial selection. In general, there appears to be natural selection acting on chromosome size and/or estimated DNA content which favors the substitution of most of the D‐genome chromosomes from hexaploid wheat for the much larger, homocologous rye chromosomes whose DNA contents exceed those of any wheat chromosome. The order and frequency of fixation of the seven possible homoeologous D(R) genome substitutions during the history of triticale breeding appears not to occur on a tandem basis. The observed pattern of fixation reflects artificial selection pressure for desirable agronomicharacters, and may also have been determined by natural selection pressure distinguishing between the different DNA content of individual rye chromosomes.