Tetraploid hybrids from crosses between diploid and tetraploidDactylis and their significance

Chromosome counts on the progeny of crosses between diploid and tetraploid races ofDactylis show that tetraploid hybrids are produced as well as the expected triploids. The relative proportions of 4x and 3x hybrids vary greatly in different crosses, and the data suggest that parental geno-type influences the result. Overall, the frquencies of 3x and 4x hybrids are about equal, with no indication of a difference between the reciprocal 2x×4x and 4x×2x cross-combinations except perhaps in the case of diploids and autotetraploids of the same subspecies. Rare triploid hybrids are found in crosses between diploid subspecies ofDactylis. The mechanisms by which a diploid plant could donate two genomes to its offspring are discussed in relation to theDactylis situation, and the evolutionary significance of 4x hybrids formed in this way is considered.