On the Inheritance of Apomixis in Parthenium argentatum

Abstract

1. In feral guayule the diploids (2n = 36) reproduce sexually, but polyploids are facultative apomicts. A fertile polyhaploid with the diploid number of chromosomes but facultatively apomictic in its reproduction was crossed with a diploid. If the polyhaploid was used as a female, the following progeny classes resulted: maternals, diploid F1, triploids, and plants with higher chromosome numbers (doubtful cases). 2. The reciprocal cross, sexual diploid x polyhaploid, produced only diploid F1 plants. Diploid F1 from both types of crosses were almost entirely sexual; an exceptional unreduced egg was produced in one instance. Production of F2 and B1 generations was impaired by incompatibility (not sterility) among F1 sibs and between them and their recessive parent. 3. Triploids from the cross apomictic polyhaploid x sexual diploid were facultative apomicts. 4. Tetraploids produced by artificially doubling the chromosome number of diploids reproduced sexually on the whole-one exceptional maternal plant in the progeny may have arisen either by selfing or by apomixis. 5. It is concluded that the genes for apomixis in guayule are recessive but that dominance is reversed where two apomictic genomes combine with one sexual one. While apomixis, on the whole, is determined by the genotype, chromosome numbers may have modifying quantitative effects.