On the Inheritance of Apomixis in Parthenium argentatum

Abstract

The genetics of apomixis can be analyzed in very few higher plants because sexuality and apomixis are usually associated with different chromosome numbers. In feral guayule (P. argentatum) also diploids reproduce sexually and polyploids are facultative apomicts. However polyhaploids with the diploid chromosome number occur in the progeny of tetraploids. When pollinated by a sexual diploid the progeny of one such polyhaploid consisted of maternals, diploid F1 plants, and triploids. It was therefore an apomict like its polyploid ancestor, exhibiting independently facultative non-reduction and facultative non-fertilization. Two diploid F1 plants and 14 resulting from the reciprocal cross sexual x apomict were tested for their type of inheritance by pollinating with P. stramonium; all the 575 offspring were interspecific hybrids and all but two were diploids. Chromosome reduction and fertilization are therefore dominant. Self- and cross-incompatibilities involving all F1 plants and the recessive parent made the production of F2 and B1 difficult. F2 could only be obtained by mass-isolating 15 F1 plants; their Mendelian analysis is still outstanding. The triploids from the cross apomict x sexual proved facultative apomicts, presumably because they combined two doses of genes for apomixis with only one dose for sexuality. Artificial tetraploids, produced from sexual diploids, were sexual, unlike natural tetraploids. Apparently apomixis in guayule is detd., on the whole, by the genotype, while the level of polyploidy may have modifying quantitative effects determining the extent of non-reduction and non-fertilization taking place in facultative apomicts.