Nature and Inheritance of Apomixis in Bothriochloa and Dichanthium

Abstract

All apomicts so far studied in the Bothriochloa-Dicanthium complex had some sexual potential, but only a part, and sometimes none, of this potential was realized. The degree of sexuality of an apomict depends upon the synchronization of the various embryological phenomena, almost certainly under complex genetic control. Apomixis or lack of it, however, is probably rather simply inherited and is controlled by no more than one gene per genome. Apomixis was dominant to, but independent of, sexuality. Apomixis and sexual reproduction are not alternative modes of reproduction, either genetically or operationally, but are simultaneous and independent phenomena. The genes controlling normal sexual reproduction are not allelic to those controlling apomixis in the conventional sense. But, in apomicts, the genotype Aa may be assigned in the sense that the A allele induces four-nucleate asexual embryo sac formation and a allele does not. The autonomous parthenogenetic development of embryos in reduced sexual embryo sacs can take place independently of the primary apomictic mechanism. When combined with apospory, however, parthenogenesis tended to induce precocious embryos in the sexual sacs, thereby reducing the realization of sexual potential. The haploid embryos could seldom compete successfully with aposporous ones. We had here three independent phenomena operating simultaneously: normal sexual reproduction, parthenogenetic autonomous development of embryos, and nucellar apospory. Of the tetraploid plants that were studied in some detail, the apomictic ones could be assigned the genotype AaAa and the sexual ones the genotype aaaa. The fact that plants which behave as obligate apomicts could, nevertheless, be shown to be heterozygous for sexual reproduction suggests the possibility that all the naturally occurring apomicts in the group were heterozygous for these loci. Some sort of balanced heterozygote system seems to be operating here.