Causes of Apomixis
Apomictic species are generally hybrids or polyploids, as a consequence, there is irregular meiosis. Apomixis appears to be controlled by a set of genes. The trait is genetically inherited. The genes controlling sexual reproduction are non-allelic to those of apomixis. Accordingly, any line of dissent carrying the genes for apomixis will produce both types, apomicts as well as sexually reproducing plants. It has been proposed that apomixis is governed by recessive genes. The three genes (AABBCC) determine the breeding behavior.
In homozygous condition a forms unreduced eggs, b prevents fertilization, and c promotes egg development without fertilization. Thus, aaBBCC will have unreduced egg but cannot develop without fertilization, AAbbCC produces reduced egg but no embryo development take place because fertilization is prevented, and AABBcc will show normal sexual behaviour because the gene C has no effect in the presence of A and B. As a consequence of apomixis, genetic variability in such species is frozen as they have the same genotypes as parents. However, facultative apomicts have an advantage as they have retained both kinds of reproduction.