Apomixis in cassava: advances and challenges.

Cassava is the most important staple crop in the Tropics and Subtropics. Apomixis may revolutionize its production due to various attributes. These potential advantages include production by true seed, maintaining cultivar superiority over generations without segregation, and avoiding contamination by bacteria and viruses. Historically, apomixis was initially observed by International Institute of Tropical Agriculture researchers, in the 1980s, in homogenous progeny of hybrid crosses. Later, from 1980 through 2010, apomixis was extensively studied by Universidade de Brasília, in order to determine contributing mechanisms and occurrence. Apomixis genes occur naturally at low frequencies in cultivated cassava and can be transferred by crosses with wild species. Apparently, apomixis in cassava is controlled by more than one recessive gene, which act in an additive form. Aneuploidy is associated with apomixis in cassava and can provide the double dosages necessary for recessive gene action. By using molecular techniques, genetic homogeneous progeny has been demonstrated, while embryonic exams have shown nucellar multiembryos. Polyploidy was found to increase apomixis percentage. From an evolutionary viewpoint, polyploidy has contributed to production of new species, when combined with apomixis. Recently, somatic embryos have been detected in the integument, revealing a rare model of apomixis that has only been documented in cassava.