Harnessing Apomixis for Heterosis Breeding in Crop Improvement

Apomixis is an asexual mode of reproduction through seeds where embryo develops without undergoing meiosis and fertilization of gametes. Majority of natural apomicts are polyploids and thought to have evolved through hybridization and polyploidization. Apomixis is highly desirable for agriculture as it fixes hybridity or heterosis. Apomicts form huge polyploid complexes in nature which are the results of their facultative nature. They harbor enormous amount of variability resulting in cytotypes. Majority of the crop plants do not reproduce through apomixis although few wild relatives of crop plants such as Pennisetum glaucum and Zea mays reproduce asexually. Harnessing apomixis for heterosis breeding of crop plants through introgression of this trait from tertiary to primary gene pool was not possible due to imprinting barriers. Deviation in endosperm balance number from the male and female parents during introgression caused poor seed set in Pennisetum and Zea mays hybrids. Apomicts exhibit three major developmental variations from normal sexual reproduction, viz. apomeiosis, parthenogenesis, and autonomous endosperm development. Initial studies indicated that all the three components are governed by a single or a few genes which was later refuted owing to recombinants showing independent events. Thus, genetics of apomixis is very complex and is often riddled with large-scale segregation distortions. In many apomictic grasses, transmission of apomixis is through a physically large, hemizygous, non-recombining genomic region. One of the genes from an apospory-specific genomic region (ASGR) of Pennisetum squamulatum, namely BABY BOOM LIKE, elicited parthenogenetic development of embryo in the sexual pearl millet. Unraveling of genetic and molecular mechanisms controlling apomixis could revolutionize the way agriculture is practiced.