Rise of the clones: apomixis in plant breeding

Abstract

In flowering plants the transfer of traits from one generation to the other involves fertilization of female gametophyte with sperm cells delivered by pollen tube, and the subsequent reassortment of traits (alleles) in the developing progeny. In nature DNA recombination and segregation of traits to the progeny prevents the accumulation of deleterious genes and loss of fitness; however for breeding purposes it is advantageous to fix superior trait combinations (genotypes) and by-pass sexual reproduction. The formation of sexual seed without fertilization of the egg is called apomixis and is considered the holy grail of plant breeding because top-performing varieties can be reproduced indefinitely without changes in the genotypes themselves or in their expression patterns. Apomixis is a dominant trait and consists on several processes working in tandem, separately each process is detrimental for plants, however, as a single unit they allow development of embryo and endosperm from unfertilized eggs. The three processes that constitute apomixis are: 1) apomeiosis, or cell division without DNA recombination in pollen and eggs, 2) parthenogenesis, autonomous development of eggs into fully formed embryos, and 3) stable development of the endosperm, the part of the seed, needed for the embryo to grow. Unfortunately full expression and transmission of apomixis is affected by DNA recombination, therefore introgression of the trait from wild relatives into commercial varieties is extremely difficult. In this review we present genes that have been identified to regulate each step of apomixis and discuss strategies to allow transmission of the trait in full using tools from molecular biology.