Reverse breeding: A modern plant breeding approach for hybrid recreation

Reverse breeding is a modern plant breeding method for producing complementing parental lines for any heterozygous plant through achaismatic meiosis (meiosis without crossovers). The achiasmatic meiosis leads to univalent segregation at meiotic metaphase-I and the generation of aneuploid gametes. These gametes are then regenerated as doubled-haploid (DH) plants. Each DH carries combinations of its parental chromosomes, and complementing DH pairs can be crossed to reconstitute the initial hybrid. In reverse breeding, the suppression of meiotic crossovers in a hybrid ensures the transmission of non-recombinant chromosomes to haploid gametes. The PAIR2 gene is required for homologous chromosome synapsis at meiosis-I in plants. An insertional mutation in the rice PAIR2 gene, the ortholog of Arabidopsis thaliana ASY1, results in a defect in homologous chromosome pairing during meiosis, display univalents at metaphase-I. Essentially, reverse breeding follows an approach akin to the generation of a DH population from an F1 hybrid, carrying a dominant-acting transgene that down-regulates the expression of Disrupted Meiotic cDNA1 (DMC1), resulting in inhibition of crossover recombination and thereby enabling intact-chromosome inheritance. In earlier reports on reverse breeding in A. thaliana, a hybrid was constructed of using two of its natural ecotypes (Col-0 and Laer-0), carrying an RNAi transgene targeting the meiotic recombinase (RecA homolog) DMC1 that prevented the formation of meiotic crossover recombination. This method mainly included steps: (i) the generation and selection of RNAi: DMC1 transformed lines; (ii) the generation of achiasmatic hybrids; (iii) the crossing of achiasmatic hybrids to GFP-tailswap to generate haploid chromosome substitution lines (CSLs); (iv) the generation of DHs by spontaneous doubling of haploid CSLs; and (v) the crossing of complementing CSLs to recreate the initial hybrid. The scope of reverse breeding could be envisioned for the improvement of agricultural crops, as it may enable the generation of parental breeding lines for the recreation of hybrid.