REVERSE BREEDING: ADVANCING BREEDING PARADIGMS

Reverse breeding is an innovative plant breeding technique designed to produce complementary parental lines for any heterozygous plant through a mechanism called achiasmatic meiosis, which occurs without crossovers.This process leads to univalent segregation during meiotic metaphase-I and the production of aneuploid gametes, which are then regenerated as doubled-haploid (DH) plants.Each DH plant carries combinations of parental chromosomes, and complementary DH pairs can be crossed to reconstruct the original hybrid.In reverse breeding, the inhibition of meiotic crossovers in a hybrid ensures the transmission of non-recombinant chromosomes to haploid gametes.The PAIR2 gene, necessary for homologous chromosome synapsis during meiosis-I in plants, plays a crucial role in this process.Mutations in the PAIR2 gene, such as an insertional mutation in rice or orthologous mutations in other species like Arabidopsis thaliana (e.g., ASY1), lead to defects in chromosome pairing during meiosis, resulting in the formation of univalents at metaphase-I.Reverse breeding essentially follows a strategy similar to generating a DH population from an F1 hybrid, incorporating a dominant-acting transgene that down-regulates the expression of Disrupted Meiotic cDNA1 (DMC1), thereby inhibiting crossover recombination and facilitating intact-chromosome inheritance.In previous studies on reverse breeding in A. thaliana, a hybrid was created using two natural ecotypes (Col-0 and Laer-0), with an RNAi transgene targeting the meiotic recombinase DMC1 to prevent meiotic crossover recombination.This method involved several steps: (i) generating and selecting RNAi: DMC1 transformed lines; (ii) producing achiasmatic hybrids; (iii) crossing achiasmatic hybrids with GFP-tailswap to generate haploid chromosome substitution lines (CSLs); (iv) generating DHs by spontaneously doubling haploid CSLs; and (v) crossing complementary CSLs to recreate the original hybrid.The potential of reverse breeding extends to the improvement of agricultural crops by enabling the generation of parental breeding lines for hybrid reconstruction.