Practice question

Cybridization primarily useful for transfer of cytoplasmic traits encoded by mitochondrial or plastid genomes which are generally maternally inherited and cannot be introgressed through nuclear hybridization alone. Key agricultural examples include cytoplasmic male sterility CMS caused by chimeric mitochondrial open reading frames atp6 orf138 orf79 interfering with pollen development essential for hybrid seed production in maize rice sunflower Brassica onion; triazine herbicide resistance due to psbA D1 protein mutation in chloroplast photosystem II; and traits like cold tolerance linked to mitochondrial respiration efficiency. By fusing protoplast of elite high yielding cultivar nucleus donor with CMS donor cytoplasm inactivated nucleus donor protoplast, cybrid combines elite nuclear background determining yield quality with sterile cytoplasm enabling controlled cross pollination without self pollination. Mitochondrial genomes often recombine after fusion generating novel CMS patterns, while chloroplasts normally segregate maintaining single parental type. This avoids linkage drag associated with sexual backcrossing introducing undesirable nuclear genes. Thus cybridization revolutionized hybrid breeding providing male sterile female lines at commercial scale for many vegetable and oilseed crops, facilitating hybrid vigor exploitation.