Practice question

Plants produced via androgenesis are initially haploid bearing single chromosome set n derived from meiotic reduction of diploid mother cells into microspores. After meiosis each microspore contains recombined haploid genome; when cultured without fertilization embryogenic divisions maintain this n number, so tissues leaves roots show halved DNA content compared to diploid control confirmed by flow cytometry and chromosome counting revealing for example 7 chromosomes in barley haploid versus 14 diploid. Such haploids exhibit reduced vigor smaller organs small stomatal guard cells and complete male and female sterility due to failure of homologous pairing during meiosis producing unbalanced gametes. For breeding utility spontaneous or induced chromosome doubling restores diploidy creating doubled haploid line homozygous at all loci capturing single meiotic recombination event. Spontaneous doubling occurs at low frequency through endomitosis nuclear fusion, but artificial doubling usually applied using colchicine or oryzalin which inhibit spindle fiber polymerization during mitosis causing chromosome duplication without cell division. Haploid phase is prerequisite step linking gametic selection to instant homozygous diploid production accelerating cultivar development.