Glossary

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Haploid breeding

Production of homozygous lines

Crop plant breeders strive to produce homozygous or true-breeding lines. The desired trait will be carried through unchanged to subsequent generations only if the trait is present in homozygous form (i.e. having two identical alleles for a given trait).

To obtain a homozygous breeding line, the breeding line would have originally been developed by self-pollination over six to eight generations, which is a very time-consuming and costly process.

Nowadays homozygous lines of some plant species (e.g. tobacco, barley, potatoes, rape and wheat) can be produced from gametes, which contain only one set of chromosomes (haploid). In most cases unripe pollen is placed on a suitable culture medium, where it develops into plants with a single set of chromosomes (haploid androgenesis). Ova may also be used as the source material, although this is less common (haploid parthenogenesis).

Following a cultivation period of three to four weeks, the haploid plantlets are treated with colchicine, a toxin found in meadow saffron. Colchicine inhibits cell division: duplication of the chromosomes occurs, but the subsequent division into two daughter cells is suppressed. The resulting cells produce “double haploid”, fully homozygous plants which produce identical offspring.