Haploids, hybrids, cybrids and virus-free plants

Haploid plants have only one set of chromosomes. They are grown from anthers, pollen or ovaries. Treating them with colchicine doubles their chromosomes, giving double haploids: fully homozygous plants (both copies of every gene identical). This shows up recessive traits at once, and saves plant breeders years of inbreeding. It is used in broccoli, brassicas, sorghum, rice and tobacco.

Somatic hybrids. Two distant species usually cannot be crossed sexually: the species barrier stops it. Tissue culture can get round this:

• Enzymes, cellulases and pectinases, dissolve the cell walls, leaving naked protoplasts.
• Protoplasts of the two species are fused, using PEG.
• The fused cells are grown into whole plants: somatic (or parasexual) hybrids.

Carlson made the first inter-specific somatic hybrids in 1972.

Cybrids. Some useful traits are carried not in the nucleus but in organelles: cytoplasmic male sterility (useful for making hybrid seed) lies in the mitochondria, and some photosynthesis proteins in the chloroplasts. In sexual reproduction, the cytoplasm comes mostly from the mother. Protoplast fusion can combine the nucleus of one parent with the organelles of the other: cybridisation, which gives cybrids (cytoplasmic hybrids).

Virus-free plants. Crops propagated by cuttings or tubers suffer badly from viruses: every clone of an infected stock carries the virus. But the apical and axillary meristems (growing tips) are generally free of virus. So a tiny meristem tip, less than 1 mm long, is cut out and cultured, giving virus-free plants. This has been done with sugarcane, banana and potato.