by Robert Connell Clarke
Asexual Propagation
Asexual propagation (cloning) allows the preservation of genotype because only normal cell division (mitosis) occurs during growth and regeneration. The vegetative (non-reproductive) tissue of Cannabis has 10 pairs of chromosomes in the nucleus of each cell. This is known as the diploid (2n) condition where 2n = 20 chromosomes. During mitosis every chromosome pair replicates and one of the two identical sets of chromosome pairs migrates to each daughter cell, which now has a genotype identical to the mother cell. Consequently, every vegetative cell in a Cannabis plant has the same genotype and a plant resulting from asexual propagation will have the same genotype as the mother plant and will, for all practical purposes, develop identically under the same environmental conditions.
In Cannabis, mitosis takes place in the shoot apex In Cannabis, mitosis takes place in the shoot apex (meristem), root tip meristems, and the meristematic cambium layer of the stalk. A propagator makes use of these meristematic areas to produce clones that will grow and be multiplied. Asexual propagation techniques such as cuttage,layerage, and division of roots can ensure identical populations as large as the growth and development of the parental material will permit. Clones can be produced from even a single cell, because every cell of the plant possesses the genetic information necessary to regenerate a complete plant.
Asexual propagation produces clones which perpetuate the unique characteristics of the parent plant. Because of the heterozygous nature of Cannabis, valuable traits may be lost by sexual propagation that can be preserved and multiplied by cloning. Propagation of nearly identical populations of all-pistillate, fast growing, evenly maturing Cannabis is made possible through cloning. Any agricultural or environmental influences will affect all the members of that clone equally.
The concept of clone does not mean that all members of the clone will necessarily appear identical in all characteristics. The phenotype that we observe in an individual is influenced by its surroundings. Therefore, members of the clone will develop differently under varying environmental conditions. These influences do not affect genotype and therefore are not permanent. Cloning theoretically can preserve a genotype forever. Vigor may slowly decline due to poor selection of clone material or the constant pressure of disease or environmental stress, but this trend will reverse if the pressures are removed. Shifts in genetic composition occasionally occur during selection for vigorous growth. However, if parental strains are maintained by infrequent cloning this is less likely. Only mutation of a gene in a vegetative cell that then divides and passes on the mutated gene will permanently affect the genotype of the clone. If this mutated portion is cloned or reproduced sexually, the mutant genotype will be further replicated. Mutations in clones usually affect dominance relations and are therefore noticed immediately. Mutations may be in duced artificially (but without much predictability) by treating meristematic regions with X-rays, colchicine, or other mutagens.
The genetic uniformity provided by clones offers a control for experiments designed to quantify the subtle effects of environment and cultural techniques. These subtleties are usually obscured by the extreme diversity resulting from sexual propagation. However, clonal uniformity can also invite serious problems. If a population of clones is subjected to sudden environmental stress, pests, or disease for which it has no defense, every member of the clone is sure to be affected and the entire population may be lost. Since no genetic diversity is found within the clone, no adaptation to new stresses can occur through recombination of genes as in a sexually propagated population.
In propagation by cuttage or layerage it is only necessary for a new root system to form, since the meristematic shoot apex comes directly from the parental plant. Many stem cells, even in mature plants, have the capability of producing adventitious roots. In fact, every vegetative cell in the plant contains the genetic information needed for an entire plant. Adventitious roots appear spontaneously from stems and old roots as opposed to systemic roots which appear along the developing root system originating in the embryo. In humid conditions (as in the tropics or a green house) adventitious roots occur naturally along the main stalk near the ground and along limbs where they droop and touch the ground.