Creating tetraploids This requires the use of mutagenic (potentially cancer causing) chemicals under strict laboratory conditions. I cannot overemphasise how dangerous this procedure is, as the most effective chemical to date is colchicine. Do not try to use this or any other mutagenic chemical at home under any circumstances. Colchicine
This is a very toxic chemical, an alkaloid of meadow saffron, Colchicum autumnale, which should only be used under strict laboratory conditions. Even small amounts, especialy aerosols from working with the solution or the powder, can cause lung and liver cancer. It is used therapeutically for relief of gout and overdose can result in multiple organ failure. Tetraploids Some tetraploids occur naturally, but ones such as some of those created by Dr. Roland Fischer have been induced in the laboratory using colchicine, which promotes polyploidy. Colchicine causes the chromosomes to double up within each cell by allowing the nucleus of each cell to produce more copies of chromosomes but prevents mitosis i.e. the splitting of the nucleus to form new cells. Colchicine is applied to the growing tips of emerging seedlings and only a small percentage subsequently thrive. Dr Roland Fischer Roland comments further on the Passiflora e-group, ''The main reason for tetraploidizing plants is to get a set of homologous chromosomes. When there are 2 homologs of each kind, pairing during meiosis is easy and allows division into gametes. The result is that a sterile diploid hybrid is again fertile.
As soon as there is any imbalance in the genome, such as when a chromosome does not have a homologous partner or when a chromosome has 2 or 3 homologous partners, the correct pairing during meiosis is obstructed.
Autopolyploids:
This means that primary autopolyploids (from pure species) are often bad. They have 4 homologous chromsomes of each kind and little genetic variation (A’A’A*A*). If one crosses two different autologous tets by normal pollination, the offspring is already genetically improved because of more variation: (A’A’A*A*) x (A”A”A°A°) = (A’A*A”A°). It still has four homologous chromosomes, but they differ much more because of different grandmothers and grandfathers. This F1 openes better chances for genetic recombination by crossovers and allows selection of best traits during evolution or breeding.
Allopolyploids:
If one has primary allotetraploids (from crossing two species and treating the offspring in the lab), pairing during meiosis is no problem, there are two homologous of each kind (AABB). However, the primary tets have little genetic variation too and again F1 offspring is the start of evolution and a new species, AA’BB*, is born, an established example is oilseed (Brassica napus). An example from passiflora could be P. ‘Inspiration’ x P. ‘Temptation’, which are both primary tets of incarnata x cincinnata.
Imbalanced Polyploids:
As early as 1940 Kihara & Matsumura found, that from imbalanced genomes the number of gametes with imbalanced chromosomes differ: Gametes with a correct number of homologous chromosomes have a much better chance. Univalents are lost in so called micronuclei. Therefore continued backcrosssing of an imbalanced allotetraploid hybrid to a secondary autoploid gives a rapid loss
of the imbalanced chromosomes and the true species has a huge selective advantage. This advantage is of course increased, when there is a selective pressure which gives advantage to this species (e.g. frost resistance).'' |
Passiflora tetraploids
