The Passiflora Cultivar Registrar Dr Les King on the left & myself studying one of my new Passiflora hybrids - a very unusual polyploid cross. In nature plants commonly have multiple occurences of chromosomes, called polyploidy. A single set is called haploid, a double diploid, a triple triploid & four sets tetraploid. Some plants like many commercially grown strawberries have as many as eight sets, which is called octoploid. Tetraploids of other plants have occured as a spontaneous mutation naturally in the wild. There may be some wild Passiflora tetraploids (P. suberosa?) but for the moment it is unclear. Tetraploidy is of interest to hybridizers as it can give rise to larger flowers, fruit & leaf differences and more vigorous and hardy plants.
Passiflora like ourselves are diploid, meaning that they have 2 copies of each chromosome. For most Passiflora, but not all, 2n (the number of chromosomes) =18, so their pollen or egg cells are half that, n=9. Tetraploid plants have 4n=36 chromosomes & their pollen usually 2n=18.
Thanks to Dr. Knight (USDA) we have a tetraploid hybrid, P. 'Byron Beauty'. More recently Dr. Roland Fischer has also created tetraploid species selections such as P. 'Clear Sky' & P. 'Emil Kugler' which are both selections of P. caerulea and also some spectacular tetraploid hybrids such as P. 'Jara' & P. 'Inspiration'. I have also managed some polyploid hybrids myself with plenty more to come I hope!
Sometimes not all the tissues in a plant will have the same increase in chromosomes, which can cause some oddities e.g runners reverting back to diploid from tetraploid. The first triploids are also now starting to appear.
Dr Roland Fischer
''Hybrids often produce unreduced egg cells (2n=18) which would give 4n=36 seeds with pollen from tetraploids, that means they would be true tetraploids.''
This means that a number of diploid hybrids, including ones previously thought sterile may be able to cross with tetraploids. It is also likely that some hybrids and species will produce a small number of unreduced pollen grains which could be used to set fruit on tetraploids to produce fruit containing tetraploid seed. Dr Roland Fischer, Dr. Les King, others and myself are doing work on this - see papers below.
of Passiflora hybrids using flow cytometry
by L.A. King and M.S. Irvine