Rhododendron 'Countess of Derby', which is a 1913 Henry White hybrid documented as 'Pink Pearl' x 'Cynthia', has been determined to be a tetraploid using flow cytometry by
Dr. João Loureiro, Dr. Silvia Castro, José Cerca, and Mariana Castro
Plant Ecology and Evolution Group,
Centre for Functional Ecology,
Department of Life Sciences,
Faculty of Science and Technology,
University of Coimbra, Portugal.
Rhododendron 'Countess of Derby' is the oldest known tetraploid large leaf elepidote. 'Pink Pearl' and 'Cynthia' are both triploids.
Rhododendron 'Countess of Derby' has as ancestors 'Broughtonii', 'Pink Pearl', and 'Cynthia' and has as descendants 'Trude Webster', 'Platinum Pearl', 'Gentle Giant', and 'Grand Slam'.
In the case of the tetraploid 'Countess of Derby', a triploid crossed with a triploid produced a tetraploid indicating that reduced meiosis occurred for either or both triploid parents. In other words, 'Countess of Derby' having 52 chromosomes either 1) received something similar to 26 chromosomes from each triploid parent or 2) received something similar to 39 chromosomes from one triploid parent and 13 chromosomes from the other triploid parent.
Summary of Ploidy Levels
Triploid 'Broughtonii',
Triploid 'Cynthia',
Triploid 'Pink Pearl',
Triploid 'Platinum Pearl',
Tetraploid 'Countess of Derby',
Tetraploid 'Gentle Giant',
Tetraploid 'Grand Slam',
Tetraploid 'Trude Webster'
Source: Van Veen Nursery
John and Sally Perkins
