The Application of Ooubled Haploid Plants to Population ... - MSpace

2.7
There is potential for loss of desirable characteristics if not induded in the initial
selection criteria. Mass and pedigree selection are only useful in improving B. tapa if
there is enough diversity within the selected populations to avoid inbreeding
depression. Boü, of these procedures require many cydes of selection to obtain a
stable population with desirable characteristics
Synthetic varieties are developed by crossing parents seleded based on their
performance and general combining ability as determineci by the performance of
progeny from different combinations (Becker, 1988). The synthetic is established by
mixing seed and allowing interpollination for a number of generations. Parents can be
clones. inbred lines or small populations and must be maintained in isolation so that the
variety can be reconstituted perbdically. The number of parents selected to create the
synthetic must be large enough to minimize inbreeding depression and small enough to
incorporate only individuals that will maintain the mean performance of the population
(Becker. 1988). The optimum number of generations of random interpollination will
result in the maximum heterotygosity and potential for heterosis as shown in B. napus
(Schuster, 1982).
Synthetic varieties have been developed in B. mpa to explol heterosis for yield
using two or more cultivars or inbred lines mixed in equal proportions and grown in
isolation (Buna, 1995). Development of synthetics is not currently used in B. napus
breeding because it is difficult to predict the proportion of outcrossing between the
parental lines based on the genetic variation and environmentally induced variation in
outcrossing rates in B. napus (Rakow and Woods. 1987). Synthetics in B. repa have