LIBRO-CONGRESO-CITRUS

mandarin cultivars such as `Fortune´ in Spain. Ranges of varietal susceptibility have been established for most of
these diseases and tolerant parents are selected in some breeding programs.
Conventional breeding in citrus has important limitations due to the complex reproductive biology of these
species. Most genotypes are apomictic and adventitious embryos develop directly from nucellar cells limiting
or precluding the development of zygotic embryos. This limits the recovery of large sexual populations and in
practice apomictic genotypes are avoided as female parents in many programs. Several high quality genotypes
have pollen and /or ovule sterility and thus cannot be used as parents in breeding programs. Self and crossincompatibility
are relatively common among many genotypes, also limiting the possibilities to select parents
for specific crosses. They have a very long juvenile phase and in most species at least five years are required
to star flowering and many more to completely loose the undesirable characters associated with juvenility. All
this aspects, together with large progeny size hamper setting up breeding schemes over several generations
and are the main reason for the relatively low success of conventional breeding programs carried out so far.
Citrus breeders, taking advantage of vegetative propagation, put their main effort in the research or
induction of polymorphism on one cycle from which they made clonal selection. Selection relates thus, either
to spontaneous mutation identified in the orchards, or on genotypes obtained by hybridizations, induced
mutagenesis or after recourse to biotechnologies that have been strongly developed in citrus in part to solve
the problems found in conventional breeding. The main methodologies used are:
• Mutants’ induction
Selection of spontaneous mutations is the oldest citrus breeding method and most of the varieties
cultivated worldwide arose from this process. In Spain, Morocco and Corsica, it has provided good results
for clementine, by extending the production period and enhancing fruit size and color. On a much longer
time scale, similar results have been obtained for Satsuma mandarins in Japan.
A number of experiments of induced mutagenesis were carried out since 1935 for cultivar improvement.
Gamma irradiation has been the most common method of mutagenesis. Examples of mandarin cultivars
obtained by irradiation programs are the low-seeded selection of `Murcott`, called `Mor´, low-seeded
selection of `Clemenules´ clementine named `Nulessin´, and the seedless mandarin `Tango´ produced by
irradiation of `Afourer´ (Nadorcott) budwood.
• Sexual breeding at diploid level
Sexual breeding is mainly used for diversification in mandarins. The main limitation of this strategy is that
most of the diploid hybrids are fertile and thus seedy. The selection of seedless cultivars displaying high
quality and good yield requires the evaluation of very large progenies. Moreover, if seedlessness of these
new hybrids is based on self-incompatibility or male sterility, important problems should be encountered
in areas where self-incompatible varieties such as the clementine are the predominant production.
• Seedlessness and ploidy manipulation for triploid creation
The selection of triploid lines is a very interesting way to develop seedless cultivars. Indeed triploidy is
generally associated with both male and female sterility. Thus, most of the trees of a triploid progeny under
field evaluation present these characters and an efficient selection can be carried out in other traits.
Several methods have been developed for triploid citrus creation. One of them exploit natural events
of polyploidization such as 2n gametes, using embryo rescue and flow cytometry to select triploids in
2x X 2x crosses. The most classical strategy is to cross diploid non-apomictic females with tetraploid
males. Such tetraploid plants can be found in apomictic seedlings (natural doubling of the chromosome
stock of nucellar cells) or are created by somatic hybridisation. Tetraploid non-apomictic lines have been
obtained by colchicine treatment of shoot tips grafted in vitro and, these tetraploids open the avenue to
4x X 2x crosses with tetraploid female parent. As an example, in the Spanish mandarin triploid breeding
program more that 15,500 triploids have been recovered using the three crossing strategies and using
routinely in vitro embryo rescue and flow cytometry to determine the ploidy level of regenerated plants.
In the last 10 years several new triploid cultivars resulting mostly from the oldest breeding strategy
(diploid x autotetraploid sexual crosses) have been released in Italy, USA, and Japan, and some of these
cultivars are now being produced for the market. Recently, four new triploid mandarin cultivars recovered
after 2x x 2x crosses have been released in Spain.
XII INTERNATIONAL CITRUS CONGRESS 2012 - 15