Introduction. Seedlessness is a major characteristic for the fresh-fruit market and may have impor- tance for the juice industry because the presence of seeds in the fruit

Seedlessness is a major characteristic for the fresh-fruit market and may have impor- tance for the juice industry because the presence of seeds in the fruit could be asso- ciated with unfavourable aromatic com- pounds and bitterness. A lot of breeding work has been done to develop new seed- less cultivars.

The seedlessness of citrus varies among accessions and, sometimes, environ- mental conditions. For instance, ‘Mukaku kishiu’ is completely seedless under any conditions. Navel orange and satsuma man- darin are usually seedless, but occasionally they produce seeds when pollinated. On the other hand, with cross-pollination, the fruit of some pummelos produce more than 100 seeds while they are seedless in self- pollination conditions. The number of seeds of almost all accessions is less than the number of ovules. Therefore, it is prob- able that most of the commonly cultivated accessions have some degree of ovule or pollen sterility (Frost, 1948).

Strong sterility coupled with partheno- carpy is necessary for the production of stable seedless fruits. Sterility could be

divided into three types: female sterility, male sterility and self-incompatibility.

Yamamoto et al. (1995) proposed that the degree of female fertility/sterility should be rated on the basis of the average number of seeds per fruit obtained through hand pollination. High positive correlation (r = 0.93) was found between the number of seeds of open pollinated fruits and those of hand-pollinated fruits. This result indi- cated that female sterility is directly related to seediness.

The degree of male (pollen) sterility is variable in cultivated citrus, and usually pollen-sterile accessions produce seedless or few seedy fruits when cultivated in solid blocks. Male-sterile or self-incompatible accessions have the ability to produce seed- less fruits when cross-pollination is pre- vented. Even in mixed planting with male-fertile accessions, male sterility and self-incompatibility reduce the seed pro- duction and increase the percentage of seedless fruits because those accessions have a smaller chance of fertilization than self-compatible accessions (Yamamoto et al., 1995). However, those accessions may produce seedy fruits in mixed planting par- ticularly if pollinating insects are frequent.

© CAB International 2007. Citrus Genetics, Breeding and Biotechnology (ed. I.A. Khan) 197

This implies that for diversifi cation pro- grammes, male and female sterilities must be selected, particularly for an area such as the Mediterranean Basin where the main seedless easy peel cultivar is the self- incompatible ‘clementine’.

Some male-sterile and self-incompati- ble accessions cannot produce seedless fruits because of a lack of parthenocarpy. Thus, parthnenocarpy is an indispensable trait for seedless fruit production and this character seems to be widely present in Citrus germplasm. Some seedy accessions can also produce parthenocarpic fruit (Nagai and Tanikawa, 1928; Sykes and Possingham, 1992). Autonomic partheno- carpy, where seedless fruit is produced without any external stimulation (pollina- tion), is the main type of parthenocarpy in citrus, such as in navel or satsuma man- darin. Stimulative parthenocarpy has also been reported (Vardi et al., 1988). It was suggested that the autonomic partheno- carpy found in satsuma mandarin depends on three dominant complementary genes.

In Citrus, self-incompatibility seems to be of the gametophytic type, while male and female sterility may be due to different genetic factors such as sterility genes, chro- mosomal abnormalities (structural het- erozygosity, inversion, translocation) and triploidy.

This chapter presents a review on the mechanism and genetic factors underlying seedlessness in Citrus and on their exploita- tion for new seedless cultivar breeding. It focuses mainly on the aspect of ploidy manipulation for triploid seedless breeding which has received much attention at the international level and for which new avenues have been opened up by biotech- nology.