Identification and evaluation of diplogyny in clementine (Citrus clementina) for use in breeding

Seedlessness is an important trait in citrus cultivars for the fresh fruit market. It can be obtained by selection of self-incompatibility and sterility genes or by triploid methodology. Auto-incompatibility phenomena induce sterility but cross pollinations restore fertility. INRA and CIRAD-FLHOR are exploring triploid methodology to create new hybrids for diversification of the easy peeler citrus group. One of the possibilities is to select for spontaneous triploids occurring naturally in citrus progenies. These result from gametic cell fusions between a haploid gamete and an unreduced diploid gamete. We crossed three selections of clementine and a mandarin with different citrus species. Considering that the unbalanced level of ploidy between endosperm cells and embryo cells could affect seed development, we screened seeds, and extracted embryos which were then cultivated in vitro. From 99 triploid hybrids, we saved 88 by macro-grafting. We determined the parental origin of diplogamy for 72 triploid hybrids using isozymes. We have found that 2n ovum production (diplogyny) is very frequent, and 2n pollen production (diploandry) is a rare event (less than 2%). A few triploid hybrids have started to fruit. Comparison of seed yield per fruit for triploid hybrids and their genitors showed a dramatic decrease of female fertility leading most of the time to total sterility. Pollen of triploid hybrids was totally sterile.