Recovering triploid citrus hybrids from 2x × 2x sexual crosses with the aid of embryo rescue and flow cytometry in Turkey.

BACKGROUND: Turkey is one of the major exporters of mandarins in the Mediterranean region. Seedlessness in citrus, which is one of the most desired fruit quality traits, especially in fresh mandarin export markets, can be obtained via triploidy as in many fruit species. Triploid plants can be recovered by 2x × 2x hybridizations in citrus, as well as 2x × 4x and 4x × 2x crosses. Hence, we aimed to develop local triploid hybrids by using the embryo rescue technique in five crosses using eight different citrus varieties in the present study. METHODS AND RESULTS: Embryos isolated from abortive seeds derived by 135 days after pollinations were cultured on modified Murashige and Tucker (MT) medium by adding different levels of GA3 to achieve a high germination rate. A population of 574 plants was developed as a result of embryo rescue. After screening the ploidy levels of this 574-plant population with the aid of flow cytometry, 4 triploids from 'Encore' × 'Murcott', 8 triploids from 'Fortune' × 'Willow leaf', 1 triploid from 'Kiyomi' × 'Murcott', and 1 triploid from 'Ortanique' × 'Murcott' hybridization were recovered. Triploid hybrid plants and related parents were analyzed with SSR markers heterozygotic for parental mandarin varieties. In addition, we evaluated stomatal characteristics of diploid and triploid hybrids obtained from different crosses. Stomatal traits of diploid and triploid hybrids in all crosses significantly differed except the stomata index. CONCLUSIONS: Genotyping of triploid plants confirmed using Simple Sequence Repeat (SSR) molecular markers and five SSRs were able to identify three alleles of triploid hybrids. Selected triploid mandarin hybrids have been grafted on several rootstocks for field trials and are in the process of yield and quality performances.

Somatic hybridization for citrus rootstock breeding: an effective tool to solve some important issues of the Mediterranean citrus industry.

The prevalence of sour orange rootstock in the southern and eastern part of the Mediterranean Basin is presently threatened by the spread of Citrus Tristeza Virus (CTV) and its main vector Toxoptera citricida, combined with abiotic constraints such as drought, salinity and alkalinity. The search for alternative CTV-resistant rootstocks that also withstand the other constraints is now considered an urgent priority for a sustainable citrus industry in the area. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Poncirus and Citrus genera. The production of somatic hybrids allows cumulating all dominant traits irrespective of their heterozygosity level, and would appear to be an effective way to solve the rootstock challenge facing the Mediterranean citrus industry. This paper presents the results obtained during a regional collaborative effort between five countries, to develop new rootstocks by somatic hybridization. New embryogenic callus lines to be used for somatic hybridization have been created. Protoplast fusions have been performed at CIRAD and IVIA laboratories, focusing on intergeneric combinations. Analysis of ploidy level by flow cytometry and molecular markers confirmed the acquisition of new interesting tetraploid somatic hybrids for six combinations. Diploid cybrids with intergeneric (Citrus × Poncirus) nucleus and C. reticulata or C. aurantifolia mitochondria were also identified for four combinations. The agronomical performance of a pre-existing somatic hybrid between Poncirus trifoliata and Citrus reticulata was validated in calcareous soils in Morocco. Somatic hybridization is now integrated into the breeding programs of the five Mediterranean countries.