Proceedings of the XLV Italian Society of Agricultural Genetics - SIGA Annual Congress

Salsomaggiore Terme, Italy - 26/29 September, 2001

ISBN 88-900622-1-5

Oral Communication Abstract

SOMATIC HYBRIDIZATION: STRATEGIES AND RESULTS IN CITRUS CULTIVAR AND ROOTSTOCK IMPROVEMENT

TUSA N., SCARANO M.T., ABBATE L., FERRANTE S., FATTA DEL BOSCO S.

Istituto di Ricerca per la Genetica degli Agrumi (IRGA-CNR), Via delle Scienze, Palermo

ntusa@unipa.it

protoplast fusion, somatic hybrids, cybrids, breeding

Citrus cultivar and rootstock improvement programs are strongly hampered by the complexity of Citrus and related genera reproductive biology. Biotechnological methods are offering several opportunities that can be used in breeding of Citrus and relatives; among different approaches, somatic hybridization by protoplast fusion provides an effective tool to obtain new genotypes (somatic hybrids) that may improve horticultural performance or may be better adapted to specific ecosystems. This approach provides the opportunity to bypass barriers to sexual hybridization, transferring and integrating genes for important traits from different genotypes into new cultivars.

The IRGA Citrus scion development project started 12 years ago; it was mainly focused on the introduction of genes for resistance/tolerance to biotic stress in lemon (Citrus limon L. Burm f.), the enhancement of fruit quality and field performance of sweet orange (C. sinensis L. Osbeck) and mandarin (C. reticulata Blanco) introducing traits for seedlessness, peel color and peelability, juice quantity and quality.

The IRGA Citrus rootstock development project is focused on the obtainment of new genotypes showing improved traits compared with Sour orange (C. aurantium L.), the main rootstock for italian citriculture.

The value of somatic hybrids for scion improvement will be realized with their utilization as tetraploid breeding parents in crosses with diploids to generate triploid progeny. Several somatic hybrids have been produced in a effort to combine the good quality and performance of ‘Femminello’ lemon and ‘Key lime’ with the mal secco tolerance of ‘Valencia’ sweet orange and ‘Milam’ lemon hybrid. The allotetraploid somatic hybrids “Valencia + Femminello”, “Milam + Femminello” and “Key lime + Valencia” were used as tetraploid parents in crosses with diploid lemons to generate different populations of seedless triploid lemon types with adequate mal secco tolerance. Triploid offspring have been propagated on sour orange in specific experimental fields since 1995.

In several somatic hybridization experiments, plants with morphology and zymotype of the non-embryogenic parent were recovered. These plants have been characterized as cybrids, genotypes showing the nucleus of leaf parent and the mitochondrial genome of callus donor parent. Several lines of diploid and tetraploid “Valencia + Femminello” cybrids, which are also very interesting in order to investigate the nuclear and mithocondrial interrelationship, have been involved in lemon improvement programs.

All the genotypes obtained from simmetrical fusion, spontaneous asymmetrical fusion (cybrid) and backcrosses have passed the juvenile stage and actually provide data on mal secco tolerance and horticultural characteristics.

Allotetraploid somatic hybrids may find direct utility as rootstock because undesirable fruit characteristics are irrelevant for this purpose. The goal is to maintain and combine the expression of traits that are either controlled by dominant genes in one parent or recessive genes in both parents (Grosser et al., 1990). We have obtained the following somatic hybrids and cybrids to be used in rootstock improvement, as an alternative to Sour orange: “Milam lemon + Sour orange”, “Calamondin + Keen sour” and “Cleopatra mandarin + Sour orange”. All these genotypes have been propagated and are under evaluation for disease, nematode and pest resistance/tolerance and horticultural performance.

Concluding, it is proved that the biotechnological approaches are fully integrated into the classical breeding methods enhancing genetic variability for the constitution of new genotypes.