Cytogenetics and immature embryo culture at Embrapa Trigo breeding program: transfer of disease resistance from related species by artificial resynthesis of hexaploid wheat (Triticum aestivum L. em. Thell)

Worldwide wheat (Triticum aestivum L. em. Thell, 2n = 6X = 42, AABBDD) bree ding programs aim to reorganize genotypes to achieve better yields, environ mental adaptation and food quality. The necessary interdisciplinarity for b reeding purposes requires an accurate choice of the most appropriate cellul ar and/or molecular strategies available to be integrated with agronomic ap proaches in order to overcome the genetic limitation of each cultivated spe cies, at each agroecosystem. Cytogenetics has given a great contribution to wheat genetic studies and breeding, due to viability of chromosomal varian ts because of homoeology among genomes in this allohexaploid species and th e genus Triticum. The level of development of cytogenetic techniques achiev ed over the last 60 years has set wheat apart from other cereal crops in te rms of possibilities to introduce genetic material from other species. Cyto genetic approaches have been extensively used in chromosomal mapping and/or resistance gene transference from tribe Triticeae-related species. Monosom ic analysis, entire chromosomes engineered through single additions and/or substitutions, reciprocal translocation through radiation or manipulation o f homoeologous pairing, as well as synthesis of new amphiploids to allow ho mologous recombination by chiasmata evolved considerably since the past dec ades. The association of tissue culture and molecular biology techniques pr ovides bread wheat breeding programs with a powerful set of biotechnologica l tools. However, knowledge on genetic system components, cytotaxonomical r elationships, cytogenetic structure and evolutionary history of wheat speci es cannot be neglected. This information indicates the appropriate strategy to avoid isolation mechanisms in interspecific or intergeneric crosses, ac cording to the genome constitution of the species the desired gene is to be transferred from. The development of amphiploids as "bridge" species is on e of the available procedures to facilitate gene flow between wheat and rel ated species. Since the environment at the center of origin of wheat in Sou thern Asia is quite different from subtropical environments, Brazilian bree ding programs overcome more challenges to adapt wheat crop to biotic and ab iotic stresses than some other countries. The germplasm bank of Embrapa Tri go has about 1000 registered entries of Triticum relatives, Aegilops, Secal e and Agropyron species supplied from several germplasm banks distributed o ver the world which were multiplied and/or selected for naturally occurring or artificially inoculated fungal diseases. Since Aegilops squarrosa L. en tries showed very good performance, the genetic variability observed in thi s species was firstly exploited. It is reported here the strategy used for transferring useful genes from Ae. squarrosa (DI), 2n = 14): crossing with tetraploid species (AABB, 2n = 28), rescue and in vitro culture of immature embryos for regeneration of the trihaploid (ABD, 2n = 21) hybrid, and colc hicine treatment for genome duplication resulting in the artificial synthes is of hexaploid wheat lines (AABBDD, 2n = 42). Results of 10,739 artificial pollinations involving 28 cross combinations amongst eight T durum L., T d icoccum and T cartlicum tetraploid entries used as female parents and ten s elected Ae. squarrosa sources of resistance as male parents are presented h ere. Immature embryos from 18 cross combinations were recovered and culture d in vitro. Green plantlets from 13 combinations were regenerated. Fertile amphiploids were recovered only from crosses among entries of tetraploid T durum and diploid Ae. squarrosa. They originated 11 fertile synthetic amphi ploid lines from seven different combinations. Useful stem and leaf rust as well as powdery mildew resistance for future u se in breeding programs were obtained.