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)

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Abstract
Worldwide wheat (Triticum aestivum L. em. Thell, 2n = 6X = 42, AABBDD) breeding programs aim to reorganize genotypes to achieve better yields, environmental adaptation and food quality. The necessary interdisciplinarity for breeding purposes requires an accurate choice of the most appropriate cellular and/or molecular strategies available to be integrated with agronomic approaches in order to overcome the genetic limitation of each cultivated species, at each agroecosystem. Cytogenetics has given a great contribution to wheat genetic studies and breeding, due to viability of chromosomal variants because of homoeology among genomes in this allohexaploid species and the genus Triticum. The level of development of cytogenetic techniques achieved over the last 60 years has set wheat apart from other cereal crops in terms of possibilities to introduce genetic material from other species. Cytogenetic approaches have been extensively used in chromosomal mapping and/or resistance gene transference from tribe Triticeae-related species. Monosomic analysis, entire chromosomes engineered through single additions and/or substitutions, reciprocal translocation through radiation or manipulation of homoeologous pairing, as well as synthesis of new amphiploids to allow homologous recombination by chiasmata evolved considerably since the past decades. The association of tissue culture and molecular biology techniques provides bread wheat breeding programs with a powerful set of biotechnological tools. However, knowledge on genetic system components, cytotaxonomical relationships, cytogenetic structure and evolutionary history of wheat species cannot be neglected. This information indicates the appropriate strategy to avoid isolation mechanisms in interspecific or intergeneric crosses, according to the genome constitution of the species the desired gene is to be transferred from. The development of amphiploids as "bridge" species is one of the available procedures to facilitate gene flow between wheat and related species. Since the environment at the center of origin of wheat in Southern Asia is quite different from subtropical environments, Brazilian breeding programs overcome more challenges to adapt wheat crop to biotic and abiotic stresses than some other countries. The germplasm bank of Embrapa Trigo has about 1000 registered entries of Triticum relatives, Aegilops, Secale and Agropyron species supplied from several germplasm banks distributed over the world which were multiplied and/or selected for naturally occurring or artificially inoculated fungal diseases. Since Aegilops squarrosa L. entries showed very good performance, the genetic variability observed in this species was firstly exploited. It is reported here the strategy used for transferring useful genes from Ae. squarrosa (DD, 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 colchicine treatment for genome duplication resulting in the artificial synthesis of hexaploid wheat lines (AABBDD, 2n = 42). Results of 10,739 artificial pollinations involving 28 cross combinations amongst eight T. durum L., T. dicoccum and T. cartlicum tetraploid entries used as female parents and ten selected Ae. squarrosa sources of resistance as male parents are presented here. Immature embryos from 18 cross combinations were recovered and cultured 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 amphiploid lines from seven different combinations. Useful stem and leaf rust as well as powdery mildew resistance for future use in breeding programs were obtained.