Jacob Manisterski

Nuclear DNA amount and genome downsizing in natural and synthetic allopolyploids of the genera Aegilops and Triticum

Recent molecular studies in the genera Aegilops and Triticum showed that allopolyploidization (interspecific or intergeneric hybridization followed by chromosome doubling) generated rapid elimination of low-copy or high-copy, non-coding and coding DNA sequences. The aims of this work were to determine the amount of nuclear DNA in allopolyploid species of the group and to see to what extent elimination of DNA sequences affected genome size. Nuclear DNA amount was determined by the flow cytometry method in 27 natural allopolyploid species (most of which were represented by several lines and each line by several plants) as well as 14 newly synthesized allopolyploids (each represented by several plants) and their parental plants. Very small intraspecific variation in DNA amount was found between lines of allopolyploid species collected from different habitats or between wild and domesticated forms of allopolyploid wheat. In contrast to the constancy in nuclear DNA amount at the intraspecific level, there are significant differences in genome size between the various allopolyploid species, at both the tetraploid and hexaploid levels. In most allopolyploids nuclear DNA amount was significantly less than the sum of DNA amounts of the parental species. Newly synthesized allopolyploids exhibited a similar decrease in nuclear DNA amount in the first generation, indicating that genome downsizing occurs during and (or) immediately after the formation of the allopolyploids and that there are no further changes in genome size during the life of the allopolyploids. Phylogenetic considerations of the origin of the B genome of allopolyploid wheat, based on nuclear DNA amount, are discussed.

Genome size in natural and synthetic autopolyploids and in a natural segmental allopolyploid of several Triticeae species

Nuclear DNA amount (1C) was determined by flow cytometry in the autotetraploid cytotype of Hordeum bulbosum, in the cytologically diploidized autotetraploid cytotypes of Elymus elongatus, Hordeum murinum subsp. murinum and Hordeum murinum subsp. leporinum, in Hordeum marinum subsp. gussoneanum, in their progenitor diploid cytotypes, and in a newly synthesized autotetraploid line of E. elongatus. Several lines collected from different regions of the distribution area of every taxon, each represented by a number of plants, were analyzed in each taxon. The intracytotype variation in nuclear DNA amount of every diploid and autotetraploid cytotype was very small, indicating that no significant changes have occurred in DNA amount either after speciation or after autopolyploid formation. The autotetraploid cytotypes of H. bulbosum and the cytologically diploidized H. marinum subsp. gussoneanum had the expected additive amount of their diploid cytotypes. On the other hand, the cytologically diploidized autotetraploid cytotypes of E. elongatus and H. murinum subsp. murinum and H. murinum subsp. leporinum had considerably less nuclear DNA (10%-23%) than the expected additive value. Also, the newly synthesized autotetraploid line of E. elongatus showed similar reduction in DNA as its natural counterpart, indicating that the reduction in genome size occurred in the natural cytotype during autopolyploidization. It is suggested that the diploid-like meiotic behavior of these cytologically dipolidized autotetraploids is caused by the instantaneous elimination of a large number of DNA sequences, different sequences from different homologous pairs, leading to differentiation of the constituent genomes. The eliminated sequences are likely to include those that participate in homologous recognition and initiation of meiotic pairing. A gene system determining exclusive bivalent pairing by utilizing the differentiation between the two groups of homologues has been presumably superimposed on the DNA reduction process.

Resistance to Leaf Rust, Stripe Rust, and Stem Rust in Aegilops spp. in Israel

In all, 1,323 single plant accessions of Aegilops bicornis, A. kotschyi, A. longissima, A. ovata, A. searsii, A. sharonensis, A. speltoides, and A. variabilis collected from 18 regions in Israel and 2 adjacent regions in Lebanon and Egypt were evaluated for leaf rust (Puccinia triticina) and stripe rust (P. striiformis) resistance in field plots and for seedling resistance to leaf rust and stem rust (P. graminis f. sp. tritici) in greenhouse tests. Nearly all accessions of A. speltoides were highly resistant to leaf rust, stripe rust, and stem rust. A. longissima and A. ovata were highly resistant to stripe rust, whereas A. bicornis and A. kotschyi were highly susceptible. A. searsii was highly susceptible to stem rust, but 24 to 51% of accessions of A. bicornis, A. longissima, A. ovata, and A. variabilis were resistant to stem rust. Except for A. ovata and A. speltoides, more than 95% of the Aegilops accessions were susceptible to leaf rust caused by P. recondita alternating on Anchusa spp. Only Aegilops ovata was susceptible to P. recondita from Echium spp. A. bicornis, A. koschyi, and A. searsii were highly susceptible as seedlings to common wheat leaf rust caused by P. triticina. Most accessions of A. variabilis and about half of the accessions of A. longissima had good seedling resistance to P. triticina. Few accessions of A. ovata showed seedling resistance to the P. triticina population in Israel, but 30% were resistant to U.S. isolates. In field tests, A. bicornis showed high susceptibility to common wheat leaf rust, but more than 90% of the accessions of the other Aegilops spp. developed little or no leaf rust on adult plants. The Aegilops spp. in Israel and adjoining countries provide a rich and varied source of rust resistance for wheat breeding.

Leaf Rust and Stem Rust Resistance in Triticum dicoccoides Populations in Israel

A total of 742 single plant accessions of Triticum dicoccoides were collected from 26 locations in Israel. All accessions were evaluated for leaf rust (Puccinia triticina) resistance in field plots at Tel Aviv, and subsets of 284 and 468 accessions were tested in the greenhouse in Tel Aviv and St. Paul, MN, respectively, for seedling resistance to leaf rust; 460 accessions were also tested for seedling resistance to stem rust (Puccinia graminis f. sp. tritici) in St. Paul. One accession was highly resistant to leaf rust in seedling tests in Tel Aviv, and 21 others had moderately susceptible to moderately resistant seedling resistance. Four accessions were highly resistant to leaf rust in seedling tests in St. Paul, and 11 were resistant to at least one stem rust race. Adult resistance to leaf rust was more common than seedling resistance among the accessions; 21 accessions had less than 25% leaf rust severity in field plots compared with 80 to 90% severity for highly susceptible accessions. Most of the accessions with effective adult plant resistance came from two nearby locations in Upper Galilee, a region where populations of T. dicoccoides are most extensive and genetically diverse. These accessions may provide valuable new partial resistance genes for durable protection against leaf rust in cultivated wheat.

Distribution of Sharon goat grass (Aegilops sharonensis Eig) in Israel

Sharon goat grass (Aegilops sharonensis Eig) is endemic to the Coastal Plain of Israel and south Lebanon where it grows exclusively on sandy soils. Plants of this species were reported in many locations, but most of these reports are old and many of these locations have undergone urban and agricultural development in recent decades. This study is aimed at determining the current area and mode of distribution of this species. In a systematic survey of the Coastal Plain in Israel, we have located 42 disjunct populations in the area between Acre in the north and Gaza in the south. Physical and ecological data of the populations were recorded. The possibility of diminishing genetic diversity of this species due to extinction of populations is discussed.

Pathogen race determines the type of resistance response in the stripe rust –Triticum dicoccoides pathosystem

Wild relatives of crop plants may serve as a promising source for screening for new disease resistance genes that can be utilized in breeding programs. Triticum dicoccoides, the wild progenitor of most cultivated wheats, was shown to harbor many resistance genes against the major diseases attacking cultivated wheat. Stripe rust is a devastating fungal disease that attacks wheat in many regions of the world. New races of Puccinia striiformis Westend. f. sp. tritici, the causative agent of stripe rust, have overcome most of the known Yr resistance genes in wheat. Therefore, there is a need to search for new resistance genes in the T. dicoccoides gene pool. A set of 120 T. dicoccoides accessions, collected from 13 populations representing different habitats in Israel and vicinity, was tested for resistance to three prevalent stripe rust races (38E134, 6E16 and 6E0). Of these 120 accessions, 14, 8 and 12% were resistant to races 38E134, 6E16 and 6E0, respectively, while 57, 2 and 4% were moderately resistant to these races, respectively. A unique resistance was found in the population of Mt Hermon where >80% of the accessions showed resistance to all races. Distribution of infection types (ITs) of race 38E134 showed a normal distribution that can fit a quantitative pattern of response, while the distributions of ITs of races 6E16 and 6E0 had excess of extreme values and therefore showing a qualitative pattern of response. anova testing the main factor effects and interaction showed significant effects of population, race and their interaction on IT. Significant positive correlations were obtained between the resistance to races 6E16 and 6E0 and humidity variables of the collections sites, while resistance to race 38E134 was positively correlated with temperature variables. These results show that the pathogen race can determine the type of resistance response, qualitative or quantitative, in the stripe rust-T. dicoccoides pathosystem. The obtained results also reveal that the distribution of resistance to different pathogen races can be affected by different climatic factors.

Resistance of Aegilops Species from Israel to Widely Virulent African and Israeli Races of the Wheat Stem Rust Pathogen

Widely virulent races of the stem rust pathogen (Puccinia graminis f. sp. tritici) such as those isolated from Africa (e.g., TTKSK, isolate synonym Ug99) threaten wheat production worldwide. To identify Aegilops accessions with effective resistance to such virulent stem rust races, up to 10 different species from Israel were evaluated against African races TTKSK, TTKST, and TTTSK and the Israeli race TTTTC as seedlings in the greenhouse. A wide diversity of stem rust reactions was observed across the Aegilops spp. and ranged from highly resistant (i.e., infection type 0) to highly susceptible (infection type 4). The frequency of resistance within a species to races TTTTC and TTKSK ranged from 7 and 14%, respectively, in Aegilops searsii to 98 and 100% in AE. speltoides. In all, 346 accessions were found resistant to the three African races and 138 accessions were resistant (or heterogeneous with a resistant component) to all four races. The species with broadly resistant accessions included Ae. longissima (59 accessions), Ae. peregrina (47 accessions), Ae. sharonensis (15 accessions), Ae. geniculata (9 accessions), Ae. kotschyi (5 accessions), and Ae. bicornis (3 accessions). Few geographical trends or correlations with climatic variables were observed with respect to stem rust resistance in the Aegilops spp. The exception was Ae. longissima infected with race TTTTC, where a high frequency of resistance was found in central and northern Israel and a very low frequency in southern Israel (Negev desert region). This geographical trend followed a pattern of annual precipitation in Israel, and a significant correlation was found between this variable and resistance in Ae. longissima. Although difficult, it is feasible to transfer resistance genes from Aegilops spp. into wheat through conventional wide-crossing schemes or, alternatively, a cloning and transformation approach. The broadly resistant accessions identified in this study will be valuable in these research programs.

Transfer to Wheat of Potentially New Stem Rust Resistance Genes from Aegilops speltoides

Stem rust resistance genes have been found in four different sources of Aegilops speltoides. These include diploid accessions AEG357-4 and AEG874-60 and the amphiploids Chinese Spring/Ae. speltoides TA8026 and TS01. Stem rust resistance was mapped to the 2S chromosomes derived from each of these lines. The previously reported 2B-2S#3 translocation derived from AEG357-4 was found to carry two stem rust resistance genes, here temporarily named SrAes2t and SrAes3t. The resistance genes found on the 2S chromosomes each derived from TA8026, TS01 and AEG874-60 are named SrAes4t, SrAes5t and SrAes6t, respectively. Lines carrying genes SrAes2t and SrAes3t are being distributed to wheat breeding programs around the world.