Introduction
Utilizing wheat wild relatives offers a great opportunity for expanding the genetic base to combat climatic challenges and biotic stresses. Synthetic hexaploid wheats (2n = 6x =42; AABBDD), derived from crosses between durum wheat (2n = 4x = 28; AABB) and Aegilops tauschii syn. Ae. squarossa (2n = 2x = 14; DD), are widely accepted as important source of useful traits in wheat breeding (Ogbonnaya et al., Reference Ogbonnaya, Abdalla, Mujeeb-Kazi, Kazi, Xu, Gosman and Lagudah2013). Recent studies have proven the value of synthetics in breeding for root traits (Becker et al., Reference Becker, Byrne, Reid, Bauerle, McKay and Haley2016) and resistance to multiple insect pests and diseases (El Bouhssini et al., Reference El Bouhssini, Ogbonnaya, Chen, Lhaloui, Rihawi and Dabbous2013; Jighly et al., Reference Jighly, Alagu, Makdis, Singh, Singh, Emebiri and Ogbonnaya2016).
CIMMYT started developing synthetic spring wheats in the mid-1980s by crossing elite spring semi-dwarf durum wheat germplasm with a large collection of Ae. tauschii (Mujeeb-Kazi et al., Reference Mujeeb-Kazi, Gul, Farooq, Rizwan and Ahmad2008). The resulting spring primary synthetics were successfully utilized by breeders (Trethowan and Mujeeb-Kazi, Reference Trethowan and Mujeeb-Kazi2008). Development of winter synthetics started at CIMMYT in 2004, when winter durum wheat germplasm from Ukraine and Romania was crossed with winter Ae. tauschii accessions from the Caspian Sea basin. The chromosome number of the amphiploid plants was not controlled and, hence, 42-chromosome plants were not selected. F3 populations were shipped to Turkey in 2008 and were subjected to rigorous pedigree selection under the dry, cold, disease-affected environment of the Central Anatolian Plateau during 2009–2015. The wide segregation for height, morphological traits (including awns and spike colour) and partial sterility observed in 2009 gradually decreased. By 2016, most of the F8 progenies demonstrated high floret fertility and good agronomic performance. This study aimed to identify superior synthetics to be used as breeding material.
Experimental
The synthetics used in the study (249 lines) originated from 6 winter durum parents and 10 Ae. tauschii accessions from Azerbaijan, Iran and Russia (Table 1; online Supplementary Fig. S1). In 2016, comprehensive evaluations were conducted at multiple locations in Turkey, Azerbaijan, Kazakhstan, Lebanon, Morocco and Russia. Field evaluation for rusts was conducted in disease hotspots in Turkey under artificial inoculation: stripe rust (Puccinia striiformis) near Ankara, stem rust (Puccinia graminis) near Kastamonu (Kast.), leaf rust (Puccinia recondite) in Sakarya, Terter (Azerbaijan), Shortandy (Kazakhstan) and Omsk (Russia). Common bunt reaction was evaluated using artificial inoculation in Eskisehir (Turkey). Resistance to cereal cyst nematodes (Heterodera filipjevi) and crown rot (Fusarium culmorum) was evaluated under artificial inoculation in growth rooms in Eskisehir. Screening for sunny pest (Eurygaster integriceps) was conducted under artificial infestation in Lebanon. The synthetics were screened for Hessian fly (Mayetiola destructor) resistance using a population from Morocco (effective genes H5, H11, H13, H14, H15, H21, H22, H23, H25 and H26). Screening for Russian wheat aphid (Diuraphis noxia) and barley yellow dwarf virus (BYDV) was conducted under heavy natural field pressure in Konya and Sakarya (Turkey), respectively. Growth habit was evaluated by planting material in the field in early May in Russia and Turkey. A field trial for grain yield was conducted under drought conditions in Konya (110 lines, unreplicated, 5 m2 plots). Spike productivity traits were evaluated by harvesting five random spikes from synthetics grown at Sakarya and Kastamonu. Spike threshability (% of threshed grains) was evaluated using manual threshing. Turkish rain-fed varieties Gerek-79 and Karahan-99 were used as checks.
Table 1. Pedigree of primary hexaploid synthetics and frequency of lines resistant to diseases
BYDV, barley yellow dwarf virus; CCN, cereal cyst nematode.
a Data from Haymana station near Ankara; stripe rust-resistant lines were identified as those with 30% or less severity. Stripe rust population was avirulent on genes Yr5, Yr8, Yr10, Yr15, Yr24, Yr26 and Yr27.
b Data from Kastamonu province; stem rust-resistant lines were identified as those with 30% or less severity. Stripe rust population was avirulent on genes Sr13, Sr24, Sr31, Sr36 and Sr38.
c Data from Eskisehir province; common bunt-resistant lines were identified as those with <5% infected spikes (population avirulent on genes Bt1, Bt5, Bt8, Bt9, Bt10, Bt11 and Bt13); CCN-resistant lines from groups 1 and 2 based on number of cysts/plant and crown rot-resistant lines from groups 1 and 2 based on the visual score.
d Data from Sakarya province; BYDV-resistant lines were identified with score 1 based on visual evaluations.
Discussion
The majority of the lines studied (219–87.9%) had winter growth habit (Table 1). The highest number of resistant synthetics was identified for stem rust (183–73.5%), followed by stripe rust (118–47.3%), common bunt (92–36.9%), cereal cyst nematodes (64–25.7%), crown rot (57–22.9%) and BYDV (11–4.4%). Most lines combined resistance to more than one pathogen. Both durum and Aegilops parents influenced resistance of synthetic wheats. For instance, durum wheat line Leuc. 84693 crossed with Ae. tauschii (409) produced a higher frequency of genotypes with individual or combined resistance to diseases, compared with crosses between Pandur and the same Ae. tauschii (409). Breeding line Ukr.-Od. 1530.94 was crossed with five accessions of Aegilops; the most successful was the cross with Ae. tauschii (1027), which resulted in the highest number of lines having high productivity combined with resistance to stripe rust, stem rusts, common bunt and soil pathogens.
Disease resistance – along with spike productivity and other traits – was used to select 85 elite synthetic lines listed in online Supplementary Table S1. The best lines (Table 2) combined high spike productivity traits with resistance to stripe and stem rust, tall stature (100–110 cm), and variable reactions to common bunt and soil borne pathogens. More than 100 lines were identified with resistance to leaf rust. Screening against Hessian fly and sunny pest identified seven and eight resistant synthetic lines, respectively, including three lines with resistance to both insects. Confirmation of this resistance is underway. Ten lines (plots 14–17; 22; 41; 44; 81; 114; 142) showed high resistance to Russian wheat aphid under severe pest pressure.
Table 2. Superior primary hexaploid synthetics with combination of disease resistance and productive spikes, Turkey, 2016
Kast., Kastamonu; CCN, cereal cyst nematode; LSD, least significant difference.
LSD value was computed based on augmented design with replicated checks (Gerek and Karahan) using PROC GLM in SAS 9.4.
Synthetic lines’ spikes were longer than the checks, with more spikelets per spike. The number of grains per spike varied from 20 to 58, though many exceeded the checks (40 grains/spike). Thousand kernel weight of many synthetic lines exceeded 50 g, while checks rarely reached 40 g. Threshability of synthetic lines varied from 0 to 95%, demonstrating genetic variation for this important domestication trait. The best five lines demonstrated grain yield exceeding 400 g/m2 (online Supplementary Fig. S2).
Developing primary synthetics normally involves selection of plants with 42 chromosomes, resulting in a stable uniform genotype per cross. The primary synthetics developed in this study underwent the gradual process of chromosome stabilization. Plants with chromosome irregularities were eliminated by natural and artificial selection. Genetic material was exchanged between the chromosomes, resulting in diversity within each population for morphological and agronomic traits, which enabled targeted selection for resistance to diseases, pests and abiotic stresses. This resulted in a diverse set of valuable germplasm (120 lines) with resistance to multiple diseases and pests, while maintaining superior spike productivity. Several studies are underway to reveal the synthetics’ genetic diversity and genetic basis of resistance to diseases and insects. The germplasm represents a new, unique winter bread wheat parental pool and is available to breeding/research programmes upon request.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S147926211700017X.
Acknowledgments
The International Winter Wheat Improvement Program (IWWIP) is supported by Turkey's Ministry of Food, Agriculture and Livestock and CRP WHEAT. CIMMYT thanks the Bill and Melinda Gates Foundation (BMGF) and UK Department for International Development (DFID) for providing financial support through the grant OPP1133199. Germplasm evaluation in Russia was supported by the Russian Science Foundation (Project No. 16-16-10005 signed 10.05.2016). Technical editing by Emma Quilligan is highly appreciated.
