Abstract
Hybrid necrosis sometimes appears in triploid hybrids between tetraploid wheat and Aegilops tauschii Coss. Two types of hybrid necrosis (type II and type III ) were observed when cultivar Langdon was used as female parent for hybrid production. Type II necrosis symptoms occurred only under low temperature conditions, whereas bushy and dwarf phenotypes were observed under normal temperature conditions. The developmental plasticity might be related to a temperature-responsive alteration of meristematic activity at the crown tissue of triploid hybrids. Epistatic interaction between the AB and D genomes induced not only upregulation of a number of defense-related genes, but also extensive changes in plant architecture in the type II necrosis hybrids. Such phenotypic plasticity was also observed in other cross combinations between cultivated tetraploid wheat and type II necrosis-induced Ae. tauschii accessions. Wild tetraploid wheat, Triticum turgidum subspecies dicoccoides, did not induce type II necrosis in the triploid hybrids, indicating the possibility of identifying the chromosomal location of a causal gene for type II necrosis in the AB genome.
Key words: autoimmune response, allopolyploid speciation, common wheat, epistatic interaction, hybrid necrosis, post-zygotic hybridization barrier
Common wheat (Triticum aestivum L.) is an allohexaploid species (AABBDD genome), derived through endoreduplication of an interspecific triploid hybrid between cultivated tetraploid wheat Triticum turgidum L. (AABB genome) and a wild diploid relative, Aegilops tauschii Coss (DD genome). Natural hybridization of the parental species, avoidance of hybrid breakdown, and formation of unreduced gametes are essential for hexaploidization.1 The breakdown of wheat triploid hybrids was first reported in Nishikawa's pioneer works.2–4 It was recently found that cultivar Langdon of T. turgidum subspecies durum is an efficient AB genome parent for production of hexaploid wheat synthetics,5 allowing us to produce a number of synthetic hexaploid wheat lines with D genomes derived from various accessions of Ae. tauschii.6,7 In the process of synthetic wheat production, we found several types of hybrid abnormalities including hybrid necrosis and hybrid chlorosis,8,9 as Nishikawa reported previously in references 2–4.
Hybrid necrosis is one of the post-zygotic hybridization barriers between two diverging lineages within the same species or in two closely related species.10 The Dobzhansky-Muller model simply explains the process for generating hybridization barriers.11,12 In diploid species, post-zygotic hybridization barriers, including hybrid necrosis, function in a positive manner to accelerate establishment of a new diploid species. Recent reports showed that the causal genes for hybrid necrosis are defense response-related genes in higher plants such as Arabidopsis and lettuce.13–16 These hybrid necrosis genes act to accelerate genetic differentiation among genetically related accessions in the same species and between two relative species. On the other hand, the hybrid necrosis observed in wheat triploid hybrids apparently inhibits hexaploid wheat formation, indicating that hybridization barriers act negatively in allopolyploid speciation. Allopolyploid speciation is one of the major mechanisms for producing new species, especially in higher plants;17 wheat and its related species are good materials for the study of allopolyploid speciation. One of our interests is whether the same mechanism observed in Arabidopsis underlies the incompatibilities between the wheat AB and D genomes.
The abnormal growth phenotypes observed in triploid hybrids between Langdon and Ae. tauschii accessions are divided into four types: two types of hybrid necrosis (type II and type III), hybrid chlorosis and severe growth abortion.9 Cell death occurs gradually beginning with older tissues in hybrid lines showing type III necrosis, whereas type II necrosis lines show a necrotic phenotype under low temperature conditions.2,9 Transcriptome analysis showed that expression of defense-related genes was significantly increased in lines showing type II and type III necrosis.9,18 Based on cytological observations, an autoimmune response-like reaction may be associated with necrotic cell death in the two types of hybrid necrosis.9,18 Epistatic interaction between the AB and D genomes induces upregulation of a set of defense-related genes in the necrosis-exhibiting triploids, which implies that hybrid necrosis in wheat triploid hybrids shares similar responses with those observed in Arabidopsis and lettuce. In addition, type II necrosis lines exhibit distinct phenotypes from those showing type III necrosis, and is a marked growth repression induced by low temperature.18 This repression was supported by observation of a significant decrease in cell cycle- and division-related gene expression at the crown tissues including shoot apical meristems. Interestingly, tillering number is dramatically increased at normal temperatures in type II necrosis, although plant height is significantly shorter.18 This temperature-dependent developmental plasticity might be due to an abnormality in meristematic activity at the crown tissues. Therefore, epistatic interaction between the AB and D genomes greatly influences plant architecture in type II necrosis plants, probably through the alteration of meristematic activity.
In intraspecific crosses of common wheat cultivars, type I hybrid necrosis is controlled by complementary genes Ne1 and Ne2, which are respectively located on chromosomes 5B and 2B.19,20 Our recent studies showed that causal genes for type II and type III necrosis in the D genome can be assigned to the short arms of chromosomes 2D and 7D, respectively.9,18 Chromosome assignment analyses clearly indicated that the causal genes for hybrid necrosis in wheat originated independently. Nishikawa4 assumed that complementary genes located on the AB and D genomes, named Net1 and Net2, respectively, control type II necrosis. Net2 was confirmed to be located on chromosome 2DS,18 whereas the chromosomal location of Net1 remains unknown. To verify whether type II necrosis conforms to the Dobzhansky-Muller model, chromosome assignment of Net1 is important. Type II necrosis was also observed in F1 triploid hybrids from other tetraploid parental accessions (Fig. 1). Table 1 summarizes the phenotypes of interspecific hybrids from some crossed combinations of tetraploid wheat and Ae. tauschii. In total, four T. turgidum accessions, excepting Langdon, were used as female parent. Three cultivated accessions, KU-9270, KU-125 and KU-138, induced type II necrosis in hybrids with the three Ae. tauschii accessions, IG 126489, KU-2003 and KU-2025, as well as Langdon, whereas no abnormal phenotype was observed in triploid hybrids between the wild tetraploid wheat accession KU-8736A and the Ae. tauschii accession, KU-2025. This finding allowed us to identify the chromosomal location of Net1, and suggested that most T. turgidum cultivars possess the gene. However, Net1-less accessions may be found in a wild tetraploid wheat population of subspecies dicoccoides.
Figure 1.
Phenotype of triploid F1 plants obtained from crosses between tetraploid wheat and Ae. tauschii. (A) Comparison of two triploid hybrids in winter; the one generated from an interspecific cross between T. turgidum (KU-9270) and Ae. tauschii (KU-2001) showed wild-type phenotype, and another cross between T. durum (KU-125) and Ae. tauschii (KU-2025) exhibited type II necrosis. (B) Type II necrosis phenotype in triploid plant from a cross between T. durum (KU-125) and Ae. tauschii (KU-2025) before appearance of necrotic symptoms in winter. Under low temperature (4°C), stem elongation was greatly repressed and expansion of new leaves was delayed. (C) Magnified image of the leaf in (B). Before the appearance of necrotic symptoms, the abaxial face of the leaves whitened. (D) Marked increase in the tiller number and reduction in the culm length of two type II necrosis plants in spring. The plant shown on the left is a triploid hybrid between T. turgidum (KU-9270) and Ae. tauschii (IG 126489), and the plant shown on the right is one between T. turgidum (KU-9270) and Ae. tauschii (KU-2003).
Table 1.
Phenotype of triploid F1 obtained from crosses between tetraploid wheat accessions and Ae. tauschii
| Parental tetraploid wheat accession (Triticum turgidum) | Parental Ae. tauschii accession | Phenotype of triploid plants |
| ssp. durum cv Langdon | PI 476874 | wild type |
| KU-2001 | wild type | |
| IG 126489 | type II necrosis | |
| KU-2003 | type II necrosis | |
| KU-2025 | type II necrosis | |
| ssp. turgidum KU-9270 | PI 476874 | wild type |
| KU-2001 | wild type | |
| IG 126489 | type II necrosis, bushy and dwarf | |
| KU-2003 | type II necrosis, bushy and dwarf | |
| ssp. durum KU-125 | KU-2025 | type II necrosis |
| ssp. carthlicum KU-138 | KU-2025 | type II necrosis, bushy and dwarf |
| ssp. dicoccoides KU-8736A | PI 476874 | wild type |
| KU-2025 | wild type |
The taxonomic classification is according to a monograph by Van Slageren.23 KU, Plant Germ-Plasm Institute, Faculty of Agriculture, Kyoto University, Japan; PI, National Small Grains Research Facility, USDA-ARS, USA; IG, International Center for Agricultural Research in the Dry Areas (ICARDA).
The triploid hybrids between two T. turgidum accessions (KU-9270 and KU-138) and Ae. tauschii (KU-2025) survived over winter and produced selfed seeds at low frequencies (less than 1% selfed-seed fertility), although the typical phenotype of type II necrosis was observed. These hybrids then showed a bushy dwarf phenotype in spring (Fig. 1D). This observation indicated that developmental plasticity is found in triploid hybrids derived from interspecific crosses using other tetraploid accessions. Recently, the association of microRNA networks with developmental plasticity in higher plants has been discussed.21 In maize, Corngrass1 (Cg1) mutations showing an extremely bushy dwarf phenotype are caused by the overexpression of miR156, which induces altered expression of transcription factor genes including teosinte glume architecture 1.22 Moreover, penetrance of Cg1 seems to be dependent on environment and inbred background. It is unknown whether microRNA-regulated pathways are related to the growth abnormalities in type II necrosis, and which microRNAs are responsive to low temperature conditions at the crown tissues of wheat. To answer these questions, the molecular nature of Net1 and Net2 should first be elucidated.
Disclosure of Potential Conflicts of Interest
No potential conflicts of interest were disclosed.
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