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Hybrid Incompatibility in Higher Plants

For successful speciation through allopolyploidization, normal growth and fertility of interspecific hybrids are essential. However, hybrid plants frequently fail to produce a next generation due to lethality and sterility. Such hybrid incompatabilities are considered a postzygotic reproductive barrier, and play important roles in differentiation and establishment of new genealogical lineages in plants. The Dobzhansky-Müller (DM) model simply explains the process for generating genetic incompatibility in hybrids between two diverging lineages (Bomblies and Weigel 2007). This model proposes that reduction of fitness in hybrids generally occurs due to interaction between at least two epistatic loci derived from divergent parents.

The molecular nature of the causal genes for DM-type hybrid incompatibilities, including hybrid sterility and hybrid lethality, was recently elucidated in some plant species (Bomblies and Weigel 2007). A nucleotide binding leucine rich repeat-type disease resistance (R) gene is necessary for induction of hybrid necrosis in some intraspecific crosses of Arabidopsis thaliana L. (Bomblies et al. 2007; Alcázar et al. 2009). The epistatic interaction of RPP1, the NB-LRR-type R gene, and SRF3, a receptor-like protein kinase gene, corresponds to the DM relationship for induction of hybrid necrosis in Arabidopsis (Alcázar et al. 2010). Therefore, it has been postulated that hybrid necrosis is caused by particular alleles of the R locus inducing autoimmune-like responses when interacting epistatically with particular alleles of genes elsewhere in the genome (Bomblies and Weigel 2007). On the other hand, gene duplication followed by reciprocal gene loss sometimes results in DM-type hybrid incompatibilities. Loss or silencing of different copies of a duplicated gene in genealogically separated populations contributes to reduced fitness of F2 progeny derived from F1 hybrids, which functions to accelerate genetic differentiation between the genealogically separated populations (Taylor et al. 2001). In intraspecific hybrids of A. thaliana, arrested embryo development and root growth impairment are induced by a lack of both duplicated gene copies (Bikard et al. 2009). Similarly, pollen sterility is induced by independent disruption of each copy of two paralogs in intersubspecific hybrids between Oryza sativa L. subspecies indica and japonica (Mizuta et al. 2010). These epistatic interactions of the paralogous DM genes result in segregation distortion in the progeny.

Some reproductive barriers in interploidy crosses are established in the endosperm, and parent-of-origin specific gene expression in the endosperm is related to reproductive barriers (Schatlowski and Köhler 2012). A maternally expressed WRKY transcription factor controls hybrid lethality during seed development in interploidy crosses between a tetraploid accession of A. thaliana and a diploid accession of Arabidopsis arenosa L. (Dilkes et al. 2008). Embryo arrest occurs, accompanied by abnormal proliferation of endosperm in interspecific crosses between this tetraploid accession of A. thaliana and diploid accession of A. arenosa, and increased expression of target genes of an imprinted Polycomb group protein gene is observed during seed development in the incompatible crosses (Walia et al. 2009). Therefore, precise expression of imprinted genes in the endosperm plays an important role in successful development and establishment of hybrid seeds, and seed arrest in incompatible crosses is explained by deregulation of the imprinted gene expression during endosperm development (Schatlowski and Köhler 2012). In addition, recent studies have shown association of small RNAs with failure of seed development in Arabidopsis hybrids (Ng et al. 2012). Paternal expression of the ATHILA retrotransposon is related to maternally expressed p4-siRNAs in developing endosperm (Mosher et al. 2009), which is essential for normal seed development in interploidy hybrids of Arabidopsis (Josefsson et al. 2006). Thus, disruption of parental genome balance in interploidy crosses could result in developmental failure of endosperm through imbalanced regulation of siRNA-mediated transcripts (Ng et al. 2012).

Abnormal Phenotypes in Wheat Hybrids

Wheat type I necrosis, caused by Ne1-Ne2 complementary genes, is a good example of a DM incompatibility (Tsunewaki 1960). Necrotic cell death induced by the Ne1-Ne2 epistatic interaction is accompanied with generation of reactive oxygen (Sugie et al. 2007). Because the Ne2 gene seems to be located in a chromosomal region closely linked to the R gene against rust fungus, it has been postulated that type I necrosis is due to autoimmune-like responses triggered by the Ne1-Ne2 interaction (Bomblies and Weigel 2007). The Ne1-Ne2 interaction results in segregation distortion of molecular markers around the Ne1 and Ne2 chromosomal regions in mapping populations of common wheat even if the necrotic effect is weak (Takumi et al. 2013; Iehisa et al. 2014).

Sometimes ABD triploid hybrids between tetraploid wheat and wild diploid Aegilops tauschii Coss. show abnormal growth phenotypes such as germination failure, hybrid necrosis and hybrid sterility (Matsuoka et al. 2007). In particular, the abnormal growth phenotypes in hybrids between the tetraploid wheat cultivar Langdon and Ae. tauschii accessions have mainly been divided into the following four types: two types of hybrid necrosis (type II and type III), hybrid chlorosis, and severe growth abortion (Mizuno et al. 2010). In hybrid lines showing type III necrosis, cell death occurs, gradually beginning with older tissues, as observed in type I necrosis. Type III necrosis is presumed to be due to interaction of Nec1 and Nec2 complementary genes located on the D and AB genomes, respectively. Plants exhibiting type II necrosis show necrotic symptom and marked growth repression only under low temperature. A previous report assumed that complementary genes located on the AB and D genomes, respectively named Net1 and Net2, trigger type II necrosis (Nishikawa 1962). A hypersensitive response-like reaction might be associated with necrotic cell death in type II and III necrosis (Mizuno et al. 2010, 2011). Therefore, the two types of hybrid necrosis in wheat triploid hybrids at least partly share similar responses.

In addition to necrotic symptom, a significant decrease in cell cycle- and division-related gene expression occurs at the crown tissues including the shoot apical meristem (SAM) of plants displaying type II necrosis (Mizuno et al. 2011). Severe growth abortion, which is hybrid lethality with developmental arrest at the early seedling stage in ABD wheat hybrids, might be caused by abortion of mitotic cell division and meristematic activity at the SAM (Hatano et al. 2012). In severe growth abortion, the related cell death induced by an autoimmune response might be a secondary event; arrest of cell division at SAM seems to occur prior to the autoimmune response (Hatano et al. 2012). Thus, dramatic alteration of gene expression profiles at the SAM induced by the AB and D genome interaction could be significantly associated with the growth abnormalities in triploid wheat hybrids.

Interestingly, tiller number is dramatically increased at normal temperatures in type II necrosis, although plant height is significantly shorter (Mizuno et al. 2011). An extremely bushy dwarf phenotype, called grass clump, can also be induced by epistatic interaction of Net1 and Net2. Therefore, phenotypic effects of the Net1-Net2 interaction at the crown tissues show plasticity strongly dependent on plant growth temperature (Takumi and Mizuno 2011; Fig. 17.1). At the normal growth temperature, transcriptome analysis of the crown tissues of plants with type II necrosis showed downregulation of wheat APETALA1-like MADS-box genes, which are considered to act as flowering promoters (Matsuda et al. in preparation). The downregulation of the MADS-box genes corresponds with the delayed flowering phenotype in plants showing type II necrosis. On the other hand, disease resistance-related genes are not upregulated under normal temperature conditions (Matsuda et al. in preparation). Thus, dramatic alteration of gene expression profiles at the SAM induced by DM gene interaction could be significantly associated with the growth abnormalities in triploid wheat hybrids.

Fig. 17.1
figure 1

Putative mechanism underlying the temperature-dependent phenotypic plasticity observed in ABD wheat hybrids

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Wheat microRNAs and Their Association with Hybrid Incompatibility

Recently, the association of microRNA (miRNA) networks with developmental plasticity in higher plants has been discussed (Rubio-Somoza and Weigel 2011). miRNAs have been defined as a highly conserved class of small non-coding RNA molecules acting in post-transcriptional gene repression (Bartel 2009). The mature miRNA coupled with an RNA-induced silencing complex directs repression of mRNAs containing the complementary sequence, usually by mRNA cleavage in higher plants. Modified expression levels of small RNAs including miRNAs have been reported in the allopolyploidization process of Arabidopsis suecica Fries (Ha et al. 2009; Ng et al. 2012). A number of miRNA molecules have been identified in common wheat by next generation sequencing of small RNA molecules (Kanter et al. 2012; Yao and Sun 2012). The percentage of small RNAs corresponding to miRNAs increases with wheat polyploidy level, with the abundance of most miRNA species similar to midparent values in an interspecific wheat hybrid between tetraploid wheat and Ae. tauschii (Kenan-Eichler et al. 2011). Levels of accumulation of some miRNAs, such as miR168, miR156 and miR390, respective to the midparent values were distinct in ABD hybrids (Kenan-Eichler et al. 2011), though a direct relationship between the altered miRNA levels and phenotypic changes during allopolyploid evolution of wheat has not been demonstrated.

To clarify temperature-dependent changes in expression profiles of miRNAs in type II necrosis plants, we conducted deep sequencing using small RNAs isolated from crown tissues (Matsuda et al. in preparation). A comparative study of miRNA expression profiles showed that growth temperature dramatically changed the expression profiles of miRNAs, and that more than 200 (15 %) of the identified 1,600 miRNAs were differentially expressed between the wild type and type II necrosis plants. Among the differentially expressed miRNAs, miR156 was upregulated in the crown tissues of type II necrosis plants under normal temperatures. In maize, the grass clump phenotype of Corngrass1 mutants is caused by the overexpression of miR156, which induces altered expression of SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) transcription factor genes (Chuck et al. 2007). SPL genes, post-transcriptionally regulated by miR156, control tillering in higher plants (Xie et al. 2006; Fu et al. 2012). Overexpression of miR156 was also observed in synthetic hexaploid wheat relative to the miR156 midparental value (Kenan-Eichler et al. 2011). These observations imply significant association of miRNAs with temperature-dependent phenotypic plasticity in the Net1-Net2 interaction at the crown tissues. In fact, transcript accumulation of some wheat SPLs containing the miR156 target site were significantly reduced in the crown tissues of type II necrosis plants only at the normal temperature (Matsuda et al. in preparation). Therefore, we presumed that, at the normal temperature, Net1-Net2 epistatic interaction increased the miR156 level, and that the enhanced levels of miR156 led to digestion of SPL transcripts, resulting in an excessive increase in tiller numbers in type II necrosis (Fig. 17.1). Therefore, gene expression profiles including miRNAs in SAM in response to growth temperature could be dramatically altered in wheat hybrids and allopolyploids, resulting in phenotypic plasticity. Further studies of interspecific hybrids in Triticum and Aegilops species should offer new knowledge about hybrid incompatibility.