Species origin of genomic factors in Nicotiana nudicaulis Watson controlling hybrid lethality in interspecific hybrids between N. nudicaulis Watson and N. tabacum L

Fine mapping of Ne1, the hybrid necrosis gene complementary to Ne2 in common wheat (Triticum aestivum L.)

Apart from confinement of Ne1 to a 4.45 Mb genomic segment, markers closely linked to Ne2 were identified and incomplete dominance of both genes in conditioning necrosis severity was shown. Hybrid necrosis in plants is characterized by premature death of leaves or plants in F₁ hybrids. Interaction of two complementary dominant genes Ne1 and Ne2 in wheat (Triticum aestivum L.) is known to cause hybrid necrosis. However, the mechanism underlying this necrosis is still elusive. To obtain markers closely-linked to these two genes, Ne1-carrying cultivar Zheng891 was crossed with Ne2-carrying cultivar Pan555. Using BC₁F₁ plants derived from crosses of the F₁ plants with the two parental lines, Ne1 and Ne2 were mapped to a 2.2 cM interval and a 2.3 cM interval with newly developed markers, respectively. Ne1 was further delimited to a 0.19 cM interval using 2015 Ne2-carrying F₂ plants. Xwgrc3146, Xwgrc3147 and Xwgrc3150, three of the four markers co-segregating with Ne1, were all Zheng891-dominant, suggesting that, compared with Pan555, Ne1 is located in a region with substantial sequence diversity. The Ne1 interval is syntenic to chromosomes 5H, 4, 9 and 2 of barley, Brachypodium distachyon, rice and sorghum, respectively, and corresponds to a 4.45 Mb Chinese Spring sequence. Variations in necrosis severity of the F₂ plants differing in Ne1 and Ne2 genotypes implied that these two genes are incompletely dominant in determining the timing and severity of necrosis.

HD-AGPs as Speciation Genes: Positive Selection on a Proline-Rich Domain in Non-Hybridizing Species of Petunia, Solanum, and Nicotiana

Transmitting tissue-specific proteins (TTS proteins) are abundant in the extracellular matrix of Nicotiana pistils, and vital for optimal pollen tube growth and seed set. We have identified orthologs from several species in the Solanaceae, including Petunia axillaris axillaris and Petunia integrifolia. We refer to TTS proteins and their orthologs as histidine domain-arabinogalactan proteins (HD-AGPs). HD-AGPs have distinctive domains, including a small histidine-rich region and a C-terminal PAC domain. Pairwise comparisons between HD-AGPs of 15 species belonging to Petunia, Nicotiana, and Solanum show that the his-domain and PAC domain are under purifying selection. In contrast, a proline-rich domain (HV2) is conserved among cross-hybridizing species, but variant in species-pairs that are reproductively isolated by post-pollination pre-fertilization reproductive barriers. In particular, variation in a tetrapeptide motif (XKPP) is systematically correlated with the presence of an interspecific reproductive barrier. Ka/Ks ratios are not informative at the infrageneric level, but the ratios reveal a clear signature of positive selection on two hypervariable domains (HV1 and HV2) when HD-AGPs from five solanaceous genera are compared. We propose that sequence divergence in the hypervariable domains of HD-AGPs reinforces sympatric speciation in incipient species that may have first diverged as a consequence of pollinator preferences or other ecological factors.

Tumorigenesis inheritance from the putative progenitor species of Nicotiana rustica

Hybrid seedlings from crosses of Nicotiana rustica×N. langsdorffii and N. rustica×N. alata show tumors including teratomas and vitrification. In the present study, we attempted to elucidate the genetic background leading to tumorigenesis and vitrification from the viewpoint of the amphidiploidy of N. rustica. The species N. undulata, N. paniculata, and N. knightiana have been suggested to be the progenitors of N. rustica or closely related to its progenitors. We tested tumorigenesis in interspecific hybrids between these putative progenitors of N. rustica and N. langsdorffii or N. alata, which are the species in section Alatae. The hybrid seedlings were cultured in test tubes and their morphological characteristics were observed. According to previous reports, most of the hybrid seedlings from the crosses N. rustica×N. langsdorffii and N. rustica×N. alata formed tumors and showed vitrification. In crosses with every putative progenitor of N. rustica, a portion of hybrid seedlings formed tumors and showed vitrification. These observations suggested that N. rustica inherited the factors leading to expression of abnormal symptoms from its putative progenitors. We also observed the influence of high temperature on the expression of abnormal symptoms of hybrid seedlings from the cross N. rustica×N. alata. While these hybrids developed teratomas and other tumors at 28°C, when cultured at 34°C, they did not show any abnormalities. This is the first report to show that phenotypic abnormalities in hybrid seedlings of N. rustica×N. alata are temperature sensitive.