Genetic Cause of Hybrid Lethality Observed in Reciprocal Interspecific Crosses between Nicotiana simulans and N. tabacum.

Hybrid lethality, a type of postzygotic reproductive isolation, is an obstacle to wide hybridization breeding. Here, we report the hybrid lethality that was observed in crosses between the cultivated tobacco, Nicotiana tabacum (section Nicotiana), and the wild tobacco species, Nicotiana simulans (section Suaveolentes). Reciprocal hybrid seedlings were inviable at 28 °C, and the lethality was characterized by browning of the hypocotyl and roots, suggesting that hybrid lethality is due to the interaction of nuclear genomes derived from each parental species, and not to a cytoplasmic effect. Hybrid lethality was temperature-sensitive and suppressed at 36 °C. However, when hybrid seedlings cultured at 36 °C were transferred to 28 °C, all of them showed hybrid lethality. After crossing between an N. tabacum monosomic line missing one copy of the Q chromosome and N. simulans, hybrid seedlings with or without the Q chromosome were inviable and viable, respectively. These results indicated that gene(s) on the Q chromosome are responsible for hybrid lethality and also suggested that N. simulans has the same allele at the Hybrid Lethality A1 (HLA1) locus responsible for hybrid lethality as other species in the section Suaveolentes. Haplotype analysis around the HLA1 locus suggested that there are at least six and two haplotypes containing Hla1-1 and hla1-2 alleles, respectively, in the section Suaveolentes.

Identification of Nicotiana tabacum linkage group corresponding to the Q chromosome gene(s) involved in hybrid lethality.

BACKGROUND: A linkage map consisting of 24 linkage groups has been constructed using simple sequence repeat (SSR) markers in Nicotiana tabacum. However, chromosomal assignments of all linkage groups have not yet been made. The Q chromosome in N. tabacum encodes a gene or genes triggering hybrid lethality, a phenomenon that causes death of hybrids derived from some crosses. METHODOLOGY/PRINCIPAL FINDINGS: We identified a linkage group corresponding to the Q chromosome using an interspecific cross between an N. tabacum monosomic line lacking the Q chromosome and N. africana. N. ingulba yielded inviable hybrids after crossing with N. tabacum. SSR markers on the identified linkage group were used to analyze hybrid lethality in this cross. The results implied that one or more genes on the Q chromosome are responsible for hybrid lethality in this cross. Furthermore, the gene(s) responsible for hybrid lethality in the cross N. tabacum × N. africana appear to be on the region of the Q chromosome to which SSR markers PT30342 and PT30365 map. CONCLUSIONS/SIGNIFICANCE: Linkage group 11 corresponded to the Q chromosome. We propose a new method to correlate linkage groups with chromosomes in N. tabacum.

[Necessity of delivering the calibration coefficient of a dosimeter for low energy X-rays used in mammography X-ray units].

The calibration coefficient currently supplied by the National Institute of Advanced Industrial Science and Technology (AIST) is measured with an X-ray unit equipped with a W target/Al filter. We compared the calibration coefficient, which is measured with the X-ray unit equipped with a W target/Al filter and the X-ray unit for mammography. For the calibration coefficient, which is measured at the calibration fields of the X-ray units for mammography, the difference in the calibration coefficient measured with the X-ray unit equipped with a W target/Al filter that assumed a difference in the quality index-value of less than 1.5% was 24.2% at the maximum. In addition, many differences of more than unsureness 5% of the calibration coefficient were accepted. Thus, the correct dosimetry is not being performed for mammography. To improve the precision of dosimetry for mammography, at least, it is desirable to employ the calibration coefficient using the Mo target supplied by AIST. Furthermore, the indication of quality of radiation should be made severe, too. To perform equal dosimetry with a diagnosis domain, it is necessary to supply the calibration coefficient of the other target/filters.