Polyploidy exist in plants as well as many animals and it is a universal phenomenon in natural conditions [1] . Polyploidization or whole-genome duplication (WGD) is regarded as a driving force in the processes of evolution [2] [3] . Polyploidization can trigger rapid genomic rearrangements, genomic downsizing, and movement of genetic elements across genomes, epigenetic remodeling such as DNA methylation changes and of foreign genetic materials into the polyploid genome, suggesting the complex dynamics of polyploid genomes [4] . These extensive changes in the genome of polyploidy may further give rise to new phenotype, transgressive traits and high resistance, etc. [5] . Lai et al. [6] analyzed one of the three hybrid species, Helianthus deserticola, by microarray, and revealed that approximately 2% of genes had extreme or transgressive expression in the hybrid species. These findings also showed that polyploidy may play an important role in the processes of evolution. Likewise, Soltis and Soltis [7] indicated that polyploidy played a major role in the evolution of many eukaryotes. About 70% of all angiosperms have experienced one or more episodes of polyploidization based on recent estimates [8] . Recent studies have focused on the high growth vigor in the polyploid which had been widely studied [9] [10] .

Early studies on aneuploidy began with the Drosophila and Datura by Calvin Bridges who noted that individuals could have changes in chromosome number and was confirmatory of the chromosome theory of inheritance [11] . Compared with polyploidy, Aneuploidy plays an important role in the evolution of plants as well [12] . Some studies indicated that polyploidy may promote the formation of aneuploidy and these two are often intertwined in the evolution processes [13] [14] . In the genome of aneuploidy, loss of one or more chromosomes in aneuploidy can lead to dramatically changes of the stability of the genome, the effect of gene dosage and gene expression [15] . The meiosis of aneuploidy exhibits inordinate and the extra chromosome in aneuploidy may undergo a phenomenon of centromere misdivision [16] . Due to a series of changes in the genome of aneuploidy, aneuploidy is sterile and less fit than euploidy in many species under natural conditions, and even these are fatal [17] [18] . However, recent findings showed that aneuploid not only can generate many new phenotypes but also can revert back to stabilize euploidy, so they were regarded as the valuable materials by many breeders [19] . Although aneuploidy has an important role in the evolution processes and breeding as polyploidy does, we still know little about the mechanisms of the genomic changes and phenotype variations compared with polyploidy.

Tobacco is an annual herbaceous plant belonging to Solanaceae. Cultivated tobacco (Nicotiana tabacum L.) is not only an important economic crop but also a model system used in numerous studies of plant biology [20] . Since tobacco is 3 - 4 months per cycle, thousands of seeds per cross, large phenotypic diversity, and the cultivated tobacco is allotetraploid, it is an ideal material to investigate the mechanisms of genomic changes at the initial stage of aneuploidy formation.

In order to investigate the segregation pattern of chromosome among the aneuploidy hybrids, we used the model plant tobacco as material to generate an aneuploidy population, and then using 20 SSR markers to trace the distribution of different chromosome in the aneuploidy hybrids. The SSR markers were linkage to different chromosome and were obtained from the first linkage map of tobacco (http://solgenomics.net/cview/map.pl?map_id=15) which was released in 2007. By counting the number of bands of each SSR marker in the aneuploidy, the segregation ratios of the aneuploidy hybrids were calculated. We also had measured some morphological traits of flowers to further verify the segregation ratios of SSR markers and the result showed that the segregation of SSR markers and morphological traits of flowers were correlated.

Tobacco is not only an important agronomic crop in the large family of Solanaceae, but also a model plant frequently used in plant biology [25] . SSR markers have been developed and adopted to evaluate genetic diversity in a tobacco collection by Wu et al. [26] . Polyploid segregation ratios have been detected at 4 microsatellite loci in the diploid and polyploid Ipomoea [27] . These studies proved that the SSR markers are stable and applicable.

In this study, the correlation between the segregation of SSR loci and the segregation of the six morphological traits of flowers in aneuploid tobacco, has

been established. To our knowledge, our result represents the first report on the close accordance of segregation based on SSR markers with that based on the morphological traits in polyploid progeny.

Polyploidy is a prominent and significant force in plant evolution, at temporal scales ranging from ancient to contemporary, and with profound effects at scales ranging from molecular to ecological [28] . Hybrids are formed by cross-fertilization between different individuals, strains, or species. Hybrids often grow more vigorously and produce more seeds and offspring than the parents. This phenomenon is known as hybrid vigor or heterosis [29] . Findings made in the last two decades have revealed that the early stages of allopolyploidy are often associated with rapid and extensive genomic instabilities, which are thought to be important for initial stabilization of the newly formed allopolyploids as well as contribute to their establishment as competitive new species [30] [9] . The materials used in this study came from a series of polyploid hybridization. What is more, most of the hybrids are aneuploids and thus a variety of morphological traits may appear in the population. Therefore, these hybrids are very precious and suitable for studying genomic evolution of the polyploidy.

SSR loci were used to investigate gametic segregation [31] . In fact, hexaploid tobacco can be treated as triploid. Because N. tabacum tetraploids are disomic, therefore hexaploids are trisomic. Trivalents between homeologs from one progenitor genome are expected during meiosis. Homeologs from different progenitor genomes do not pair in meiosis. Otto and Whitton [32] reported that triploid plants generally give rise to aneuploidy gametes which exhibit very low fertility. So the hexaploid may produce different types of the gametes in theory. Based on the linkage analysis of the F₂ population of hexaploid tobacco, the 42 loci isolated were subject to Chi-square test to probe the segregation of the progeny. The resulting segregation was 1:1 to 2:1.

The segregation ratios of the six traits were fallen into the theoretic value with only one exception, the width of the calyx, which apparently showed matroclinous and/or gene interactive effect of the progenitors, as has been reported by Yu et al. [33] . Wissemann [34] indicated the primary cause for matroclinous inheritance: hybridization caused asymmetric meiosis, the reason for heterogamy is asymmetrical, mostly matroclinous inheritance of characters and character states, but not vice versa. The apparently higher mutation rate found in the gene flanking regions in the progenitors might be associated with the complex polyploid nature of these species, which could induce chromosome pairing aberrations [35] . The polyploid has a tendency of being diploidized. The higher mutation may also be related with the progeny’s maintaining of the chromosome balance [36] . Reasons for the segregation ratio of the width of the calyx not consistent with the theoretic value might be the change of gene function or nucleotide sequence variations in the expressed regions [37] .

The phenotypes exhibiting in hybrids that are extreme or novel relative to the parental lines are called transgressive traits [38] . The progeny plants studied in this paper all displayed transgressive characters in all six traits examined. Chen [5] had proposed two models at gene expression level to explain heterosis in allopolyploids and hybrids: the dominance model and the overdominance model. The dominance model suggests that the expression of genes in the hybrids is a result of combined or additive expression of two alleles in the parents (e.g., 1 + 1 = 2), whereas the overdominance model indicates that allelic interactions in the hybrids lead to nonadditive expression of the alleles derived from the parents (1 + 1 ≠ 2). If the interactions lead to positive effects or gene activation, the outcome is expected to be 1 + 1 > 2. If the interactions result in negative effects or gene repression, the expected outcome would be 1 + 1 < 2. Nonadditive expression explains positive as well as negative epistatic interactions. Otherwise, dosage regulation may also affect the phenotypic variation [39] . It has been suggested that additive effects in polyploidys lead to increase or decrease levels of gene expression and phenotypic variation. The reasons for a variety of morphological traits appeared in the hexaploid progeny may be even more intricate than the discussions shown above.

Schwarzbach et al. [40] studied 41 morphological traits using the hybrid Helianthus anomalus obtained from the crossing of H. annuus with the H. petiolaris, and found that in Helianthus anomalus, only 1 trait was morphologically intermediate (2.4%), 23 traits, parents-like (56.1%), and 17 traits, transgressive (41.5%). Rieseberg et al. [41] gave four possible predictions to explain transgression, based on the assumption that complementary gene action is the main cause for transgression. Baack and Rieseberg [42] suggested that transgressive segregation could occur when parental species contain alleles with opposing effects. From the evolutionary point of view, the transgressive characters might also be very beneficial to the adaptive evolution [43] .

It should be noted that although in this paper the correlation between SSR markers segregation and flower morphological traits segregation in aneuploid tobacco has been established, the correlation of SSR markers segregation with other morphological traits need further investigation.

This work was supported by Breeding of Special Flavored, Excellent Quality and Disease-resistant Novel Flue-cured Tobacco Varieties in Chongqing Mountainous Region (No. 2012044). The authors are thankful to Professor Qiao He, Misses Hui Luo and Tianjiao Wu (College of Horticulture and Landscape Architecture, Southwest University) for their assistance in constructing the F₂ population and the measurement of the morphological traits.

The authors declare that they have no conflict of interest.

Liu, C., Dang, J.B., Wang, L.L., Chen, Z., Wang, Y., Li, G.R. and Liang, G.L. (2018) Segregation Correlation of SSR Markers and Flower Traits in Aneuploid Tobacco. World Journal of Engineering and Technology, 6, 225-240. https://doi.org/10.4236/wjet.2018.62013