Hibiscus, a genus in the Malvaceae family, contains ≈300 species of trees, shrubs, and herbs with various growth habits. Due to their impressive flowers and long flowering season, many Hibiscus species have been used as ornamental plants. Most Hibiscus species are tropical, but Hibiscus syriacus is widely used in temperate regions due to its broad adaptability and variable ornamental characteristics. H. syriacus is the most commonly used hardy Hibiscus species in temperate to subtropical zones due to its cold tolerance and persistent, woody stems (Lawton, 2004). Consumer demand for novelty and color has driven most of the breeding efforts toward improving the floral traits of H. syriacus and other ornamentals.
Interspecific hybridization is an important breeding strategy that has been used for many ornamental crops (van Tuyl and Lim, 2003); in addition, it has been a common strategy for cultivar development of Hibiscus with novel phenotypes (Ha et al., 2014, 2015; Klips, 1999; Kuligowska et al., 2016; Pounders and Sakhanokho, 2016; van Huylenbroeck et al., 2000; van Laere et al., 2007). Several North American herbaceous perennial species of Hibiscus, including H. coccineus, H. laevis, and H. moscheutos, are reportedly compatible (Klips, 1999; Kuligowska et al., 2016). A multiple-species hybrid cultivar Blue Angel (H. ×moscheutos) that showed wider adaptability and a longer flowering season (from early summer to late fall) compared with its parents (Malinowski et al., 2012) was released. In addition to interspecific crosses within a section, H. mutabilis has been used for intersectional crosses with H. moscheutos. Three interspecific hybrid cultivars of H. mutabilis × H. moscheutos that were recently released showed an extended flowering season, better cold adaptability, increased vigor, and infertility (Pounders and Sakhanokho, 2016).
Interspecific hybridization has been applied to improve H. syriacus. Two related species, H. sinosyriacus and H. paramutabilis, are compatible with H. syriacus (van Huylenbroeck et al., 2000; van Laere et al., 2007). The two species were used to increase flower size, alter leaf morphology, and increase vigor (van Laere et al., 2007). Early research reported that interspecific hybridization between H. syriacus and the other two species could be accomplished only if H. syriacus was used as the female parent (van Laere et al., 2007); however, recent reports showed that this barrier is not impregnable. Daewangchun is a cultivar with a unique eyespot that was created and released using H. syriacus ‘Samchully’ as the female parent and H. sinosyriacus ‘Seobong’ as the male parent (Ha et al., 2015). Another new interspecific hybrid cultivar, Tohagol Red, resulted from the reciprocal cross combination (Ha et al., 2014). Both hybrid cultivars showed some advantages attributable to interspecific hybridization; however, crosses between H. sinosyriacus and H. syriacus do not appear to be a good strategy for creating infertile cultivars because Daewangchun and Tohagol Red are both fertile (Ha et al., 2014, 2015). However, interspecific hybridization between H. syriacus (2n = 4x = 80) and H. paramutabilis (2n = 4x = 82) was viable only when H. syriacus was used as the female parent (van Laere et al., 2007; van Tuyl and Lim, 2003). Furthermore, F1 hybrids were observed to have low self-fertility, and only a few F2 progeny were obtained (van Laere et al., 2007). Based on acetocarmine staining, F1 hybrids had 65% to 88% stainable pollen, which was slightly lower than that of the parents, which had 90% to 99% stainable pollen (van Laere et al., 2009); however, the in vivo pollination success rate and male fertility level of the F1 hybrids (H. syriacus × H. paramutabilis) remain unknown.
The fertility information of cultivars and breeding materials is important. For breeding projects, fertility information helps breeders to design the breeding programs and hybridization strategies. When breeding ornamental plants, breeders invest much effort in creating low-fertility cultivars to reduce the possibility of escaping cultivation or becoming invasive (Gagliardi and Brand, 2007; Oates et al., 2014). In addition, infertile plants might spend more energy on vegetative growth and bloom instead of producing seeds, which are a priority after flowering. Based on their morphological characteristics, Hibiscus cultivars like Lohengrin, Resi, and Tosca are believed to be hybrids of H. syriacus × H. paramutabilis (Dirr, 2009). These three hybrid cultivars are all notable for their large flowers, large leaves, and low fertility, with almost no fruit set. However, the male fertility of these hybrid cultivars is unknown. Pollen staining using acetocarmine has been regularly used in studies of woody plants, even though it is understood that it provides an overestimation. For example, Contreras et al. (2007) observed that allotetraploid Rhododendron had 68% pollen staining but 45% in vitro pollen germination. Regardless of the potential for overestimation, acetocarmine staining is a useful tool for rapidly and simply testing male fertility, and it is most useful when compared with a fully fertile control.
Flower size, petal number, flower color, and flower shape are important ornamental plant attributes because these traits directly influence the consumer appeal. Independent selection of two correlated floral traits may lead to biased and unpredictable or low selection rates. For example, selection based on two independent thresholds of negatively correlated traits might result in a very low selection rate because plants with one excellent trait tend to have poor other traits; therefore, very few plants would satisfy both thresholds and be selected. In this case, a simultaneous selection threshold using a regression line of correlated traits may overcome this problem. However, using simple regression to build a selection threshold can be problematic. The simple linear model is only useful for building a threshold with a selection rate of 0.5 because simple linear regression is based on mean values of the response variable such that 50% of data points are expected to be above the regression line. However, for different breeding purposes, various selection rates might be required; generally, they will be much lower than 50%. Manually shifting the regression line until the required proportion of data points pass the threshold may fail to correct this problem, particularly when the simple linear regression model assumptions are violated. For example, when the residual variation is not constant, the shifted simple linear regression line might not be useful because the threshold is influenced by data points with larger variances, which is very common in breeding projects.
Quantile regression is a useful method of setting thresholds for various selection rates because it is more resistant to model assumption violations. Quantile regression was first introduced by Koenker and Bassett (1978), and it has been recently used in ecological and agricultural research to estimate relationships or effects between two variables (Cade and Noon, 2003; Casagrande et al., 2010). Unlike simple linear regression that only estimates the relationship between an explanatory variable and the conditional mean values of the response variable, quantile regression can estimate all possible relationships between an explanatory variable and various quantiles of the response variable (Cade and Noon, 2003). Furthermore, quantile regression has been recommended for analyzing biological data with unequal variance (Cade and Noon, 2003). As such, quantile regression could be a useful tool in datasets with a complex distribution and/or when the assumptions of normality required for simple linear regression are violated.
The objectives of the current report were to assess the male fertility of interspecific hybrid Hibiscus cultivars Lohengrin and Resi and to develop appropriate selection methods to optimize simultaneous selection for larger flowers and increased petal numbers.
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