Michelia L. is a large genus from the Magnoliaceae family and consists of about 80 tree- or shrub-habit species which can be characterized by axillary flowers, lateral dehiscent anthers, and obvious gynoecium handle (Law, 2004). The flowers of Michelia are usually white or yellow, excepting M. crassipes Law, M. maudiae Dunn var. rubicunda Yiet. J.C. Fan (Fan et al., 2009) and M. rubriflora Law et R.Z. Zhou (Wang et al., 2005), whose flowers are purple or red. Crossbreeding can create colorful hybrid cultivars. Thus, M. crassipes is widely cultivated and highly desirable in breeding new cultivars for its unique purple blooms. In recent decades, a total of nine new Michelia interspecific hybrid varieties were derived from M. crassipes (Gong et al., 2003b; Han et al., 2014; Li et al., 2013; Shao et al., 2015a, 2015b; Shao et al., 2016). Intriguingly, in the above studies, M. crassipes was often treated as paternal parent, with the two cultivars Chilongzhua and Yanzhizui as exceptions, which employed M. calcicola as paternal and M. crassipes as maternal parent (Han et al., 2014). In addition, previous interspecific hybridization studies within Michelia also reported that incompatibility existed between the maternal M. crassipes and the other species in the genus (Wang et al., 2003), suggesting some interspecific hybridization with respect to M. crassipes may be unilateral (Lewis, 1958), videlicet, the cross using M. crassipes as a parent is only successful in one direction, whereas the reciprocal cross always fails.
We have carried out numerous artificial hybridization combinations and obtained a series of intra- and intergeneric hybrids from Magnoliaceae plants since 1996. One of the cross combinations was Michelia yunnanensis × M. crassipes, its three new horticulture cultivars Yujin, Danxin, and Qinfang have been evaluated and registered at Kunming by the forestry department of Yunnan province in 2002. The horticultural characters, cultivation and propagation techniques, and cytological identification of the new cultivars were recorded during crossbreeding following previous studies (Fu and Sun, 2006; Gong et al., 2001, 2003a; Ji and Sun, 2011; Pan et al., 2008). However, the karyotypes of M. yunnanensis × M. crassipes offspring were markedly different from the expected ones according to the parental karyotypes, which might result from the structural variation of the parental chromosome during the hybridization (Pan et al., 2008). Moreover, there was few reported study regarding the hybridization processes and heterozygote embryogenesis in Michelia, especially for the possible unilateral incompatibility of M. crassipes.
In this study, we further investigated the interspecific hybridization processes between M. yunnanensis and M. crassipes and their heterozygote embryogenesis, and we hereby addressed three questions: 1) What are the hybridization processes of the direct cross and reciprocal cross between M. yunnanensis and M. crassipes? 2) Is there unilateral incompatibility for M. crassipes in this combination, which could result in this phenomenon? 3) What theoretical evidence and practical guidance will the study provide for future interspecific hybridizations on M. crassipes?
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