Sweetbox (Sarcococca confusa) is an evergreen shrub with attractive foliage in Buxaceae that requires shade and is relatively compact, making it an excellent choice for foundation plantings. Its white flowers are apetalous, sweetly fragrant, borne during winter, and give rise to glossy black drupes. In addition to its numerous ornamental attributes, it is regarded as highly drought tolerant (Dirr, 2009), which allows it to thrive in dry shade, which is a challenging landscape site for most plants. More compact forms of this species may be preferred for modern landscapes or container gardens. However, reports of apomixis in Sarcococca present challenges to breeding. Dirr (2009) reported that seedlings of sweetbox grew true to type, such that all seedlings were identical to their seed parent. This anecdotal observation suggests either 1) self-pollination of a relatively homozygous plant resulting in a homogenous population or 2) apomixis. During early preliminary studies, seeds sown from open-pollinated plants of S. confusa at Oregon State University (OSU) resulted in one, two, or three seedlings (Fig. 1), indicating apomixis occurs, at least facultatively, in the species.
Apomixis was first reported in Sarcococca ruscifolia (Orr, 1923), and a number of studies listed in the work by Nygren (1954) followed that confirmed adventive embryony (nucellary polyembryony) in several other species of Sarcococca. More recent studies have provided more in-depth studies to document specific mechanisms and activities during the process of apomixis in Sarcococca (e.g., Naumova, 1979; 1980; Naumova and Willemse, 1983).
Seedlings arising through apomixis are genetically identical to the seed parent and, in cases of polyembryony, when one seedling is sexually derived and one or more seedlings develop through apomixis, it can be difficult to identify the sexually derived seedling. Regardless of the mode of apomixis, this phenomenon presents a major obstacle to traditional breeding. Based on the presence of apomixis in the genus and our observations of polyembryony, we chose to introduce variation in this species using mutagenesis.
Mutation breeding is an important tool to improve plants exhibiting apomixis (Lapins, 1983). Although S. confusa appears to be a facultative apomict, it is inefficient and cumbersome to identify and separate sexually derived seedlings from nucellar embryos. As such, the objectives of our study were 1) to expose seeds of S. confusa to EMS at varying concentrations and exposure durations to evaluate the effects on seed germination, 2) to evaluate growth effects from various treatments, and 3) to determine whether the relative frequency of polyembryony is affected by EMS treatment.
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Denaeghel, H., Van Laere, K., Leus, L., Van Huylenbroeck, J. & Van Labeke, M.-C. 2017 Interspecific hybridization in Sarcococca supported by analysis of ploidy level, genome size and genetic relationships Euphytica 213 149
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