Plant Breeding and Horticulture Research

Genome Size and Ploidy Level Survey of Styrax (Snowbells)

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Styrax (Styracaceae) is a genus of approximately 130 species of woody, dicotyledonous, often resinous trees and shrubs with a wide distribution spanning temperate and tropical regions of the Americas, the Mediterranean, and East and Southeast Asia. Surveys of public garden collections reveal that Styrax japonicus, Styrax obassia, Styrax americanus, and Styrax grandifolius are by far the most commonly cultivated and accessioned species. Several other ornamental species such as Styrax officinalis, Styrax hemsleyanus, and Styrax wilsonii are also somewhat represented though rare, and certainly worthy of greater use. The ornamental potential of the group is vast and largely underused and may be improved by selective breeding and hybridization techniques.

Selective breeding and hybridization requires an understanding of the number of sets of chromosomes possessed by each species. As with many woody plants, chromosomes of Styrax are small and difficult to count using traditional light microscopy techniques. The base number for Styrax appears to be 8 (2n=2x=16). Tetraploidy is known from at least one species, Styrax grandifolius Aiton (2n=4x=32). One report notes tetraploidy for Styrax hookeri. Pentaploidy has been historically reported for Styrax japonicus, and additionally, hexaploidy has been recently published in Styrax japonicus var. tomentosus (2n=6x=48). Styrax japonicus also has a published count of 2n=16, implying that multiple ploidy levels exist over the range of the species.

With the development and application of flow cytometry in the plant sciences over the past few decades, there have been a growing number of genome size and ploidy surveys in woody plants.  Gaining a greater understanding of genome size and ploidy in more diverse taxa answers the call put forth in 2011 by Galbraith et al. for a Coordinated Global Census of all data relating to the genomes of flowering plants.

These surveys also provide invaluable information to plant breeders working with Styrax. As an ornamental flowering tree group with diverse characters in form, flower color, and foliage, Styrax has been growing as a crop of interest at both academic institutions and commercial nurseries. Breeding programs centered on the improvement of Styrax exist in Oregon, Tennessee, and Georgia. Knowledge of genome size and ploidy can help breeders to develop targeted objectives and increased efficiency.  

Use of flow cytometry, combined with some traditional light microscopy techniques, will be used to better understand the base number of the genus, as well as the presence of ploidy series.


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