Butternut hybridizes with English walnut (Juglans regia L.) to produce J. x quadrangulata (Carr.) Rehd. It also crosses with Japanese walnut J. ailantifolia Carr. to produce J. x bixbyi Rehd. Butternut is also reported to successfully hybridize with little walnut (J. microcarpa Berland.) and Manchurian walnut (J. mandshurica Maxim.) (6,14). Reports of crosses between butternut and black walnut have not been substantiated. Butternut is thought to have a haploid chromosome number of 16.
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