Cercis Inheritance Study by David Roberts

The genus Cercis, commonly known as redbud, is a valuable commodity to the landscape industry in North America and can be found growing in numerous regions across the globe. Cercis consists of approximately 10 species, often cultivated as medium sized trees or shrubs possessing a variety of interesting morphological characteristics. Careful examination of these traits and how they are inherited can help plant breeders better understand the genetic mechanisms that lead to specific phenotypes and allow for a greater degree of control while manipulating these characteristics in future breeding programs. David Roberts thesis involves three major areas of research; examining the genome size of more than 30 Cercis taxa, determining chlorophyll and carotenoid concentrations in the leaf tissue of yellow leaf Cercis cultivars and investigating the modes of inheritance for various morphological traits found in select Cercis cultivars.

Most recently, David Roberts completed a working draft of my inheritance study, which documents several intriguing occurrences within our populations of interest. This study was possible thanks to the extensive breeding populations of Cercis previously established by Dr. Dennis Werner. Dr. Werner’s impressive collection of Cercis has allowed research modes of inheritance that underlie numerous ornamental phenotypes including weeping architecture, purple leaf color, gold leaf color and variegated leaf type. Characteristics like purple leaf color and weeping architecture have proven to be the result of recessive traits, inherited in a simple Mendelian fashion while the variegated phenotype of C. ‘Floating Clouds’ can be attributed to maternally inherited genes. The most challenging (and interesting) aspect of Mr. Roberts research so far, has come from investigating the inheritance of a gold leaf cultivar, C. ‘Hearts of Gold’. It was originally hypothesized that the gold leaf color found in C. ‘Hearts of Gold’ was simply inherited and recessive in nature. However, F2 plants derived from C. ‘Covey’ (green leaf, weeping habit) x C. ‘Hearts of Gold’ (yellow leaf, upright habit) do not fit any known segregation ratio and exhibit unexpected phenotypes. In addition, an unusually high number of albino mutants were recovered from the F2 progeny of this cross. Interestingly, when the albino mutants and gold leaf mutants were combined into a single category (yellow), 10 out of 13 families fit a 3:1 ratio for green leaf : yellow leaf phenotypes (P<.05). While the exact reason behind this distortion in segregation is unknown, one possible explanation could come in the form of transposons. A large number progeny recovered from crosses involving C. ‘Hearts of Gold’ demonstrate a type of unstable variegation that is often attributed to transposon activity (fig. 1). While the transposon theory might explain how the unexpected phenotypes occurred, it does not adequately explain the high proportion of albino progeny.  The most likely explanation is that the gold-leaf trait in Cercis is polygenic in nature and that through recombination, lethal mutations occur at a relatively high frequency. One of the most mysterious aspects of my research has also proven to be one of the most educational. David Robers quandary has led to discussions with professors of not just horticulture and genetics but crop science, plant biology and forestry as well. While Mr. Roberts does not currently (and may never) have an explanation for this phenomenon, trying to fit the pieces of this puzzle together has been a truly enlightening experience.

Cercis Inheritance

a) ‘Covey’ x ‘Hearts of Gold’ F2 cotyledon variegation, b) ‘Rising Sun’ x ‘Hearts of Gold’ F1, 7 months old showing 50/50 leaf variegation c) ‘Covey’ x ‘Hearts of Gold’ F2 cotyledon variegation, d) ‘Covey’ x ‘Hearts of Gold’ F2 showing 50/50 cotyledon variegation. Arrows indicate variegated sectors.