Genetic Analysis of the Evolution of Petaloid Bracts in Dogwoods.

Abstract

The genus Cornus is best known for beautiful flowering dogwood trees that have large petaloid bracts. Another group of species within Cornus, the dwarf dogwoods, also bear petaloid bracts, and whether there were one or two origins of petaloid bracts in Cornus is debated. Since the discovery of MADS-box floral organ identity genes in model organisms, the molecular evolution of this gene family has been investigated and implicated in floral evolution, especially in origins of petals/petaloidy and resulting perianth diversity. We hypothesize that ectopic expression of petal organ identity genes may play a key role in the transition of inflorescence bracts from small and leaf-like to large and petal-like in two clades of Cornus. Here we identify A, B, and E class MADS-box genes from across the genus and investigate their expression in bracts, flowers, receptacles and leaves of four species of dogwood representing the four clades of the genus. Our results of real-time quantitative PCR show that A, B and E class genes are significantly expressed in bracts of the flowering dogwood C. florida and are not significantly expressed in bracts of the dwarf dogwood C. canadensis or the non-showybracted C. mas; this difference in genetic formulas for petaloidy indicates that it may have originated twice in the genus. Our results also show that A and E class genes are highly expressed in receptacle tissue of bractless and non-showy-bracted dogwoods; this expression may have been a pre-adaptation for petaloidy.