Hybridization, Genome Duplication, And Chemical Diversification In The Evolution Of Calendula L. (Compositae)

Abstract

Hybridization and polyploidy are common in plants. Both processes can have extensive genomic consequences, and resulting morphological, biochemical, and reproductive changes may drive speciation. The effects of hybridization and polyploidy on speciation and biochemical diversity were explored in Calendula, a small, circum-Mediterranean genus in Compositae. Calendula officinalis (pot marigold), the best known species, has been cultivated for centuries for ornamental and medicinal use. Calendula is remarkable for the wide range of chromosome numbers (2n=14, 18, 30, 32, 44, and ~85), likely resulting from hybridization between species with different chromosome numbers followed by genome duplication. Hypotheses of species origins were tested using evidence from three non-coding chloroplast regions (atpIatpH, petLpsbE, and ndhFrpl32), nuclear ribosomal DNA (ITS), and two putatively low-copy nuclear markers (Chs and A39). Analyses of these markers provided support for a division of the genus into annual and perennial polyploid complexes, multiple origins of most polyploid taxa, and a single origin of C. officinalis. A39 was duplicated once in Calendula. Nine or more duplications of Chs were inferred from analyses of Calendula sequences with others from Compositae. Variation of four classes of phenolic (caffeic acid derivatives and flavonoid glycosides) and isoprenoid (monoterpenes and sesquiterpenes) compounds was investigated within the context of the annual and perennial polyploid complexes. Arising from different biosynthetic pathways, and highly diverse in plants, phenolics and isoprenoids offered different perspectives on the effects of speciation, hybridization, and polyploidy on chemical diversification. All four classes varied quantitatively and/or qualitatively across Calendula species. For three of four compound classes, proportions of compounds within each class were relatively steady within taxa or within tissue types but varied across taxa or tissue types, suggesting that particular blends of compounds may have evolutionary and ecological significance in Calendula. Neither the number of compounds detected nor bulk concentration of any compound class increased consistently with ploidy, but both were higher in floral tissue than in leaf tissue. Phenolic profiles were more consistent with hypotheses of evolutionary relationships than were isoprenoid profiles. Compound profiles in the annual polyploids showed additive compound complements or intermediate proportions of dominant compounds relative to putative progenitors.