2006b), pale flowers set more seed when hawkmoths are present (Miller, 1981), and in manipulative studies hawkmoths preferentially visit pale flowers (Hodges et al., 2002). To consider trends in color shifts when anthocyanin production is maintained, we inferred the color produced at ancestral nodes in the Aquilegia phylogeny by using the inferred pollination syndrome as a guide (Whittall and Hodges, 2007). Given that hummingbird-pollinated flowers in general tend to be red (Cronk and Ojeda, 2008) and all but 1 hummingbird-pollinated species of Aquilegia have red flowers (Aquilegia flavescens has yellow flowers), we inferred ancestral nodes reconstructed as hummingbird-pollinated anthocyanin producers to be red-flowered (Fig. 2.2). Similarly, given that bee-pollinated flowers tend to be blue and all but 1 bee-pollinated species of Aquilegia have blue flowers (Aquilegia laramiensis has white flowers) we inferred ancestral nodes reconstructed as bee-pollinated anthocyanin producers to be blue-flowered (Fig. 2.2). These reconstructions suggest that there have been 2 shifts from blue to red flowers and 2 shifts from red to blue. Thus, color shifts in Aquilegia, where anthocyanin production is maintained, do not exhibit an evolutionary trend.
The significant trends in pollinator evolution in Aquilegia, together with field experimentation on the functional relevance of specific traits, make variation in spur length, flower orientation, and flower color obvious targets for understanding the genetic basis of adaptive traits and traits affecting reproductive isolation. However, focusing on the genetics of flower color evolution is also advantageous for practical reasons. Since Mendel’s first experiments, flower color has been a primary system for dissecting the genetics of a biochemical pathway because the phenotype is readily observable and relatively simple. Many genes in the anthocyanin biosynthetic pathway (ABP) and their regulators have been described, thus facilitating efforts to identify the likely targets of selection. Reds and blues of most flowers result from the accumulation of anthocyanin pigments in the vacuoles of floral tissue cells (Fig. 2.1A, D, and G). When anthocyanins are absent, flowers are yellow or white, largely depending on, respectively, the presence or absence of carotenoid pigments (Fig. 2.1C and E). Anthocyanin biosynthetic genes are a central core to the larger flavonoid biosynthetic pathway and are expressed in multiple tissues other than flowers (Grotewold, 2006). The production of all anthocyanins involves 6 enzymes [in functional order chalcone synthase (CHS), chalcone flavone isomerase (CHI), flavanone-3-hydroxylase (F3H), dihydroflavonol 4-reductase (DFR), anthocyanidin synthase (ANS), and UDP flavonoid glucosyltransferse (UFGT); Fig. 2.3]. There are 3 basic types of anthocya-