Sequential paedomorphosis among the endemic Hawaiian Lobelioideae (Campanulaceae)

Summary Recently, attention has been focused on the evolutionary role of heterochrony (phyletic change in the timing of development). Heterochrony results in the condition of paedomorphosis, the retention of juvenile features of an ancestor in adults of a descendent. Relatively few examples of evolution via heterochrony are known among flowering plants, due to the vague demarcation of juvenile and adult stages. Recent systematic studies among the endemic Hawaiian Lobelioideae (Campanulaceae) have revealed the occurrence of a significant example. The endemic genus Cyanea includes several species that produce morphologically distinct juvenile forms. The four species of the C. solanacea complex form a monophyletic lineage in which the adults of each species resemble juveniles of the next more primitive species, i.e., adults of the more derived species are progressively more juvenile in appearance. It is hypothesized that the C. solanacea complex is a paedomorphic lineage, in which successive heterochronic events have produced a sequence of progressively more paedomorphic species. The publication of Ontogeny and Phylogeny (Gould, 1977) renewed interest in the role of heterochrony in evolution. Gould defined heterochrony as phyletic change in the timing of development, i.e., as an acceleration or retardation of the rate of development in a descendent as compared to that of an ancestor. As such, evolution proceeds not necessarily through major changes in the genome, but rather by changes in the timing of gene action. Acceleration of development in reproductive structures is termed progenesis, while retar- dation of development in somatic organs is termed neoteny. Either of these heterochronic processes results in the condition of paedomorphosis, the retention of juvenile features of an ancestor in adults of a descendent. Thus, paedomorphic species are often described as permanently juvenile. Changes in the timing of development might simply form part of a species' normal phenotypic variation. However, if the resulting changes in morphology are favored by selection, these paedomorphic individuals might become established and re- productively isolated, with speciation as the eventual result (McNamara, 1982). Although paedomorphosis long has been recognized in the animal kingdom, heterochrony rarely has been invoked as a major evolutionary process among plants (Takhtajan, 1972). Carlquist (1962a) demonstrated the paedomorphic nature of secondary xylem. More recent studies have suggested that heterochrony has been operative in the evolution of cleistog- amous flowers (Lord and Hill, 1987), ornithophilous flowers (Guerrant, 1982), and mosses (Mishler, 1986). Perhaps one reason that clear botanical examples of paedomorphosis have been lacking is that plants seldom produce distinct juvenile and adult stages. The concept of adulthood has two aspects: onset of sexual reproduction and cessation of somatic growth (Guerrant, 1982). In plants, due to their perpetually embryonic meristems, these two aspects are not as tightly linked as in animals. Typically, onset of reproduction is associated instead with the attainment of some minimum size, and somatic growth continues long afterward. The ephemeral nature of plant reproductive structures, as compared to those of animals, also vitiates the strict demarcation of juvenile and adult in plants. For these reasons, certain species of the endemic Hawaiian genus Cyanea Gaudich. (Campanulaceae: Lobelioideae) are of interest for the production of morphologically distinct juveniles.