The Genetic Basis of Morphological Differences between Plant Species

Evolutionists have long stressed the need for a better understanding of the evolution of complex morphological features (e.g., Stebbins 1974; Bachmann 1983). How many mutations are required to initiate major differences in structure, shape, and architecture, and what are the developmental consequences of these mutations? In the conventional view, derived from the synthetic theory of evolution (Mayr and Provine 1980), complex morphological characters have a complex genetic basis, and thus major evolutionary changes require the cumulative additive effects of many genes (Coyne and Lande 1985). However, a number of plant evolutionists (Hilu 1983; Gottlieb 1984) have recently challenged this view, contending that the "open, less integrative, and plastic pattern of morphogenesis in plants" (Gottlieb 1984) may permit mutations with large morphological effects. In contrast, animal developmental systems are thought to be more tightly integrated, and mutations of large effect are considered less likely because of disharmonious pleiotropic-effects. Understanding the genetic basis of morphological change is of particular importance to the field of plant taxonomy because morphological characters provide the foundation for species descriptions and play a central role in systems of classification (Bachmann 1983). The justification for the use of these characters is the assumption that they have a complex genetic basis and are therefore likely to be invariant within a particular taxonomic group. A better understanding of the number of loci responsible for species differences would greatly assist in assessing a priori the reliability of particular morphological characters, e.g., reproductive morphology, frequently employed for phylogenetic and taxonomic inference. In this issue, Vlot et al. (1992) investigate the genetic basis of variation in the number of pappus parts in the wild-plant genus, Microseris (Asteraceae). Pappus parts are thought to be modified sepals that are attached to the ovary in most composite flowers and often serve as agents of wind dispersal. Characteristics of the pappus are frequently used to distinguish taxa at many levels in the Asteraceae. In addition, pappus numbers are representative of a type of morphological character frequently used in angiosperm taxonomy-numerically constant features. A wellknown example of this type of character is the fixed number of cotyledons that differentiate monocots from dicots. Clearly, understanding how numbers of plant parts can change, or alternatively, why they remain invariant, would enhance the ability of systematists to infer evolutionary relationships from these types of characters. The basic strategy Vlot et al. (1992) employ to determine the genetic basis of pappus part number involves a cross between a strain of Microseris douglasii, which is characterized by a pappus part number of three, and a strain of M. bigelovii, which possesses an average of slightly more than five pappus parts. Segregation of pappus part numbers was then monitored in F2 and F3 progenies. In addition to simple segregation analysis, Vlot et al. employ a relatively new approach to the estimation of the number of genes controlling morphological traits. They found a molecular marker locus associated with the major gene controlling pappus part number and examined segregation for both loci simultaneously. This allowed them to make much more precise estimates of the number of genes controlling pappus part number than might normally have been possible. One weakness of many previous genetic studies of quantitative characters has been a lack of understanding regarding the developmental basis of the morphological differences studied. This is particularly critical for numerically constant characters because the invariance observed appears to result from developmental buffering or "numerical canalization," not from an inherent lack of genetic variation for the invariant feature. To compensate for the effect of numerical canalization, Vlot et al. employed an empirical transformation, which allowed them to interpret the segregation data in a biologically realistic manner. The results from the study were quite unexpected. Previous studies of variation in the number of pappus parts in intraspecific crosses in M. douglasii by Bachmann and co-workers had revealed polygenic control of the number of pappus parts (referenced in Vlot et al. 1992). Thus, it was predicted that interspecific differences in pappus part number would have an even more complex genetic basis. This was not the case. Rather, Vlot et al. (1992) demonstrate that a single major gene accounts for most of the differences in average pappus part numbers observed between the parental strains. The remaining variation was accounted for by the presence of several additional, but minor, modifying loci and by phenotypic plasticity. The contribution of Vlot et al. (1992) adds to a growing body of literature which shows that species differences often result from a few mutations with major effects, commonly with many epistatic modifiers (Grant 1 975; Templeton 198 1; Gottlieb 1984), based on the theory that the novel