What Can Molecular and Morphological Markers Tell Us About Plant Hybridization?

Abstract

The study of hybrids and their evolutionary significance is often based on a number of tacit assumptions regarding character expression in hybrids. This article examines morphological, chemical, and molecular character expression in hybrids to determine whether traditionally recognized properties of hybrid plants, such as hybrid intermediacy and character coherence, are actually supported by empirical evidence, and also examines the impact of hybrids on phylogenetic analyses. We show that hybrids are a mosaic of both parental and intermediate morphological characters rather than just intermediate ones, and that a large proportion of first (64%) and later generation hybrids (89%) exhibit extreme or novel characters. Chemical character expression in hybrids is more predictable, with predominantly additive or complementary expression for both first generation hybrids (68%) and hybrid taxa (54%). Likewise, the genetic basis, and thus the expression of molecular characters, is well-worked out and predictable, although non-Mendelian inheritance has been reported in a few instances for molecular markers as well. There is even less empirical support for the concept of character coherence than hybrid intermediacy. Although morphological character coherence has been reported in both artificial and natural hybrid populations, it appears to be the exception rather than the rule. This idiosyncratic relationship among morphological characters is shown to be the norm for the relationship of morphological to molecular characters, as well as for the relationship among molecular characters. Hybrids are shown to have little impact on the topology of nonhybrid taxa in phylogenetic trees. However, the expression of primitive vs. derived character states, the placement of hybrids in cladograms, the number of equally parsimonious trees produced, and the effects of hybrids on phylogenetic topology do not appear to be predictable. Thus, cladistic identification of hybrid taxa is difficult and may not be possible based on phenotypic data, regardless of the analytical tools used.