WHAT CAN MOLECULAR AND MORPHOLOGICAL MARKERS TELL US ABOUT PLANT HYBRIDIZATION

The study of hybrids and their evolutionary significance is often base d on a number of tacit assumptions regarding character expression in h ybrids. This article examines morphological, chemical, and molecular c haracter expression in hybrids to determine whether traditionally reco gnized properties of hybrid plants, such as hybrid intermediacy and ch aracter coherence, are actually supported by empirical evidence, and a lso examines the impact of hybrids on phylogenetic analyses. We show t hat hybrids are a mosaic of both parental and intermediate morphologic al characters rather than just intermediate ones, and that a large pro portion of first (64%) and later generation hybrids (89%) exhibit extr eme or novel characters. Chemical character expression in hybrids is m ore predictable, with predominantly additive or complementary expressi on for both first generation hybrids (68%) and hybrid taxa (54%). Like wise, the genetic basis, and thus the expression of molecular characte rs, is well-worked out and predictable, although non-Mendelian inherit ance has been reported in a few instances for molecular markers as wel l. There is even less empirical support for the concept of character c oherence than hybrid intermediacy. Although morphological character co herence has been reported in both artificial and natural hybrid popula tions, it appears to be the exception rather than the rule. This idios yncratic relationship among morphological characters is shown to be th e norm for the relationship of morphological to molecular characters, as well as for the relationship among molecular characters. Hybrids ar e shown to have little impact on the topology of nonhybrid taxa in phy logenetic trees. However, the expression of primitive vs. derived char acter states, the placement of hybrids in cladograms, the number of eq ually parsimonious trees produced, and the effects of hybrids on phylo genetic topology do not appear to be predictable. Thus, cladistic iden tification of hybrid taxa is difficult and may not be possible based o n phenotypic data, regardless of the analytical tools used.