Molecules, morphology and maps: New directions in evolutionary genetics.

Abstract With the characterization of a complete nucleotide sequence of a plant genome within sight, we are getting access to the ultimate source of data for evolutionary genetics. This achievement could hardly have been imagined twenty years ago. The rapid progress in methods is generating novel results that require a reassessment of our ideas about the relationship between the evolution of genomes and organisms, and of the relationship between genotype and phenotype. The widespread incongruence between chloroplast and nuclear (organismal) phylogenies was not previously suspected. It shows that introgression based on ‘wide crosses’ is a regular process in natural populations, and geneflow among species and genera must influence the distribution of genes and alleles as much as linear, phyletic inheritance. Speciation from hybrids at the diploid level seems to be stabilized by the creation of new epistatic interactions. Speciation via allopolyploidy also involves more than the additive action of the combined genomes; rearrangements and homogenization of non-coding repetitive sequences seems to occur quickly, and repetitive rDNA sequences of the combined genomes are homogenized to one of the parental sequences or to a recombinant sequence. Diploidization of allopolyploid genomes involves the evolution of redundant genes. This has profound effects on morphological evolution. Extrapolation from the developmental genetics of model systems (e.g. Arabidopsis) has shown that single mutations in regulatory genes can explain major differences in diagnostic characters between taxa. This seems to be in conflict with the general assumption that morphological evolution proceeds gradually by the selection of allelic polymorphisms in populations. Where plants with major morphological character differences can be crossed to produce fertile offspring, genetic analysis aided by molecular marker maps confirms that major regulatory genes are involved. At the same time, such experiments suggest a hypothesis integrating saltatory evolution and population genetics. The key factor is genetic redundancy through duplicate or modifier genes that reduce the phenotypic penetrance of the major gene. A low incidence of the new, at first unadapted phenotype, possibly mainly under environmental stress conditions, facilitates selection for the proper genetic background and for a new ‘integrated genotype’. Such episodes of saltatory evolution are likely to be linked to reticulate evolution; wide hybrids are likely to break up the protective modifier networks and expose major gene effects, while polyploidy immediately generates genetic redundancy. [ABSTRACT FROM AUTHOR]