Biodiversity and Dollo's Law: To What Extent can the Phenotypic Differences between Misopates orontium and Antirrhinum majus be Bridged by Mutagenesis?

摘要

According to Dollo's law, evolution is irreversible. Yet, of the eight derived features essentially distinguishing Misopates orontium from its closely related Antirrhinum majus, five differences have phenotypically been clearly diminished or fully overcome by mutant genes, so that Misopates orontium outwardly approaches, meets or even overlaps the features of Antirrhinum majus or vice versa (aspects of the life cycle, leaf form, flower size, flower colour and mode of fertilization). However, to datethe morphological key distinguishing feature between the two genera, the strongly elongated sepals in Misopates (itself a feature being at odds with Dollo's law), could not be reduced to that of the length of Antirrhinum nor could the development of theshort Antirrhinum sepals be extended to that of the length of Misopates, in spite of extensive mutagenesis programmes with both species (agreeing with Dollo's law as to the stasis of this difference). Also, the long sepal character strongly dominated almost all homeotic Misopates mutants. After a general discussion of Dollo's law, its relevance for our mutants (and vice versa) is examined according to different evolutionary viewpoints. Furthermore, two concerns are raised: (1) To what extent can the hypothesis be substantiated such that the long and short sepals could really constitute genuinely persistent ("immutable") characters? (2) To what magnitude can the unexpected constancy of a feature distinguishing genera like the sepal difference be generalized for systematics and paleontology? Moreover, four basic genetic explanations (losses of gene functions, redundancy, the origin of new genes and chromosome rearrangements) are examined in this connection, and their relevance for some pivotal questionson the origin of species is investigated. As far as the authors are aware, this is the first thorough paper on Dollo's law in botany.