WALBOT, VIRGINIA. Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020. - Accelerating the pace of allelic diversity: the role of transposable elements.
Allelic diversity is the "raw material" for natural
selection. Consequently mechanisms that modulate allelic diversity
should play and important evolutionary role, and these mechanisms
should themselves be subject to stringent natural selection. Maize
alleles are remarkably polymorphic, and much of the diversity can be
attributed to transposable element activity. Transposons remain
resident in many alleles, proven to confer novel regulatory properties
in some cases. Insertion-deletion mutations in allele comparisons are
similar in size to the footprints created by transposon
insertion-deletion events. Because transposons cause mutations, most
of which are predicted to be deleterious, their activities are
regulated at the cellular, organismal, and populational levels.
Retrotransposons comprise the majority of the maize genome; these
elements can insert but never excise. Today, these retroelements are
restricted mainly to intergenic regions; sporadic amplification of
specific retroelements distinguish closely related grass genomes, but
few retroelement are active today. In contrast, DNA transposons are
active in a few individuals in every population, and they show
preferential insertion into transcription units. Such insertions often
create null alleles in host genes, but a wide spectrum of functional
alleles can be generated after transposon excision. Some transposons
generate both somatic and germinal excision alleles (Ac/Ds,
Spm/dSpm in maize), while others generate primarily somatic
diversity (MuDR/Mu elements in maize). MuDR/Mu elements
are the most aggressive known in terms of forward mutation, and these
elements are under stringent developmental regulation. Somatic
excisions are restricted to the terminal cell divisions of organ
development, resulting in a fine mosaic of diverse revertant alleles.
Germinal revertants are rare, although the MuDR/Mu elements
proliferate during meiosis and in the gametophyte. Because plants have
an alternation of generations in which genetic "quality" is
tested in the gametophyte, individuals carrying deleterious or lethal
mutations are eliminated in the haploid phase.
Key words: genome structure, mutation, transposons