Polyploidy is an important evolutionary mechanism in plants
that can lead to novel genetic and phenotypic variation, conferring heterosis
or the ability to exploit new environmental niches and undergo speciation. In order to observe the early events
following polyploidy formation, fifty resynthesized lines of Brassica napus derived from doubled haploid
homozygous diploid parents of Brassica oleracea and Brassica rapa were created and selfed to the S5
generation. The lines were
compared to the parents and assayed for changes in genome structure, gene
expression, and phenotype at both the S0 and S5 level
using a combination of RFLPs, SSRs, cDNA-AFLPs, and RT-PCR SSCP. Genome structure and gene expression
appeared quiescent and additive immediately following polyploidization and
phenotypic variability was low.
However, by the S5 generation numerous and variable genome
changes, non-additive patterns of gene expression, and phenotypic divergence
was identified among the lines.
Estimated mapping locations of observed genome changes were obtained
using maps of the parental chromosomes.
In support of previous data, structural genome changes, including DNA
loss and chromosomal rearrangements, may be an important source of de novo
variation in newly resynthesized Brassica
polyploids. In addition, the
location of chromosomal rearrangements in these resynthesized Brassica allopolyploids is in agreement with
studies that have found non-reciprocal transpositions in natural genotypes of Brassica
napus, suggesting that certain regions of
this allopolyploid genome are more likely to undergo structural changes as seen
in Helianthus hybrids. Various mechanisms such as
translocations, transpositions, and gene conversion will be discussed in light
of these results and in comparison to rapid genomic changes seen in
resynthesized allopolyploid wheat.