Probing plant parentage proves previous postulate
In my walks alongside the estuary of the
River Exe near my home, a small stand of Spartina
anglica reminds me of the importance of allopolyploidy in plant speciation and evolution. This
involves hybridization followed by genome duplication that restores the ability
of chromosomes to pair at meiosis. With S. anglica
the parenthood is well known. However, for many species allopolyploidy
has had to be deduced from chromosome numbers and overall plant characteristics.
An example of this is Iris versicolor which,
with its chromosome number 2n = 108, has been regarded as an
allopolyploid derived from hybrids between I. virginica
(2n = 70) and I. setosa (2n = 38). This idea about the parental
origins of I. versicolor has now been
beautifully confirmed by Lim et al., from London, UK
and Brno, Czech Republic* (pp. 219–224). The authors made fluorescent probes for the whole genomes of both
species (for GISH: genomic in situ hybridization) and for
specific ribosomal RNA genes (for FISH: fluorescent in situ
hybridization). GISH clearly showed that chromosome sets from both the putative
parents were present in I. versicolor, thus confirming the long-held hypothesis on
the parentage of this species. However, FISH showed some uniparental
loss of specific genes. The 26S and 18S ribosomal RNAs
are transcribed from a gene that is repeated many times within the plant
genome. In Iris versicolor, only the 26S–18S rRNA genes from I. virginica are present; those from I.
setosa having been lost during evolution of the
hybrid. By contrast, the repeated genes encoding 5S rRNA
from both parents are present, but there has been a partial loss of these genes
when derived from I. setosa. The loss of rRNA
genes derived from one parent, although by no means universal, has been shown
in other allopolyploids and may occur following epigenetic silencing, for
example because of uniparental imprinting.
*A most appropriate location for studying
plant genetics, following the example of Gregor
Mendel.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
