| |
Evolution
Evolution of a grass and its fungal
symbiont
Evolution of the Bioprotective
Fungal Symbiont of Tall Fescue
One of the most complex genetic systems in nature is
the grass, tall fescue (Lolium arundinaceum = Festuca
arundinacea), with the seed-transmissible fungal symbiont,
Neotyphodium coenophialum. Both the grass host and its
symbiont are complex hybrids. The origin of hexaploid (2n=6x=42
chromosomes) tall fescue was the hybridization of meadow fescue
(2n=2x=14 chromosomes) and L. arundinaceum ssp. fenas
(2n=4x=28 chromosomes). Both tall fescue and meadow fescue have well
characterized and ubiquitous symbionts, N.coenophialum and
N. uncinatum, respectively. We determined that the origin of
N. coenophialum is a series of hybridizations involving N.
uncinatum and related sexual fungi of genus Epichloe.
Apparently, the original tall fescue inherited the symbiont of its
meadow fescue ancestor. Later, a tall fescue-N. uncinatum
symbiotum was infected with an Epichloe species, and the
coexisting fungi hybridized and combined nuclei. A repeat of this
process led to the double hybrid, N. coenophialum.
The modern tall fescue-N. coenophialum symbiotum has
nuclear genomes from two grass ancestors, a Neotyphodium and
two Epichloe ancestors, plant and fungal mitochondrial
genomes, and a chloroplast genome. All but the plant nuclear
genomesare maternally transmitted.
Coevolution and hybridization in
symbiont evolution.
The phylogenetic relationship of
grass tribes mainly fit those of their symbiotic
Epichloe species. One exception
to this pattern is Epichloe glyceriae (species VIII), a pathogen of Glyceria striata
(tribe Meliceae). The origin of species VIII apparently involved
hybridization of at least two ancestral Epichloe species.
The Epichloe species
with mutualistic or mixed symbioses with their hosts are shown in
boldface (E. festucae, E. amarillans, E. elymi, E.
sylvatica, and E. brachyelytri (species III, VII,and IX,
resp.). The other three species are highly pathogenic to their
hostplants.
Updated: September 29 2004
|
