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James F. White Jr. et al.
form stromata, sexual recombination does not occur. Seed transmission is
the result of growth of the endophyte in inflorescence primordia. When
ovules differentiate in the primordia, the fungus is incorporated into tissues
of the nucellus. When the embryo differentiates within the nucellus, it is
invaded by endophytic mycelium and the next generation of host has been
effectively colonized (White and Cole 1986).
7.2 The Hypotheses
Two hypotheses have been proposed to explain loss of the sexual cycles by
species of Epichloë. In the ‘hybridization hypothesis’ it is suggested that
hybridization between two different species of Epichloë results in ‘hybrids’
that cannot undergo sexual reproduction due to meiotic incompatibility of
the two sets of chromosomes (Schardl and Wilkerson 2000). The frequent
occurrence of multiple sets of genes in some asexual endophytes supports
this hypothesis (Leuchtmann and Clay 1990; Tsai et al. 1994; Cabral et al.
1999). The occurrence of asexual forms such as the endophyte of Lolium
rigidifolium that do not show multiple copies of genes is problematic for the
hybridization hypothesis (Moon et al. 2000). The second problem with the
hybridization hypothesis is that it suggests a very unlikely scenario. It suggests
that haploid spermatia of one species fuse with haploid mycelium on a
stroma of an opposite species to produce a dikaryotic mycelium. The next
steps would involve formation of perithecia, asci, and ascospores. Within the
asci the two nuclei from different species of Epichloë would fuse to become a
diploid which would be immediately followed by meiosis to result in production
of the haploid ascospores. Without formation of ascospores, the hybrid
would be unable to spread. If the two genomes were meiotically incompatible
as the ‘hybridization hypothesis’ suggests that first meiosis would not occur
and ascospores could not be produced. This hypothesis invokes meiotic
incompatibility, yet demands that meiosis occurred at least once following
hybridization. It seems unlikely that hybridization and meiotic incompatibility
account for the origins of asexual Epichloë endophytes. It should be noted
that speciation by hybridization does work in plants. However, in plants meiosis
does not occur immediately after hybridization, instead, a diploid forms.
The diploid may reproduce clonally for a time (Grant 1977).
The plurality of gene copies present within many asexual endophytes may
be an indication of a parasexual process that is acting in asexual endophytes
to produce variation. To evaluate whether multiple gene copies reflect parasexual
recombinations within populations of asexual endophytes, it is necessary
to conduct populational studies on gene variation. To this point studies
examining gene variation in asexual endophytes have involved only a few
isolates. It will be important to determine whether this parasexual recombination
(hybridization) is a populational phenomenon and occurring rela-