there is evidence for a chromosome drive
resembling the scenarios in plants and animals.
Frequently observed aneuploidy of accessory
chromosomes in fungi and the particular meiotic inheritance of these chromosomes could
represent interesting starting points to further
dissect the mechanism of meiotic transmission
of fungal accessory chromosomes. In general,
the non-essential fungal accessory chromosomes represent interesting models for studies
of meiotic mechanisms. The presence/absence
polymorphism allows for the comparative analysis of transmission studying the same chromosomes when paired or unpaired. Hence pairing
of homologous chromosomes—which is central in meiosis and other closely related
mechanisms like recombination and DNArepair—can be addressed using fungal accessory chromosomes as model systems.
VII. Concluding Remarks
Accessory chromosomes are a diverse set of
genomic entities that are widespread in plants,
animals, and fungi. There is neither a single
sequence characteristic that is shared among
all accessory chromosomes between these kingdoms nor a unifying sequence characteristic
that has been identified among the accessory
chromosomes in fungi. However, fungal cells
appear to be able to differentiate between core
and accessory chromosomes. Many fungal
accessory chromosomes show differences in
their mitotic and meiotic transmission compared to core chromosomes. However, the
transmission of accessory chromosomes during
meiosis and its effect on the maintenance of
these chromosomes are not well understood.
The meiotic drive identified for the accessory
chromosomes of Zymoseptoria tritici appears
to be responsible for the maintenance of these
chromosomes despite conferring a fitness costs
during host interaction. This may be indicative
for similar mechanisms in other fungal systems
where accessory chromosomes confer fitness
costs, like the accessory chromosomes of Leptosphaeria maculans and Magnaporthe oryzae,
which contain avirulence genes. Interestingly,
in both organisms the accessory chromosomes
also show a non-Mendelian transmission during meiosis (Balesdent et al. 2013; Chuma et al.
2003; Orbach et al. 1996; Rouxel et al. 2011;
Rouxel and Balesdent 2017). Thus, a more
detailed analysis of their meiotic transmission
pattern could be highly relevant.
The underlying mechanisms responsible
for the distinctly different meiotic and mitotic
transmission of accessory chromosomes are
unknown. How does the fungal cell distinguish
between accessory and core chromosomes? In
the absence of common sequence characteristics of fungal accessory chromosomes, epigenetic marks could be promising targets of
future research. The involvement of the histone
mark H3K27me3 in the transmission fidelity of
the accessory chromosomes of Z. tritici could
be indicative of a mechanism that connects the
transmission pattern to the localization of the
accessory chromosomes in the nucleus. To
date, the epigenetic landscapes for a very limited number of accessory chromosomes have
been determined. In those cases the histone
mark H3K27me3 has been identified as being
enriched on the accessory chromosomes compared to the core chromosome (Galazka and
Freitag 2014; Schotanus et al. 2015; Fokkens
et al. 2018; Soyer et al. 2018). However, the
functional effect of this H3K27me3 enrichment
on the meiotic and mitotic transmission of the
accessory chromosomes is unknown for most
species.
In conclusion, we propose that fungal
accessory chromosomes provide an intriguing
opportunity to dissect novel chromosome drive
mechanisms and may serve as models to
improve our understanding of the underlying
mechanisms of meiotic and mitotic transmission of accessory as well as core chromosomes.
References
Ahn JH, Walton JD (1996) Chromosomal organization
of TOX2, a complex locus controlling hostselective toxin biosynthesis in Cochliobolus carbonum. Plant Cell 8:887–897. https://doi.org/10.1105/
tpc.8.5.887
40
M. Habig and E. H. Stukenbrock
resembling the scenarios in plants and animals.
Frequently observed aneuploidy of accessory
chromosomes in fungi and the particular meiotic inheritance of these chromosomes could
represent interesting starting points to further
dissect the mechanism of meiotic transmission
of fungal accessory chromosomes. In general,
the non-essential fungal accessory chromosomes represent interesting models for studies
of meiotic mechanisms. The presence/absence
polymorphism allows for the comparative analysis of transmission studying the same chromosomes when paired or unpaired. Hence pairing
of homologous chromosomes—which is central in meiosis and other closely related
mechanisms like recombination and DNArepair—can be addressed using fungal accessory chromosomes as model systems.
VII. Concluding Remarks
Accessory chromosomes are a diverse set of
genomic entities that are widespread in plants,
animals, and fungi. There is neither a single
sequence characteristic that is shared among
all accessory chromosomes between these kingdoms nor a unifying sequence characteristic
that has been identified among the accessory
chromosomes in fungi. However, fungal cells
appear to be able to differentiate between core
and accessory chromosomes. Many fungal
accessory chromosomes show differences in
their mitotic and meiotic transmission compared to core chromosomes. However, the
transmission of accessory chromosomes during
meiosis and its effect on the maintenance of
these chromosomes are not well understood.
The meiotic drive identified for the accessory
chromosomes of Zymoseptoria tritici appears
to be responsible for the maintenance of these
chromosomes despite conferring a fitness costs
during host interaction. This may be indicative
for similar mechanisms in other fungal systems
where accessory chromosomes confer fitness
costs, like the accessory chromosomes of Leptosphaeria maculans and Magnaporthe oryzae,
which contain avirulence genes. Interestingly,
in both organisms the accessory chromosomes
also show a non-Mendelian transmission during meiosis (Balesdent et al. 2013; Chuma et al.
2003; Orbach et al. 1996; Rouxel et al. 2011;
Rouxel and Balesdent 2017). Thus, a more
detailed analysis of their meiotic transmission
pattern could be highly relevant.
The underlying mechanisms responsible
for the distinctly different meiotic and mitotic
transmission of accessory chromosomes are
unknown. How does the fungal cell distinguish
between accessory and core chromosomes? In
the absence of common sequence characteristics of fungal accessory chromosomes, epigenetic marks could be promising targets of
future research. The involvement of the histone
mark H3K27me3 in the transmission fidelity of
the accessory chromosomes of Z. tritici could
be indicative of a mechanism that connects the
transmission pattern to the localization of the
accessory chromosomes in the nucleus. To
date, the epigenetic landscapes for a very limited number of accessory chromosomes have
been determined. In those cases the histone
mark H3K27me3 has been identified as being
enriched on the accessory chromosomes compared to the core chromosome (Galazka and
Freitag 2014; Schotanus et al. 2015; Fokkens
et al. 2018; Soyer et al. 2018). However, the
functional effect of this H3K27me3 enrichment
on the meiotic and mitotic transmission of the
accessory chromosomes is unknown for most
species.
In conclusion, we propose that fungal
accessory chromosomes provide an intriguing
opportunity to dissect novel chromosome drive
mechanisms and may serve as models to
improve our understanding of the underlying
mechanisms of meiotic and mitotic transmission of accessory as well as core chromosomes.
References
Ahn JH, Walton JD (1996) Chromosomal organization
of TOX2, a complex locus controlling hostselective toxin biosynthesis in Cochliobolus carbonum. Plant Cell 8:887–897. https://doi.org/10.1105/
tpc.8.5.887
40
M. Habig and E. H. Stukenbrock