tion of accessory chromosomes (Ma et al.
2010). Further examples of chromosome transfers include the asexual plant pathogen Colletotrichum gloeosporioides that infects a wide
variety of crops. The supernumerary chromosome of C. gloeosporioides is considered to have
been transferred by vegetative fusion, which
can also be observed under laboratory conditions (He et al. 1998; Masel 1996). Similarly,
chromosome transfer occurs in Alternaria
spp. Here, pathogenic strains cause leaf spots
and blights on different host plants (Rotem
1994). Phylogenetic analysis of toxin-encoding,
conditionally dispensable chromosomes supports chromosome transfer between different
strains (Akagi et al. 2009a). This hypothesis
could be confirmed under laboratory conditions (Akagi et al. 2009b). In addition, transfer
of accessory chromosomes was suggested by
comparative analyses of transposon or repeat
content and codon usage of the accessory and
the core chromosomes of Alternaria arborescens (Hu et al. 2012). Although the origin of
the conditionally dispensable chromosomes in
Alternaria spp. is still unknown, these potential
horizontal transfers would be an example for a
possible mechanism by which pathogens could
acquire novel determinants of host specificity
(Mehrabi et al. 2011).
It is, however, important to note that the observed
differences in sequence composition between core and
accessory chromosomes do not provide a proof of a
horizontal chromosome transfer. Different recombination and mutation rates, the differences in transposable
element content, and higher frequencies of pseudogenes could likewise explain some of the observed differences in sequence composition (Galazka and Freitag
2014). This is exemplified by the wheat pathogen Fusarium poae which carries several accessory chromosomes
(Fekete and Hornok 1997). Here, the accessory genome
was shown to be lacking repeat-induced point mutations (RIP). RIP is a genome defense mechanism initially described in Neurospora crassa that acts to mutate
repetitive elements to prevent their further propagation
in the genome (Selker 1990). Interestingly, in F. poae
RIP occurs on the core chromosomes (Vanheule et al.
2016). The sole absence of RIP on the accessory may
confer differences in sequence composition between
core and accessory chromosomes. Similarly, sequences
on the accessory chromosomes of Z. tritici show signatures of accelerated evolution, which could also account
for the observed sequence differences between the core
and the accessory chromosomes (Stukenbrock et al.
2010; Stukenbrock et al. 2011).
Although accessory chromosomes are
thought to be non-recombining with the core
chromosomes, transfer of genetic material
from accessory to core chromosomes appears
to happen frequently. In several cases an accessory chromosome became part of a core chromosome (Ma et al. 2010; Schotanus et al. 2015;
Vanheule et al. 2016). In Z. tritici, the distal
0.865 Mb segment of the long right arm of the
core chromosomes 7 shares characteristics like
histone modifications, gene content, and gene
organization with the accessory chromosomes
(Schotanus et al. 2015). This chromosome segment is therefore considered to be either a
translocated fragment or an entire accessory
chromosome fused to a core chromosome
(Schotanus et al. 2015). This type of chromosomal fusion could be the result of BFB cycles
as already observed in the genome of Z. tritici
(Croll et al. 2013) (Fig. 2.1b). Similarly, the right
arms of the core chromosomes 1 and 2 in F.
oxysporum f. sp. lycopersici share sequence
characteristics with the lineage-specific chromosomes, and hence these chromosomal
regions are also considered to be lineagespecific (Ma et al. 2010). In addition, the core
chromosome of F. poae contains large blocks
(>200 kb) of sequences that were recently
translocated from the accessory genome (Vanheule et al. 2016). Hence, sequence exchange
between accessory and core chromosomes
occurs, but the extent of this exchange and the
functional and evolutionary consequences
remain to be elucidated.
V. Accessory Chromosomes Are
Mitotically Instable
Accessory chromosomes often show distinct
patterns of mitotic transmission compared to
the core chromosomes. These transmission differences can also affect their meiotic transmission if cells of the germline are affected (e.g.,
non-disjunction during first pollen mitosis of
rye B chromosomes) (Houben 2017). Further
examples of non-equal transmission during
34
M. Habig and E. H. Stukenbrock
2010). Further examples of chromosome transfers include the asexual plant pathogen Colletotrichum gloeosporioides that infects a wide
variety of crops. The supernumerary chromosome of C. gloeosporioides is considered to have
been transferred by vegetative fusion, which
can also be observed under laboratory conditions (He et al. 1998; Masel 1996). Similarly,
chromosome transfer occurs in Alternaria
spp. Here, pathogenic strains cause leaf spots
and blights on different host plants (Rotem
1994). Phylogenetic analysis of toxin-encoding,
conditionally dispensable chromosomes supports chromosome transfer between different
strains (Akagi et al. 2009a). This hypothesis
could be confirmed under laboratory conditions (Akagi et al. 2009b). In addition, transfer
of accessory chromosomes was suggested by
comparative analyses of transposon or repeat
content and codon usage of the accessory and
the core chromosomes of Alternaria arborescens (Hu et al. 2012). Although the origin of
the conditionally dispensable chromosomes in
Alternaria spp. is still unknown, these potential
horizontal transfers would be an example for a
possible mechanism by which pathogens could
acquire novel determinants of host specificity
(Mehrabi et al. 2011).
It is, however, important to note that the observed
differences in sequence composition between core and
accessory chromosomes do not provide a proof of a
horizontal chromosome transfer. Different recombination and mutation rates, the differences in transposable
element content, and higher frequencies of pseudogenes could likewise explain some of the observed differences in sequence composition (Galazka and Freitag
2014). This is exemplified by the wheat pathogen Fusarium poae which carries several accessory chromosomes
(Fekete and Hornok 1997). Here, the accessory genome
was shown to be lacking repeat-induced point mutations (RIP). RIP is a genome defense mechanism initially described in Neurospora crassa that acts to mutate
repetitive elements to prevent their further propagation
in the genome (Selker 1990). Interestingly, in F. poae
RIP occurs on the core chromosomes (Vanheule et al.
2016). The sole absence of RIP on the accessory may
confer differences in sequence composition between
core and accessory chromosomes. Similarly, sequences
on the accessory chromosomes of Z. tritici show signatures of accelerated evolution, which could also account
for the observed sequence differences between the core
and the accessory chromosomes (Stukenbrock et al.
2010; Stukenbrock et al. 2011).
Although accessory chromosomes are
thought to be non-recombining with the core
chromosomes, transfer of genetic material
from accessory to core chromosomes appears
to happen frequently. In several cases an accessory chromosome became part of a core chromosome (Ma et al. 2010; Schotanus et al. 2015;
Vanheule et al. 2016). In Z. tritici, the distal
0.865 Mb segment of the long right arm of the
core chromosomes 7 shares characteristics like
histone modifications, gene content, and gene
organization with the accessory chromosomes
(Schotanus et al. 2015). This chromosome segment is therefore considered to be either a
translocated fragment or an entire accessory
chromosome fused to a core chromosome
(Schotanus et al. 2015). This type of chromosomal fusion could be the result of BFB cycles
as already observed in the genome of Z. tritici
(Croll et al. 2013) (Fig. 2.1b). Similarly, the right
arms of the core chromosomes 1 and 2 in F.
oxysporum f. sp. lycopersici share sequence
characteristics with the lineage-specific chromosomes, and hence these chromosomal
regions are also considered to be lineagespecific (Ma et al. 2010). In addition, the core
chromosome of F. poae contains large blocks
(>200 kb) of sequences that were recently
translocated from the accessory genome (Vanheule et al. 2016). Hence, sequence exchange
between accessory and core chromosomes
occurs, but the extent of this exchange and the
functional and evolutionary consequences
remain to be elucidated.
V. Accessory Chromosomes Are
Mitotically Instable
Accessory chromosomes often show distinct
patterns of mitotic transmission compared to
the core chromosomes. These transmission differences can also affect their meiotic transmission if cells of the germline are affected (e.g.,
non-disjunction during first pollen mitosis of
rye B chromosomes) (Houben 2017). Further
examples of non-equal transmission during
34
M. Habig and E. H. Stukenbrock