American Journal of Botany 92(4): 761–767. 2005.
GENOME
EVOLUTION AMONG CRUCIFEROUS PLANTS: A
LECTURE FROM THE COMPARISON OF THE GENETIC
MAPS OF THREE DIPLOID SPECIES—CAPSELLA RUBELLA,
ARABIDOPSIS LYRATA SUBSP. PETRAEA,
A. THALIANA1
MARCUS A. KOCH2
AND
AND
MARKUS KIEFER
University of Heidelberg, Heidelberg Institute of Plant Science, Im Neuenheimer Feld 345, D-69120 Heidelberg, Germany
Comparative mapping in cruciferous plants is ongoing, and recently two additional genetic maps of diploid Capsella and Arabidopsis
lyrata subsp. petraea have been presented. We compared both maps with each other using the sequence map and genomic data
resources from Arabidopsis thaliana as a reference. The ancestors of the species pair Capsella–Arabidopsis diverged from one another
approximately 10–14 million years ago (mya), whereas Arabidopsis thaliana and Arabidopsis lyrata have been separated since roughly
5–6 mya. Our analysis indicated that among diploid Capsella and Arabidopsis lyrata all eight genetic linkage groups are totally
colinear to each other, with only two inversions significantly differentiating these two species.
By minimizing the number of chromosomal rearrangements during genome evolution, we presented a model of chromosome evolution involving all three species. From this scenario, it is obvious that Arabidopsis thaliana underwent a dramatic genome reconstruction, with a base chromosome number reduction from five to eight and with approximately 1.3 chromosomal rearrangements per
million years. In contrast, the terminal lineage leading to Capsella has only undergone less than 0.09 rearrangements per million years.
This is the same rate as calculated for Arabidopsis lyrata since its separation from the Capsella lineage 10–14 mya. These results are
in strong contrast to all overestimated rates calculated from comparisons of the systems Arabidopsis thaliana and Brassica, and our
data demonstrate the problematic nature of both model systems.
Key words: Arabidopsis lyrata subsp. petraea; Arabidopsis thaliana; Brassicaceae; Capsella rubella; Cardaminopsis petraea;
comparative genomics; genetic maps; genome evolution.
The Brassicaceae family consists of approximately 340 genera with 3350 species. While this species number is not higher
than in many other angiosperm families, systematics and evolution of this family remains unsettled. Numerous efforts are
underway to solve critical questions and to integrate the conflicting traditional, morphology-based tribal systems (Prantl,
1891; Hayek, 1911; Schulz, 1936; Janchen, 1942) with molecular evidence (Koch et al., 1999, 2000, 2001; Hall et al.,
2002; O’Kane and Al-Shehbaz, 2003).
A detailed overview of the research status is given in Koch
(2003), Koch et al. (2003) and Mitchell-Olds et al. (2004).
Most information for the analysis of crucifer evolution and
phylogenetics originated from DNA sequencing (Koch, 2003),
which is used to integrate traditional data on morphology and
cytology as compiled by Appel and Al-Shehbaz (2003).
Much less attention has been given to chromosome and genome evolution in Brassicaceae. There are mostly two reasons
for this. First, crucifer chromosomes are small and thus crucifers never became attractive cytogenetic models, except economically important Brassica species. Second, chromosome
number varies greatly from n 5 4 in Physaria and Stenopetalum to n 5 128 in Cardamine concatenata (see Al-Shehbaz,
1984), and enormous variation in base and total chromosome
numbers occurred frequently in parallel making it difficult to
follow evolutionary trends without a (molecular) phylogenetic
framework.
However, the agronomically importance of the Brassica lineages resulted in the early reconstruction of genetic maps of
several Brassica species (Kowalski et al., 1994; Lagercrantz
and Lydiate, 1996; Lagercrantz, 1998). Subsequently, these
maps have been studied in direct comparison to Arabidopsis
thaliana (Kowalski et al., 1994; Grant et al., 1998; Cavell et
al., 1998; Lagercrantz, 1998; Axelsson et al., 2000; Grant et
al., 2000; Lan et al., 2000; O’Neill and Bancroft, 2000; Quiros
et al., 2001; Babula et al., 2003; Lukens et al., 2003). In several of these cited studies, the authors speculated about a palaeopolyploid origin of the modern diploid Brassica species;
and in agreement with this assumption, the recently established
powerful approach of chromosome painting among various
cruciferous plant taxa (Lysak et al., 2003) demonstrated genome triplications among the representatives of the whole tribe
Brassiceae (Lysak et al., in press). Genetic maps for two additional Brassiceae, Moricandia arvensis (Beschorner et al.,
1999) and Raphanus (Bett and Lydiate, 2003, 2004) have also
been developed.
Given the availability of the entire genomic DNA sequence
of A. thaliana (Arabidopsis Genome Initiative [AGI], 2000),
comparative genome analysis can be used (1) to define chromosomal map position and copy number of any given sequence in Arabidopsis or in related species, which provide
additional information about orthologous and/or paralogous
sequences, (2) to explore gene content and density in any selected region, and (3) to test if loci under study may be positioned in duplicated chromosomal segments in the A. thaliana genome (Boivin et al., 2004). The use of A. thaliana as
Manuscript received 26 August 2004; revision accepted 1 February 2005.
This work has been supported in part by a research grant from the FWF—
Austrian Science Fund (Ref. P15609) to M. Koch. We also are grateful to
two anonymous reviewers whose comments improved the manuscript substantially.
2
Author for correspondence (e-mail: [email protected])
1
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a resource to develop molecular markers facilitates comparative mapping experiments within the Brassicaceae. However,
these studies are focusing on crop plants for the most part
(Paterson et al., 2000).
Since Lagercrantz (1998, p. 1218) concluded that ‘‘. . . to
elucidate the relationship between the uniquely small genome
of A. thaliana and highly replicated Brassica genomes, there
is a need for additional comparative mapping,’’ most work has
focused on Brassica. Ultimately, it has taken more than five
years for genetic mapping experiments to be completed in nonBrassiceae: Arabidopsis lyrata subsp. petraea (conspecific
with Cardaminopsis petraea subsp. petraea) (Kuittinen et al.,
2004) and Capsella grandiflora 3 C. rubella (Boivin et al.,
2004). Both studies used markers, which have been well characterized in A. thaliana, and their position as well as copy
number are known from the A. thaliana sequence map.
The genus Capsella comprises only three species. Tetraploid
Capsella bursa-pastoris, (2n 5 4x 5 32) is thought to be a
hybrid established between the two ancestral diploid (2n 5
16) taxa C. rubella and C. grandiflora (Hurka and Neuffer,
1997). This genus is more closely related to A. thaliana and
members of the former genus Cardaminopsis than to the Brassica lineage, which is not only obvious from phylogenetic reconstruction using gene trees (Koch et al., 2000, 2001), but
also from the occurrence of plastidic trnF pseudogenes (Koch
et al., 2005).
Because Arabidopsis thaliana has been described within a
broadly defined genus Arabidopsis (DC.) Heynh., several attempts have been undertaken to provide a phylogenetic framework for these taxa. The results of these studies are compiled
in Koch et al. (1999) and O’Kane and Al-Shehbaz (2003) and
can be summarized as follows: (1) Arabidopsis as treated traditionally does not exist and is polyphyletic, and (2) the genus
Cardaminopsis is the closest sister group to Arabidopsis thaliana. A new taxonomical concept has been proposed merging
members of the former genus Cardaminopsis into Arabidopsis
(O’Kane and Al-Shehbaz, 1997). However, it should be noted
here that phylogenetic relationships within Cardaminopsis are
still unsettled and are the focus of ongoing research (M. A.
Koch, unpublished data).
In this study we used available mapping information (Boivin et al., 2004; Kuittinen et al., 2004) and estimates of divergence times between the different species calculated from
synonymous substitution rates (KS values) of plastid matK and
nuclear chs (Koch et al., 2001) to answer the following questions: (1) To what extent are the genetic maps of Capsella and
A. lyrata colinear to one another? (2) Do the Capsella and A.
lyrata lineages differ in the rate of chromosomal rearrangements? (3) Does an annual life cycle such as in A. thaliana or
Capsella explain an increase in the chromosomal mutation
rate?
MATERIALS AND METHODS
Mapping populations—Arabidopsis lyrata subsp. petraea—The A. lyrata
mapping population is explained in detail in Kuittinen et al. (2004). This
mapping population originated from a cross between two individuals from
populations from Mjällom (Sweden) and Karhumäki (Russia) (van Treuren et
al., 1997; Kuittinen et al., 2004). Because A. lyrata represents a self-incompatible species, a three-generation pedigree was used with two F1 interpopulational hybrids as starting material for subsequent reciprocal crosses to obtain seed material to raise the F2 generation. This complex crossing scheme
has been used to avoid possible effects of inbreeding depression in the final
F2. The 72 markers used in the development of the map have been enumerated
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consecutively in this study (from linkage group AL1 to AL8) following the
order of Table 2 in Kuittinen et al. (2004).
Capsella grandiflora 3 C. rubella—This mapping population was generated, tested, and established by M. Koch in 1997 (Max Delbrück Laboratory,
Max Planck Institute for Plant Breeding, Cologne, Germany). This population
has been already used to demonstrate genome colinearity and synteny on a
small scale between C. rubella and A. thaliana (Acarkan et al., 2000), and a
comprehensive comparative genetic map in relation to A. thaliana has been
published recently (Boivin et al., 2004). However, in both studies only limited
information is given on the source material and the crossing scheme. Therefore, we will add this information here.
Capsella rubella is a self-compatible annual, whereas C. grandiflora is a
self-incompatible annual to biennial. These two species are diploid with n 5
8, and seeds were obtained from the Brassicaceae germ plasm collection in
Osnabrück, Germany (H. Hurka, Institut für Spezielle Botanik, Osnabrück
University, personal communication). Individuals were raised from wild collected seeds (Capsella rubella: accession number 774, Gargano, Italy; Capsella grandiflora 936/15/11 and Capsella grandiflora 922/3/1 from Igoumenitsa and the mountain Pantokrator on Korfu, Greece). We tested all reciprocal
crosses between plants from all three populations. However, in 1997 only one
of the six possible crosses with C. rubella as pollen donator and C. grandiflora
922/3/1 as mother plant produced viable seeds. These seeds were sown in
June 1997. Of the 17 seedlings that developed, 12 represented true hybrids
and five resembled selfed C. grandiflora, which had succeeded to overcome
their self-incompatibility system. The identity of the hybrids was supported
by data from allozyme analysis of the parents and their offspring, generated
by selfing of the original hybrids, using the systems of PGI, AAT, LAP and
PGM, for which a detailed genetic analysis was available (Hurka et al., 1989;
Hurka and Neuffer, 1997). The F2 was sown in November 1997 using individual ‘‘number 11’’ of the 12 true hybrids from the original F1 as seed
donator. Of the 50 plants that were cultivated, most were fully self-compatible.
This population showed some variation in leaf morphology, and the specimens
of the corresponding leaves have been deposited in the Herbarium of the
Heidelberg Institute of Plant Sciences (HEID).
Marker integration—For all markers used in the two mapping populations,
the absolute position within the A. thaliana sequence map (AP1 map, Arabidopsis genome resources at TAIR database, web address: http://
www.arabidopsis.org) were scored. With this information, genetic maps for
both taxa, A. lyrata and Capsella, were drawn to scale in a comparative way.
One hundred thirty-three markers from the Capsella map and 72 markers from
the A. lyrata map were considered. Because neither of the marker sets were
identical to one another, the integration of both maps into one map was performed under the assumption of a high degree of colinearity and synteny
among the genomes under study, which was shown previously on a small
scale (Acarkan et al., 2000).
RESULTS
Map integration—The integration of the Capsella and A.
lyrata maps resulted in a nearly identical chromosomal organization. The eight Capsella linkage groups (A–H) had complete colinearity with the eight A. lyrata linkage groups (AL1–
AL8) (Fig. 1). In both taxa, the number of linkage groups
correlated with the haploid chromosome number (n 5 8). For
A. thaliana chromosomes, I–III the colinearity is as follows
(Fig. 1): Capsella linkage group A 5 A. lyrata linkage group
AL1, B 5 AL2, C 5 AL3, D 5 AL4, and E 5 AL5. In case
of AL5, the A. lyrata chromosome comprises not only A. thaliana chromosome III, but also a minor portion of the distal
part of the short arm of chromosome II. Here the Capsella
map provides no information, and one might argue that the
same region is a part of Capsella linkage group E, at a similar
position as in A. lyrata. The remaining Capsella linkage
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KOCH
AND
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EVOLUTION IN CRUCIFEROUS PLANTS
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Fig. 1. Integration of the genetic maps of Capsella and Arabidopsis lyrata into the physical A. thaliana (Arabidopsis genome, [AG]) sequence map (AtI–
AtV). Markers from Capsella (Boivin et al., 2004) have been indicated by their position only (linkage groups A–H). Genetic markers for A. lyrata have been
enumerated from 1 to 72 according to Table 1 from Kuittinen et al. (2004), and the corresponding linkage groups AL1 to AL8 have been indicated. The positions
of the seven inversions are shown as well as the three fusion/breakage events. (cM: centimorgan; Inv.: inversion; Mbp: Mega basepairs)
groups (G, F, and H) and A. lyrata (AL7, AL6, and AL8)
correspond to one another. In one case only, limited marker
density and missing markers did not allow coverage of the
total corresponding A. thaliana chromosome (distal part of the
short arm of chromosome II). Markers covering this region
(numbers 37 and 38 from the A. lyrata map) have not been
mapped in Capsella (Boivin et al., 2004).
Chromosomal rearrangements—Combining the results
from both genetic maps, we have to consider eight inversions,
two reciprocal translocations, and three fusion/breakage events
to explain differences among all three maps (Figs. 1, 2). Here,
we did not follow the description of Boivin et al. (2004) for
chromosomal rearrangements, because the explanation by
Kuittinen et al. (2004) is simpler and most likely reflects the
true number of chromosomal mutation events. However, three
of the eight inversions described herein (inversions 1–3) were
detected by Boivin et al. (2004). We searched for evidence of
these inversions in the A. lyrata map. In the case of inversion
1 (linkage groups B or AL2), we found no evidence for such
an inversion in the A. lyrata map (Fig. 1). Inversion 2 (F, AL6,
markers 48–50) includes the proximal part of the long arm of
Arabidopsis chromosome IV, which is inverted compared to
Capsella and A. lyrata. Inversion 3 (H) occurred in Capsella
only and was not detected in the corresponding linkage group
(AL8) of A. lyrata. However, the marker density for the bottom arm of chromosome V is lower in the A. lyrata map compared to that of Capsella (Fig. 1), and we can speculate that
increased marker density in this region might allow the recognition of this inversion in A. lyrata, too. One must keep this
source of error in mind when calculating absolute mutational
rates.
Inversion 4 (AL7, marker numbers 66, 67) differentiates A.
lyrata and A. thaliana (Fig. 1), and Capsella showed the same
marker order as A. thaliana. Three additional inconsistent
marker orders (inversions 5–8) comparing A. lyrata with A.
thaliana are found at AL1 (markers 1–2, 5–6, and 9–10) and
AL6 (Kuittinen et al., 2004). However, it has been concluded
by Kuittinen et al. (2004) that the actual order in A. lyrata
could well be colinear with the situation in A. thaliana, because of the close marker proximity and possible mapping
errors. For this reason, we performed calculations with these
five extra and much less significant structural mutations separately.
Divergence time estimates—The divergence time (T) between the Capsella and the Arabidopsis lineages has been calculated previously with a synonymous mutation rate of 1.4 3
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Fig. 2. Most parsimonious explanation for chromosomal evolution among Capsella rubella, Arabidopsis lyrata, and A. thaliana. Inversion 3 can be placed
at two alternative positions. The definition of reciprocal translocations and fusion/breakage follow Kuittinen et al. (2004). More detailed depictions are provided
for the two reciprocal translocations. (AL: A. thaliana subsp. petraea linkage group; cM: centimorgan; Inv.: inversion; Mbp: Mega basepairs; mya: million
years ago)
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1028 for Chs and 1.7 3 1029 for matK using the simple equation: KS/2T 5 synonymous mutation rate (Koch et al., 2001).
The corresponding values are 11–14 million years ago (mya)
(Chs) and 10–13 mya (matK). A simulation approach (Koch
et al., 2000) for Chs and Adh data provided slightly differing
values with 95% confidence limit (Chs, 7.0 , 11.0 , 17.4
mya; and Adh, 5.7 , 9.0 , 14.6 mya). However, all values
are in the same order of magnitude and herein were calculated
with a mean of 11 mya. The same simulation approach for
Adh and Chs provided a divergence time estimate for the split
of the A. lyrata (based on four sequences of former Cardaminopsis species): A. thaliana lineage of approximately 5.2
mya (Chs, 3.1 , 5.1 , 8.3 mya; and Adh, 3.3 , 5.4 , 9.0
mya) (Koch et al., 2000). Use of the simple equation KS/2T 5
synonymous mutation rate provides corresponding estimates
from 5.2 to 5.5 mya (Chs) (Koch et al., 2001). Again, the
values are in the same order of magnitude, and we performed
further calculations under the assumption of a divergence time
of this lineage of 5.3 mya.
DISCUSSION
Reconstruction of the chromosomal evolution—The high
degree of colinearity between the Capsella and the A. lyrata
genomes enabled us to reconstruct a most parsimonious scenario of chromosomal rearrangements (Fig. 2). Mapping differences in chromosome organization on to the currently accepted Brassicaceae phylogeny (Koch et al., 2000, 2001;
Koch, 2003) suggests that the eight A. lyrata and Capsella
linkage groups are ancestral to the derived A. thaliana condition.
The distribution of the three significant inversions can be
explained in a most parsimonious way by inversion 1 and 2
occurring during the evolution of A. thaliana chromosomes I
and IV, respectively, after separation of the lineages A. lyrata
and A. thaliana. The optimization of inversion 3 is ambiguous
as given by the currently available data (Fig. 2). Both optimizations result in the same number of mutational changes,
and we did not find an argument to favor one or the other
hypothesis. Inversions 4–8 might have occurred within the A.
lyrata lineage after separation from A. thaliana (Fig. 2). Given
the available data, this appears to be the only supported hypothesis. However, as outlined here and concluded by Kuittinen et al. (2004), there is some uncertainty about marker order
in the A. lyrata map, and the actual marker order could well
be consistent with the order in A. thaliana.
All remaining chromosomal changes characterize A. thaliana (Fig. 2): two reciprocal translocation involving AL3/AL5
(C, E) and AL6/AL7 (F, G); the three fusions occurred among
AL1 and AL2 (B, A; joined together at the longer arm of A.
thaliana chromosome I), among AL4 and AL3 (C, D; joined
together at the long arm of A. thaliana chromosome II), and
among AL6 (part), AL7 (part) and AL8 (F [part], G [part] and
H; joined together at the longer arm of A. thaliana chromosome V).
Rate of chromosomal changes—A total of seven structural
mutations optimize to the A. thaliana terminal branch (Fig. 2).
This is in strict contrast to the distribution of mutational events
along the Capsella and the A. lyrata lineages. Considering that
the A. lyrata–A. thaliana split occurred 5.3 mya, we obtain a
chromosomal mutation rate of 1.3 mutations per million years.
If we assume that inversion 3 occurred either during the evo-
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lutionary history of the genus Capsella (Fig. 2) or prior to the
separation of A. lyrata and A. thaliana, the other lineages are
characterized by only one to zero mutations. In this case we
obtain a mutational rate of 0.09 chromosomal mutations per
million years. These results are remarkable because the comparison of the three genomes allows us to distinguish between
varying evolutionary rates. Based on the A. lyrata–A. thaliana
comparison only, and under the assumption of a constant mutation rate, Kuittinen et al. (2004) estimated a mutation rate of
0.6 structural mutations per million years. This means that the
interpretation of these values not only depends on map resolution, orthology criteria, or divergence time estimates, but
also on the knowledge of the evolutionary history and the
ancestral structure of the genomes under study. If we consider
also the uncertain inversions 4–7 detected in A. lyrata only,
this particular lineage shows an intermediate evolutionary rate
of 0.75 mutations per million years since its separation from
A. thaliana.
The estimates presented here have serious implications for
comparisons of genome structure made using Brassica vs. A.
thaliana. Assuming 90 chromosomal rearrangements differentiating the genomes of B. nigra and A. thaliana (Lagercrantz, 1998) and a divergence time between both lineages of
14–24 mya (Yang et al., 1999; Koch et al., 2000), we have to
assume 3.2–1.9 rearrangements per million years per genome
(under the assumption of rate constancy). If we now apply our
data favoring a conserved ‘‘ancestral’’ genome with n 5 8
similar to the Capsella genome (10–14 mya, see node A in
Fig. 2) and a rate of 0.09 mutations per million years down
to the node separating Brassica from Capsella/A. lyrata/A.
thaliana (14–24 mya), more than 80 mutations must have occurred along the Brassica lineage. This would result in a greatly increased rate of 5.7–3.8 mutations per million years in this
particular lineage, which is still 3–4 times higher that inferred
for the A. thaliana lineage. However, we know from a recent
study (Lysak et al., in press) that along this lineage an ancestral
genome triplication occurred between 14 and 8 mya, which is
characterizing all members of the tribe Brassiceae. This important structural mutation might have resulted in that highly
increased mutional rate, and, consequently, the evolutionary
rate among the Brassica species would be even much higher
(9.7–6.5 mutations per million years). It should be also noted
here that evolution of the several Brassica species was characterized by reticulation and hybridization and involved also
several other genera. Since U. N. (1935) hypothesized the classical ‘‘Brassica triangle’’ demonstrating relationships between
three diploids and their putative three-tetraploid hybrid, it has
become clear that transmission of chloroplasts through this
lineage resulted in two different clades, namely ‘‘Nigra’’ and
‘‘Rapa/Oleracea’’ (Warwick and Black, 1991, 1993, 1997;
Warwick et al., 1992) . Both clades include also numerous
other genera from tribe Brassiceae, and, not surprisingly, the
phylogenetic hypothesis derived from nuclear markers such as
the internal transcribed spacers (ITS) 1 and 2 failed to resolve
each of the two groups (Warwick and Sauder, 2005). Most
recently, it has been calculated that the two lineages within
tribe Brassiceae (nigra lineage vs. rapa-oleracea lineage) diverged approximately 7.9 mya. If we assume that the genome
triplication during or prior to the evolution of tribe Brassiceae
14–8 mya did not cause course changes in mutational rates,
but reticulation during evolution of the genus Brassica itself
occurred within the last 7.9 my, we have to speculate about a
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largely increased mutational rate of up to 10 mutations per
million years during this shorter period of time.
There are a few possible arguments that have been used to
explain increased rates of genome rearrangements. First, polyploidization might cause an enhanced rate of structural mutations during establishment of a new genome. We are not able
to test this hypothesis from our data, but we have no evidence
for similar increased rates of chromosomal rearrangements as
in the polpyloid Brassica species in polyploid Capsella (C.
bursa-pastoris, tetraploid) and some species from the former
genus Cardaminopsis (tetraploids frequently occur in C. arenosa [5 Arabidopsis arenosa], rarely in A. lyrata subsp. petraea). Second, an annual, rapid life cycle might increase the
overall evolutionary dynamics of a particular genome. Here,
again, all Capsella species are annuals, and, in contrast, all
members of the former genus Cardaminopsis are biennial or
perennials, indicating that just a rapid life cycle does not necessarily result in increased evolutionary rates. Third, an annual
and ephemeral life cycle in combination with polyploidization,
as it is the case in many wild Brassica relatives, might favor
rapid fixation of even those rearrangements that otherwise
would be partially deleterious. However, the self-incompatibility system of the several Brassica species might counteract
or retard this process to some extent. In Capsella we have an
annual obligate outcrosser (C. grandiflora) and a tetraploid
selfer (C. bursa-pastoris), and in the genus Cardaminopsis we
have selfers (e.g., A. arenosa) as well as outcrossers (A. lyrata), and both are represented by diploids and tetraploid (see
Dobeš and Vitek, 2000). This means that even the simple combination of these two traits does not necessarily account for
increased chromosomal mutational rates.
In summary, we have to conclude that none of these simple
observations can explain the differing dynamics of karyotype
evolution. Future comparative maps and karyotype studies employing comparative chromosome painting (Lysak et al., 2003,
in press) in cruciferous species outside the Brassica lineage
and the Capsella/Arabidopsis lineage will help to solve these
questions. In addition, genome-wide research in recently constituted hybrids and their offspring will provide a deeper understanding of the mechanisms and the direction of genome
evolution (e.g., Rieseberg and Buerkle, 2002; Burke et al.,
2004; Livingstone and Rieseberg, 2004).
Finally, our findings have far reaching implications for
Brassicaceae evolutionary biology. It is not appropriate to use
only the model systems Brassica and Arabidopsis thaliana in
a comparative way, but other taxa from unrelated lineages
have to be considered as well in order to obtain serious conclusions.
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