Evolution of Genomes and Genome Relationship among the Rapeseed and Mustard
Reports regarding the chromosomal architecture of Brassica genomes and genome relationship among various species of the genus appeared in the second quarter of the 20th century AD. In the second quarter genome relationship was elaborated through secondary associations, pachytene and somatic karyotypes. Genome analysis and preferential pairing were extensively utilized for characterizing Brassica genomes in the third quarter. In last quarter of the century molecular characterization supplemented the conventional analytical tools for understanding the infrastructure of Brassica genomes. Whether it is the breeding system analysis, chromosome morphology, meiotic associations or molecular characterization proves that Brassica is monophyletic origin and it descended from an unknown six chromosomal prototype. Scientific developments made in Brassica genetics, from genome analysis to genomics during the 20th century AD are generalized in this paper.
Phylogenetically Brassica is the nearest related genera to the recently
sequenced Arabidopsis thaliana (Anonymous, 2000). Eight species of Brassica
are reported from Pakistan (Nasir and Ali, 1973), among which B. tournefortii
and B. deflexa are reported only from the wild and the rest are cultivated
as important agricultural crops. Brassica rapa and B. napus among
the agriculturally important species are conventionally grouped as rapeseed
(Khan and Munir, 1986), cultivated mainly for the extraction of rapeseed and
canola oil. Brassica juncea, B. carinata and B. nigra are placed
in the mustard group (Khan et al., 1987). Besides their major role in
the production of mustard oil, they are also cultivated for their demand as
commercial spices (Hemingway, 1976). Brassica oleracea, the 6th agriculturally
important species is cultivated for its use as vegetable and fodder under the
common names of cabbage, cauliflower, broccoli, brussels sprouts and marrowstem
The successful demonstration of Raphanobrassica (Karpechenko, 1929)
a classical intergeneric hybrid and the artificial resynthesis of Brassica
napus (U, 1935) during the second quarter of the 20th century inclined the
scientists to resolve the genome constitution of Brassica species. Genome
relationship among the species were primarily elaborated either through the
analysis of secondary association during microsporogenesis of diploids (Alam,
1936; Catcheside, 1937) or somatic karyotyping (Richharia, 1937). Meiotic karyotyping
(Robbelen, 1960; Prakash, 1973), preferential pairing in haploids and amphihaploids
(Prakash, 1974; Armstrong and Keller, 1981; Robbelen, 1960; Attia and Robbelen,
1986) were later on used for in genome analysis. Though hardships are still
there in distinguishing Brassica chromosomes (Hasterok and Maluszynska,
2000), developments in the field of genomics during the recent past, however
proved very effective in producing successfully high yielding and pest resistant
genetically engineered RR Canola (Phillipson, 2001). This review will provide
an insight of the genome biology and other genetic developments of the in Brassica,
especially the rapeseed and mustard.
Wide hybridization: Till the first quarter of the 20th century AD, Brassica
species were considered as fixed taxonomic entities and the alien gene transfer
among different genomes, practically seemed impossible. Sinskaia (1927) was
the first who tried to cross Brassica species and succeeded to change
the long lasting belief of the taxonomic boundaries regarding the crossability
of Brassica species. Karpechenko (1928) successfully synthesized Raphanobrassica,
the first intergeneric fertile hybrid of Raphanus sativus and Brassica
oleracea. His demonstration further strengthened the idea of genome manipulation
through wide hybridization. Besides their use in breeding system analysis the
interspecific crosses were employed in different ways to elaborate their genome
relationship also. Hence Morinaga (1934) analyzed meiotic pairing in B. nigra
X B. juncea hybrids and concluded thereby that the genus Brassica
is a polyploid complex having both the elementary (diploid) and amphidiploid
genomes. Brassica rapa, B. nigra and B. oleracea are represented
by genomes AA, BB and CC respectively are diploid. Whereas B. carinata, B.
nigra and B. napus are represented by the genomes BBCC, AABB and
AACC respectively, are amphidiploid, originated through the intercrossing of
the elementary species. These studies were later on verified by the artificial
resynthesis of B. napus from B. oleracea X B. rapa (U,
1935). It was later on verified that Brassica species are monophyletic
in origin and have been evolved from an obscure six chromosomal prototype (Alam,
1936), either through the process of secondary polyploidy (Richharia, 1937)
aneuploidy (Prakash, 1973; Prakash, 1974) or a combination of modified tertiary-and-compensatory
trisomy (Armstrong and Keller, 1982). These elaboration encouraged breeders
to integrate species diversity through wide hybridization for improved oilseed
(Prakash, 1980), vegetable (Nishi, 1980; Lange et al., 1989), fodder
(Namai, 1971) and forage Brassica crops (Hosoda, 1950, 1953). All the
developments were only possible after the primary elucidation of the basic architecture
of the Brassica genomes through the refined cytotechnological procedures,
still want a lot of explanation (Lan et al., 2000; Soltis and Soltis,
Interspecific relationship: It is clear from the above discussion that
the cultivated Brassica has two types of genomes i.e. the diploid (elementary)
and amphidiploid genomes. The elementary species includes Brassica rapa (AA,
2n=20), B. nigra (BB, 2n=16) and B. oleracea (CC, 2n=18). The
amphidiploid species includes B. carinata (AABB, 2n=34), B. juncea
(BBCC, 2n=36) and B. napus (AACC, 2n=38). These relationships among
Brassica were elucidated with the artificial resynthesis of B. juncea,
B. carinata and B. napus (Morinaga, 1934; U, 1935; Ramanujam and
Srinivasachar, 1943; Prakash, 1973) through hybridizing B. rapa-B.
nigra, B. rapa-B. nigra and B. rapa-B. oleracea,
respectively. Comparative maps of Arabidopsis thaliana, an ideal plant
for genetics and molecular studies (Meyerwitz and Somerville, 1994) and Brassica
are becoming popular (Lan et al., 2000) for understanding the Brassica
phylogenetic relationships. The comparative maps of Arabidopsis thaliana
and Brassica nigra (Lagercrantz, 1998) shows that the diploid Brassica
species are descended from a hexaploid ancestor and that the A. thaliana
genome is similar in structure and complexity to those of each of the hypothetical
diploid progenitors of the proposed hexaploid. Furthermore according to Lagercrantz
(1998) the Brassica lineage probably went through a replication after
the divergence of die lineages leading to A. thaliana and B. nigra.
However the findings of hexaploid origin of Brassica was neither verified
by its preferential (Ahmad, 2001), nor through its isozyme analysis (Warwick,
1999). Variation in isozymes, chromosome numbers and the related systematic
relationships of tribe Brassiceae (Warwick, 1999) reveal that aneuploidy and
segmental polyploidy have played a more significant role in the evolution of
amphidiploid Brassica. Furthermore the widespread isozyme duplication
in Brassica (Warwick, 1999) and the frequently occurring multivalents
in the intergenomic haploids, colchiploids and species hybrids (Ahmad, 2001)
determines extensive gene duplication resulting from the polyploidization of
the common ancestor of the Brassiceae tribe, prior to the aneuploid divergence
of the species.
Karyotypic overview: The Brassica chromosomes are very small and poorly differentiated. Their identification through the ordinary cytogenetic techniques is extremely difficult. Progress in molecular analysis of Brassica species still needs their proper karyotyping, the chromosome-specific markers for differentiating the particular homologous pair (Hasterok and Maluszynska, 2000) is an encouraging development in this direction.
Karyotypic investigations of the Brassica genomes started with the analysis of secondary association in diploids (Catcheside, 1934; Alam, 1936). The secondary pairing revealed the monophyletic origin of Brassica from an obscure 6 chromosomal prototype. Somatic karyotyping based upon the chromosomal length and centromeric position in (Richharia, 1937) also recognized six to seven types of chromosomes. Richharia (1937) designated the genomic formulae ABCDDEEFFF and ABBBCDDEF to Brassica rapa and Brassica oleracea respectively. Robbelen (1960) analyzed pachytene chromosomes of Brassica rapa, B. nigra and B. oleracea and allotted the genomic formulae AABCDDEFFF, ABCDDEFF and ABBCCDEEF respectively, to them. Besides meiotic associations in Brassica (Catcheside, 1937; Alam, 1936; Robbelen, 1960) also confirmed the findings of Morinaga (1934) and U (1935) regarding the monophyletic origin of Brassica from a six chromosomal obscure prototype.
A number of efforts were made in the second half of the 20th century AD for making standard karyotypes for different members of the polyploid series of the genus but is still not established scientifically. The use of fluorescence in situ hybridization (FISH) and differential staining (Hasterok and Maluszynska, 2000; Armstrong et al., 1999; Fukui et al., 1998) has recognized some markers for different chromosome pairs in genome A, B and C; of B. rapa, B. nigra and B. oleracea respectively. It will hopefully be helpful for the successful karyotyping of Brassica genomes. Comparative data on quantitative trait loci (QTLs) are also successfully used in karyotypic analysis of Brassica (Osborn et al., 1997).
Genome analysis: Whether meiosis or mitosis, the homologous pairing is controlled genetically (Schulz-Schaefer, 1980; Sybenga, 1975). Partial pairing of homoeologous chromosomes is observed both among haploid and the amphihaploid genomes, during gametogenesis. It usually provides precise information regarding the evolutionary relationships of chromosomes within the genome and is therefore successfully employed in understanding the genome relationship among various taxonomic groups. Genome analysis (Stace, 1980) is the resolving of genome relationships through preferential pairing within the haploid or through partial pairing among the allied amphihaploid genomes.
In genome analysis the information got from homoeologous association is generally
concealed with the role of genetic factors in meiotic pairing. In Brassica
the presence or absence of genetic factors for suppressing the homoeologous
pairing is still not established. Some of the authorities (Richharia, 1937;
Busso et al., 1987) reported that in F1s (of genomes A and B) the pairing
frequency among two homoeologous genomes enhances with the addition of a third,
the apparently distant genome (i.e. genome B of B. nigra). They are reported
that Brassica has no genetic factors for suppressing homoeologous pairing.
On the other hand authorities like Harberd (1950), Prakash (1974) and Harberd
(1976) through their series suggested the presence bear genetic factors for
suppressing the homoeologous pairing Brassica. Harberd (1976) observed
that low pairing among the chromosomes of genomes AB and BC was due to the genetic
regulations of pairing control. But structural differentiation of chromosomes
in B. nigra, both form B. rapa and B. oleracea, was the
only reason for low pairing (Armstrong and Keller, 1982).
|| Genome constitution of the cultivated
|Source: Morinaga (1934)
The predominant allosyndesis among the chromosomes of genomes A and C than
genome B (Attia et al., 1987; Yang and Robbelen, 1994) further clarified
findings that no genetic factors for suppressing the homoeologous pairing were
present in Brassica. It was thus recognized that neither any genetic
factor for suppressing pairing (Yang and Robbelen, 1994) nor cytoplasmic factor
for regulating pairing exists in Brassica (Busso et al., 1987).
Primarily the work on genome analysis in Brassica started with the observation
of consistently occurring 8 IIs in B. juncea x B. nigra hybrids
(Morinaga, 1934). These associations were attributed due to the allosyndesis
of 8 chromosomes both from B. nigra and B. juncea and this homologous
genome with 8 chromosomes both in B. nigra and B. juncea was designated
as "genome B". Whereas the remaining 10 unpaired chromosomes of B. juncea,
were designated as "genome A" B. chinensis (2n=20) or B.
rapa type. Through his detailed observations Morinaga (1934) was able to
classify the agriculturally important Brassica species into six cytogenetical
groups as given in Table 1. These findings made the foundation
for the genome analysis of Brassica. Results of the genome analysis among
Brassica and its allied genomes in Synapis, Eruca and Raphanus
concluded that all the genomes were partially homologous, they were secondary
polyploids and originated from a common unidentified genome (Mizushima, 1950).
The findings of Morinaga (1934) were soon confirmed by the artificial resynthesis
of B. napus (U, 1935), B. juncea (Ramanujam and Srinivasachar,
1943) and other means (Robbelen, 1960; Soltis and Soltis, 1999; Hasterok and
Maluszynska, 2000). The idea of the origin of Brassica from an unknown
six-chromosomal prototype (Catcheside, 1934; Alam, 1936) is verified through
pachytene karyotyping (Robbelen, 1960), chromosomal behavior at M1 (Attia and
Robbelen, 1986; Prakash, 1973; Attia et al., 1987) and molecular analysis
of the species (Warwick, 1999). The acceptable genomic formulae, their karyotypes
and mode of origin of various taxons is still unresolved (Armstrong et al.,
1999; Fukui et al., 1998; Truco et al., 1996).
Preferential pairing in haploids and amphidihaploids: As it is clear from the preceding section, preferential pairing is important for understanding the chromosome homoeology and it is successfully employed in determining the affinity among chromosomes of various genomes and phylogenetic relationships of Brassica species. Preferential pairing in haploids has also unfolded interesting information regarding the genome relationship of various taxons including Brassica (Ramanujam and Srinivasachar, 1943; Thompson, 1956; Prakash, 1973). The presence of two bivalents in the haploid B. nigra were thought to be due to its origin from a six chromosomal prototype and the two chromosomes in its 8-chromosomal gametic complement (X= 8) is due to the duplication two original chromosomes of the progenitor genome (Prakash, 1973).
During the course of evolution the synaptic ability of the duplicated chromosomes
remained no more intact. But still these chromosomes retained some traces of
genetic equivalence resulting in allosyndetic associations in haploids. Thus
from the preferential pairing in haploids of B. nigra (Prakash, 1973),
B. tournefortii (Prakash, 1974) and B. rapa (Armstrong and Keller,
1981) the genome relationship sketch of Morinaga (Morinaga, 1934) and the secondary
association theorem of Catcheside (1937) regarding the origin of these genomes,
was verified. The origin of Brassica from a six chromosomal obscure prototype
(Robbelen, 1960; Alam, 1936; Catcheside, 1937; Venkateswarlu and Kamala, 1971).
Though the genome relationship of Morinaga (Morinaga, 1934) is widely accepted,
the secondary association theorem is criticized by a number of scientists. For
example the genomic formula of Richharia (1937) is not in accordance with that
of Catcheside (1937) and Robbelen (1960). Ramanujam and Srinivasachar (1943)
reported 1II in haploids of B. rapa and was not in a position to conform
the secondary pairing theorem. Similarly in other studies (Thompson, 1956; Ahmad
and Khan, 1994) the scientists were not even able to confirm the secondary association
in Brassica rapa, on which the secondary pairing theorem (Catcheside,
1937; Robbelen, 1960) was based. However it was certainly generalized by most
of these studies (Schulz-Schaefer, 1980) that the aneuploid series in Brassica
could have been derived from a combination of modified tertiary and compensating
trisomics. Some recent studies of the linkage comparison the maps (Truco et
al., 1996) in B. rapa, B. nigra and B. oleracea showed homologous
regions shared by these species. This study also found intergenomic conserved
regions with the extensive reordering among the genomes and associated eighteen
linkage groups (from all three species) on the basis of homologous segments,
based on at least three common markers. Intragenomic homologous conservation
was also observed for some of the chromosomes of A, B and C genomes and thus
it was concluded that an ancestral genome was made up of at least five, and
not more than seven chromosomes from the observed chromosomal inter-relationships.
Moreover some other findings also revealed that the diploid Brassica species
have been descended from a hexaploid (Sybenga, 1975) ancestor. It can be concluded
from the discussion that, though a lot of work has been carried out on the genome
architecture of Brassica, it is still unresolved and needs further investigations.
Overview of genomics: The introduction of DNA analytical technologies particularly RAPD and RFLP has revolutionized the fields of genetic finger printing, isolation of genetic traits and molecular biosystematics. These technologies are also employed in the determination of genetic diversity at the intra and intergenomic levels. RAPD analysis are successfully employed in detecting extensive intragenomic polymorphism (Rabbani et al., 1998) but its use at interspecific level is still limited (Quiros, 1998; Thormann, 1994). During the last 10 years mapping in Brassica has however been focused on B. napus, B. nigra, B. oleracea and B. rapa (Quiros, 1998). More recently, mapping has been expanded to include B. juncea. The maps produced in Brassica crops are mainly based on Fz progenies developed independently by various laboratories, which will require their integration for a more efficient use in future. The marker maps are being used to locate genes determining traits of economic interest, including quantitative trait loci for utilization in applied genetics and breeding of the numerous Brassica crops. Another important application of the maps is the study of structure, origin and evolution of the Brassica genomes. Arabidopsis sequencing program puts the Brassica crops in an advantageous position because of the immediate application of these information in its genomics.
Genetic linkage map of Brassica juncea on the cDNA markers of B.
napus (Cheung et al., 1997) showed that 62% of the marker loci were
duplicated and majority of them were involved in interlinkage group duplications.
The study illustrates that complex duplication and subsequent rearrangements
have been occurred in the species after allopolyploidy. Parkin et al.
(1995) noticed that majority of the loci of genetic linkage map of a cross between
resynthesized Brassica napus and natural oilseed rape exhibited disomic
inheritance of parental alleles. The study demonstrated that the chromosomes
of genomes A and C were pairing exclusively with their recognizable homologues
of B. rapa and B. oleracea chromosomes in B. napus crosses.
This behavior identified 10 and 9 linkage groups of the genome A and C types,
respectively in B. napus. Moreover it was also concluded from the studies
that the nuclear genomes of B. napus, B. rapa, and B. oleracea have
remained essentially unaltered since the formation of amphidiploid species,
B. napus. A range of unusual marker patterns, which could be explained
by aneuploidy and nonreciprocal translocations, were observed in the mapping
population. These chromosome abnormalities were probably caused by associations
between homoeologous chromosomes at meiosis in the resynthesized parent and
the F-1 plant leading to nondisjunction and homoeologous recombination.
It can be concluded from the above discussion that Brassica species are monophyletic in origin and could sexually be employed at any ploidy level for genetic introgression or alien genetic transfer. Furthermore the diploid species are phylogenetically more close to the amphidiploids than their diploid relatives. The aneuploidy and segmental polyploidy has played important role in its genome evolution. The chromosomes of genome A (irrespective of its source) have retained more chromosomal homology as compared with their homology to the chromosomes of genomes B and C. In alloploid condition genome A prefer to form autosyndetic IIs with its homologous genome or its chromosomes will remain as univalents in the absence of genome A. Some of the chromosomes of genome A also has homoeology with the chromosomes of genomes C and B, which give rise to the formation of allosyndetic multivalents. No one among the genomes B nor C have got any genetic role in affecting the homoeologous pairing of genome A from different backgrounds.
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