PLS221                                                         Instructor: Carlos F. Quiros

Brassica spp. Cole Crops and allies

List of references (includes reading assignments)

Brassica Genome Gateway

Slides


Cole crops and allies

Genus Brassica

Cultivated species and its wild relatives

Origin and distribution

Elementary species: Diploids

Brassica oleracea and its wild relatives

Brassica rapa and cultivated types

Derived amphidiploids: the U triangle

Interspecific and intergeneric hybridizations

Aneuploidy: cytogenetic stocks

Genetic markers, mapping and their applications

Genome structure, origin and evolution

Homology among different genomes: homoeologous recombination

Haploids: anther and microspore culture

Autoploids

Male-sterility and self-incompatibility

Transgenics.

Outlook


Brassica spp.

Importance of Brassica crops

Crop Brassicas encompass many diverse types of plants, which are grown as vegetables, fodder or sources of oils and condiments.

These have been mentioned since ancient times and might have been cultivated as early as 5000 BC. The utilization of oilseed Brassica is steadily increasing. At the present time it represents 13.2% of the world's edible oil, with a production of 210 million metric tons (Carr and McDonald 1991). Vegetable Brassica crops also have great economic significance because of their wide popularity. The increased importance of these crops requires the development of basic genetic information for its application to breeding.

Taxonomy:

There is considerable confusion about the taxonomic nomenclature of Brassica species and related genera. The best approach is to classify them biologically, encompassing all the species and genera sufficiently related to the crop Brassicas as potentially capable of exchanging genes with them. These are referred in the broad sense as Brassica coenospecies. The diploids species range in genomic numbers from x=7 to x=12, when including Brassica hirta (syn. Synapis alba) in this group.

The most prominent diploid species of Brassica and those of related genera, determined by genome analysis (Mizushima 1950), can be grouped as follows on the basis of chromosome numbers:
 
 

Diploid Brassica species and related genera
 
 

x=7

x=8

x=9

 x=10

x=11

x=12

B. adpressa

B.nigra

 B.oleracea

B.campestris

B.amplexicaulis

B.hirta

B.deflexa

B.fruticulosa

B.rupestris 

B.tournefortii

B.elongata

 

D.erucoides

B.maurorum

B.macrocarpa

B.saxatilis

E. sativa

 

 

B.spinenesis

B.insularis

B.repanda

 

 

 

 

B.montana

B.barrilieri

 

 

 

 

B. cretica

 

 

 

 

 

B.hilarionis

 

 

 

 

 

R. sativus

 

 

 

B=Brassica, E=Eruca, D=Diplotaxis, R=Raphanus
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This brings us to the concept of cytodeme, developed by Harberd (1972): It is a group of species sharing the same chromosome number, that cross readily with each other, resulting in fertile hybrids. "oleracea cytodeme". 

The evolution of Brassica species and those of related genera have taken place at two levels: The first one has taken place at the diploid level, consisting on changes of chromosome numbers in the genomes, resulting in the x=7 to x=12 genomic series mentioned above. The second one has taken place at the polyploid level, involves amphiploidy, due to hybridization of some of the basic diploid species. For other species not related to Brassica, autoploidy also has occurred, resulting in diploid, tetraploid and hexaploid series. For example in the genera Lepidium and Physaria among a few others.

Natural amphiploidization has resulted in the origin of three cultivated allotetraploid species, B. napus, B. juncea and B. carinata. The origin of these amphidiploid species and the identification of their diploid parents were elucidated in the 1930's by U and Morinaga in Japan and Karpechenko in Russia. They synthesized artificial hybrids with various diploid species and by doing chromosome counts they matched expected chromosome numbers in the amphidiploids as well as morphological traits. This elegant work resulted in the postulation of the U triangle:
 
 

Origin of cultivated amphidiploids: The U triangle

The genomes of the diploid species are denominated A, B, C, for B. rapa (syn. campestris), B. nigra and B. oleracea, the three cultivated Brassica diploids. The amphidiploids being hybrids have two of these genomes. The cytoplasm of each species is denominated by lower case letters. We now know by chloroplast DNA analysis that most of these amphiploids originated by crosses in a single direction, where the reciprocal failed.

Cultivated Brassica species:The three Brassica diploid cultivated species are: 1) B. nigra (2n=2x=16, B genome), black mustard 2) B. oleracea, (2n=2x=18, C genome), cabbage, broccoli, cauliflower, Brussels sprouts, kales, kohlrabi among others, and 3) B. rapa (syn. B. campestris) (2n=2x=20, A genome) turnips, rapeseed and oriental vegetables (Prakash and Hinata 1980).

Geographical Distribution, and Diversity of Cultivated Brassica species:

The genus Brassica has mainly a Mediterranean distribution, but it expands to Africa and Asia, including India. The distribution of the cultivated species is as follows:

1) B. nigra. This species was cultivated as a spice as early as 3000 BC. Although it may originate in Asia Minor-Iran, it is now widespread through out Europe, Africa, Asia, India and the Far East (Hemingway 1979).
The closest species to B.nigra seems to be Synapis arvensis, a 2n=18 species (Truco 1993).

2) B. oleracea. Wild forms of this species are found in the Atlantic coasts of Europe, Northern France and England in particular. Related wild species are endemic of the Mediterranean basin and might have contributed genes to some of the cultivated forms by introgression. A great array of different crops have been domesticated in various western European regions, such as Greece and Rome where some of these crops might have been cultivated since the 1st century AD (Sauer 1993).  Chinese kale, (B. alboglabra) seems to have been domesticated in China.

 3) B. rapa (syn. campestris). Wild forms are found in western Europe, western USSR, Afghanistan, Pakistan, Asia, Transcaucasus and Iran (McNaughton 1979). According to Gomez-Campo (1999), this was the first domesticated Brassica species, which took place several millennia ago.  It is likely that the wild forms of this species were first domesticated as biennial crops, to later originate by selection annual crops. 

The geographical distribution of the three allotetraploids derived from the cultivated diploids are:

4) B. carinata, is found and cultivated in North Eastern Africa and Ethiopia in particular, where it is sympatric with B. nigra and cultivated kales (B. oleracea).

5) B. juncea. Central Asia-Himalayas is a primary center of diversity for this species, with migration to China, India and the Caucasus (Hemingway 1979). The Middle East, where B. rapa and B. nigra grow wild might have been the place of origin of this species, in particular Asia Minor and southern Iran.

6) B. napus: This species seems to be of relatively recent origin, in the South West and Mediterranean regions. The first reference to this species is from the 1500's, when it was developed as a winter rape crop.

Morphotypes and taxonomic varieties:

B. oleracea provides important vegetables where practically all parts of the plants can be consumed: Leaves, terminal and axially buds, stems and floral tissues.

There is a great array of horticultural types and wild relatives that cross readily with the cultivated ones. Wild species of B. oleracea and others in same cytodeme might have been involved in the evolution of these forms, which resulted in domestication in different locations.

Varieties of B. oleracea:

The oleracea varieties

 

ramosa:  thousand headed kale

viridis: borecole, collard

capitata: cabbage

gemmifera: Brussels sprouts

gongylodes: kohlrabi

botrytis: cauliflower. purple, orange,  Romanesco

italica: sprouting broccoli

alboglabra: Chinese kale 

sabellica:  scotch kale

medullosa: marrow stem kale

sabauda : savoy cabbage

costata: Portuguese tronchuda types

palmifolia: palm kale

All these varieties intercross with each other.

Subspecies of B. rapa:

ssp. chinenses, non-heading, Pak-choi type

ssp. pekinensis: Pe-tsai, head forming, chinese cabbage, nappa type. Heading in these species is also recessive.

ssp. rapifera:  turnip

ssp. oleifera: oil seed crops,

ssp. triloculares self compatible.
 

Subspecies of B. napus:

ssp. oleifera: oil seed rape

spp. rapifera: rutabaga, turnip like

 

 


 

 


Genetics of morphotypes:

Crosses among different horticultural forms results in the breakdown of the horticultural characteristics of each variety.These are determined by multiple genes and have different dominant relationships depending on the crosses.
(see Yarnell 1956, Quiros and Farhnham (in press)

Cabbage traits

Dickson and Wallace (1986) state that it is generally agreed (based on widespread experience) that heading is recessive to nonheading.  Recent work of Farnham et al (2005) in which a series of F1 hybrids formed by crossing cabbage inbreds with nonheading collard inbreds were evaluated, confirm that heading is recessive. 

Pease (1926) postulated several factors associated with cabbage heading including: a short height factor (t) recessive to tall (T); a wide leaf (W) dominant over narrow leaf (w); sessile leaf (pet) recessive to a petiolate leaf (Pet); and an entire leaf (En) dominant to a lyrate leaf (en)  In general, Pease classified cabbages  as ttWWpetpetEnEn.  

Crinckled Savoy type leaf opposed to smooth leaf is controlled by three or more genes (Dickson and Wallace 1986) and is at least partially dominant (Yarnell 1956).   Red coloration due to increased anthocyanin content is also determined by at least two dominant factors (Yarnell 1956, Dickson and Wallace 1986). 

Kohlrabi traits

The bulb or swollen stem of kohlrabi exhibits incomplete dominance in crosses with other crops with normal stem. Full expression of the swollen stem may be conditioned by at least three genes based on observed segregation of this trait in a large F2 family and subsequent observations in the F3 and backcross generations (Pease 1927). 

Brussel sprouts traits

In general, crosses of different B. oleracea crops to Brussel sprouts results in plants with intermediate levels of lateral bud formation that lack the formation of axiliary heads characteristic of Brussel sprouts (Yarnell 1956)   Thus, formation of axilary heads appears to be recessive.

Cauliflower and Broccoli Traits

Two major genes homologous to Arabidopsis Ap1-a and Cal-a were found associated with curd formation in cauliflower by Smith and King (2000).  They designated these genes BoAp1-a (A) and BoCal-a (C) and proposed a genetic model to explain the broccoli, cauliflower and intermediate phenotypes:    With their model cauliflower has a double recessive genotype for the two genes (aacc), broccoli is A-C-, whereas AAC- and aaC- are intermediate types.   Labate et al (2006) countered that the above model only explained 6% of observed phenotypic variation, concluding that other genes complementing or modifying BoAp1-a and BoCal-a must be present.

Cauliflower has four basic curd colors, white, green, purple and orange. (Crisp and Angell 1985) reported two recessive genes are responsible for green curd and they act to alter chlorophyll accumulation   However, more recent and extensive breeding work indicates that green curd is dominant to white and orange curd colors (N. Acciarri, pers. com.) The semidominat gene Or conditions orange color by altering carotenoid accumulation of curds (Dickson et al 1988). Homozygous plants for the Or gene produce small curds and have stunted growth. Lu et al (2006) cloned this gene concluding that it represents a gain-of- function-mutation conditioning enhanced b-carotene accumulation. Purple curds result from anthocyanin (cyanidine) accumulation  and this trait is controlled by a single partially dominant gene (Chiu and Li  2006).

Flower color and bolting:

White flower, typical of alboglabra, is dominant over yellow and determined by gene Wh.   With most common oleracea crops yellow flowers are more prevalent. 

An important attribute of B. oleracea crops is whether they are annuals or biennials that typically require vernalization.   It is generally accepted that annual habit is dominant over biennial and also that the trait is polygenic with strong environmental interaction.

The gene pool of B. oleracea is quite extensive, it hybridize readily with at least 6 wild perennial species of x=9 found in the Mediterranean. For example, B. rupestris, incana, insularis, montana and others. In many cases these inhabit maritime cliffs in small rocky islands. They are useful sources of germplasm largely untapped.

Wide Hybridization in Brassica

Reports of wide hybridization attempts in Brassica go back to the nineteenth century, when chromosome numbers of the species in the genus were unknown (Prakash and Chopra 1991). Karpechenko (1928), popularized wide hybridization experiments in Brassica by creating "raphanobrassica" after crossing radish and cabbage.

The earlier work on wide hybridization resulted in the resynthesis of amphiploids and later in the discovery of the triangle of U.

Confirming the U triangle by modern techniques:

These interspecific relationships can be confirmed readily by chromosome markers. For example, in our work we found that specific allozymes are characteristic of each genome. These can be followed in the natural hybrids.

Also, we found that the DNA RFLP fragments coding for rRNA are excellent markers to test U’s hypothesis. The restriction fragments of B. oleracea and B. campestris add up perfectly to account for the fragments observed in the putative allotetraploid B. napus.

Furthermore, the direction of two of the crosses in the triangle, or in other words, the cytoplasm origin in allotetraploids, which is transmitted only by the female parent in Brassica species, has been determined by Palmer et al. (1983)  and Erickson et al. (1983) on the basis of cpDNA restriction analysis.  

Synthetic allotetraploids undergo chromosomal structural changes due to homeologous recombination resulting in reduce fertility.Cytoplasmic interactions seems to direct recombination events (Szadkowski et al 2009).   Natural selection in subsequent generations stabilizes these hybrids increasing fertility and regular preferential pairing (Gaeta and Pires 2009).

 

1) Artificial B. napus:

B. rapa x B. oleracea. This hybrid is as difficult to obtain than Raphanus x B. oleracea.

This cross is most successful using rapa as female. Embryo rescue is very useful to obtain these hybrids in both directions.

The cross is easier to do at tetraploid level than at diploid level.

Turnip x kohlrabi. Unlike Karpechenko's raphanobrassica hybrid, in this case there is dominance of root and stem enlargement, so both organs express in the hybrid. (close-up)

 2) Artificial carinata:

Synthesized in a few instances. The hybrid has high fertility. nigra is used as  female. The following combinations have been tried: cabbage and nigra, and broccoli and nigra. Often the chromosomes form multivalents in the hybrids.

3) artificial B. juncea

These are obtained by crossing B. rapa x B. nigra, with B. rapa as female parent.

4) Trigenomic hybrids

These includes the A, B and C genomes together . The can be obtained by crossing the following species:

i.e. carinata x rapa  (BBCC x AA)

i.e.. juncea x oleracea (AABB x CC)

The hybrids are highly sterile, but fertility is restored by chromosome doubling.
 

Main barriers to hybridization:

1) Inability of pollen to germinate in stigmas

2) Imbalance between embryo and endosperm

Some of these problems can be circumvented by use of hormones to the ovary, by use of mixed hybridizations (mentor pollen), style excision,  and embryo culture.

Monoploids and Polyploids:

Autoploidy

The following euploid variants of importance have been produced in Brassica.

Tetraploids: These can be readily obtained by colchicine treatment.

4n rapa :Tetraploid turnips display  20% higher yields, problem with low fertility.

4n oleracea: Tetraploid fodder obtained by crossing tetraploid cabbage x kale. These are high yielding but have low fertility.

Monoploids: These have been reported to arise spontaneously in B. oleracea, B. rapa, B. juncea, and B. napus, or after intespecific hybridization due to parthenogenesis of unfertilized egg cells.

For example:

rapa x oleracea = haploid oleracea

nigra x rapa = haploid nigra

Haploids are also readily produce by anther and microspore culture in Brassica. These techniques are widely available for most of the cultivated Brassica species.

Application of haploids: Used to produce erucic acid free B. napus cultivar cv Maria Haplona, bred after colchicine doubling of a spontaneous haploid.
Doubled haploids from F1 hybrids are now extensively used for construction of homozygous recombinant inbred lines for map construction in Brassica.
 

Aneuploids

Obtained many times spontaneously or in the progeny of autotriploids, in synthetic alloploids and their backcross progenies.

Most aneuploids have been reported on progeny from tetraploids, but have been seldom characterized. There is a report on a few primary trisomics in B. rapa, produced in the progeny of addition lines containing oleracea alien chromosomes.

Monosomics: One reported recently in B. napus. The following scheme was proposed to obtain them:

napus (AACC) x rapa (AA)

F1                  AAC  x  napus AACC

          Select for monosomics in the progeny
 
 

Alien addition lines: Readily developed by crossing amphidiploids to parental diploid species, followed by backcrosses to diploid parent. These stocks are useful for constructing synteny maps and for assignment of linkage groups to chromosomes.

Crossing scheme:

AACC x AA

F1  AAC x AA

Select for hyperploids for further backcrossing to obtain complete series of alien addition lines for each of the 9 C genome chromosomes:
AA + C1 to C9

The following alien addition lines have been produced in Brassica:

C genome lines:  AA + Cn

Obtained by crossing  B. napus (AACC) x B. rapa.
 

B genome alien addition lines:  AACC + Bn  or CC + Bn

Obtained by crossing B. napus x ABC hybrid.

Obtained by crossing  BBCC x CC.

or                           CCCC x BB, then CCB x CC

Plus a few other combinations, such as Diplotaxis erucoides (x=7) - B. nigra
 

 A-genome addition lines in radish background have been reported.  
 
Budahn et al 2007 reported a complete set of B. napus + radish addition lines

 

  Genetic markers

 Although morphological markers have been used for genetic analysis in some of the Brassica species (Sampson, 1966), they have had minimal impact on gene mapping because of their small numbers. The study of Arus and Orton (1983) on the inheritance and linkage of isozyme loci in B. oleracea represented the first major attempt to develop genetic markers and their use to assemble linkage maps in Brassica. The nomenclature of isozyme loci in Brassica has been recently standardized by Chevre et al. (1995).  Later, the advent of DNA-based genetic markers (Quiros et al., 1994; Kresovich et al. 1995; Szewc-McFadden et al. 1996; Voorrips et al. 1997) provided sufficient markers to develop comprehensive maps for the Brassica genomes and related applications.

 RFLP markers: In Brassica, RFLP markers have been derived from anonymous genomic and cDNA clones and gene specific sequences. These include anonymous genomic clones, cDNA clones and gene specific probes (Quiros et al. 1994). Recently Cheung et al. 1997a) have generated RFLP probes by representational difference analysis. Cloned genes and expressed sequence tags (ESTs) from the crucifer Arabidopsis  now constitute a useful source of probes for mapping the Brassica genomes (Lagercrantz et al. 1996; Sadowski and Quiros 1998).

Degree of polymorphism: In agreement with morphological variation, extensive RFLP polymorphism for genomic clones has been detected in Brassica diploid cultivated species. This is explained in part by the almost obligate outcrossing of these species imposed by self-incompatibility. Figdore et al. (1988) in a comprehensive study concluded that the high level of polymorphism in B. oleracea and B. rapa warrant the use of only one restriction enzyme and many clones instead of several enzymes and few clones.

  In general, polymorphism in amphidiploids is less than that observed in diploid species. For example, the level of polymorphism reported for B. napus  is less than 45% (Ferreira et al. 1994; Uzunova et al. 1995; Cheung et al. 1997a,b), whereas in B. oleracea it can be higher than 80% (Cheung et al. 1997b). In B. juncea a polymorphism of approximately 60% was reported by Cheung et al. 1997b). Lower levels of polymorphism in amphidiploids is expected since they have also a lower level of out-crossing due to a weak and often non-existing self-incompatibility system.

PCR Based markers: These include the following types of markers:

RAPD markers: These have been extensively used for map construction in Brassica species.  Levels of polymorphism of RAPDs and RFLPs are roughly similar in these species.

Microsatellite or Single Sequence Repeat (SSR) markers: This type of marker based on di- tri- and tetra-nucleotide tandem repeats was first developed in Brassica napus by Kresovich et al. (1995) and Bathia et al. (1995) in B. juncea. The use of these highly polymorphic co-dominant markers in map construction, is quite limited in Brassica species. The main limitation has been lack of polymorphism across species for comparative mapping and the high cost associated to their development. They are as polymorphic as RFLP markers (Plieske et al. 1998). A number of SSR are now available for alignment of maps from different origins (Sebastian, 2000). Parida et al 2009 have reported additional SSR loci from mining unigenes (unique EST seqs) of napus and rapa Now a large number of microsatellite loci are available to the Brassica research community, although some are proprietary. Check the Brassica database in UK CropNet. 

AFLP markers: Voorrips et al. (1997) used them in B. oleracea for mapping two clubroot resistance genes. Lim et al. (1998) constructed with these markers along with RAPDs a B. rapa linkage map. AFLPs markers are also highly polymorphic but are mostly dominant and cannot be used reliably for comparative mapping.

SRAP/TRAP markers: Sequence related amplified polymorphism (Li and Quiros 2001, TAG 103:455-461). Simpler than AFLPs since there is no need to digest and ligate DNA, but more reliable than RAPDs, uses 18-20 mer primers.  Used also for cDNA to construct transcriptome map on segregating F2 population, aimed to cDNA markers (Li et al. 2003).

Other PCR-based markers: More recently ESTs (expressed sequenced tags) from Arabidopsis, a model crucifer, have been used as RFLP markers on B. oleracea for comparative alignment of the genomes of both species (Lan et al. 2000; Babula et al. 2003). Other novel markers include Amplified Consensus Gene Markers (ACGM) based on Arabidopsis sequences by construction of degenerate primers aimed to amplify conserved coding sequences of specific genes. (Fourmann et al. 1998; Brunel et al. 1999) .

Map development in  Brassica

Brief synopsis of mapping activities in Brassica

 

The use of many common markers facilitates comparisons between the maps of the different Brassica species, and can yield further insights into their genome organization and evolution.  Within a species, maps show almost complete colinearity (cf. Lydiate et al., 1993), except for small inversions that differ between some B. oleracea morphotypes (Kianian and Quiros 1992; Lan et al. 2000).  Comparison of B. rapa to B. oleracea and B. napus supports the close evolutionary relationship between the two diploids, but indicates that deletions and insertions may have occurred after divergence of the two species (Hoenecke and Chyi 1991).  The genome of resynthesized B. napus is essentially unrearranged with respect to its B. oleracea and B. rapa progenitors (Lydiate et al. 1993).  However, natural B. napus evolution (Cheung et al. 1997a; Butruille et al. 1999) has been accompanied by rearrangements that were not observed for resynthesized B. napus.

          Presently, efforts are being made to coordinate the mapping activities of many Brassica laboratories and to align the existing maps of B. oleracea and B. napus, assigning linkage groups to chromosomes and cross-referencing these to the Arabidopsis thaliana physical map. Anchor probes based on A. thaliana sequences, called GST (genomic sequence tags) probes are under development by the Brassica IGF (Investigating Gene Function) project (http://brassica.bbsrc.ac.uk/index.htm) in England.  Additionally, efforts at Cold Spring Harbor and TIGR (http://www.tigr.org/tdb/e2k1/bog1/) in association with Arabidopsis sequence annotation yielded approximately 420,000 genomic shotgun sequences for Brassica oleracea that have been posted in GenBank and Brassica data bases.

Physical mapping in Brassica has been partially accomplished by construction of synteny maps using alien addition lines for a few species (Quiros 2001). Fluorescence in situ hybridization (FISH) to chromosome spreads, which has been mastered by a few laboratories, is proving quite useful for physical mapping. This technology makes possible to identify all the chromosomes in the A, B and C genomes based on specific probes and BAC clones (Jackson et al. 2000; Hasterok et al. 2001). All A and C genome linkage groups have been assigned to their respective chromosomes.  Single gene mapping on chromosomes and extended DNA fibers has also been accomplished for the self-incompatibility related genes SLG and SRL (Fukui, 2003).  

          Synteny maps

C genome maps: These have been constructed for most of the C genome chromosomes by RFLP, isozymes and RAPD markers.  Genes for erucic acid content and a flower color gene have also been mapped.

B genome maps:  Similar to the C genome synteny maps, these have been constructed for most chromosomes of the B genome using various types of markers including traits such as erucic acid, sinigrin and blackleg resistance.

Determination of general stability of alien chromosomes:
 Basically two types of instability have been observed in alien chromosomes: Frequent terminal deletions as reported by Hu and Quiros, (1991) and Quiros et al. (1994) and intragenomic recombination. Intragenomic recombination is also quite frequent and makes difficult to obtain reliable synteny maps for the Brassica genomes. This has been observed for the C genome (Quiros et al. 1994) and more extensively for the B genome during alien addition line construction. Presence of more than one alien chromosome in the triploid resulting from the initial cross results in homoeologous pairing causing recombination. 

Brassica genome program, based on shotgun sequences of BAC clones of B .oleracea  by TIGR/Cold Spring Harbor, so far 2GB have been sequenced. Aimed to have 0.5X to 1X genome coverage. 

 Genetic maps

 

B. oleracea maps

Several maps have been developed independently for this species involving crosses between different crops, as reviewed by Quiros and Paterson (2004). Currently a “consensus map” for B. oleracea is available after perpetuating the mapping populations and making the seed available to the research community. This map was assembled by Sebastian et al. (2000) after integrating a doubled haploid (DH) population resulting from crossing cauliflower by Brussels sprouts to cauliflower with a set of DH lines of another mapping population resulting from backcrossing DH broccoli and Chinese kale (Bouhon et al. 1996; Ramsey et al. 1996). It consists of 547 RFLP, AFLP and SSR loci on nine linkage groups covering 893cM (average locus interval 2.6cM).

Gao et al (2007) reported the construction of a  high-density genetic map based on the broccoli x cauliflower F2 population used by Li et al (2003), adding over 1200 various types of PCR-based markers spanning  703cM in nine linkage groups, designated LG1-LG9. It also includes major glucosinolate genes and a downy mildew resistance.

Iñiguez-Luy et al. (2009) recently constructed a map for B. oleracea using a DH population of approximately 150 lines developed from a broccoli x rapid cycling B. oleracea cross.  A total of 120 RFLP probes and 146 SSR markers, as well as one phenotypic marker (flower color) have been mapped on this population to date. 

 

 

The assignment of B. oleracea linkage groups to their respective chromosomes was partially accomplished by alignment to synteny maps based on C-genome alien addition lines (Quiros 2001; Heneen and Jorgensen 2001; Heneen and Brismar 2001).

On the other hand, fluorescence in situ hybridization (FISH) using rDNA probes and DAPI stain has allowed the identification of virtually all mitotic chromosomes in the A and C genomes (Hasterok et al. 2001; Schrader et al. 2000; Snowdon et al. 2002). BAC clones as FISH probes are becoming increasingly useful for Brassica chromosome identification. These clones have been applied to meiotic (Ziolkowski and Sadowski 2002) and mitotic C genome chromosomes quite successfully. Howell et al. (2002) using mostly B. oleracea BAC clones as probes on mitotic chromosomes were able to assign and orient each linkage group of the consensus B. oleracea genetic map mentioned above to its respective chromosome.

 

 

           

B. rapa maps

The genome od this species is being sequenced at this time under the Multinational B. rapa Genome Sequencing Project,

In addition to the early maps developed for this crop (see Quiros 2001 for review), Lim et al. (1998, 2002) have produced an extensive AFLP, SSR and RAPD based linkage map in doubled haploids of a hybrid between two Chinese cabbage varieties. The existing maps have been used to locate genes coding for various traits, including disease resistance genes (Kole et al. 2002).

          Development of alien addition lines in this crop has been elusive, however Kaneko et al. (2001, 2002) obtained eight putative monosomic addition lines extracted from radish by B. rapa sesquidiploids and a double monosomic line. These lines allowed the location of a series of genes responsible for various morphological traits and molecular markers.

 

The most recent map produced by Choi et al (2007) in Chinese cabbage has 556 markers including SSR, which has been used to align to B. napus maps. The contigs generated by the Multinational B. rapa Genome Sequencing project will be anchored to this map.

 

A lot of progress have been done in the Korean program on the molecular cytogenetics of B. rapa not only identifying the chromosomes by FISH but also physically mapping BAC clones into some of the chromosomes.

B. nigra maps: Truco and Quiros (1994) developed a map for this species  based on a single F2 population of 83 plants, involving two parental individuals from geographically divergent populations. The map  has 67 markers arranged in eight major linkage groups which may correspond to the eight B. nigra chromosomes, plus two small groups. The markers include RFLP’s,  RAPD's and a few isozymes.  The map covers 561 cM with average intervals of 8.4 cM. Lagercrantz and Lydiate (1995) developed a RFLP map in a backcross population of B. nigra, consisting of 288 loci covering a length of 855 cM.

  It has been possible to assign only five linkage groups of the first map to their respective chromosomes by synteny mapping based on alien addition lines (Chevre et al. 1997).

B. napus maps

Much of the mapping activity in Brassica has been focused on this species because of its economic importance. This has resulted in the construction of at least six independent maps (see Quiros 2001 for review). Several genes of known function and economic importance were located on these maps (Cloutier et al. 1995). B. napus maps.  The most recent ones are those of Lowe et al (2004) and Piquemal et al (2005) based on SSR markers. These can be used for alignment with other maps.  

 Parkin et al (2005) have produced a comparative B. napus- A thaliana map based on 1000 RFLP’s. The intense mapping activity dedicated to this crop is expected to result in the near future in the identification of genes of economic importance to expedite its genetic improvement.

 

B. juncea maps

Sharma et al. (2002) expanded an earlier map (Sharma et al. 1994) after developing recombinant inbreds. This map contains 130 RAPD markers on 21 linkage groups. Two QTLs explaining oleic acid content were resolved in this population. In addition, Cheung et al. (1997b) developed another map based on doubled haploid progeny from the F1 hybrid of a cross involving a canola quality line by a high oil content line. The map was developed with probes from B. napus, consisting of 343 loci on 18 main linkage groups. 

 

 


 

Last Modified: April 27, 2010

 

 


 
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Last modified May2011

© Carlos F Quiros, 1998