Wild hybrids

Wild Brassica and its close relatives hybridized naturally, forming polyploids. These amphidiploids together with their parental wild diploids were key building blocks from which our domesticated brassica crops have evolved. Three hybrid species are of particular interest as ancestors of the crop brassicas.

Brassica carinata (n = 17) was formed from B. oleracea (n = 9) X B. nigra (n = 8). This species is characterized by the slow steady growth of B. oleracea and the mustard oil content of B. nigra. Wild forms of B. carinata are not known, but primitive domesticated types are cultivated in upland areas of Ethiopia and further south into Kenya. This hybrid may have originated from kale land races of B. oleracea types hybridizing with wild or semi-domesticated forms of B. nigra. Both kale and carinata crops thrive in cool environments typical of the Ethiopian plateau. The local farmers grow them in 'kale gardens'. This term, translated into many languages and dialects, is commonly found throughout those rural areas using vegetables derived from B. oleracea types. Carinata crops themselves are alternatively named 'guomin', Abyssinian mustard or Ethiopian cabbage, and provide leafy vegetables and sources of oil. Cabbage itself can be a ubiquitous term used to describe cole brassicas and not necessarily synonymous with the sophisticated heads seen on today's supermarket shelves.

Brassica juncea (n = 18) is a hybrid between B. rapa (n = 10) and B. nigra (n = 8) (Fransden, 1943) producing large leaves and with the rapid growth of B. rapa and the mustard oil of B. nigra. Use of B. juncea as a source of vegetable oil is gaining importance in India; while throughout Asia, especially in China and Japan, the plant has a great diversity of cultivated forms used as staple vegetables of immense dietary importance. Reputably, wild forms are still found on the Asia Minor plateau and in southern Iran.

The third hybrid, B. napus (n = 19), has wild forms in Sweden, Denmark, The Netherlands and the UK. It developed from B. rapa (n = 10) X B. oleracea (n = 9). This hybrid may have formed as B. oleracea types expanded their range along the coasts of northern Europe and B. rapa extended from the Irano-Turanian regions. Alternatively, B. napus may have Mediterranean origins or, as seems likely, there were several centres of evolution. The wild populations of B. napus have acquired major scientific significance recently as they present

Table 1.1. Examples of the diversity and characteristics of selected natural Brassica species.

Chromosome

Geographical

Name

count

distribution

Habitat

B. amplexicaulis

n = 11

Intermountain area southeast of Algiers

Small plant, colonizes colluvial slopes, especially medium-sized gravel

B. barrelieri

n = 9

Iberian peninsula, extending to Morocco and Algeria

Common on wastelands, especially areas of more dense vegetation

B. elongata

n= 11

Plateau steppe lands of southeastern Europe and western Asia as far as Iran

Semi-arid areas

B. fruticulosa

n = 8

Found around the

Biennial or perennial

subsp. fruticulosa

Mediterranean coasts

B. fruticulosa

n = 8

especially amongst pine

Annual

subsp. cossoniana

trees; the subspecies cossoniana extends beyond the coastal zone and is found on the Saharan side of the Moyen Atlas in Morocco

Both subspecies require well-drained sites; on inland sites characteristically colonizing stony mountain slopes and alluvial areas

B. maurorum

n= 8

Endemic to North Africa; inhabits arable land in Morocco and Algeria

Colonizes stony pastures in semi-arid areas from the coast to low mountainous zones; on arable land grows to 2 m high in dense clumps similar to B. nigra

B. oxyrrhina

n= 9

Southern Portugal, Spain and northwestern Morocco

Coastal sandy habitats

B. repanda and

n = 10

Inland rocky areas

These species grow together in

B. gravinae

the lithosol in crevices of rocky outcrops.

A polypoid (n = 20) form has been found north of Biskra in Algeria

B. spinescens

n= 8

Endemic to North Africa

Coastal calcareous or siliceous cliffs.

Diminutive growth habit with small, thick glabrous leaves; B. fruticulosa, B. maurorum and B. spinescens possibly form a single cytodeme

B. tournefortii

n= 10

Coastal areas of the Mediterranean extending to western Asia as far as India

Capable of colonizing arid alluvial sand where other vegetation is sparse

After Tsunoda et al. (1984).

After Tsunoda et al. (1984).

means of determining the potential for gene flow to and from genetically modified cultivars of oilseed rape (also B. napus) (Hailles et al., 1997; Hall et al., 2000; Bond et al., 2004).

The relationships between the hybrid amphidiploids and their parental species are summarized in the gene flow triangle of U (1935) (Fig. 1.2).

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