We compared the structural, biochemical, and physiological characteristics involved in photorespiration of intergeneric hybrids differing in genome constitution (DtDtR, DtDtRR, and DtRR) between the C(3)-C(4) intermediate species Diplotaxis tenuifolia (DtDt) and the C(3) species radish (Raphanus sativus; RR). The bundle sheath (BS) cells in D. tenuifolia included many centripetally located chloroplasts and mitochondria, but those of radish had only a few chloroplasts and mitochondria. In the hybrids, the numbers of chloroplasts and mitochondria, the ratio of centripetally located organelles to total organelles, and the mitochondrial size in the BS cells increased with an increase in the constitution ratio of the Dt:R genome. The P-protein of glycine decarboxylase (GDC) was confined to the BS mitochondria in D. tenuifolia, whereas in radish, it accumulated more densely in the mesophyll than in the BS mitochondria. In the hybrids, more intense accumulation of GDC in the BS relative to the mesophyll mitochondria occurred with an increase in the Dt:R ratio. These structural and biochemical features in the hybrids were reflected in the gas exchange characteristics of leaves, such as the CO(2) compensation point. Our data indicate that the leaf structure, the intercellular pattern of GDC expression, and the gas exchange characteristics of C(3)-C(4) intermediate photosynthesis are inherited in the hybrids depending on the constitution ratio of the parent genomes. Our findings also demonstrate that the apparent reduced photorespiration in C(3)-C(4) intermediate plants is mainly due to the structural differentiation of mitochondria and chloroplasts in the BS cells combined with the BS-dominant expression of GDC.

In Brassicaceae crop breeding programs, wild relatives have been evaluated as genetic resources to develop new cultivars with biotic and abiotic stress resistance. This has become necessary because of the diversification of ecotypes of diseases and pests, changing food preferences, advances in production technology, the use of new approaches such as in vitro breeding programs, and the need for economical production of F1 seed. To produce potential new cultivars, interspecific and intergeneric hybridizations have been performed between cultivated species and between cultivated species and their wild relatives. Furthermore, interspecific and intergeneric hybrids have been successfully produced using embryo rescue techniques. In this paper, we review the interspecific and intergeneric incompatibilities between Brassicaceae crops and their wild relatives, and the production, characterization, and improvement of synthetic amphidiploid lines, alien gene introgression lines, alloplasmic lines, monosomic alien chromosome addition lines, and monosomic alien chromosome substitution lines. The goal is to provide useful materials to support practical breeding strategies and to study the genetic effects of individual chromosomes on plant traits, the number of genes that control a trait, their linkage relationships, and genetic improvement in Brassicaceae crops.

Artificial hybridization studies have been carried out between plants with different photosynthetic types to study the genetic mechanism of photosynthetic types. However, there are only few reports describing the possibility of natural hybridization between plants with different photosynthetic types. A previous cytological and morphological study suggested that a cruciferous allotetraploid species, Diplotaxis muralis (L.) DC. (2n = 42), originated from natural hybridization between D. tenuifolia (L.) DC. (2n = 22) and D. viminea (L.) DC. (2n = 20). These putative parents have recently been reported to be a C (3)-C (4) intermediate and a C (3) species, respectively. If this hybridization occurred, D. muralis should have characteristics intermediate between those of the C (3)-C (4) intermediate and C (3) types. We compared leaf structures and photosynthetic characteristics of the three species. The bundle sheath (BS) cells in D. tenuifolia included many centripetally located chloroplasts and mitochondria, but those of D. viminea had only a few organelles. The BS cells in D. muralis displayed intermediate features between the putative parents. Glycine decarboxylase P protein was confined to the BS mitochondria in D. tenuifolia, but accumulated mainly in the mesophyll mitochondria in D. viminea. In D. muralis, it accumulated in both the BS and the mesophyll mitochondria. Values of CO (2) compensation point and its response to changing light intensity were also intermediate between the putative parents. These data support the theory that D. muralis was created by natural hybridization between species with different photosynthetic types.

Breeding of Raphanus sativus‐Brassica rapa monosomic chromosome addition lines (MALs, 2n = 19) was carried out by backcrossing the synthesized amphidiploid line, Raphanobrassica (R. sativus×B. rapa, 2n = 38, RRAA, line RA89) with R. sativus cv. ‘Shogoin’ (2n = 18, RR). In the first cross of Raphanobrassica× radish, four sesquidiploidal BC1 plants (2n = 28, RRA, RA89‐36‐1, RA89‐31‐1, RA89‐31‐2, RA89‐31‐3) were successfully developed. In these plants, the chromosome configurations of 9II + 10I and 10II + 8I were observed frequently at first metaphase (MI) of meiosis in pollen mother cells (PMCs). The RA 89‐36‐1 plant produced many seeds in the reciprocal backcrosses with radish. About 50% of the BC2 plants obtained from the cross of RA89‐36‐1 plant × radish were 2n = 19 plants, followed by 2n = 18 plants (24%) and 2n = 20 plants (19%). In the reciprocal cross, 2n = 19 plants were also developed at the rate of 40%. From analysis of specific morphological traits, 2n = 19 plants were classified into eight types (a‐h). When 25 selected primers were used in polyacrylamide gel electrophoresis, random amplified polymorphic DNA (RAPD) markers derived from B. rapa for each type of MAL were detected in numbers between three for e‐type and 16 for b‐type. RAPD markers specific for each type alone were from one (OPE 05‐344) for h‐type to nine for b‐type. In the g‐type, no marker specific to this type alone was observed. However, 19 bands were common between at least two types. These MAL plants exhibited predominantly the chromosome configuration of 9II + 1I at MI of PMCs, pollen and seed fertility being the same level as the radish cv. ‘Shogoin’. From the morphological traits and DNA markers, eight different MAL types among 10 expected were identified.

By backcrossing to Raphanus sativus cv. 'Pink ball', 55 BC 2 plants were obtained from two sesquidiploidal BC 1 plants (MaRR, 2n = 32) between Moricandia arvensis (MaMa, 2n = 28) and R. sativus (RR, 2n = 18). Their somatic chromosome numbers ranged from 2n = 18 to 2n = 23, except for one hyperploid plant with 2n = 44. In the BC 3 generation, 64 plants (2n = 19) were generated from 16 BC 2 plants with 2n = 19~23. Each plant with 2n = 19 exhibited both morphological and physiological characteristics diagnostic for the presence of an added chromosome of M. arvensis genome, and showed predominantly the chromosome pairing type of 9II + 1I at metaphase I of PMCs. They were classified into twelve (a~l) types of monosomic chromosome addition lines (MALs) of alloplasmic (M. arvensis) R. sativus carrying M. arvensis cytoplasm by their morphological, physiological and cytogenetical characteristics. The mean of seed setting in the twelve types of MALs ranged from 2.88 grains (f-type) to 0.51 grains (j-type) per pollinated flower when they were backcrossed to R. sativus cv. 'Pink ball'. The transmission rates to the next generation through female gametes ranged from 32.5 % (f-and j-types) to 5.5 % (h-type) when smaller seeds were selectively grown. The specific characteristics of each type of MALs were transmitted from the BC 3 generation to the BC 4 and BC 5 ones without any modification. The MALs of the distinctive twelve types were also identified using RAPD markers in the BC 5 generation. Pollen fertility of the twelve types of MALs ranged from 85.6 % (c-type) to 3.4 % (l-type), although four types (g-, h-, i-and jtypes) exhibited complete male sterility. Furthermore, alloplasmic (M. arvensis) R. sativus plants (2n = 18) which were derived from male fertile MALs showed complete male sterility. The twelve types of MALs produced in this study should be useful materials to determine the localization of genes for agronomic traits on the individual chromosome of M. arvensis and the alloplasmic (M. arvensis) R. sativus should also be a useful material for the development of a new cytoplasmic male sterility system in R. sativus.

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For the development of a new cytoplasmic male sterility (CMS) system in Brassica rapa (2n= 20, AA), intergeneric hybridization was performed in Eruca sativa (2n= 22, EE)×B. rapa. The original amphihaploid F1 plant (2n= 21, EA) generated via embryo rescue produced a sesquidiploid F1 plant (2n= 31, EAA), from which the alloplasmic F3 plants were generated. In F3, some progenies with malformed anthers were maintained as male‐sterile lines up to the F5 generation. In the F6 and F7 generations, the alloplasmic male‐sterile plants were bred by backcrossing to several B. rapa genotypes and were then classified into the three distinctive types: petaloid, antherless and brown anther, in addition to three intermediate types between them. By southern blot analysis, each plant of the petaloid and antherless types was shown to carry the cytoplasm genome of E. sativa. These male‐sterile plants produced as many viable seeds as the corresponding male‐fertile plants, although their nectary gland development was minimal. Consequently, CMS lines of the petaloid and antherless types with enhanced seed fertility and nectary gland development could provide promising plant materials for F1 seed production in B. rapa.

Intergeneric F, hybrids between Raphanus sativus (2« = 18, RR) and Moricandia arvensis (2« = 28, MaMa) have been produced through ovary culture followed by embryo culture, when M. arvensis was used as a pistillate parent. Six BC, plants were also obtained through ovary culture followed by embryo culture in the backcross of an amphidiploid F| hybrid with R. sativus cv. 'Pink ball'. Two BCj plants were sesquidiploids (2« = 32, MaRR), and the other BC, plants were hyperploid with 2n = 55, having MaMaRRR genomes. BC, seeds were obtained by conventional pollination in the successive backcross of two sesquidiploid BCi plants with R. sativus cv. 'Pink ball'. Their seed set percentages were 12.7% and 17.0%, respectively. These novel hybrid plants and derived progenies may be valuable materials for the genetic investigation and breeding of Brassiceae, including R. sativus.cross generation -embryo rescue -intergeneric hybridsmeiotic association -ovary culture When some Brassica crops were shown to hybridize with the related wild species, several investigations for the introgression of desirable agronomic traits from these wild species into cultivated crops were carried out (Delourme et al. 1989, Agnihotri et al. 1990, Batra et al. 1990, Ripley and Arnison 1990, Leliveh et al. 1993, Inomata 1994. In these crosses, however, intrinsic cross incompatibility often disturbs the transfer of these traits. Barriers to wide hybridization may be of the pre-or postfertilization type. Techniques for overcoming the barriers as reviewed by Khush and Brar (1992) largely depend upon the combination and direction of crosses, and may be different in each case.Among the tribe Brassiceae, in which C4 species have not been found up to now, the genus Moricandia is unique because it includes five C3-C4 intermediate species (Hylton et al. 1988). Moricandia arvensis (L.) DC. was the first-reported C3-C4 intermediate species (Apel et al. 1978, Holady et al. 1981 and has attracted breeders' attention to introgressing C3-C4 intermediate traits into cultivated Brassica crops. Apel et al. (1984) investigated the photosynthetic properties ofthe F, hybrid and BC, plants between Brassica alboglabra and M. arvensis and suggested that the C3-C4 intermediate traits, especially low photo-respiration activity, were valuable for the breeding of Brassica crops. Other intergeneric hybrids between M. arvensis and five Brassica species have been obtained by ovary culture (Takahata 1990, Takahata and Takeda 1990, Takahata et al. 1993. Somatic hybrid plants via protoplast fusion have also been developed in M. arvensis + B. oleracea (Toriyama et al. 1987), and B. juncea + M. arvensis (Kirti et al. 1992).In this study, aspects of the production of the intergeneric hybrid between Raphanus sativus and M. arvensis are reported. Materials and MethodsM. arvensis (L.) DC. {2n = 28, MaMa), strain 4, and 11 cultivars of R. sativus L. (In = 18, RR) were used as parents in reciprocal crosses. M. arvensis strain 4, an accession of Cruciferae genetic stocks of the Laboratory of ...