Patterns of genetic variability within and among populations of wild radish,Raphanus raphanistrum (Brassicaceae)

The genetic structure of populations is an important determinant of the evolutionary potential of a species. Colonizing plants tend to be characterized by low within- and high among-population variability. Genetic differentiation of both floral traits and isozymes was studied in six populations of wild radish (Raphanus raphanistrum). Evidence for differentiation in both sets of traits was found, but patterns of differentiation of floral traits did not coincide with isozyme differentiation. Contrary to most colonizing species, wild radish showed high within- and only moderate among-population variability at isozyme loci. In addition, levels of differentiation did not correspond to geographic distance between the populations. These results are likely due at least in part to the self-incompatibility system of this species, long-distance movement of large numbers of wild radish seeds by humans, and introgression from cultivated radish (R. sativus).     

Selecting for floral character associations in wild radish,Raphanus sativus L.

ESS models of reproductive allocation have been used extensively to explain patterns of floral diversity in angiosperms. These theoretical explorations assume that proportional allocation to pollen, ovules, and seeds, as well as to secondary features such as showy petals and nectar rewards, can evolve independently within the limits set by total resource availability. In populations of California wild radish, we have shown previously that petal size, a strong determinant of visitation by honey bee pollinators, is positively correlated with both pollen and nectar production, but not with ovule or seed number per flower. These phenotypic associations may reflect selection, environmental correlation, and/or genetic constraint. By exerting selection on the petal size : pollen number ratio over two generations, we eliminated the positive correlation between petal size and pollen production, with both characters showing significant change after a single selection episode. Once these two floral traits became uncoupled, nectar sugar production was significantly correlated only with petal size. Our results suggest that natural selection could readily alter reproductive allocation in these flowers, and that the phenotypic correlations observed in nature may be maintained by selection for effective reproductive phenotypes.     

Mechanisms of constraints: the contributions of selection and genetic variance to the maintenance of cotyledon number in wild radish

The evolutionary mechanisms underlying the maintenance of invariant traits are poorly understood, partly because the lack of variance makes these mechanisms difficult to study. Although the number of cotyledons that plant species produce is highly canalized, populations of plants frequently contain individuals with abnormal cotyledon numbers. In a garden study with 1857 wild radish plants from 75 paternal half-sibling families, 89 (almost 5%) had cotyledon numbers less or greater than two. We found evidence for direct selection on cotyledon number, but no evidence for additive genetic variation for cotyledon number. In spite of the very large sample size, our power to detect variation and selection was hampered by the small number of individuals (10) producing more than two cotyledons. Thus, our results provide support for both a lack of genetic variation and selection as reasons for the current lack of variation in wild radish cotyledon number.     

Direct and indirect responses to selection on pollen size in Brassica rapa L.

Pollen size varies little within angiosperm species, but differs extensively between species, suggesting the action of strong selection. Nevertheless, the potential for genetic responses of pollen size to selection, as determined by additive genetic variance and genetic correlations with other floral traits, has received little attention. To assess this potential, we subjected Brassica rapa to artificial selection for large and small pollen during three generations. This selection caused significant divergence in pollen diameter, with additive genetic effects accounting for over 30% of the observed phenotypic variation in pollen size. Such heritable genetic variation suggests that natural selection could effect evolutionary change in this trait. Selection on pollen size also elicited correlated responses in pollen number (–), flower size (+), style length (+), and ovule number (+), suggesting that pollen size cannot evolve independently. The correlated responses of pollen number, flower size and ovule number probably reflect the genetically determined and physically constrained pattern of resource allocation in B. rapa. In contrast, the positive correlation between pollen size and style length may represent a widespread gametic‐phase disequilibrium in angiosperms that arises from nonrandom fertilization success of large pollen in pistils with long styles.     

Spontaneous hybridizations between oilseed rape and wild radish

The occurence of spontaneous hybridization between Brassica napus (oilseed rape) and Raphanus raphanistrum (wild radish) was investigated under different density conditions in cages and open-field experiments. Hybrids with wild radish as the seed parent were identified by screening for herbicide resistance belonging to rape. Small seed size and intermediate morphology were used to screen for hybrids with rape as the seed parent. Leaf isozyme patterns and flow cytometry provided confirmation of hybrids. Wild radish in an oilseed rape field produced as many as three interspecific hybrids per 100 plants. This is the first report of such a spontaneous event. The frequency of hybrids is expected to range from 0.006 to 0.2% of the total seed produced, at P = 0.05. Male-sterile oilseed rape plants surrounded by wild radish can produce up to 37 hybrids per plant. Seed production of the F₁ hybrids and their F₂ descendants was up to 0.4% and 2%, respectively, of that of wild radish. Gene escape from transgenic oilseed rape to wild related species is discussed.     

Hybridisation between Brassica napus L. and Raphanus raphanistrum L. under agronomic field conditions

The frequency of hybridisation between Brassica napus L. and Raphanus raphanistrum L. under agronomic conditions was assessed in field experiments, where R. raphanistrum were randomly planted at two different densities into large plots of B. napus. An acetolacate synthase (ALS)-inhibiting herbicide-resistant trait was used to detect potential hybrid individuals. No hybrids were detected amongst 25,000 seedlings grown from seed collected from R. raphanistrum plants. Two hybrids were obtained from more than 52-million B. napus seedlings. Both hybrids were characterised as amphidiploids (AACCRrRr, 2n = 56) and were fertile. The frequency of hybridisation into B. napus in this experiment using male-fertile B. napus was 4 × 10^–8. Received: 31 August 2000 / Accepted: 23 January 2001     

我国部分白菜型油菜 RAPD 的研究

本研究用RAPD技术和统计学方法,对以湖南和湖北省油菜为主的34个白菜型油菜品种的遗传多样性进行了分析。根据离差平方和,用Ward's聚类方法进行聚类。结果表明：白菜型油菜的遗传变异与生态地理分布有密切的关系;湖南和湖北两省的白菜型油菜品种存在广泛的遗传变异;在DNA水平上可将所分析的34个品种分成8个类群。作者对研究结果在遗传育种中的应用进行了讨论。     

Development of the herbivore Pieris rapae and its endoparasitoid Cotesia rubecula on crucifers of field edges

Abstract:  Several studies have reported that flowering herbs, which grow naturally or are sown adjacent to agricultural fields, may be an important source of nutrients for natural enemies. Many parasitoids readily feed on plant exudates such as floral nectar, which contain different types of sugars that enable the insects to optimize their longevity, mobility and reproductive success. However, leaf tissues of plants grown in the margins of agricultural fields may also provide food for immature stages of insect herbivores, such as caterpillars, that are in turn attacked by parasitoids. Herbivores and their parasitoids may later disperse into the crop, so the nutritional quality of surrounding plants, as this affects herbivore and parasitoid fitness, may also influence the success of biological control programmes, especially later in the season. Here, we compare the suitability of three species of cruciferous plants (Brassicaceae) on the development of Pieris rapae L. (Lep., Pieridae) and its solitary endoparasitoid, Cotesia rubecula Marshall (Hym., Bracondiae). Insects were reared on a feral population of cabbage, Brassica oleracea , on radish Raphanus sativus , which is widely sown in agricultural margins, and on hedge mustard, Sisymbrium officinale , a wild crucifer which often grows in medium to large stands along road verges and field edges. Development time in both the herbivore and parasitoid were extended on R. sativus , compared with the other two species, whereas C. rubecula completed its development most rapidly on B. oleracea . Moreover, adult butterflies and parasitoids were significantly smaller when reared on R. sativus plants. Our results reveal that differences in the quality of plants growing adjacent to agricultural fields can affect the development of key herbivores and their parasitoids. This should be borne in mind when establishing criteria for the selection of floral biodiversity.