Search in patchy media: Exploitation-exploration tradeoff.

How to best exploit patchy resources? We introduce a minimal exploitation-migration model that incorporates the coupling between a searcher's trajectory, modeled by a random walk, and ensuing depletion of the environment by the searcher's consumption of resources. The searcher also migrates to a new patch when it takes S consecutive steps without finding resources. We compute the distribution of consumed resources F_{t} at time t for this non-Markovian searcher and show that consumption is maximized by exploring multiple patches. In one dimension, we derive the optimal strategy to maximize F_{t}. This strategy is robust with respect to the distribution of resources within patches and the criterion for leaving the current patch. We also show that F_{t} has an optimum in the ecologically relevant case of two-dimensional patchy environments.

Universality classes of foraging with resource renewal.

We determine the impact of resource renewal on the lifetime of a forager that depletes its environment and starves if it wanders too long without eating. In the framework of a minimal starving random-walk model with resource renewal, there are three universal classes of behavior as a function of the renewal time. For sufficiently rapid renewal, foragers are immortal, while foragers have a finite lifetime otherwise. In the specific case of one dimension, there is a third regime, for sufficiently slow renewal, in which the lifetime of the forager is independent of the renewal time. We outline an enumeration method to determine the mean lifetime of the forager in the mortal regime.

Molecular and functional characterization of Slide, an Ac-like autonomous transposable element from tobacco.

A new transposable element of tobacco, Slide, was isolated from the tl mutant line, which shows somatic instability, after its transposition into a locus encoding nitrate reductase (NR). The Slide-124 element is 3733 bp long and its coding sequences show similarities with conserved domains of the transposases of Ac, Tam3 and hobo. Excision from the NR locus is detectable in somatic leaf tissues and Slide mobility is triggered by in vitro tissue culture. Slide excision events create footprints similar to those left by Ac and Tam3. Tobacco lines derived from the tl mutant line seem characterized by unmethylated copies of a few members of the highly repetitive Slide family. Slide mobility was monitored in transient expression assays. In wild-type tobacco protoplasts, the complete Slide element, as well as a defective copy, is able to excise. The complete Slide element, but not the defective version, is able to excise in protoplasts of the heterologous species lettuce (Lactuca sativa). These results show that Slide carries the functions required for its own mobility, and represents the first autonomous Ac-like element characterized in Solanaceae species.

Regeneration and characterization of plants produced from mature tobacco pollen protoplasts via gametosomatic hybridization.

Mature pollen protoplasts (n) isolated from kanamycin resistant plants of Nicotiana tabacum (2n = 4x = 48) were fused with somatic mesophyll protoplasts (2n) of Nicotiana plumbaginifolia (2n = 20) to produce plants. A total of 3.6·10(6) mature pollen protoplasts were fused with 7·10(6) mesophyll protoplasts using a PEG/Ca(2+) method. Mature pollen protoplasts did not divide in our culture conditions, and N. plumbaginifolia protoplasts stopped dividing when the protoplast-derived colonies were transferred to a selection medium containing paromomycine (20 mg·l(-1)). A total of 133 actively growing colonies were recovered on the selection medium containing kanamycin (100 mg·l(-1)). Plants from twenty resulting cell lines were confirmed as hybrids (17) or cybrids (3) based on leaf and floral morphology and fertility analysis. Isozyme pattern analysis confirmed the nuclear hybrid and cybrid nature, respectively, for 2 and 3 typical gametosomatic selected plants. Root tip squashes of 6 of the gametosomatic hybrid plants revealed chromosome numbers ranging from 44 to 68; the 3 selected cybrid plants had 48 chromosomes. Evidence for organelle transmission from the mesophyll partner in the gametosomatic plants is shown. From the analysis it can be concluded that the gametosomatic fusion involving mature pollen protoplasts (n) carrying a dominant selection marker can be convenient for synthesis of either hybrids or cybrids. Such gametosomatic fusion is therefore considered as a new approach towards the production of androgenetic plants with a choosen cytoplasm.

A Lactuca universal hybridizer, and its use in creation of fertile interspecific somatic hybrids.

A Lactuca sativa cv. Ardente line heterozygous for a gene encoding resistance to kanamycin, a positive and dominant trait, was crossed with cv. Girelle, which is heterozygous for a recessive albinism marker. The resulting seeds yielded 25% albino seedlings, of which 50% were also resistant to kanamycin. Such plantlets (KR, a) grown in vitro were used for preparation of universal hybridizer protoplasts, since green buds that can develop on kanamycin containing-medium should result from fusion with any wild-type protoplast. To test the practicability of this selection scheme, we fused L. sativa KR, a protoplasts with protoplasts derived from various wild Lactuca as well as various other related species. Protoplast-derived cell colonies were selected for resistance to kanamycin at the regeneration stage. Green buds were regenerated after fusion with protoplasts of L. tatarica and of L. perennis. So far, 9 interspecific hybrid plants have been characterized morphologically. In addition, random amplified polymorphic DNA (RAPD) analysis with selected primers confirmed that these plants are indeed interspecific hybrids. Some plants are female-fertile and production of backcross progenies with L. sativa is in progress. Since many desirable traits such as resistances to viruses, bacteria and fungi (Bremia lactucae) have been characterized in wild Lactuca species, the use of somatic hybridization in breeding programmes now appears a practical possibility.

Improved flax regeneration from hypocotyls using thidiazuron as a cytokinin source.

The effects of thidiazuron, benzyladenine and zeatin were tested with respect to bud regeneration of different flax explants from hypocotyls, cotyledons and apices of two fibre varieties (Ariane, Viking) and one linseed variety (Antarès). These three cytokinins were tested either alone or in combination with naphthalene acetic acid, indole acetic acid or 2,4-dichlorophenoxyacetic acid.Hypocotyls were the most responsive explants. Thidiazuron was significantly the most effective followed by benzyladenine, and then zeatin, in inducing organogenesis from hypocotyl segments. The optimal thidiazuron concentration for bud regeneration from hypocotyls was 0.1-0.3 µM in combination with 0.01 µM of naphthalene acetic acid. Six days after plating, shoot initials began to appear on hypocotyl sections compared with ten to fifteen days when using benzyladenine or zeatin.