Molecular Cascades in UV Induced Melanogenesis: A Central Role for Melanotropins?

When human skin is exposed to ultraviolet (UV) light, a highly complex cascade of events ensues that culminates, among other things, in increased skin melanin content. From analyses at the tissue and cellular level, it has been shown that following exposure to UV light there is an increase in the number of active melanocytes in the basal layer of the epidermis, and individual melanocytes are stimulated to produce more melanin. In addition, the rate of transfer of melanosomes from melanocytes to keratinocytes is apparently increased, although the role of UV light in this process remains to be demonstrated. Recent biochemical evidence is reviewed on factors that regulate these processes. A plausible explanation for the effects of UV on pigmentation is that there are mechanisms in the skin for the orderly, regulated reception of UV signals that are then transduced to initiate the cascade. The signals involve both melanocytes and keratinocytes, and avail-able evidence supports a model in which melanotropins and their receptors play a central role in the process.     

Shoot and root physiological responses to localized zones of soil moisture in cultivated and wild lettuce (Lactuca spp.)

Cultivated crisphead lettuce (Lactuca sativa L.) has a shallower root system than its wild relative, Lactuca serriola L. The effects of localized soil water, at depth, on plant water relations, gas exchange and root distribution were examined in the two species using soil columns with the soil hydraulic-ally separated into two layers, at (0–20 cm and 20–81) cm, but permitting root growth between the layers. Three treatments were imposed on 7-week-old plants, and maintained for 4 weeks: (i) watering, both layers to field capacity; (ii) drying the upper layer while watering the lower layer to field capacity, and (iii) drying both layers. Drying only 0–20 cm of soil had no effect on leaf water status, net photosynthesis, stomatal conductance or biomass production in L. serriola compared to a well-watered control, but caused a short-term reduction (10 d) in leaf water status and photosynthesis in L. sativa that reduced final shoot production. The different responses may be explained by differences in root distribution. Just before the treatments commenced, L. serriola had 50% of total root length at 20–80 cm compared to 35% in L. sativa. Allocation of total biomass to roots in L. serriola was approximately double that in L. sativa. The wild species could provide germplasm for cultivated lettuces to extract more soil water from depth, which may improve irrigation efficiency.     

The reproductive biology of the Mediterranean endemic Cyclamen balearicum Willk. (Primulaceae)

Knowledge of the reproductive biology of endemic plants improves our understanding of how mating system may be related to patterns of species abundance and provides a basis for the development of rational conservation programmes. In this paper we present natural population data on the floral biology and reproductive ecology of the endemic Mediterranean species Cyclamen balearicum Willk. This is a long-lived, diploid perennial herb which occurs in southern France in five fragmented and isolated regions and on the Balearic Islands of Mallorca, Menorca, Ibiza, Cabrera and Draponera. Our observations indicate a particularly scarce pollinator activity (rare syrphid visits) and dispersal by ants over small distances. A controlled pollination experiment in a natural population showed that in southern France C. balearicum is fully self-compatible and that selfing is autonomous and probably delayed (i.e. following opportunities for outcrossing). The proximity of stigmas and anthers will favour autonomous selfing. The high pollen/ovule ratio indicates nevertheless that C. balearicum has a mixed mating system. Patterns of variation in stigma-anther separation and pollen production per flower suggest that not only has the current mating system of the species evolved from an outcrossing ancestor but that due to the fragmentation and isolation of populations greater levels of selfing have evolved in southern France (and to an intermediate degree on Ibiza and Menorca). On the island of Mallorca where larger continuous belts of C. balearicum habitat still exist the species has floral traits indicating a more outcrossed mating system. To our knowledge this is the first paper to document such trends in floral traits in the endemic component of the Mediterranean flora.     

Planktonic green algae in western Lake Erie: the importance of temporal scale in the interpretation of change

1. Total densities of planktonic Chlorophyceae collected in weekly sampling of the Kingsville (Ontario) municipal water intake in western Lake Erie were evaluated for potential effects of the recent zebra mussel ( Dreissena polymorpha ) invasion and for the longer term effects of the Lake Erie phosphorus loading control programme.
2. At a relatively small temporal scale of about 10 years, an apparent zebra mussel-related impact was clearly revealed in 1988 as an inflection point on the cumulative sum chlorophyte density curve. However, at a temporal scale of nearly three decades, this inflection point was not distinct. There was a steady decline in total Chlorophyceae throughout the 1970s which accelerated during the early 1980s; this corresponds to declining western Lake Erie phosphorus loading rates and phosphorus concentrations reported by others over the same period. In the absence of zebra mussels, average annual chlorophyte density decreased by 94% between the early 1970s and the mid-1980s.
3. The dramatic long-term decline of planktonic chlorophytes in western Lake Erie reveals the success of the phosphorus control programme, places the recent impact of the zebra mussels in its proper perspective and underscores the need for and value of long-term limnological data for management of the Laurentian Great Lakes.