External factors determining seasonal breeding in a subtropical population of the shore crab Pachygrapsus transversus (Gibbes, 1850) (Brachyura,Grapsidae)

Summary

Monthly samples of the shore crab Pachygrapsus transversus from two distinct annual periods showed that ovigerous females are present throughout the year in the population. However, the relative ovigerous abundance of specimens among sexually mature changes from less than 10% during the winter months to almost 80% in summer. Linear correlations for each year revealed that both temperature and photoperiod were positively associated with relative abundance of ovigerous females. Multiple regression analyses suggested that photoperiod was the main factor affecting breeding in this species. Timing of observed reproductive pattern may enhance larval survival because of particular oceanographic conditions in the study region and favor early juvenile development due to certain species-specific growth features.     

Phytoplankton distribution along trophic gradients within and among reservoirs in Catalonia (Spain)

1. Longitudinal gradients in the epilimnetic waters of stratified reservoirs provide a useful database to study changing environmental conditions. The spatial distribution, assemblage structure and specific adaptations of phytoplankton assemblages can be analysed along these gradients over short time scales. 2. Four reservoirs with a similar typology, located along an altitudinal gradient in the same eco‐region, were sampled along their longitudinal axes. In total, 19 sampling stations provided a trophic spectrum, ranging from oligo‐mesotrophy to hypertrophy, which was quantified by calculating the trophic state index of each sampling station in the four reservoirs. 3. Several patterns in phytoplankton assemblage structure were detected. Total chlorophyll‐a (Chl‐a), biovolume, abundance and the relative biomass contribution of the main algal groups (chlorophytes, cyanobacteria, cryptophytes and diatoms) were highly correlated with their location along the trophic gradient. 4. We also adopted the functional classification of Reynolds et al. (2002) : this effectively summarized differences among phytoplankton assemblages under varying resource‐limiting combinations, especially nutrients and underwater light climate. 5. In terms of relationships with the trophic gradient, diatoms and cyanobacteria exhibited significant opposing trends in both their relative chlorophyll contribution to total Chl‐a and biovolume. Chlorophytes were more abundant at an intermediate position along the trophic spectrum. 6. The identified patterns are consistent with models of self‐organization of phytoplankton assemblages. In particular, light availability was a strong determinant of size and shape diversity, especially in hypertrophic conditions, where ‘R‐strategist’, needle shaped species, dominated the system. In contrast, under decreased availability of nutrients and higher light extinction coefficients (K_d), the system was co‐dominated by C‐ and S‐strategist species, having shapes with a higher surface/volume ratio.     

Chromosome numbers in Brazilian species of Crotalaria (Leguminosae, Papilionoideae) and their taxonomic significance

Chromosome numbers were counted for 23 species of Crotalaria native to Brazil. Among these data there were new counts for 15 taxa, and some confirmed previous reports or represented numbers that were different from those cited previously. The chromosome numbers most frequently found were 2 n  = 16 and 2 n  = 32. Only C. incana L. had 2 n  = 14 and C. tweediana Benth. had 2 n  = 54. The counts 2 n  = 32 and 54 were found in species of section Calycinae and 2 n  = 16 and 14 in species of section Chrysocalycinae . The data revealed the importance of chromosomal parameters in the characterization of sections Calycinae and Chrysocalycinae in Brazil. We discuss the systematic significance and evolutionary aspects for the genus, comparing the results with the two sections that are native in Brazil.  © 2006 The Linnean Society of London, Botanical Journal of the Linnean Society , 2006, 151 , 271–277.     

Positive effects of native shrubs on three specially protected cacti species in Durango,México

The spatial association between the specially protected cacti Coryphanta durangensis, Echinocereus longisetus and Peniocereus greggii and potential nurse plants was evaluated, as was their relative position to the sun under the crown of the latter in the southern Chihuahuan Desert. The soil temperature under potential nurse plants was lower than under direct sunlight. There was more nitrogen and organic matter in the soil under Prosopis laevigata trees than in soil under direct sunlight. There were 68 plants of C. durangensis, 59 plants of E. longisetus and 157 of P. greggii. Only one individual of C. durangensis and one of E. longisetus grew outside the shade of the crown of other plants. Echinocereus longisetus was not associated with any particular species, but grew more often than expected by chance in the northern segment of the crown (i.e. the area most shaded in the afternoon). Coryphanta durangensis and P. greggii grew more often under P. laevigata than expected by chance. The effect of other plants, and P. laevigata in particular, on the facilitation of growth of protected cacti species should be considered in management plans of the Chihuahuan Desert, where mesquite (P. laevigata) is often harvested for charcoal production.     

Selection of low investment in sex in a cyclically parthenogenetic rotifer

Cyclical parthenogens, which combine asexual and sexual reproduction, are good models for research into the ecological and population processes affecting the evolutionary maintenance of sex. Sex in cyclically parthenogenetic rotifers is necessary for diapausing egg production, which is essential to survive adverse conditions between planktonic growing seasons. However, within a planktonic season sexual reproduction prevents clonal proliferation. Hence, clones with a low propensity for sex should be selected, becoming dominant in the population as the growing season progresses. In this context, we studied the dynamics of the heritable variation in propensity for sexual reproduction among clones of a Brachionus plicatilis rotifer population in a temporary Mediterranean pond during the period the species occurred in plankton. Clonal isolates displayed high heritable variation in their propensity for sex. Moreover, the frequency of clones with low propensity for sex increased during the growing season, which supports the hypothesized short‐term selection for low investment in sex within a growing season. These results demonstrate (1) the inherent instability of the cyclical parthenogenetic life cycle, (2) the cost of sexual reproduction in cyclical parthenogens where sex produces diapausing eggs and (3) the role of the association between sexual reproduction and diapause in maintaining sex in these cyclical parthenogens.     

Cl‐ homeostasis in includer and excluder citrus rootstocks: transport mechanisms and identification of candidate genes

To reveal specific Cl^‐ transport activities in the symplastic pathway, uptake, long‐distance transport and distribution of Cl^‐ have been investigated in the citrus rootstocks Carrizo citrange (CC, Cl^‐ includer) and Cleopatra mandarin (CM, Cl^‐ excluder). Using an external concentration of 4.5 mm Cl^‐, both species actively transported Cl^‐ to levels that exceeded the critical requirement concentration by one and two orders of magnitude in the excluder and the includer rootstocks, respectively. Both CC and CM modulated Cl^‐ influx according to the availability of the nutrient as uptake capacity was induced by Cl^‐ starvation, but inhibited after Cl^‐ resupply. Net Cl^‐ uptake was higher in the includer CC, an observation that correlated with a lower root‐to‐shoot transport capacity in the excluder CM. The patterns of tissue Cl^‐ accumulation indicated that chloride exclusion in the salt‐tolerant rootstock CM was caused by a reduced net Cl^‐ loading into the root xylem. Genes CcCCC1, CcSLAH1 and CcICln1 putatively involved in the regulation of chloride transport were isolated and their expression analysed in response to both changes in the nutritional status of Cl^‐ and salt stress. The previously uncharacterized ICln gene exhibited a strong repression to Cl^‐ application in the excluder rootstock, suggesting a role in regulating Cl^‐ homeostasis in plants.