Specific cellular localization of tyrosinase mRNA during Ciona intestinalis larval development

A Ciona intestinalis cDNA clone that encodes a protein highly homologous to other tyrosinases was isolated. Northern blot analysis showed that expression of Ciona tyrosinase starts at the early neurula stage and continues throughout the tail-bud and tadpole larval stages. The earliest tyrosinase expression was detected, by in situ hybridization, at the neural plate stage, in pigment precursor cells located along the two neural folds, in the animal region of the embryo. In the course of embryonic development the strong hybridization signal was always localized, within the rostral part of the developing brain, in the pigment precursor cells and was later detected in the otolith and ocellus. These results are discussed in relation to tyrosinase as an early marker of neural induction.     

The relation between dispersal and survival of Lobesia botrana larvae and their density in vine inflorescences

Dispersal and survival of Lobesia botrana Den. & Schiff. (Lepidoptera: Tortricidae) larvae in a simulated first generation and the relationship with their density on vine inflorescences were studied under field conditions. Artificial infestations with neonate larvae were conducted at densities of 5, 10, 15, 20 and 25 individuals per vine inflorescence. Larvae had a considerable dispersal capacity on the vine espalier and were able to reach several inflorescences around those artificially infested. Dispersal downwards (63.1%) was significantly more frequent than upwards (36.9%), probably because larvae move down the vine plant using silk threads. However, the fact that there was upwards dispersal provides evidence that larvae are capable of active locomotion upwards on the vine plant structure. Mean distances covered by larvae ranged between 10 and 30 cm, with a maximum as far as 45 cm. The longest displacements were not associated with the downward dispersal. The maximum distance covered by larvae was positively correlated with larval density. Mean distance and larval density were not correlated, but mean distances covered at high larval densities were significantly higher than at low densities. At higher larval densities, the proportion of larvae which established in the artificially infested inflorescences decreased whereas the proportion of dispersing individuals increased. However, as a result of the balance between establishment and dispersal, larval survival did not differ significantly among larval densities (26–44%). The results obtained suggest that larval dispersal must be taken into account when preimaginal stages are sampled to determine whether damage thresholds are reached in an integrated pest management program.     

THE PONTIA DAPLIDICE-ED USA HYBRID ZONE IN NORTHWESTERN ITALY

The pierid butterflies Pontia daplidice and P. edusa, parapatrically distributed in southern Europe, have very similar morphologies and life histories, but show fixed differences at four allozyme markers. We sampled these allozymes in a 28-population transect north of Genoa in Italy, through the hybrid zone where these taxa meet. We used the numerical techniques developed for hybrid zone analysis to study the patterns of genetic differentiation and their underlying evolutionary causes. The hybrid zone is characterized by a very short and steep central region, flanked by broad tails of introgression extended up to 100 km in either direction. From mean two-locus disequilibium of D = 0.148 (maximum-likelihood two-unit support limits 0.139-0.153), and after accounting for minor differences in the center locations of the single-locus clines, which act to bias the dispersal estimate, we estimated a dispersal rate of σ = 4.4 (3.7-5.5) km/gen^1/2. The effective selection needed to maintain the steep central portion is strong, 0.47 < s? < 0.64, when combined over potential intrinsic (genetic background) and extrinsic (ecological) sources of selection. The clines in allozyme loci showed variation that was significantly different between the most divergent shapes, and the differences are attributable to different degrees of introgression on the edusa side of the zone. The average selection acting on individual allozyme loci was high at s_???e  1.5%, but because of the narrowness of the central region of the cline, we suspect that this estimate is somewhat biased by selection on loci closely linked to the allozyme markers. A common question for taxa that show fixed allozyme differences in parapatry is whether or not they are genetically isolated. A fairly general measure of genetic isolation across hybrid zones is the time, T, that it takes a neutral allele to cross the hybrid zone and recombine into the opposite genetic background, given by T = (β/σ)², where β is the barrier strength of the hybrid zone. Genetic isolation in the Pontia zone is weak, with T  25 generations for most allozyme markers. By this measure, populations of daplidice and edusa on opposite sides of the hybrid zone share more identical-by-descent alleles than do populations of phenotypically pure daplidice in, say, France and Morocco. Accordingly, we think it best for systematists to consider edusa as a well-marked subspecies of P. daplidice.     

PHYLOGEOGRAPHY OF THE SARDINES (SARDINOPS SPP.): ASSESSING BIOGEOGRAPHIC MODELS AND POPULATION HISTORIES IN TEMPERATE UPWELLING ZONES

Sardines (Sardinops spp.) occupy temperate upwelling zones in the coastal regions of the Indian and Pacific Oceans, including locations in Japan, California, Chile, Australia, and South Africa. East and West Pacific populations are separated by vast expanses of open ocean, and northern and southern hemisphere populations are separated by tropical waters which are lethal to sardines. The relative importance of these barriers has been the focus of a longstanding debate between vicariance and dispersal schools in biogeography. Comparisons of a 500 bp fragment of the mitochondrial (mt) DNA control region reveal strong geographic structuring of mtDNA lineages but shallow divergence both within and between regional populations. Regional populations are related to one another in a stepping-stone pattern, the apparent result of a series of Pleistocene dispersal events around the continental margins of the Indian-Pacific Basin. These mtDNA data, combined with an electrophoretic survey of variability at 34 nuclear loci (Grant and Leslie 1996), indicate that the five regional forms of Sardinops (considered separate taxa by most authorities) probably diverged within 500,000 years BP, a much shorter timeframe than predicted by vicariance models based on plate tectonics. High mtDNA haplotype diversity, coupled with an excess of rare alleles in the protein electrophoretic dataset, may indicate exponential growth from a small ancestral population. The mtDNA and allozyme data are concordant with climate records and fossil evidence in portraying regional populations as recent, unstable, and ephemeral. Regional populations of sardines have probably been extinguished and recolonized over short evolutionary timescales in response to changes in climate and the oceanography of coastal upwelling zones.     

Patterns of wing size variation in seeds of the lily Cardiocrinum cordatum (Liliaceae)

We examined the patterns of variation in wing-loading and its related characteristics in Cardiocrinum cordatum to clarify the factors that determine the variation in seed dispersal ability in this species. The square root of wing-loading of a seed of a plant was not significantly correlated with basal stem diameter of a plant, indicating that large plants did not necessarily produce seeds with high dispersal ability. This result was inconsistent with the hypothesis that large plants produce seeds with high dispersal ability to avoid high mortality of seeds and seedlings in the vicinity of the parents. On the other hand, the square root of wing-loading of a seed of a fruit was negatively dependent on seed number of a fruit. Thus, many-seeded fruits produced seeds with high dispersal ability. This was because the projected surface area per seed was large in large fruits and large fruits contained large numbers of seeds. The cost per seed of producing fruit structures was small for many-seeded fruits. Thus, high dispersal ability of seeds in many-seeded fruits may be a result of an effective resource allocation pattern in which a high proportion of resources are allocated to those many-seeded fruits, enabling seeds to develop large wings and thus reducing the structural cost of fruits per seed.     

Do ectomycorrhizal fungi exhibit adaptive tolerance to potentially toxic metals in the environment ?

The effects of potentially toxic metals on ectomycorrhizal (ECM) fungi and their higher plant hosts are examined in this review. Investigations at a species and community level have revealed wide inter- and intraspecific variation in sensitivity to metals. Adaptive and constitutive mechanisms of ECM tolerance are proposed and discussed in relation to proven tolerance mechanisms in bacteria, yeasts and plants. Problems with methodology and research priorities are highlighted. These include the need for a detailed understanding of the genetic basis of tolerance in the ECM symbiosis, and for studies of ECM community dynamics in polluted sites.     

Local colonization and extinction of field layer plants in a deciduous forest and their dependence upon life history features

Abstract. In this study we used species inventory data collected in 1970 and 1993 from 132 plots in a 14.5ha deciduous forest to examine local extinction and colonization processes among 45 field layer species. Local colonization rate was positively related to both seed size and seed dispersal features. Local extinction rate was negatively related to seed size. Growth form (clonal vs. non-clonal) and presence of a seed bank were not found to be associated with local dynamics. Despite an overall constancy in species composition during this period, plants exhibited a considerable mobility among the 132 plots. This pattern conformed to a suggested ‘carousel model’ of species mobility in grasslands. A tentative suggestion is that this mobility acts on a comparatively broader spatio-temporal scale in deciduous forests as compared with grasslands. Additional data are presented indicating that species abundance (frequency) among field layer plants in deciduous forest communities is consistent among forest fragments, and when comparing local and regional scales. The main conclusion is that life history features of the seed dispersal/recruitment phase, particularly seed size, are causally associated with abundance patterns at least at a local scale, but possibly also on a broader regional scale.     

Continuing dispersal of freshwater triclads (Platyhelminthes; Turbellaria) in Britain with particular reference to lakes

1. A survey of lakes in Anglesey and Snowdonia, Wales, in the summer of 1997 recorded changes in the triclad fauna which had occurred since earlier studies in the 1950s and in 1973. Two native species, Polycelis tenuis and Dugesia polychroa, and two immigrants, Planaria torva and especially Dugesia tigrina, have increased their range. The triclad fauna has remained unchanged with time in eleven out of sixteen lakes in Snowdonia, in contrast to only one out of fourteen lakes on Anglesey. This supports the hypothesis produced in the 1950s, that the triclad fauna of the ion-poor, unproductive lakes in Snowdonia would change little with time, being maintained by ecological interactions, whereas the ion-rich, productive lakes on Anglesey, from which native Dugesia spp. and Dendrocoelum lacteum are historically absent, would increase in triclad diversity. 2. An examination of national records for the distribution of D. tigrina and Pl. torva indicated their continuing dispersal in both still and running waters in mainland Britain, with the former species having the greater frequency of occurrence. 3. The reasons for this may be that D. tigrina is (a) more easily dispersed, (b) a more opportunistic, catholic feeder, (c) a more vigorous competitor, (d) able to reproduce asexually and (e) able to prey to some extent on other triclad species, as shown in the laboratory but not yet confirmed in the field. The absence of D. tigrina and Pl. torva from unproductive lakes may be explained in terms of low temperature and feeding mode, and a low standing crop of gastropods, respectively. 4. Seven surveys (1961–1997) of the triclad fauna (six species) of Colemere, England, revealed that D. tigrina was confined to one small area of the littoral zone until the mid-1980s, after which it spread fairly rapidly to occupy the entire shore by 1997. Its expansion has been to the detriment of all the native species, particularly P. tenuis, except Dd. lacteum. The possible extinction of Pl. torva may be linked to a numerical reduction in snails, particularly Potamopyrgus jenkinsi. 5. Dugesia tigrina has an adverse effect on the native triclad fauna in lakes, particularly when only a few species are present. This has been explained in terms of the availability of a wider variety of food. The presence of a larger number of triclad species denies the immigrant food items, particularly crustaceans and snails, the food refuges of Dd. lacteum and Dugesia spp., respectively. However, the long-term study of Colemere has shown that more diverse triclad faunas may also be vulnerable to this immigrant. The effect of Pl. torva on native triclads remains uncertain.