ALBERT

Notes: Short-period (Sim/Gen)N superlattices (SSLs) are grown step by step on a Si(001) substrates by solid source molecular beam epitaxy. Using the step-graded SSLs as buffer layers, 2000 Å uniform Si0.75Ge0.25 alloy layers are grown on the same substrates. The growth temperature of the SSLs and uniform layers is 500 °C. In the SSLs layers, m and n are the number of monolayers of Si and Ge, respectively. N is the period of (Sim/Gen) bilayers. The samples grown are characterized by x-ray diffraction, atomic force microscopy (AFM), and transmission electron microscopy (TEM) as a function of the step number of SSL layers. The SSLs show very smooth surfaces [the root-mean-square (rms) surface roughness is between 7 and 12 Å]. A dramatic decrease in roughness is observed in the uniform Si0.75Ge0.25 alloy layers, when even a one-step SSL is used as a buffer layer. A noticeable increase in rms roughness is seen in both SSL and alloy layers when the number of Ge monolayers is changed from one to two. AFM observation shows that the rms surface roughness behavior of the SSLs is reflected to their corresponding top alloy layers. The residual strains in alloy layers are considerably lower, with a maximum relaxation rate of about 80% for the sample with a seven-step SSL buffer. Cross-sectional TEM images show that strained SSL buffer layers effectively deflect threading dislocations in the substrate or confine the dislocations in the SSL buffer layers. © 2001 American Institute of Physics.

Notes: We have studied the magnetoresistance in an inclined magnetic field of a two-dimensional electron gas modulated by a two-dimensional array of thin cobalt nanostructures. At low fields, hysteretic behavior due to the magnetization of the elements was observed, and at higher fields magnetic commensurability (Weiss) oscillations were clearly seen. These magnetoresistance features only occurred when the in-plane component of the field was in the current flow direction. The commensurability oscillations behave exactly as predicted in a recent theory of two-dimensional surface superlattices. © 2001 American Institute of Physics.

Notes: Measurements of photoluminescence intensity from GaAs and InGaAs quantum well heterostructures have been used as a noninvasive probe of the damage distribution due to very low-power dry etching. Samples were etched using SiCl4 reactive-ion etching. Comparative studies were made on samples bombarded by the separate constituent ions of a SiCl4 discharge using a mass-resolving ion implanter fitted with a deceleration lens. We also examined the influence of more complex multicomponent discharges. We found that molecular ions contribute less to deep damage than do atomic ions. The reason is that molecular ions fragment upon impact and the secondary atomic ions do not channel very far. Studies of laser illumination on the sample during etching show that a form of radiation-enhanced diffusion can modify the damage distribution. The net picture emerges of a complex process underlying dry-etch damage penetration at very low energies. © 2001 American Institute of Physics.

Notes: Testicular and ovarian fragments of the protogynous Pacific wrasse Haliochoeres trimaculatus were incubated in vitro with [3H]pregnenolone ([3H]P5), [3H]17-hydroxyprogesterone ([3H]17OHP4), non-radioactive (nr) 17β-oestradiol (nrE2) or nrP5 to identify the major gonadal steroidogenic pathways and steroid products in females and in the two male variants of this species, the terminal phase (TP) and initial phase (IP) males. Both testis and ovarian tissues exhibited 7 hydroxylase activity resulting in the formation of 7α-hydroxypregnenolone (7OHP5) from [3H]P5, and many HPLC peaks were identified as products of testicular (c. 29) and ovarian (c. 23) steroidogenesis, and only c. 50% of these metabolites co-eluted with authentic reference standards; only very small amounts of conjugated steroid were synthesized from any of the precursors. [3H]P5 was converted by testis mainly to 7αOHP5, and two unknown steroids, whereas [3H]17OHP4 metabolism gave rise to [3H]17,20β-dihydroxy-4-pregnen-3-one (DHP), 11-ketotestosterone (11KT), and two unknown steroids. For ovarian tissues, [3H]17OHP4 and [3H]P5 were metabolized to form E2, oestrone (E1), androstenedione (A4), 20α- and 20β-dihydroprogesterone (20αDHP and 20βDHP), 7αOHP5 (from [3H]P5) and a major unknown. The HPLC steroid profiles for testis incubations for IP and TP males were similar, however, the steroidogenic response of the testis of TP males to human chorionic gonadotrophin, in vitro (determined by hormone assay), was significantly higher than that of IP males.

Notes: In a long-term growth trial, transgenic tilapia Oreochromis niloticus L. showed a 2·5-fold increase in growth compared with non-transgenic siblings. At 7 months, mean mass of transgenic tilapia was 653 g compared with 260 g for non-transgenic siblings. A significant increase (P 〉0·01) in head: total length ratio, viscera-somatic index and hepato-somatic index was observed in transgenic fish. Female gonado-somatic index (I G) was found to be lower in transgenics than non-transgenic siblings in both mixed and separate culture conditions. Transgenic male I G values were found to be higher in mixed culture and lower in separate culture than that of their non-transgenic siblings. Food conversion efficiency was more than 20% greater in the transgenic fish. In a second shorter-term growth performance trial, the transgenic fish grew to about four times the size of their non-transgenic siblings. A digestibility trial suggested that transgenic tilapia were more efficient utilizers of protein, dry matter and energy. Apparent digestibility of protein and apparent energy digestibility were higher in the transgenic fish.

Notes: Intact ovarian follicles, obtained from untreated and human chorionic gonadotropin (HCG) treated Japanese yellowtail Seriola quinqueradiata during different maturational stages, were incubated with radioactive [3H]pregnenolone, [3H]17-hydroxyprogesterone or [14C] androstenedione and steroid metabolites identified by thin layer chromatography (TLC) followed by recrystallization to constant specific activity. In untreated late vitellogenic (0 h) follicles, androstenedione was the major product with smaller amounts of testosterone and oestradiol-17α. In post-vitellogenic (12 h post-injection) intact follicles, androstenedione predominated, and although testosterone and oestradiol-17α were not produced, there were small amounts of 17, 20β-dihydroxy-4-pregnen-3-one (17,20β-P) and 17,21-dihydroxy-4-pregnene-3, 20-dione (11-deoxycortisol). In HCG-treated fish, a steroidogenic shift resulted in the disappearance of testosterone and oestradiol-17 coinciding with the appearance of 17, 20β-P. During early and late final oocyte maturation FOM (24 and 36 h post-injection), there was a five- to seven-fold increase in the production of 17, 20β-P, whereas production of 11-deoxycortisol remained almost the same. During FOM, in addition to 17,20β-P, its 5β-reduced metabolite, 17,20β-dihydroxy-5β-pregnan-3-one (5β-17,20β-P) was synthesized, suggesting a decrease in maturation-inducing 17,20β-P activity. 17, 20β,21-Trihydroxy-4-pregnen-3-one (20β-S) was not synthesized by ovarian fragments in Japanese yellowtail at any maturational stage. The metabolites identified on TLC during FOM were tested to evaluate their maturation-inducing activity in an in vitro bioassay. Of the steroids tested, 17,20β-P was the most effective inducer of germinal vesicle breakdown (GVBD), followed by 5β-17,20β-P. Timely synthesis of 17,20β-P immediately prior to and during FOM as well as its great potency in inducing GVBD in vitro supports the evidence for a physiological role of 17,20β-P as a maturation-inducing hormone in Japanese yellowtail.

Notes: A polyculture experiment with the large carp rohu, catla and either mrigal or common carp (as cash crop fish), and the small indigenous fish punti (as food for the farmer's family) was carried out at Bangladesh Agricultural University, Mymensingh. The main objectives were to compare polycultures of large carp in which the bottom feeder is either the native mrigal or the exotic common carp, and to assess the effects of adding the small indigenous species punti to those polycultures. The results of fish–fish interactions and overall fish production have already been reported. The present paper presents the effects on the water quality, and discusses fish–environment interactions. The main conclusions are: time changes in the pond environment were stronger than fish composition effects. The main practice affecting water quality was liming, that incresed alkalinity, pH and water transparency and decreased ammonia. Rain affected photosynthesis and the match-mismatch of the two steps of nitrification. The more that bottom feeding fish species disrupt the mud bottom, the stronger their effects on pond environment. Common carp produce the strongest disruption of the mud bottom, followed by punti and then by mrigal. Mud disruption produced by common carp leads to a stronger liming effect, nutrient release into the water, and provides more particles that rain-floods wash out, facilitating the mismatch of the two steps of nitrification, and increased phosphorus adsorption into the mud bottom. Mud disruption by punti is only enough to improve the liming effect. Mud disruption by mrigal is the least, hence less particles are resuspended, nitrification is not affected during floods and relatively more phosphate remains in the water available for photosynthesis. The bottom feeder common carp can be seen not only as a target-cultured fish but also as a management tool. Farmers can get double benefit in introducing common carp in the ponds as it enhances the effectiveness of lime application and increases the availability of nutrients to phytoplankton. Through the manipulation of species in the polyculture alone, farmers can maintain the environment better and also reduce input costs.

Notes: The inheritance of petal (flower) colour and seed colour in Brassica rapa was investigated using two creamy-white flowered, yellow-seeded yellow sarson (an ecotype from Indian subcontinent) lines, two yellow-flowered, partially yellow-seeded Canadian cultivars and one yellow-flowered, brown-seeded rapid cycling accession, and their F1, F2, F3 and backcross populations. A joint segregation of these two characters was examined in the F2 population. Petal colour was found to be under monogenic control, where the yellow petal colour gene is dominant over the creamy-white petal colour gene. The seed colour was found to be under digenic control and the yellow seed colour (due to a transparent coat) genes of yellow sarson are recessive to the brown/partially yellow seed colour genes of the Canadian B. rapa cvs.‘Candle’ and ‘Tobin’. The genes governing the petal colour and seed colour are inherited independently. A distorted segregation for petal colour was found in the backcross populations of yellow sarson × F1 crosses, but not in the reciprocal backcrosses, i.e. F1× yellow sarson. The possible reason is discussed in the light of genetic diversity of the parental genotypes.

Notes: Yellow-seeded Brassica napus was developed from interspecific crosses between yellow-seeded Brassica rapa var.‘yellow sarson’ (AA), black-seeded Brassica alboglabra (CC), yellow-seeded Brassica carinata (Bbcc) and black-seeded B. napus (AACC). Three different interspecific crossing approaches were undertaken. Approaches 1 and 2 were designed directly to develop yellow-seeded B. napus while approach 3 was designed to produce a yellow-seeded CC genome species. Approaches 1 and 2 differed in the steps taken after trigenomic interspecific hybrids (ABC) were generated from B. carinata×B. rapa crosses. The aim of approach 1 was to transfer the yellow seed colour genes from the A to the C genome as an intermediate step in developing yellow-seeded B. napus. For this purpose, the ABC hybrids were crossed with black-seeded B. napus and the three-way interspecific hybrids were self-pollinated for a number of generations. The F7 generation resulted in the yellowish-brown-seeded B. napus line, No. 06. Crossing this line with the B. napus line No. 01, resynthesized from a black-seeded B. alboglabra x B. rapa var.‘yellow sarson’ cross (containing the yellow seed colour genes in its AA genome), yielded yellow-seeded B. napus. This result indicated that the yellow seed colour genes were transferred from the A to the C genome in the yellowish-brown seed colour line No. 06. In approach 2, trigenomic diploids (AABBCC) were generated from the above-mentioned trigenomic haploids (ABC). The seed colour of the trigenomic diploid was brown, in contrast to the yellow seed colour of the parental species. Trigenomic diploids were crossed with the resynthesized B. napus line No. 01 to eliminate the B genome chromosomes, and to develop yellow-seeded B. napus with the AA genome of ‘yellow sarson’ and the CC genome of B. carinata with yellow seed colour genes. This interspecific cross failed to generate any yellow-seeded B. napus. Approach 3 was to develop yellow-seeded CC genome species from B. alboglabra×B. carinata crosses. It was possible to obtain a yellowish-brown seeded B. alboglabra, but crossing this B. alboglabra with B. rapa var.‘yellow sarson’ failed to produce yellow seed in the resynthesized B. napus. The results of approaches 2 and 3 demonstrated that yellow-seeded B. napus cannot be developed by combining the yellow seed colour genes of the CC genome of yellow-seeded B. carinata and the AA genome of ‘yellow sarson’.