Are workers of Atta leafcutter ants capable of reproduction?

Summary. Workers of most eusocial Hymenoptera can produce sons after queen loss, which (posthumously) benefits the queen and increases worker inclusive fitness. However, the evolutionary loss of worker ovaries has occurred in several lineages, while workers in other taxa may be infertile despite having ovaries. Workers of Atta leafcutter ants only lay trophic eggs in queenright colonies. Although Atta colonies are commonly kept at universities, museums, and zoos, no reports of worker sons in orphaned colonies exist, suggesting that Atta workers are infertile. To explicitly test this, we created eleven orphaned laboratory nests of Atta cephalotes, A. sexdens, and A. colombica, and maintained them for 3–6 months after queen loss. Eight colonies did not produce any brood, but three nests produced 1–4 worker-derived male larvae and pupae. Microsatellite genotyping indicated that these were worker sons. However, all males were tiny (3.5–9 mm long) compared to normal queen sons (16 mm long), and would almost certainly be unable to mate. We also found reproductive eggs, but most of these had no yolk and were thus inviable. We conclude that Atta workers are not completely infertile, but that worker fertility is low compared to the sister genus Acromyrmex, where workers routinely produce normally-size males after queen loss in the laboratory. We hypothesize that worker reproduction in orphaned Atta field colonies is almost never successful because the last workers die before their sons can be raised to adulthood, but that the importance of worker-laid trophic eggs for queen feeding has precluded the evolutionary loss of worker ovaries. Received 17 January 2005; revised 12 September 2005; accepted 3 October 2005.     

The persistence of calcareous grassland species in the soil seed bank under developing and established scrub

The relationships between the composition of the soil seed bank, the field layer vegetation, and the scrub canopy were investigated along a 69 m transect, grading from incipient woodland, through scrub, into intensively rabbit-grazed calcareous grassland. The results are used to assess the persistence of species associated with open calcareous grassland in the seed bank under developing scrub. Scrub age, composition and density, changed along the transect from the woodland to open grassland. A total of 35 forb and grass species were found in the field layer. The pattern evident in the scrub layer was also reflected in the herbaceous vegetation. The field layer in the most closed portion of the transect, where the scrub was oldest, was dominated by shade-tolerant species normally associated with woodland habitats. The abundance of these species decreased along the transect as the scrub age declined, and the field layer became increasingly dominated by species typical of open grassland. A total of 47 species germinated from the seed bank. Few species were recorded in the seed bank along the entire length of the transect. Overall, the seed bank was dominated by Hypericum perforatum and Centaurium erythraea, which accounted for 38.2% and 28.6% of emerging seedlings respectively. As with a number of similar studies, the composition of the seed bank had a low correspondence with the composition of the field layer vegetation. The results also emphasise that the composition of the seed bank can be viewed as an ecological palimpsest, with germinable seed of species from each stage of the old-field succession occurring in the soil. The seed bank is an important component in the re-vegetation of an area after disturbance such as scrub removal. This study supports the findings of previous research in showing that relatively few characteristic calcareous grassland species form persistent seed banks. The soil seed bank would therefore appear to be of limited value in the restoration of such grassland following scrub removal.     

一个玉米矮秆突变体K123d的遗传鉴定

矮秆已被广泛用于改良作物的抗倒伏性状,培育理想株型,从而提高作物产量。玉米矮秆突变体K123d由自交系K123自然突变产生。本研究比较该突变体与野生型主要农艺性状差异及其对赤霉素的敏感性;用K123d与株高不同的3个自交系分别构建F1、BC和F2群体,分析矮秆性状的遗传模式;以K169/K123d-F2为定位群体,采用集团分离分析法(BSA),运用SSR标记定位矮秆基因d123;参照br-2序列信息分段设计特异引物,同源克隆d123。结果表明,与野生型相比K123d株高降低35.59%,穗位高降低、节间缩短、叶片较直立,但结实率差,对赤霉素敏感;在F2群体和BC1群体中,正常植株与矮秆植株分离比例分别符合3∶1和1∶1,说明矮秆性状受1对隐性基因控制;其矮秆基因d123定位于第一条染色体上SSR标记umc1278和bnlg1564之间,遗传距离分别为12.8 c M和7.3 c M;同源克隆显示d123与br-2存在12个碱基替换,其中第4个外显子编码的一个谷氨酸被替换为赖氨酸。由此可见,矮秆突变体K123d为br-2的一个突变类型,对矮化育种具有进一步研究利用价值。     

Long-term changes in vegetation and soil chemistry in a calcareous and sandy semi-natural grassland

Calcicole plant species are vulnerable to acidification and fertilization, caused by deposition and changes in land use, since they are adapted to nutrient-poor calcareous conditions. In this study we used vegetation data (vascular plants, bryophytes and lichens) from 1964 and 1985 and stored soil samples from 1966 to investigate long-term soil chemistry and vegetation changes in a semi-natural, sandy calcareous grassland in southern Sweden. In the re-investigation in 2008 we found that increased decalcification due to acidification could not be verified. The plant community had changed from stress-tolerant calcareous grassland towards a community promoted by higher nutrient availability. Furthermore, the cover of species indicating calcareous conditions had decreased. A decline in the cover of species adapted to alkaline, phosphorus-poor conditions may be due to increased nutrient availability, but there were also indications that the vegetation had changed due to overgrowth by woody plants. This long-term impoverishment of the plant community highlights the need for appropriate management of calcareous grasslands, in order to limit the nutrients available in the soil and prevent overgrowth by shrubs and trees.     

Plants with longer-lived seeds have lower local extinction rates in grassland remnants 1950–1985

We investigated whether plant species with longer-lived seeds in the soil had lower rates of local extinction between 1950 and 1985 than species with shorter-lived seeds in 26 intact remnants of extensively used calcareous grasslands. In a previous study we had found higher rates of local extinction for smaller populations, for species with shorter life cycles, and for species with higher habitat specificity. We compiled information on seed longevity from recent literature. Seed longevity had a highly significant effect on the rate of population extinction. Rates of local extinction were lower for species with seed longevity >5 years (33.8%; 31 species) compared with species with shorter-lived seeds (1–5 years: 60.7%, 46 species; <1 year: 58.5%, 81 species). The integration of seed longevity into our analysis did not change our previous conclusions on the dependence of local extinction rates on habitat specificity and on life form. Species in our data set with higher habitat specificity tended to have shorter-lived seeds than species with lower habitat specificity. Thus, most characteristic species of calcareous grassland do not persist in the seed bank and can not rely on this mechanism as a buffer against local extinction. Received: 23 December 1998 / Accepted: 29 May 1999     

Prediction of genetic gain and gene diversity in seed orchard crops under alternative management strategies

Genetic gain and the gene diversity of seed crops from clonal seed orchards were formulated considering genetic selection, fertility variation and pollen contamination, and compared for five different management strategies. Genetic response was studied as a function of orchard management tactics. Management variables included the proportion of clones left after genetic thinning and/or selective seed harvesting. Formulae were derived to calculate gene diversity (expressed as group coancestry or status number) based on the sex ratio in an orchard population. The influence of having different sets of clones serving as seed parents, or pollen parents, or as both, was analysed. In addition, the impact on genetic gain and the gene diversity of seed crops was studied quantitatively as a function of the quantity and quality of gene flow from outside the orchard. The negative impact of fertility variation among orchard genotypes on the gene diversity of the seed crop was quantified. Numerical examples were given to illustrate the impact of orchard management alternatives on genetic gain and gene diversity. The formulae and results of this study can be used for identifying favourable alternatives for the management of seed orchards. Received: 16 December 2000 / Accepted: 13 March 2001     

Differential Indirect Effects of Excluding Livestock and Rabbits from Chalk Heath on the Associated Leafhopper (Hemiptera: Auchenorrhyncha) Fauna

Preliminary results are presented of sampling the leafhopper assemblages on a field experiment designed to examine the differential effects of rabbits and livestock (mainly sheep) on the vegetation of chalk heath in southern England. Experimental plots that excluded livestock either allowed entry by rabbits or excluded them. Results were compared with those from plots grazed by both livestock and rabbits. After 7 years, exclusion of grazing herbivores had resulted in predictable increases in vegetation height, but no major changes were detected in the species composition of the vegetation. As expected, ungrazed plots had higher species richness and greater abundances of several individual leafhopper species. However, plots grazed only by rabbits had a leafhopper assemblage that was distinct from either ungrazed or mixed grazing plots. It is suggested that rabbit grazing may have subtle effects on grassland invertebrate assemblages that are not necessarily predictable from an examination of the species composition of the vegetation. Chalk heath vegetation contains an unusual mixture of calcicole and calcifuge plant species, but the leafhopper assemblage included a restricted number of calcareous grassland specialist species and only one species strongly associated with acidic grasslands; most leafhoppers recorded were generalist grassland species.     

Food plant density,patch isolation and vegetation height determine occurrence in a Swedish metapopulation of the marsh fritillary <Emphasis Type="Italic">Euphydryas aurinia</Emphasis> (Rottemburg, 1775) (Lepidoptera,Nymphalidae)

The occurrence pattern of the marsh fritillary was studied within a patch network on the Baltic island Öland, Sweden. Presence/absence was established for potentially suitable habitat patches (n = 158) on calcareous moist grassland and analyzed in a multiple logistic regression model where patch area, patch isolation and nine habitat quality variables were included as explanatory variables. Larval food plant density was positively, and patch isolation negatively, correlated to the presence of Euphydryas aurinia. Area did not contribute to the explanation of the occurrence pattern. Significant interactions between larval food plant density times patch isolation, and larval food plant density times vegetation height, show that with low food plant density the butterfly primarily occurs in patches with a vegetation height of 4–10 cm, within a distance of 250 m from nearest occupied patch. In patches with a high food plant density the butterfly occurs in patches where the vegetation height is higher, 4–16 cm, and the distance to nearest occupied patch can be longer, up to 1.4 km. This study supports earlier findings in other regions, suggesting that a network of adjacent patches with a high food plant density and a vegetation height within the preferred threshold, despite their size, is an apparent conservation goal.     

Quantifying and testing coexistence mechanisms arising from recruitment fluctuations

Temporal fluctuations in recruitment are involved in two distinct coexistence mechanisms, the storage effect and relative nonlinearity of competition, which may act simultaneously to stabilize species coexistence. It is shown that comparisons of recruitment variation between species at high versus low densities can test whether these mechanisms are responsible for stable coexistence. Moreover, under certain circumstances, these comparisons can measure the total coexistence stabilizing effect of the mechanism. These comparisons are clearest for the situation of an invader (a species perturbed to low density) in the presence of its competitors, termed residents. Then average invader-resident differences in the variances of log recruitment, potentially weighted by adult survival rates and species' sensitivities to competition, are proportional to the overall stabilizing effect of the storage effect and relative nonlinearity of competition. Less effective comparisons are available for species naturally at high and low densities or with substantial mean differences in average fitness. These developments lead also to a technique of partitioning the long-term low-density growth rate of a species into community average measures of stabilizing mechanisms, deviations from these measures, and other factors. The community average measure is argued as most appropriate for understanding the ability of a coexistence mechanism to stabilize coexistence. Individual species' deviations from the community average indicate the ways in a which a coexistence mechanism may affect average fitness differences between species either enhancing or diminishing the ability of a given set of species to coexist, depending on other factors. This approach provides a general new tool for analyzing species coexistence.     

A field study of the effects of elevated CO2 on plant biomass and community structure in a calcareous grassland

The effects of elevated CO₂ on plant biomass and community structure have been studied for four seasons in a calcareous grassland in northwest Switzerland. This highly diverse, semi-natural plant community is dominated by the perennial grass Bromus erectus and is mown twice a year to maintain species composition. Plots of 1.3 m² were exposed to ambient or elevated CO₂ concentrations (n = 8) using a novel CO₂ exposure technique, screen-aided CO₂ control (SACC) starting in March 1994. In the 1st year of treatment, the annual harvested biomass (sum of aboveground biomass from mowings in June and October) was not significantly affected by elevated CO₂. However, biomass increased significantly at elevated CO₂ in the 2nd (+20%, P = 0.05), 3rd (+21%, P = 0.02) and 4th years (+29%, P = 0.02). There were no detectable differences in root biomass in the top 8 cm of soil between CO₂ treatments on eight out of nine sampling dates. There were significant differences in CO₂ responsiveness between functional groups (legumes, non-leguminous forbs, graminoids) in the 2nd (P = 0.07) and 3rd (P < 0.001) years of the study. The order of CO₂ responsiveness among functional groups changed substantially from the 2nd to the 3rd year; for example, non-leguminous forbs had the smallest relative response in the 2nd year and the largest in the 3rd year. By the 3rd year of CO₂ exposure, large species-specific differences in CO₂ response had developed. For five important species or genera the order of responsiveness was Lotus corniculatus (+271%), Carex flacca (+249%), Bromus erectus (+33%), Sanguisorba minor (no significant CO₂ effect), and six Trifolium species (a negative response that was not significant). The positive CO₂ responses in Bromus and Carex were most closely related to increases in tiller number. Species richness was not affected by CO₂ treatment, but species evenness increased under elevated CO₂ (modified Hill ratio; P = 0.03) in June of the 3rd year, resulting in a marginally significant increase in species diversity (Simpson's index; P = 0.09). This and other experiments with calcareous grassland plants show that elevated atmospheric CO₂ concentrations can substantially alter the structure of calcareous grassland communities and may increase plant community biomass. Received: 12 July 1997 / Accepted: 14 September 1998     

Genetic diversity and structure of the noxious alien grass Praxelis clematidea in southern China

Praxelis clematidea (Asteraceae), a plant species native to South America, is a noxious weed in southern China. We examined the genetic variation and population structure of 12 populations (76 individuals) of P. clematidea from Fujian, Guangdong, and Hainan Provinces in China using inter-simple sequence repeat (ISSR) analysis. From an initial set of 69 ISSR primers, 10 were selected which yielded 80 reproducible bands. Polymorphic bands (P) were 100%, Shannon's information index (I) was 0.4226, and Nei's gene diversity (H) was 0.2791. We infer that the high levels of genetic diversity exhibited by P. clematidea may have contributed to its invasiveness. Gene flow among populations was 2.4930, which has led to homogenization. The coefficient of population differentiation (Gst = 0.1671) indicated low levels of genetic variation among populations and high levels of genetic polymorphism within populations. There was a negative correlation between population elevation and genetic diversity, while there was a significant positive correlation between genetic distance and geographic distance based on a Mantel test (r = 0.5820, P < 0.01). Some populations from different provinces clustered together in principal coordinate and UPGMA analyses indicating that human-mediated events may have contributed to the dispersal of the species.     

Effectiveness of selection methods in chickpea Cicer arietinum under different environments

Summary Three selection procedures, single-seed-descent, selection of the best 5% in both seed size and seed yield per plant were followed in two divergent crosses of chickpea under four environments in order to study the improvement of seed yield. Analysis of variance showed wide ranges of variability for all characters except days to maturity. Seed size (SSB) was found to be superior to single seed (SSD), followed by seed yield (YB), for developing high yielding lines. Fertility and spacing had no influence on the efficiency of any selection procedure indicating that selection under any environment is equally good. A part of the Ph.D. Thesis submitted to J. N. Krishi Vishwa Vidyalaya, Jabalpur-482 004, Madhya Pradesh, India     

The roles of seed mass and persistent seed banks in gap colonisation in grassland

Gaps in grassland created by animals are often sites for species regeneration. The persistent seed banks of ant-hills and surrounding soils in a calcareous grassland in southern Britain were compared and analysed in terms of seed mass and longevity. The relative abundance of species on ant-hills compared to the pasture was highly correlated with seed abundance in ant-hill soil. The abundance of plant species on ant-hills compared to the pasture was significantly negatively correlated with seed mass, and positively correlated with seed longevity. These results suggest that germination from the seed bank in ant-hill soils, as well as limited seed dispersal and vegetative invasion, contributes to the distinctive ant-hill vegetation. The same may apply to other gaps created by animals in grasslands.     

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ABSTRACT

Australian fur seals Arctocephalus pusillus doriferus are colonial breeding animals forming dense social groups during the breeding season. During this time, males establish and defend territories through physical conflicts, stereotyped posturing and vocalisations. While vocalisations are suggested to play an important role in male recognition systems, it has received little attention. Recordings of nine adult male Australian fur seals were made during the 2000 and 2001 breeding seasons at Kanowna Island (39° 10′S, 146° 18′ E), Bass Strait, Australia. The in-air bark vocalisations of territory-holding males were used to characterise the Bark Call and to determine whether males produce individually distinct calls, which could be used as a basis for vocal recognition. Seventeen frequency and temporal variables were measured from a total of 162 barks from nine individual males. The Bark Series was more reliably classified (83%) to the correct caller compared to the Bark Unit. This was assigned with less certainty (68%), although the classification was still relatively high. Findings from this study indicate that there is sufficient stereotypy within individual calls, and sufficient variation between them, to enable vocal recognition in male Australian fur seals.     

棉花矮化突变体的遗传分析

陆地棉科遗２号×中棉完紫的种间杂交衍生后代群体中分离出一株矮秆小叶突变体,经多年选择育成了矮早棉１号。在北京气候条件下,矮早棉１号成熟时,株高只有４５ｃｍ,不到正常陆地棉的１／２。遗传分析揭示矮早棉１号的矮化早熟特性系由两对隐性基因控制,其基因符号定名为ｄ_１和ｄ_２,矮早棉１号为双隐性纯合子,基因型为ｄ_１ｄ_１ｄ_２ｄ_２。正常陆地棉ＴＭ－１、中棉所１２及中棉所１６均为显性纯合子,基因型为Ｄ_１Ｄ_１Ｄ_２Ｄ_２。控制棉花株高的两对基因Ｄ_１／ｄ_１和Ｄ_２／ｄ_２间表现重叠作用。矮早棉１号在棉花早熟育种中有重要价值。