The effect of fire and ants on the seed-bank of a shrub in a semi-arid grassland

Abstract Invasion of grasslands by woody plants following the introduction of domestic stock is a worldwide phenomenon. Burning is frequently recommended as a remedial measure but for a pastoral enterprise it is costly and the frequency of the fires required is of critical economic importance. The size and longevity of the soil seed-bank is an essential part of the response of shrub populations to prescribed fire regimes. In this study the seed-bank of the semi-arid zone shrub Dodonaea attenuata in Eragrostis eriopoda tussock grassland was examined in relation to harvesting by ants and the burning history of the sites. On unburnt sites, more than 3500 seeds per m2 entered the seed-bank in the summer 1984–85 but sites burnt 5 years previously produced less than one-third of that number. Burnt shrubs did not flower for 5 years and no seeds survived in the soil through to 1985. Burning immediately prior to seed ripening destroyed the seed crop but burning after seed-fall stimulated a greater germination in the following spring than on unburnt treatments. Ants rapidly harvested most of the seeds produced and after 20 months the combined effects of ant harvesting and germination had reduced the seed-bank at unburnt sites to 8–21 seeds per m2 and at sites burnt 7 years previously to less than two seeds per m2. Seeds were initially harvested in summer by Pheidole spp. of ants for their elaiosome and then discarded in middens outside the entrance to the ant nest. During the subsequent cool season the seeds were taken back into the nest and stored at depths ranging from 2 to 30cm. It was concluded that ants provided short-range dispersal (< 10 m) and promoted the contagious distribution of D. attenuata, which is advantageous for a fire-susceptible, arid-zone shrub invading a grassland liable to be burnt: seedlings derived from seeds in ant storage chambers near the soil surface and in sparse grass situations caused by competition from shrubs may obtain some survival advantage. Prescribed fire has potential as a management tool for controlling population density of D. attenuata because, depending upon season and frequency, it reduces seed rain by killing shrubs, suppresses flowering activity and destroys seed crops on the plant. Under the influence of a regime of regular burning, such as prevailed prior to the European pastoral industry, the limited soil seed-bank would have been a major constraint on D. attenuata populations.     

中国顶丝藻科新记录

继续报导中国顶丝藻科新记录６种。渐尖旋体藻Ａｕｄｏｕｉｎｅｌａａｔｅｎｕａｔａ,球状旋体藻Ａ．ｇｌｏｂｏｓａ,纤细旋体藻Ａ．ｇｒａｃｉｌｉｓ,无柄旋体藻Ａ．ｈｙｐｎｅａｅ,偏枝旋体藻Ａ．ｓｅｃｕｎｄａｔａ,连续旋体藻Ａ．ｓｅｒｉａｔａ。     

Finding needles in a genomic haystack: targeted capture identifies clear signatures of selection in a nonmodel plant species

Teasing apart neutral and adaptive genomic processes and identifying loci that are targets of selection can be difficult, particularly for nonmodel species that lack a reference genome. However, identifying such loci and the factors driving selection have the potential to greatly assist conservation and restoration practices, especially for the management of species in the face of contemporary and future climate change. Here, we focus on assessing adaptive genomic variation within a nonmodel plant species, the narrow‐leaf hopbush (Dodonaea viscosa ssp. angustissima), commonly used for restoration in Australia. We used a hybrid‐capture target enrichment approach to selectively sequence 970 genes across 17 populations along a latitudinal gradient from 30°S to 36°S. We analysed 8462 single‐nucleotide polymorphisms (SNPs) for F_ST outliers as well as associations with environmental variables. Using three different methods, we found 55 SNPs with significant correlations to temperature and water availability, and 38 SNPs to elevation. Genes containing SNPs identified as under environmental selection were diverse, including aquaporin and abscisic acid genes, as well as genes with ontologies relating to responses to environmental stressors such as water deprivation and salt stress. Redundancy analysis demonstrated that only a small proportion of the total genetic variance was explained by environmental variables. We demonstrate that selection has led to clines in allele frequencies in a number of functional genes, including those linked to leaf shape and stomatal variation, which have been previously observed to vary along the sampled environmental cline. Using our approach, gene regions subject to environmental selection can be readily identified for nonmodel organisms.     

DEVELOPMENTAL PATTERN OF THE BONY FALX AND BONY TENTORIUM OF SPOTTED DOLPHINS (STENELLA ATTENUATA) AND THE RELATIONSHIP BETWEEN DEGREE OF DEVELOPMENT AND AGE

The bony falx (BF) and bony tentorium (BT) of spotted dolphins ( Stenella attenuata ) at different growth stages were observed. The BF and BT were observed to have the shape of the entire falx cerebri (FC) and tentorium cerebelli (TC), respectively, in adults, but were not seen in fetuses at all. Partially formed BF and BT were observed in the young. The BF and BT of spotted dolphins are formed by ossification in the FC and TC in the course of aging. Based on age determination by means of dentinal growth layers of the maxillary teeth, the ossification in the FC and TC of spotted dolphins begins by about 1 yr of age, and is completed, except for the passage of nerves and blood vessels, by about 9 yr of age. The BF and BT of spotted dolphins are formed possibly by continual dietary intake of large amounts of calcium, phosphorus and vitamin D.     

The post-pollination ethylene burst and the continuation of floral advertisement are harbingers of non-random mate selection in Nicotiana attenuata

The self-compatible plant Nicotiana attenuata grows in genetically diverse populations after fires, and produces flowers that remain open for 3 days and are visited by assorted pollinators. To determine whether and when post-pollination non-random mate selection occurs among self and non-self pollen, seed paternity and semi-in vivo pollen tube growth were determined in controlled single/mixed pollinations. Despite all pollen sources being equally proficient in siring seeds in single-genotype pollinations, self pollen was consistently selected in mixed pollinations, irrespective of maternal genotype. However, clear patterns of mate discrimination occurred amongst non-self pollen when mixed pollinations were performed soon after corollas open, including selection against hygromycin B resistance (transformation selectable marker) in wild-type styles and for it in transformed styles. However, mate choice among pollen genotypes was completely shut down in plants transformed to be unable to produce (irACO) or perceive (ETR1) ethylene. The post-pollination ethylene burst, which originates primarily from the stigma and upper style, was strongly correlated with mate selection in single and mixed hand-pollinations using eight pollen donors in two maternal ecotypes. The post-pollination ethylene burst was also negatively correlated with the continuation of emission of benzylacetone, the most abundant pollinator-attracting corolla-derived floral volatile. We conclude that ethylene signaling plays a pivotal role in mate choice, and the post-pollination ethylene burst and the termination of benzylacetone release are accurate predictors, both qualitatively and quantitatively, of pre-zygotic mate selection and seed paternity.     

干旱胁迫对坡柳等抗旱树种幼苗膜脂过氧化及保护酶活性的影响

以金沙江干热河谷主要树种坡柳、银合欢、山毛豆实生幼苗为材料,通过盆栽苗自然干旱胁迫,同时以浇水处理为对照,研究了干旱胁迫对坡柳、银合欢、山毛豆3个树种丙二醛含量、膜相对透性及保护酶活性的影响。结果表明,干旱胁迫下3个树种幼苗的细胞膜透性、MDA及SOD, POD酶活性都发生了变化,只是变化的幅度和进程不同。干旱胁迫对银合欢膜系统损伤生成的主要降解产物不是MDA;山毛豆清除活性氧毒害作用主要不是通过SOD和POD的作用;通过叶片相对保水力测定及膜透性、MDA相对含量、酶活性变化情况的分析,3个树种中坡柳耐旱性最强,其次为银合欢,山毛豆居后。     

PHYSIOLOGICAL AND BEHAVIORAL DEVELOPMENT IN DELPHINID CALVES: IMPLICATIONS FOR CALF SEPARATION AND MORTALITY DUE TO TUNA PURSE-SEINE SETS

Tuna purse-seiners in the eastern tropical Pacific (ETP) capture yellowfin tuna by chasing and encircling herds of associated dolphins. This fishery has caused mortality in 14 dolphin species (20 stocks) and has led to significant depletions of at least three stocks. Although observed dolphin mortality is currently low, set frequency remains high and dolphin stocks are not recovering at expected rates. Mortality of nursing calves permanently separated from their mothers during fishery operations may be an important factor in the lack of population recovery, based on the recent discovery that calves do not accompany 75%–95% of lactating females killed in the purse-seine nets. We assessed age-specific potential for mother–calf separations and subsequent mortality of calves by reviewing and synthesizing published data on physiological and behavioral development in delphinids from birth through 3 yr postpartum. Results indicate that evasive behavior of mothers, coupled with the developmental state of calves, provides a plausible mechanism for set-related mother–calf separations and subsequent mortality of calves. Potential for set-related separation and subsequent mortality is highest for 0–12-mo-old dolphins and becomes progressively lower with age as immature dolphins approach adult stamina and attain independence.     

Stomatal conductance in a tropical xerophilous shrubland at a lava substratum

Diurnal variation in leaf stomatal conductance (g _s) of three xerophilous species (Buddleia cordata, Senecio praecox and Dodonaea viscosa) was measured over a 10-month period during the dry and wet seasons in a shrubland that is developing in a lava substratum in Mexico. Averaged stomatal conductances were 147 and 60.2 (B. cordata), 145 and 24.8 (D. viscosa) and 142.8 and 14.1 mmol m^–2 s^–1 (S. praecox) during the wet and dry season respectively. Leaf water potential () varied in a range of –0.6 to –1.2 (S. praecox), –0.6 to –1.8 (B. cordata) and –0.9 to –3.4 MPa (D. viscosa) during the same measurement periods. Stomata were more sensitive to changes in irradiance, air temperature and leaf–air vapour pressure difference in the rainy season than the dry season. Although stomatal responses to were difficult to distinguish in any season (dry or rainy), data for the entire period of measurement showed a positive correlation, stomata tending to open as increased, but there is strong evidence of isohydric behaviour in S. praecox and B. cordata. A multiplicative model relating g _s to environmental variables and to accounted for 79%–83% of the variation of g _s in three sites (pooled data); however, the performance of the model was poorer (60%–76%) for individual species from other sites not included in the pooled data.     

SPOTTED DOLPHIN EVASIVE RESPONSE IN RELATION TO FISHING EFFORT

Spotted dolphins in the eastern Pacific Ocean associate with yellowfin tuna. During the chase and encirclement phases of purse-seining for tunas, dolphin attempt to evade encirclement with the purse-seine net. We used data on evasive behavior (1982–2001) and numbers of purse-seine sets (1959–2001) to study the relationship between evasion and fishing effort. Results show that in nearshore areas first exploited by the fishery in the early 1960s, dolphins exhibited high evasion, but with a limited correlation between evasion and cumulative effort. In areas farther offshore next exploited in the mid-to late-1960s, dolphins showed high evasion and a significant correlation between evasion and cumulative effort. Dolphins in far-western and southern areas, first exploited in the late 1960s to early 1970s, exhibited low evasion, with little relationship to cumulative effort. We hypothesize that this spatial pattern is the result of two types of pressure from fishing: early effort in nearshore areas with a high risk of mortality that generated a lasting evasive response, followed by a longer period of even greater effort but with lower risk of mortality that generated evasion by longer-term learning.