Current versus historical population sizes in vertebrate species with high gene flow: a comparison based on mitochondrial DNA lineages and inbreeding theory for neutral mutations

Using inbreeding theory as applied to neutral alleles inherited maternally, we generate expected probability distributions of times to identity by descent for random pairs of mitochondrial genotypes within a population or within an entire species characterized by high gene flow. For comparisons with these expectations, empirical distributions of times to most recent common ancestry were calculated (by conventional mtDNA clock calibrations) from mtDNA haplotype distances observed within each of three vertebrate species--American eels, hardhead catfish, and redwinged blackbirds. These species were chosen for analysis because census population size in each is currently large and because both genetic and life-history data are consistent with the postulate that historical gene flow within these species has been high. The observed molecular distances among mtDNA lineages were two to three orders of magnitude lower than predicted from census sizes of breeding females, suggesting that rate of mtDNA evolution is decelerated in these species and/or that long-term effective population size is vastly smaller than present-day population size. Several considerations point to the latter possibility as most likely. The genetic structure of any species is greatly influenced by historical demography; even for species that are currently abundant, mtDNA gene lineages appear to have been channeled through fairly small numbers of ancestors.      

Secondary succession in coastal dune fynbos: variation due to site and disturbance

Succession after fire and bushcutting in coastal dune fynbos was monitored for two and a half years and comparisons were made with adjacent, mature (13 year-old) fynbos. Sixty-two to 68% of pre-disturbance species, including all the dominants, were found in the successional communities 1.5 yr after disturbance: the patterns thus fitted the initial floristic composition model. On the more mesic south facing slope, post-fire succession differed from the north-facing (burnt) and bushcut sites in that ordinations showed a clear separation between the mature and successional communities. This difference was due to the post-fire abundance on the southfacing slope site of short-and medium-lived species not present in the mature fynbos. One and a half years after disturbance, species richness and equitability had increased relative to mature vegetation. This increase was greatest for the south facing slope where short-and medium-lived species and juveniles of pre-disturbance dominants co-occurred. In general, successional patterns were consistent with those described for other fynbos and fire-prone mediterranean shrublands.     

Canopy seed storage in woody plants

The retention of seeds in the plant canopy for one to 30 years or more is termed serotiny. It is well represented floristically and physiognomically in fire-prone, nutrient-poor and seasonally-dry sclerophyll vegetation in Australia, and to a lesser extent, South Africa followed by North America. While the seed-storing structures vary greatly, all will release their propagules following exposure to the heat of a fire (pyriscence). This phenomenon can be contrasted with seed release at maturity (non-storage) and soil storage of seeds. Although the evolutionary requirements for serotiny are clear, its adaptive advantages over other seed storage syndromes are largely the subject of conjecture in the absence of comparative experiments. Nine hypotheses were assessed here. Canopy storage maximises the quantity of seeds available for the next post-fire generation (unlike non-storage). Synchronized post-fire release satiates post-dispersal granivores (unlike non-storage and soil storage) and ensures arrival on a seed bed conducive to seedling recruitment (unlike non-storage). Canopy stored seeds are better insulated from the heat of a fire than non-stored, and probably soil-stored, seeds. Fluctuating annual seed crops, the opportunity for post-fire wind-dispersal, the possible advantages of dense stands of adults, short lifespan of the dispersed seeds and their optimal location in the soil for germination have only a limited role in explaining the advantages of serotiny. It is concluded that canopy seed storage is favoured in regions where seed production is restricted and inter-fire establishment and maturation are unlikely. In addition, these regions have a reliable seasonal rainfall and are subjected to intense fires at intervals occurring within the reproductive lifespan of the species.     

Diversity,composition and guild structure relationships between soil-stored seed banks and mature vegetation in alien plant-invaded South African fynbos shrublands

We investigated vegetation-seed bank relationships at three fynbos sites on the Cape Peninsula, South Africa, and the impacts to these sites of invasion by the alien tree Acacia saligna. Soil-stored seed banks in uninvaded fynbos were of a similar density to those previously measured in fynbos (ca. 1100–1500 seeds m^-2) and were dominated by mostly short-lived species. Lack of similarity between mature vegetation and seed banks, suggests that seed banks are poor predictors of mature vegetation composition and structure in fynbos. This lack of correspondence was attributed to the ephemerals (present only in the soil seed bank) and the dominance of serotinous (aerial seed bank) and sprouting (soil seed bank low to absent) species, in mature vegetation. Long-lived seeders were among the 10 most abundant species in the seed banks at all sites and at two sites shrub species contributed more to seed bank richness than any other growth form. Soil-stored seed banks, therefore, boost species richness and diversity both in early post-fire and later seral stages.There was a decline in fynbos species richness, diversity and abundance both in the standing vegetation and seed banks with increasing duration of invasion by the alien tree, Acacia saligna. However, the rate of decline was higher for the vegetation than the seed banks, suggesting that many fynbos species have long-term persistent seed banks. At two sites, there was no obvious shift in community composition associated with Acacia invasion: invaded sites were depauperate versions of the uninvaded site. However, at a third site, the vegetation composition shifted towards a community dominated by bird-dispersed thicket species and its seed bank shifted towards a community dominated by wind-dispersed perennials. Community composition of the soil seed banks under dense, recent Acacia was very similar to that of the corresponding uninvaded fynbos at all sites, indicating that there is good potential to return to species-rich fynbos vegetation after removal of the alien Acacia. Most seed bank species persisted in the soil seed bank of the long-invaded fynbos at low frequency and density, indicating high seed longevity in many species. We suggest that either a thick Acacia litter layer or a deep (>5 cm) burial moderated the fire and ambient temperature effects, preventing these seeds from germinating after fire and thus preventing loss from the seed bank.     

Soil nutrient dynamics and community boundaries in the Fynbos vegetation of South Africa

The relationship between changes in soil nutrient characteristics and fynbos community boundaries was investigated near Cape Agulhas, South Africa. Soil characteristics relating to total nutrient content (pH, total N and total P, organic carbon, and various cations) were assessed at sites along three transects crossing the boundaries between five plant communities. Dynamics of available N and P in soils of three communities were studied in the field over one year, using ion-exchange resins. There was a wide range in the degree of change in soil nutrient content across different community boundaries. The characteristics that varied most were pH, total N, Ca and total P. Differences in available nutrients among soils indicated that the communities in this landscape were associated with a mosaic of N and P availability. It is proposed that spatial variation in soil nutrient availability rather than total soil nutrient contents may be important in explaining landscape-level species distributions and community composition in nutrient-poor mediterranean-climate ecosystems.     

Simple cultural test for relative cellulolytic activity of fungi

A simple method is described for determining the relative cellulolytic activity of fungi. Opaque columns of an agar medium containing a partially crystalline cellulose preparation were inoculated with the fungi. Depth of the clear zone that developed beneath the growing cultures provided a visual measure of cellulolytic activity on a continuous, cumulative basis. Depth of clearing (DC) was determined for 25 species of fungi differing widely in cellulolytic activity, and compared by correlation analysis with results of three other methods for measuring cellulolytic activity. Relatively high coefficients of correlation (greater than 0.6) were obtained between DC and weight loss of cotton sliver, loss in tensile strength of cotton duck, and carboxymethyl cellulase activity in culture filtrates. In comparison with conventional assay procedures, the clearing method offered several advantages: (i) results were at least as well correlated with the capacity to utilize native cellulose as a substrate; (ii) the method measured activity of growing cultures rather than culture filtrates, thus involving less risk of losses due to product inhibition, binding, or denaturation of enzymes; (iii) repeated measurements were made on the same experimental set up, so that errors due to arbitrarily selected times of harvest were avoided conveniently; and (iv) the method required less working time and very simple equipment, making it convenient for large-scale screening tests.     

A comparison of fynbos and non-fynbos coenoclines in the lower Gamtoos River Valley,southeastern Cape,South Africa

Patterns in the relative importance of structural attributes and growth forms along fynbos and non-fynbos coenoclines were studied to test the hypothesis that there would be less structural variation in the former because the overriding influence of low levels of soil nutrients would be manifest in a great deal of structural convergence in fynbos. The coenoclines were ranged along identical environmental gradients of increasing altitude, rainfall and soil moisture and decreasing climatic variability. Results showed that along the entire fynbos coenocline vegetation was structurally a small-leaved sclerophyllous shrubland with a graminoid understorey and, usually, a large-leaved (proteoid) shrub overstorey. Fynbos structure was interpreted largely as a response to low levels of soil nutrients. Non-fynbos vegetation ranged from mixed succulent-sclerophyllous and spiny large-leaved thicket at lower altitudes to tall mesic forest at the upper end of the gradient. Non-fynbos structure was explained in terms of variations in soil moisture and climate. An analysis of the biogeographical affinities of sample floras at sites along the coenoclines showed that fynbos vegetation was dominated by taxa endemic to the Cape phytochorion, although phytochorological mixing was pronounced at the lower altitude sites. The level of local endemism in the fynbos coenocline was relatively high; nearly all endemics were Cape fynbos taxa and their incidence increased with increasing altitude. These data indicate that fynbos vegetation has had a lengthy history in the southeastern Cape and that high altitude sites would have comprised a refuge for Cape taxa during unfavourable climatic periods. Non-fynbos vegetation ranged from dry subtropical Tongaland-Pondoland thicket with a strong Karoo-Namib component to temperate Afromontane forest. Levels of endemism were lower than the fynbos coenocline and decreased with increasing altitude. The high number of karroid endemics found in both coenoclines at low altitudes suggests that karroid vegetation would have been more widespread in the past, probably during the last glacial which was considerably drier than the present Holocene interglacial.     

The nucleotide sequence of the initiator tRNA from Drosophila melanogaster.

The nucleotide sequence of Drosophila melanogaster methionine tRNAi was determined to be: pA-G-C-A-G-A-G-U-m1G-m2G-C-G-C-A-G-U-G-G-A-A-G-C-G-U-m2G-C-U-G-G-G-C-C-C-A-U-t6A-A-C-C-C-A-G-A-G-m7G-D-m5C-C-C-G-A-G-G-A-U-C-G-m1A-A-A-C-C-U-U-G-C-U-C-U-G-C-U-A-C-C-A(OH). It differs from vertebrate initiator tRNAs in only 6 out of 75 positions.