Herbivorous insect species in the tree canopy of a relict South African forest

Abstract.

• 1 The herbivorous insects on twelve species of evergreen broadleafed trees were repeatedly sampled over a period of 11 months in a small relict forest on the east coast of South Africa. This extraordinarily speciose forest patch has an unusually high proportion of endemic tree species, some of which are extremely rare.
• 2 The insect herbivore fauna (number of species) seems to be markedly depauperate compared to that reported on native, broadleafed trees from other parts of the world. Some possible reasons for this are discussed.
• 3 The total number of herbivorous insect species on each tree species was strongly correlated with the local relative abundance of the host plant species.
• 4 There was no relationship between the total number of insect herbivore species on each tree species and the relative taxonomic isolation of the trees. The proportion of seemingly unique (= specialist) herbivorous insect species (i.e. those that occurred on one tree species only) was greatest on taxonomically isolated trees.
• 5 A fundamental deficiency in the interpretation of the data in this study, and of many other similar studies that report on the number of insect species on plants, is discussed, namely the lack of clarity on the closeness of the association between individual insect herbivore species and their respective host plants.

     

Seasonal shifts in host usage in Uroleucon gravicorne (Homoptera: Aphididae) and implications for the evolution of host alternation in aphids

ABSTRACT.

• 1 Uroleucon gravicome (Patch) feeds on annual Erigeron species and on perennial Solidago species.
• 2 To test whether Solidago and Erigeron are analogous to winter and summer hosts of typical host-alternating aphids, reproductive performance, host preferences, and host associations were measured for E.strigosus, S.juncea and S.nemoralis during spring, summer and autumn.
• 3 Caged individuals can reproduce on both genera throughout the season, though colonies are rare on Solidago during summer.
• 4 Developmental rate, adult weight, and fecundity decline between May and August on all hosts; however, the drop is least on E.strigosus, intermediate on S.nemoralis, and greatest on S.juncea.
• 5 Throughout the season, reproductive performance is at least as great on Erigeron as on Solidago, the difference being least in spring.
• 6 Soluble nitrogen content shows the same seasonal trends as reproductive performance, declining in all hosts, but declining least in E.strigosus.
• 7 Preference tests and field records show that U.gravicorne moves from Solidago to Erigeron when the first alatae mature in late spring, feeds on Erigeron during the summer, and, in late summer, returns to Solidago where overwintering eggs are laid.
• 8 Solidago is not nutritionally superior during spring or autumn; its use during those seasons may result from the need for good oviposition sites.

     

Heterothallism in Saccharomyces cerevisiae isolates from nature: effect of HO locus on the mode of reproduction

Understanding the evolution of sex and recombination, key factors in the evolution of life, is a major challenge in biology. Studies of reproduction strategies of natural populations are important to complement the theoretical and experimental models. Fungi with both sexual and asexual life cycles are an interesting system for understanding the evolution of sex. In a study of natural populations of yeast Saccharomyces cerevisiae , we found that the isolates are heterothallic, meaning their mating type is stable, while the general belief is that natural S. cerevisiae strains are homothallic (can undergo mating-type switching). Mating-type switching is a gene-conversion process initiated by a site-specific endonuclease HO; this process can be followed by mother–daughter mating. Heterothallic yeast can mate with unrelated haploids (amphimixis), or undergo mating between spores from the same tetrad (intratetrad mating, or automixis), but cannot undergo mother–daughter mating as homothallic yeasts can. Sequence analysis of HO gene in a panel of natural S. cerevisiae isolates revealed multiple mutations. Good correspondence was found in the comparison of population structure characterized using 19 microsatellite markers spread over eight chromosomes and the HO sequence. Experiments that tested whether the mating-type switching pathway upstream and downstream of HO is functional, together with the detected HO mutations, strongly suggest that loss of function of HO is the cause of heterothallism. Furthermore, our results support the hypothesis that clonal reproduction and intratetrad mating may predominate in natural yeast populations, while mother–daughter mating might not be as significant as was considered.     

Survival of Breeding Pacific Common Eiders on the Yukon-Kuskokwim Delta,Alaska

ABSTRACT Populations of Pacific common eiders (Somateria mollissima v-nigrum) breeding in Alaska, USA, have declined markedly over the past 40 years. We studied survival of adult female Pacific common eiders using capture—recapture of nesting hens at 3 sites on the Yukon-Kuskokwim Delta (YKD), Alaska from 1994 to 2004. We used data consisting of 268 recapture events from 361 uniquely marked individuals to investigate temporal, geographic, and environmental variation in adult female survival. Our results suggest apparent annual survival of adult eiders from the YKD was high (0.892, SE = 0.022) and spatially and temporally invariant (σ² = 0.005), a pattern consistent with other long-lived marine birds. Moreover, our results suggest adult survival may be functionally fixed for Pacific common eiders, and at the present, adult survival may be relatively unresponsive to environmental or management perturbations. Our data did not support hypothesized variation in survival relative to mortality factors such as predation on breeding grounds, physiologic costs of reproduction, and wintering conditions. Although changes in adult survival likely have a large potential effect on prospective population growth, our results suggest viable management actions aimed at increasing survival may be extremely limited.     

Temperature gradient gel electrophoresis: detection of a single base substitution in the cattle β-lactoglobulin gene

An A in equilibrium with G transition in exon III is known to differentiate alleles A and B of the cattle beta-lactoglobulin (BLG) gene. A BLG exon III fragment containing the transition site was amplified by the polymerase chain reaction. Temperature gradient gel electrophoresis (TGGE) was then used to detect this transition and hence to genotype cattle: the AT base-pair in allele A was readily distinguished from the GC base-pair of allele B. TGGE can be used to detect any single base-pair substitution, and thus is a powerful method of detecting genetic variability.