Genetic effects of chronic habitat fragmentation on tree species: the case of Sorbus aucuparia in a deforested Scottish landscape

Sustainable forest restoration and management practices require a thorough understanding of the influence that habitat fragmentation has on the processes shaping genetic variation and its distribution in tree populations. We quantified genetic variation at isozyme markers and chloroplast DNA (cpDNA), analysed by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) in severely fragmented populations of Sorbus aucuparia (Rosaceae) in a single catchment (Moffat) in southern Scotland. Remnants maintain surprisingly high levels of gene diversity (H-E) for isozymes (H-E = 0.195) and cpDNA markers (H-E = 0.490). Estimates are very similar to those from non-fragmented populations in continental Europe, even though the latter were sampled over a much larger spatial scale. Overall, no genetic bottleneck or departures from random mating were detected in the Moffat fragments. However, genetic differentiation among remnants was detected for both types of marker (isozymes Theta(n) = 0.043, cpDNA Theta(c) = 0.131; G-test, P-value < 0.001). In this self-incompatible, insect-pollinated, bird-dispersed tree species, the estimated ratio of pollen flow to seed flow between fragments is close to 1 (r = 1.36). Reduced pollen-mediated gene flow is a likely consequence of habitat fragmentation, but effective seed dispersal by birds is probably helping to maintain high levels of genetic diversity within remnants and reduce genetic differentiation between them.

Genetic assignment methods for the direct, real-time estimation of migration rate: a simulation-based exploration of accuracy and power

Genetic assignment methods use genotype likelihoods to draw inference about where individuals were or were not born, potentially allowing direct, real-time estimates of dispersal. We used simulated data sets to test the power and accuracy of Monte Carlo resampling methods in generating statistical thresholds for identifying F-0 immigrants in populations with ongoing gene flow, and hence for providing direct, real-time estimates of migration rates. The identification of accurate critical values required that resampling methods preserved the linkage disequilibrium deriving from recent generations of immigrants and reflected the sampling variance present in the data set being analysed. A novel Monte Carlo resampling method taking into account these aspects was proposed and its efficiency was evaluated. Power and error were relatively insensitive to the frequency assumed for missing alleles. Power to identify F-0 immigrants was improved by using large sample size (up to about 50 individuals) and by sampling all populations from which migrants may have originated. A combination of plotting genotype likelihoods and calculating mean genotype likelihood ratios (D-LR) appeared to be an effective way to predict whether F-0 immigrants could be identified for a particular pair of populations using a given set of markers.

Isolation and characterization of microsatellite loci from Helicoverpa armigera Hübner (Lepidoptera: Noctuidae)

Five microsatellite loci are presented for Helicoverpa armigera. These microsatellite loci were obtained through the construction of enriched libraries, overcoming previously reported difficulties with obtaining microsatellites from H. armigera and other Lepidoptera due to the low frequency of microsatellites in their genomes. The description of a further five microsatellite loci for H. armigera makes microsatellite based population genetics studies feasible.

Association of cone thermogenesis and volatiles with pollinator specificity in Macrozamia cycads

Cone traits (volatile components and thermogenesis) of three cycad species in the genus Macrozamia were examined for differences related to their specific insect pollinators, the weevil, Tranes spp., or the thrips, Cycadothrips chadwicki. Linalool (>80% of emissions) dominated cone volatile components of M. machinii (Tranes-pollinated) and beta-myrcene was a minor component (

Monarchs across the Pacific: the Columbus hypothesis revisited

The 'Columbus hypothesis' suggests that the annual north-south return migration of Danaus plexippus in North America is a very recently evolved behaviour, less than 200 years old. This hypothesis rests, in part, on an analysis of the 19th century spread of the monarch across the Pacific that assumes a continuous east to west movement and is based predominantly on one publication. We review all the contemporary literature and present new analysis of the data. The movement of the monarch across the Pacific in the second half of the 19th century is best explained by a model which involves no more than three spot introductions, directly or indirectly aided by human movement, followed by natural spread of the monarch across island groups. Contemporary records refer to 'boom' and 'bust' population cycles on newly settled islands, which may have led to high rates of monarch movement. We see no evidence in the records to suggest an east to west sweep by monarch populations as suggested by the Columbus hypothesis. (C) 2004 The Linnean Society of London.

Does terrestrial epidemiology apply to marine systems?

Most of epidemiological theory has been developed for terrestrial systems, but the significance of disease in the ocean is now being recognized. However, the extent to which terrestrial epidemiology can be directly transferred to marine systems is uncertain. Many broad types of disease-causing organism occur both on land and in the sea, and it is clear that some emergent disease problems in marine environments are caused by pathogens moving from terrestrial to marine systems. However, marine systems are qualitatively different from terrestrial environments, and these differences affect the application of modelling and management approaches that have been developed for terrestrial systems. Phyla and body plans are more diverse in marine environments and marine organisms have different life histories and probably different disease transmission modes than many of their terrestrial counterparts. Marine populations are typically more open than terrestrial ones, with the potential for long-distance dispersal of larvae. Potentially, this might enable unusually rapid propagation of epidemics in marine systems, and there are several examples of this. Taken together, these differences will require the development of new approaches to modelling and control of infectious disease in the ocean.

Isolation and characterization of microsatellite loci from Acacia nilotica ssp. indica (Mimosaceae)

Five microsatellite loci are presented for prickly acacia, Acacia nilotica ssp. indica (Benth.) Brenan, an introduced weed of national significance in Australia. These microsatellite loci were obtained through the construction of an enriched library and their use will enable us to determine the genetic origin and extent of genetic diversity of this weed in Australia.

Isolation and characterization of microsatellite loci from Chiasmia assimilis (Warren, 1899) (Lepidoptera: Geometridae)

Twelve microsatellite loci are presented for the biological control agent Chiasmia assimilis (Warren, 1899). These microsatellite loci were obtained through the construction of an enriched library, overcoming previous reported difficulties with obtaining microsatellites from other Lepidoptera due to the low frequency of microsatellites in their genomes.

Variation in body size and sexual dimorphism across geographical and environmental space in the frogs Limnodynastes tasmaniensis and L. peronii

This study aimed to identify potential factors responsible for geographically structured morphological variation within the widespread Australian frogs Limnodynastes tasmaniensis Gunther and L. peronii Dumeril & Bibron. There was support for James's rule, and both latitude and present climate explained large amounts of the variation in body size and shape (particularly in L. peronii). There was also some support for the influence of several biogeographical barriers. Finally, both species were sexually dimorphic for body size and the degree of sexual size dimorphism (SSD) varied geographically. Climate was an important explanation for SSD variation in L. peronii, while latitude was most important for L. tasmaniensis. Geographical variations in sexual selection via male-male physical competition and climate-related resources are suggested as potential explanations for SSD variation in L. peronii. (C) 2004 The Linnean Society of London.

Limited effect of anthropogenic habitat fragmentation on molecular diversity in a rain forest skink, Gnypetoscincus queenslandiae

To examine the effects of recent habitat fragmentation, we assayed genetic diversity in a rain forest endemic lizard, the prickly forest skink (Gnypetoscincus queenslandiae), from seven forest fragments and five sites in continuous forest on the Atherton tableland of northeastern Queensland, Australia. The rain forest in this region was fragmented by logging and clearing for dairy farms in the early 1900s and most forest fragments studied have been isolated for 50-80 years or nine to 12 skink generations. We genotyped 411 individuals at nine microsatellite DNA loci and found fewer alleles per locus in prickly forest skinks from small rain forest fragments and a lower ratio of allele number to allele size range in forest fragments than in continuous forest, indicative of a decrease in effective population size. In contrast, and as expected for populations with small neighbourhood sizes, neither heterozygosity nor variance in allele size differed between fragments and sites in continuous forests. Considering measures of among population differentiation, there was no increase in F-ST among fragments and a significant isolation by distance pattern was identified across all 12 sites. However, the relationship between genetic (F-ST) and geographical distance was significantly stronger for continuous forest sites than for fragments, consistent with disruption of gene flow among the latter. The observed changes in genetic diversity within and among populations are small, but in the direction predicted by the theory of genetic erosion in recently fragmented populations. The results also illustrate the inherent difficulty in detecting genetic consequences of recent habitat fragmentation, even in genetically variable species, and especially when effective population size and dispersal rates are low.

Genetic diversity and population structure of Tasmanian devils, the largest marsupial carnivore

Genetic diversity and population structure were investigated across the core range of Tasmanian devils (Sarcophilus laniarius; Dasyuridae), a wide-ranging marsupial carnivore restricted to the island of Tasmania. Heterozygosity (0.386-0.467) and allelic diversity (2.7-3.3) were low in all subpopulations and allelic size ranges were small and almost continuous, consistent with a founder effect. Island effects and repeated periods of low population density may also have contributed to the low variation. Within continuous habitat, gene flow appears extensive up to 50 km (high assignment rates to source or close neighbour populations; nonsignificant values of pairwise F-ST), in agreement with movement data. At larger scales (150-250 km), gene flow is reduced (significant pairwise F-ST) but there is no evidence for isolation by distance. The most substantial genetic structuring was observed for comparisons spanning unsuitable habitat, implying limited dispersal of devils between the well-connected, eastern populations and a smaller northwestern population. The genetic distinctiveness of the northwestern population was reflected in all analyses: unique alleles; multivariate analyses of gene frequency (multidimensional scaling, minimum spanning tree, nearest neighbour); high self-assignment (95%); two distinct populations for Tasmania were detected in isolation by distance and in Bayesian model-based clustering analyses. Marsupial carnivores appear to have stronger population subdivisions than their placental counterparts.

Single copy nuclear DNA markers characterized for comparative phylogeography in Australian wet tropics rainforest skinks

In order to investigate population history and demography in skinks endemic to the wet tropics of Australia, multiple nuclear DNA markers were sought. The utility of 72 primers (including 63 published intron-spanning 'CATS' primers) was tested.. Seven loci were characterized and shown to be single copy by single-strand conformation polymorphism analysis. Primers to five nuclear loci were developed, four with utility in skinks and three with utility in frogs. These observations extend the available information on intron-spanning primers for amphibians and reptiles.

Fine-scale spatial genetic correlation analyses reveal strong female philopatry within a brush-tailed rock-wallaby colony in southeast Queensland

We combine spatial data on home ranges of individuals and microsatellite markers to examine patterns of fine-scale spatial genetic structure and dispersal within a brush-tailed rock-wallaby (Petrogale penicillata) colony at Hurdle Creek Valley, Queensland. Brush-tailed rock-wallabies were once abundant and widespread throughout the rocky terrain of southeastern Australia; however, populations are nearly extinct in the south of their range and in decline elsewhere. We use pairwise relatedness measures and a recent multilocus spatial autocorrelation analysis to test the hypotheses that in this species, within-colony dispersal is male-biased and that female philopatry results in spatial clusters of related females within the colony. We provide clear evidence for strong female philopatry and male-biased dispersal within this rock-wallaby colony. There was a strong, significant negative correlation between pairwise relatedness and geographical distance of individual females along only 800 m of cliff line. Spatial genetic autocorrelation analyses showed significant positive correlation for females in close proximity to each other and revealed a genetic neighbourhood size of only 600 m for females. Our study is the first to report on the fine-scale spatial genetic structure within a rock-wallaby colony and we provide the first robust evidence for strong female philopatry and spatial clustering of related females within this taxon. We discuss the ecological and conservation implications of our findings for rock-wallabies, as well as the importance of fine-scale spatial genetic patterns in studies of dispersal behaviour.

Divergent environments and population bottlenecks fail to generate premating isolation in Drosophila pseudoobscura

While the feasibility of bottleneck-induced speciation is in doubt, population bottlenecks may still affect the speciation process by interacting with divergent selection. To explore this possibility, I conducted a laboratory speciation experiment using Drosophila pseudoobscura involving 78 replicate populations assigned in a two-way factorial design to both bottleneck (present vs. absent) and environment (ancestral vs. novel) treatments. Populations independently evolved under these treatments and were then tested for assortative mating and male mating success against their common ancestor. Bottlenecks alone did not generate any premating isolation, despite an experimental design that was conducive to bottleneck-induced speciation. Premating isolation also did not evolve in the novel environment treatment, neither in the presence nor absence of bottlenecks. However, male mating success was significantly reduced in the novel environment treatment, both as a plastic response to this environment and as a result of environment-dependent inbreeding effects in the bottlenecked populations. Reduced mating success of derived males will hamper speciation by enhancing the mating success of immigrant, ancestral males. Novel environments are generally thought to promote ecological speciation by generating divergent natural selection. In the current experiment, however, the novel environment did not cause the evolution of any premating isolation and it reduced the likelihood of speciation through its effects on male mating success.

A mitochondrial phylogeny of the rainforest skink genus Saproscincus, Wells and Wellington (1984)

The phylogenetic relationships and historical biogeography of 10 currently described rainforest skinks in the genus Saproscincus were investigated using mitochondrial protein-coding ND4 and ribosomal RNA 16S genes. A robust phylogeny is inferred using both maximum likelihood and Bayesian analysis, with all inter-specific nodes strongly supported when datasets are combined. The phylogeny supports the recognition of two major lineages (northern and southern), each of which comprises two divergent clades. Both northern and southern lineages have comparably divergent representatives in mid-east Queensland (MEQ), providing further molecular evidence for the importance of two major biogeographic breaks, the St. Lawrence gap and Burdekin gap separating MEQ from southern and northern counterparts respectively. Vicariance associated with the fragmentation and contraction of temperate rainforest during the mid-late Miocene epoch underpins the deep divergence between morphologically conservative lineages in at least three instances. In contrast, one species, Saproseincus oriarus, shows very low sequence divergence but distinct morphological and ecological differentiation from its allopatric sister clade within Saproseincus mustelinus. These results suggest that while vicariance has played a prominent role in diversification and historical biogeography of Saproscincus, divergent selection may also be important. (C) 2004 Elsevier Inc. All rights reserved.