George K. Roderick

Towards writing the encyclopaedia of life: an introduction to DNA barcoding

An international consortium of major natural history museums, herbaria and other organizations has launched an ambitious project, the ‘Barcode of Life Initiative’, to promote a process enabling the rapid and inexpensive identification of the estimated 10 million species on Earth. DNA barcoding is a diagnostic technique in which short DNA sequence(s) can be used for species identification. The first international scientific conference on Barcoding of Life was held at the Natural History Museum in London in February 2005, and here we review the scientific challenges discussed during this conference and in previous publications. Although still controversial, the scientific benefits of DNA barcoding include: (i) enabling species identification, including any life stage or fragment, (ii) facilitating species discoveries based on cluster analyses of gene sequences (e.g. cox1=CO1, in animals), (iii) promoting development of handheld DNA sequencing technology that can be applied in the field for biodiversity inventories and (iv) providing insight into the diversity of life.

Long-distance dispersal: a framework for hypothesis testing

Tests of hypotheses about the biogeographical consequences of long-distance dispersal have long eluded biologists, largely because of the rarity and presumed unpredictability of such events. Here, we examine data for terrestrial (including littoral) organisms in the Pacific to show that knowledge of dispersal by wind, birds and oceanic drift or rafting, coupled with information about the natural environment and biology of the organisms, can be used to generate broad biogeographic predictions. We then examine the predictions in the context of the origin, frequency of arrival and location of establishment of dispersed organisms, as well as subsequent patterns of endemism and diversification on remote islands. The predicted patterns are being increasingly supported by phylogenetic data for both terrestrial and littoral organisms.

Genes in new environments: Genetics and evolution in biological control

The availability of new genetic technologies has positioned the field of biological control as a test bed for theories in evolutionary biology and for understanding practical aspects of the release of genetically manipulated material. Purposeful introductions of pathogens, parasites, predators and herbivores, when considered as replicated semi-natural field experiments, show the unpredictable nature of biological colonization. The characteristics of organisms and their environments that determine this variation in the establishment and success of biological control can now be explored using genetic tools. Lessons from studies of classical biological control can help inform researchers and policy makers about the risks that are associated with the release of genetically modified organisms, particularly with respect to long-term evolutionary changes.

DENSITY-RELATED MIGRATION IN PLANTHOPPERS (HOMOPTERA: DELPHACIDAE): THE ROLE OF HABITAT PERSISTENCE

We investigated the selective pressures associated with the possession of wings and migration in wing-dimorphic planthoppers (Homoptera: Delphacidae). Both migratory (macropterous) and flightless (brachypterous) forms occur. We found that levels of migration (% macroptery) in field populations decreased significantly as the persistence of their habitats increased. By using phylogenetically independent contrasts between congeners, we obtained the same result, which suggests that habitat persistence has influenced levels of migration independent of common ancestry. Also, there was a clear difference in the density-wing form response between species in temporary and persistent habitats. Macropterous forms were triggered at lower densities for species inhabiting temporary rather than persistent habitats. However, males of species in temporary habitats were macropterous at both low and high rearing densities; macroptery was positively density-dependent in the females of these species and in both sexes of species in persistent habitats. Our results suggest that habitat persistence influences the migratory capability of planthoppers in two ways: (1) by selecting for habitat escape, and (2) by dictating the availability of mates. In persistent habitats wings are less necessary for habitat escape and they are rarely required for mate location. As a consequence and because wings impose a reproductive penalty, flightlessness prevails. For species in temporary habitats, wings are favored in males to locate females at low colonizing densities and are favored in both sexes at high densities for reasons of habitat escape. We conclude that habitat persistence has played a major role in shaping patterns of migration in this group of sap-feeding herbivores.

Population structure and colonization history of the olive fly, Bactrocera oleae (Diptera, Tephritidae).

The olive fly, Bactrocera oleae, is the major pest of olives in most commercial olive‐growing regions worldwide. The species is abundant in the Mediterranean basin and has been introduced recently into California and Mexico, creating problems for quarantine protection and international trade. Here, we use nuclear microsatellite markers and mitochondrial sequences to examine the history of olive fly range expansion and colonization. Sampled populations span the current distribution of the olive fly worldwide, including South and Central Africa, Pakistan, Mediterranean Europe and Middle East, California, and Mexico. The Pakistani populations appear to be genetically well differentiated from the remaining populations, though rooting the origins of the species is problematic. Genetic similarity and assignment tests cluster the remaining populations into two genetic groups — Africa and a group including the Mediterranean basin and the American region. That Africa, and not the Mediterranean, is the origin of flies infesting cultivated olive is supported by the significantly greater genetic diversity at microsatellite loci in Africa relative to the Mediterranean area. The results also indicate that the recent invasion of olive flies in the American region most likely originated from the Mediterranean area.

Biodiversity dynamics in isolated island communities: interaction between natural and human-mediated processes

The flora and fauna of oceanic islands have inspired research since the early scientific explorations. Islands can be considered ‘natures test tubes’— simple systems with multiple replicates. Our research has used the simplicity of island systems to understand ecological community dynamics and to compare the properties of island communities with those in more complex mainland systems. Here, we present three topics: (i) current patterns of biodiversity on isolated islands of the Pacific; (ii) current patterns of disturbance and invasion on islands; and (iii) future trajectories inferred from these patterns. We examine features of islands (in particular, topography and isolation) that have allowed for given levels and distribution of endemicity. The extent to which island communities are impacted by, resist or accommodate disturbance and/or invasions by nonindigenous species appears to be dictated to a large extent by properties of the native communities and how these communities were originally assembled. Accordingly, patterns of disturbance and invasion are very different for high (montane) islands that are extremely isolated compared to those that are nearer to a source of natural migrants. As with all biotas, those on islands are dynamic entities. However, the unique aspect of islands is their isolation, and extreme isolation has largely been lost over the course of the last few centuries due to the development of transportation routes. We argue that such a modified dynamic will affect the future of the biota and the processes that gave rise to the biota. Specifically for isolated habitats, ecological processes will become increasingly more likely to generate biodiversity than evolutionary processes which have been relatively more important in the past. In the short term, island biotas and other similar biotas that occur in montane habitats may fare well as species are often abundant locally in the habitat to which they are indigenous, and may demonstrate considerable resistance and resilience to invasion. However, island biotas — and other biotas that show high local endemism — will likely not fare well in the face of prolonged disturbance. The biotas in these areas generally display a relatively low dispersal capacity; therefore, under conditions of long‐term habitat modification, isolated biotas are likely to be swamped by non‐natives, which — simply because of random processes and higher propagule pressure — will move more readily into available habitats. Thus, despite the importance of incorporating the evolutionary process into conservation efforts, we must also be careful to evaluate the likely form that the processes will take when the context (specifically, extent of isolation) has been highly modified.

The invasive coconut mite Aceria guerreronis (Acari: Eriophyidae): origin and invasion sources inferred from mitochondrial (16S) and nuclear (ITS) sequences.

Over the past 30 years the coconut mite Aceria guerreronis Keifer has emerged as one of the most important pests of coconut and has recently spread to most coconut production areas worldwide. The mite has not been recorded in the Indo-Pacific region, the area of origin of coconut, suggesting that it has infested coconut only recently. To investigate the geographical origin, ancestral host associations, and colonization history of the mite, DNA sequence data from two mitochondrial and one nuclear region were obtained from samples of 29 populations from the Americas, Africa and the Indo-ocean region. Mitochondrial DNA 16S ribosomal sequences were most diverse in Brazil, which contained six of a total of seven haplotypes. A single haplotype was shared by non-American mites. Patterns of nuclear ribosomal internal transcribed spacer (ITS) variation were similar, again with the highest nucleotide diversity found in Brazil. These results suggest an American origin of the mite and lend evidence to a previous hypothesis that the original host of the mite is a non-coconut palm. In contrast to the diversity in the Americas, all samples from Africa and Asia were identical or very similar, consistent with the hypothesis that the mite invaded these regions recently from a common source. Although the invasion routes of this mite are still only partially reconstructed, the study rules out coconut as the ancestral host of A. guerreronis, thus prompting a reassessment of efforts using quarantine and biological control to check the spread of the pest.

Male-killing bacteria trigger a cycle of increasing male fatigue and female promiscuity

Sex-ratio distorters are found in numerous species and can reach high frequencies within populations. Here, we address the compelling, but poorly tested, hypothesis that the sex ratio bias caused by such elements profoundly alters their hosts mating system. We compare aspects of female and male reproductive biology between island populations of the butterfly Hypolimnas bolina that show varying degrees of female bias, because of a male-killing Wolbachia infection. Contrary to expectation, female bias leads to an increase in female mating frequency, up to a point where male mating capacity becomes limiting. We show that increased female mating frequency can be explained as a facultative response to the depleted male mating resources in female biased populations. In other words, this system is one where male-killing bacteria trigger a vicious circle of increasing male fatigue and female promiscuity.

Influence of volcanic activity on the population genetic structure of Hawaiian Tetragnatha spiders: fragmentation, rapid population growth and the potential for accelerated evolution

Volcanic activity on the island of Hawaii results in a cyclical pattern of habitat destruction and fragmentation by lava, followed by habitat regeneration on newly formed substrates. While this pattern has been hypothesized to promote the diversification of Hawaiian lineages, there have been few attempts to link geological processes to measurable changes in population structure. We investigated the genetic structure of three species of Hawaiian spiders in forests fragmented by a 150‐year‐old lava flow on Mauna Loa Volcano, island of Hawaii: Tetragnatha quasimodo (forest and lava flow generalist), T. anuenue and T. brevignatha (forest specialists). To estimate fragmentation effects on population subdivision in each species, we examined variation in mitochondrial and nuclear genomes (DNA sequences and allozymes, respectively). Population subdivision was higher for forest specialists than for the generalist in fragments separated by lava. Patterns of mtDNA sequence evolution also revealed that forest specialists have undergone rapid expansion, while the generalist has experienced more gradual population growth. Results confirm that patterns of neutral genetic variation reflect patterns of volcanic activity in some Tetragnatha species. Our study further suggests that population subdivision and expansion can occur across small spatial and temporal scales, which may facilitate the rapid spread of new character states, leading to speciation as hypothesized by H. L. Carson 30 years ago.

Domestication of olive fly through a multi-regional host shift to cultivated olives: Comparative dating using complete mitochondrial genomes

The evolutionary history of the olive fly, Bactrocera oleae, was reconstructed in a phylogenetic and coalescent framework using full mitochondrial genome data from 21 individuals covering the entire worldwide distribution of the species. Special attention was given to reconstructing the timing of the processes under study. The early subdivision of the olive fly reflects the Quaternary differentiation between Olea europea subsp. europea in the Mediterranean area and the two lineages of Olea europea subsp. cuspidata in Africa and Asia, pointing to an early and close association between the olive fly and its host. The geographic structure and timing of olive fly differentiation in the Mediterranean indicates a clear connection with the post-glacial recolonization of wild olives in the area, and is irreconcilable with the early historical process of domestication and spread of the cultivated olive from its Levantine origin. Therefore, we suggest an early co-history of the olive fly with its wild host during the Quaternary and post-glacial periods and a multi-regional shift of olive flies to cultivated olives as these cultivars gradually replaced wild olives in historical times.